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Keratoconus: Further Reading - Appendix 2


 

Keratoconus

 

* Puberty-Onset Keratoconus
* Late-Onset Keratoconus
* Keratoconus Fruste

* Nipple-Shaped Topography
* Oval-Shaped Topography
* Globus-Shaped Topography

* Apical Thinning
* Fleischer's Ring
* Ruptures in Bowman’s Layer
* Vertical Striae
* Increased Visibility of the Corneal Nerve Fibers
* Ruptures in Descemet’s Membrane
* Endothelial Cup Reflex

* Spherical Lens Designs
* Superior Alignment Fitting Technique for Early Keratoconus
* The Intra-Palpebral 3 Point Touch Fitting Technique for Early Keratoconus
* Aspheric Lens Designs for Early Keratoconus

* Fitting Advanced Keratoconus
* Fitting the Nipple Cone
* Fitting the Oval Cone
* Fitting the Globus Cone

* Corneal Sensitivity in Early Keratoconus
* Piggyback Soft Lenses
* The Traditional Piggyback Lens System
* Custom Piggyback Lens System
* Soft Toric Lens Designs
* Custom Spherical and Aspherical Soft Lenses Designs
* Patient Selection for Soft Lenses Designs
* Fitting Custom Spherical and Aspherical Soft Lenses Designs
* Hybrid Lens Designs

* Penetrating Keratoplasty
* Lamellar Graft
*
Intrastromal Rings/Intacs-Ferrara Rings
* Laser Surgery for Keratoconus

* Pellucid Marginal Degeneration
* Management of PMD

 

Introduction

Keratoconus, also called conical cornea, results in the cornea changing from dome-shaped to cone-shaped through the progressive thinning of the cornea. Because the cornea is the part of the eye that refracts most of the light entering, the change of its shape prevents incoming light from being focused properly, ultimately distorting vision. In addition, vision may be impaired as a result of swelling and scarring of tissue. Keratoconus is a slow-developing disorder that almost always affects both eyes, though each eye may be affected differently.

 

Corneal ectasia in keratoconus

Figure1: Corneal ectasia in keratoconus

Keratoconus is a condition of obscure etiology that is characterized by the thinning and steepening of the central and/or para-central cornea. The condition usually occurs in the second or third decade of life resulting in a moderate to marked decrease in visual acuity secondary to irregular astigmatism and corneal scarring. (Figure 1.)

Keratoconus most often occurs bilaterally, however there is often asymmetry with one eye affected more than the other and generally the first eye to develop the condition has a more marked progression. (Figure 2.)

 

Bilateral keratoconus with one eye (right eye) affected more than the other.

Figure 2. Bilateral keratoconus with one eye (right eye) affected more than the other.

 

This differential rate of progression may be an important consideration when counseling patients on the progression of the condition. Cases of unilateral keratoconus are rare but can occasionally be seen in clinical practice.

The early use of the cross-linking treatment must be advised to the patient as an option for them to halt any further progression once Keratoconus has been diagnosed and progression has been detected. So that they have a chance to avoid advanced Keratoconus and all the problems that it can cause with vision correction.

The clinical management of keratoconus varies depending on the severity of the condition and can range from non-surgical options such as glasses and contact lenses to surgical interventions including Intacs, partial grafts, and penetrating keratoplasty once Keratoconus progress’s to an advanced state. This is why an early cross-linking treatment is advised as early as possible.
 

 

Corneal Collagen Cross-linking Treatment

Corneal collagen cross-linking induced by treatment of impregnating Riboflavin (B2) in to the cornea and exposure to ultraviolet-A light is a safe and effective, minimally invasive procedure both to reduce disease progression, increase the strength of the cornea and to improve upon the cornea's optical properties in eyes with early keratoconus.
 

Keratoconus
 
Picture 1. CXL
 
 

As long as keratoconus is diagnosed early enough - a stabilization of clinical findings and visual acuity can be achieved without a need for a corneal transplant or further progression.

It is the first treatment for progressive keratoconus. It treats the cause not the symptoms, therefore the number of penetrating keratoplasties can be significantly reduced in the future. The treatment is relatively easy and low in costs and is available through out the world. Avoiding advanced keratoconus because of the problems it causes (as seen below) is what the aim is with this treatment.

 

 

Keratoconus History

Early reference to keratoconus was made by Mauchart in 1748 and by Taylor in 1766, but the condition was first adequately described and distinguished from other corneal ectasias by Nottingham in 1854. At that time, the treatment of keratoconus consisted of cauterizing the conical area with silver nitrate and the instillation of miotics accompanied by a pressure dressing.

In the early months of 1888, a French ophthalmologist, Eugene Kalt, began work on a crude glass shell designed to “compress the steep conical apex thereby correcting the condition.” This was the first known application of a contact lens for the correction of keratoconus. (Figure 3.)
 

 

Keratoconus: Eugene Kalt, MD, first to propose the use of a contact lens for keratoconus.

Figure 3. Eugene Kalt, MD, first to propose the use of a contact lens for keratoconus.

 

Early reference to keratoconus was made by Mauchart in 1748 and by Taylor in 1766, but the condition was first adequately described and distinguished from other corneal ectasias by Nottingham in 1854. At that time, the treatment of keratoconus consisted of cauterizing the conical area with silver nitrate and the instillation of miotics accompanied by a pressure dressing.
In the early months of 1888, a French ophthalmologist, Eugene Kalt, began work on a crude glass shell designed to “compress the steep conical apex thereby correcting the condition.” This was the first known application of a contact lens for the correction of keratoconus. (Figure 3.)

 


Prevalence of Keratoconus in the Population

Keratoconus occurs with approximately equal gender distribution in every region and every ethnicity throughout the world. Many studies have been conducted to estimate the incidence and prevalence of the condition, and, although the incidence varies somewhat from to country to country, a 1986 population-based study in the US indicated that approximately 5 in 10,000 people have keratoconus. This Prevalence figure will increase because of the use of modern diagnostic equipment which is becoming more readily available and cheaper these days.

An early onset and increased severity of keratoconus was found in recent studies in ethnic groups. This may be related to a combination of genetic and/or environmental factors. However it also indicates that in non-western countries the prevalence of Keratoconus could be much (4.4 times) higher than what its thought to be.

If all the prevalence of corneal thinning disorders closely related to Keratoconus was all added together then this figure would be high from the outset. Keratoconus is routinely mis-diagnosed (it's confused as "normal" Astigmatism regularly) and under diagnosed. Here are some of the categories where figures are lost.

Corneal Ectasias (keratoglobus, pellucid marginal corneal degeneration, Terrien's marginal degeneration, Posterior Ectasia, forme fruste keratoconus)

Corneal Dystrophies (epithelial basement membrane dystrophy and stromal dystrophies such as granular, lattice, or macular dystrophy)

Corneal Warpage secondary to rigid contact lens wear or irregular astigmatism.

Many people in the general population wear glasses without knowing it is due to them having keratoconus or even the optician who prescribed them the glasses knowing this.  Mild forms of Keratoconus are common, and it is only in the advanced stages of the disorder a referral to an eye hospital is made for diagnoses. Only after that is their Keratoconus recorded in the statistics for the prevalence of Keratoconus. Therefore, only advanced stages of Keratoconus are noted for statistical purposes generally. This is when its too late and the Keratoconus has become problematic. The improved early detection of Keratoconus is vital, because with the advent of the cross-linking treatment milder forms of Keratoconus can be treated before they get advanced and problematic. Milder keratoconus patients are the ones who can be helped the most, to stay out of the medical loop, and therefore an earlier intervention with the Crosslinking treatment is warranted as it's been proved to be a preventative measure.

 

Primary Corneal Ectasia

This a new collective term being used to describe Keratoconus and all the other forms of corneal thinning disorders of the cornea, where Keratoconus is the most common out of all of them. The thinning characteristic’s of all the corneal thinning disorders is the common feature between them, and it’s the thinning aspect which needs to be addressed by using this term to describe it.

 

Secondary Corneal Ectasia (Also called Keratectasia or Iatrogenic Keratectasia)

This is when excessive thinning of the eye wall can cause the shape and focusing power of the eye to become unstable after Laser eye treatment as a uncommon complication rising out from it. Careful screening to detect pre-existing corneal abnormalities (Keratoconus), conservative limits on treatment (not removing too much corneal tissue), and pre-operative measurement of corneal thickness all combine to limit this risk. It should be noted that there is a reluctance to report Secondary Corneal Ectasia by the laser eye centers. This form of Ectasia can become a late complication, occurring many years after Laser eye surgery.

Every thing which has been discussed above about the prevalence of Keratoconus shows that the prevalence of Keratoconus is much more common than previously thought, and with the increasing use of modern diagnostic equipment it is even higher. The first thing a eye care specialist looks at is the front surface of the eye of a patient, which needs to be checked with the latest modern diagnostic testing equipment these days. Many of the Primary Corneal Ectasia can be picked up sooner and treated earlier,  which would not have been picked-up other-wise.

In most circumstances most practices don't even have access to a corneal topography machine, which should be a standard feature of any modern practice. The front surface of the eye is very important, because the cornea is responsible for most of the eye's total focusing power; and also it's where most medical intervention are done to the eye to treat and correct the visual errors that a patients has. Therefore it is imperative, that a practice modernizes to have the latest corneal diagnostic test equipment to-hand.

 

Etiology of Keratoconus

There has always been speculation as to the cause of keratoconus, but during the past 10 years our scientific knowledge of the condition has steadily increased. Although we now have a much better understanding of the cellular and molecular changes that occur in this condition, keratoconus remains a condition of unknown etiology.

Recent research conducted by Steven Wilson, MD, at the University of Washington suggests that keratoconus somehow accelerates the process of keratocyte apoptosis, which is the programmed death of corneal cells that occurs following injury. Minor external traumas, such as eye rubbing, poorly fitted contact lenses, and ocular allergies can release cytokines from the epithelium that stimulate keratocyte apoptosis (the earliest observable stromal response to an epithelial injury).

Although keratocyte apoptosis is virtually never detectable in the absence of epithelial injury in normal patients, a high percentage of keratoconus patients show evidence of such cell death. This typically takes place first in the anterior stroma and is manifested by breaks in Bowman’s layer (Figure 4) and later as stromal thinning (Figure 5). Wilson has also suggested that genetics may play a role in the etiology of keratoconus, in that some patients may have a genetic predisposition to chronic keratocyte apoptosis.

 

Keratoconus: Breaks in Bowman’s layer.

                                      Figure 4. Breaks in Bowman’s layer.

 

Keratoconus: Central versus mid-peripheral thickness changes in advanced keratoconus.

Figure 5. Central versus mid-peripheral thickness changes in advanced keratoconus.

 

Cristina Kenney, MD, PhD, of Cedars-Sinai Medical Center in Los Angeles, has suggested that keratoconus corneas may have increased enzyme activities and decreased levels of enzyme inhibitors. This combination results in the production of toxic by-products that bring about a cascade of events throughout the cornea, resulting in corneal thinning and scarring.

 


Genetics in Keratoconus

One of the strongest arguments for a genetic component in keratoconus etiology is that the condition can run in families. Although most patients diagnosed with keratoconus report no positive family history, the likelihood that keratoconus will be found in one or more member of the immediate family is 3.4%, which is 15 to 70 times higher than the general population rate. In addition, keratoconus has been reported in identical twins and in two or more generations of many families.

The advent of computerized corneal mapping techniques has made earlier detection of sub-clinical, and/or slowly progressive forms of the condition more precise. Therefore, it is suspected that familial prevalence rates will most likely increase. Using corneal topographical findings, in 1990 Rabinowitz, et al, showed that 50% of randomly selected family members of keratoconus patients displayed subtle topographical abnormalities somewhat suspect of keratoconus.

Studies to identify the association of keratoconus with various chromosomes have been performed. Reports on a small number of families with keratoconus show a connection between keratoconus and defects on chromosomes 21, 17, and 13. However, at this time it is unknown exactly what specific parts of these chromosomes are defective or how the defects might cause keratoconus.

Future investigations will most likely conclude that more than one gene is associated with the diverse clinical presentations seen in keratoconus. This is based on the fact that there is considerable variation among individuals with keratoconus and so multiple genes are most likely involved. For example, keratoconus manifests in many forms and degrees of severity:

  • It can be unilateral or bilateral,

  • It can affect the central or mid-peripheral cornea,

  • It can be mild or severe,

  • It can start in childhood or later in life, and

  • It can occur in more than one family member or in one individual only.

 

The Environment and Keratoconus

Although it is generally believed that keratoconus has a genetic component, there are increasing data that suggest the environment might also play a role in the development of the condition.

Cristina Kenney’s research team has discovered that corneas with keratoconus have been exposed to a number of factors that can produce reactive oxygen species (i.e., free radicals). These include ultraviolet light, atopy, mechanical eye rubbing, and poorly fitted contact lenses. They propose that susceptible corneas exhibit an inability to process reactive oxygen species because they lack the necessary protective enzymes (e.g., ALDH3 and superoxide dismutase). The reactive oxygen species result in an accumulation of toxic by-products such as MDA and peroxynitrites that can damage corneal proteins and trigger a cascade of events that disrupt the cornea’s cellular structure and function. This can result in corneal thinning, scarring, and apoptosis. (Figure 6.)

 

Keratoconus: Reactive oxygen species

 Figure 6. Reactive oxygen species within the keratoconus cornea result in an accumulation of toxic by-products that can trigger cornea thinning, scarring, and apoptosis.
 

If this theory is valid, it might be prudent for keratoconus patients to minimize the factors that can cause reactive oxygen species to be formed. It might be helpful for patients to do the following:

  • Reduce exposure to ultraviolet (UV) light by wearing UV protecting sunglasses outdoors.

  • Avoid excessive mechanical trauma such as vigorous eye rubbing and poorly fitting contact lenses.

  • Manage atopy/allergies through environmental and/or pharmacologic intervention.

  • Use preservative-free artificial tears and lens care products that are efficacious compatible with the patient’s ocular surface.


Although the environmental damage theory is interesting, at this time there is no conclusive evidence to support the fact that changes in diet, environment, or emotional state can influence the eventual course of the disease. This is supported by the fact that keratoconus occurs throughout the world in a wide range of geographic, social, and dietary conditions.


 

Associated Conditions

Keratoconus has been associated with a number of systemic conditions including Down’s Syndrome (trisomy of chromosome 21). A number of authors have reported that the incidence of keratoconus in Down’s Syndrome is between 5.5% and 15%, which is considerably higher that the incidence of approximately 5 per 10,000 (0.05%) cited for the general population. Occasionally, keratoconus is also seen in individuals with connective tissue disorders such as Osteogenesis Imperfecta, Ehlers-Danlos Syndrome, and joint hypermobility.

 

Hallmarks of Keratoconus

Although the etiology of keratoconus remains somewhat obscure, its clinical manifestations have been well documented. Some on the major hallmarks of keratoconus include:

  • A decline in visual acuity (usually greater in one eye than the other).

  • A distorted retinoscopy reflex in which there is rapid movement of the light in the periphery and slow movement in the center of the pupil. The reflex appears to spin or swirl around a point corresponding to the apex of the cone.

  • Distortion of or an inability to superimpose the bottom right keratometry mire.

  • Frequent changes in spectacle cylinder power and axis.

  • Increased myopia.

  • Squeezing of the eyelids to create a pinhole effect.

  • The appearance of halos or starbursts around light during nighttime viewing.

  • Associated atopic disease.

 

 


Types of Keratoconus

In clinical practice, three distinct forms of keratoconus have been identified, each with a unique clinical presentation. Differentiating between the three forms can be helpful in counseling patients about what to expect regarding eventual progression of the disease. However, the clinical classification of keratoconus should be viewed as only a general guide. It is important to communicate to the patient that the condition can be extremely unpredictable and that its ultimate course can only be determined with time.

 

Puberty-Onset Keratoconus

Puberty-onset keratoconus is by far the most common form of the condition, and, as its name indicates, it begins in early adolescence at about age 14 to16 years. The condition is usually bilateral with one eye affected more than the other. Following its onset, there is often a dramatic progression of the condition for the next 8 to 10 years. This is typically followed by a period in which the condition seems to stabilize. Clinical experience has shown that the earlier in life the keratoconus occurs, the more severe the condition will be. Therefore, a 12 year old, exhibiting the optical and topographical manifestations of early keratoconus is more likely to have a more severe form of the condition than an individual that shows similar findings in their late 20’s.

 

Late-Onset Keratoconus

In late-onset keratoconus, the earliest signs and symptoms begin in the late 20’s or early 30’s. Both eyes are frequently affected to a similar degree, with little or no asymmetry. This is often a more benign form of the condition, and, unlike puberty onset keratoconus, its progression is significantly less severe, rarely requiring surgical intervention in the form of a corneal transplant.

 

Keratoconus Fruste

The third form of keratoconus is called “form fruste” and was first described by Amsler in 1937. It is essentially an extremely mild form of keratoconus that can occur at anytime throughout life. The condition manifests as a central or para-central zone of irregular astigmatism of unknown etiology. The most striking hallmark of form fruste keratoconus is its lack of progression with the condition staying stable throughout the patient's life.

 

 

Use of Corneal Topography for Assessment of Keratoconus

 
In clinical practice, we use the term keratoconus to describe an entire spectrum of diversely shaped conditions in which the only common denominator may be central or paracentral corneal steepening. Yet, it is the topography beyond the central and paracentral cornea that can significantly influence the optics of the cornea and ultimately the success or failure of a contact lens design. Therefore, to better understand modern keratoconus contact lens fitting techniques, it is imperative to appreciate the diverse central and mid-peripheral topography changes associated with the condition. These are best assessed by use of a corneal topographer in which the circular light patterns (often referred to as Placido rings or mires) are projected onto the cornea and their reflections analyzed.Dekking in 1920, and later Amsler in 1932, provided the first descriptions of topographical changes that occur in keratoconus by using early photokeratoscopy techniques. The system introduced by Amsler allowed him to classify keratoconus into four stages:

 

  • Stage 1, oblique astigmatism with inequality of the keratoscopy mires

  • Stage 2, intensification of the stage 1 signs

  • Stage 3, pronounced conical shape with corneal thinning, but no opacities

  • Stage 4, opacities present at the corneal apex

 

More recently, Reynolds, Bronstein, and Rowsey have described the geometry of a keratoconic eye based on photographs obtained using the Corneascope instrument. Tomlinson, Schwartz, Townsley, and Wesley have also contributed to our understanding of the corneal topography in keratoconus through the use of a Photo Electric Keratometer. Modern advances in computer-based analysis of keratoscopy images have produced an even greater understanding of the topographic patterns manifest in early as well as late stage keratoconus. Modern photokeratoscopy and videokeratography (corneal mapping) techniques have demonstrated that in early keratoconus there is characteristically a pear-shaped pull of the central keratoscopy rings, with steepening initially occurring inferiorly, usually in the temporal quadrant. (Figure 7.) The steeper the corneal curvature, the higher the plus dioptric power of the cornea and the more closely spaced the topographic ring reflections will be. Also, rings that are not circular represent areas of optical distortion on the surface of the cornea.)

 

Keratoconus:

 Figure 7. Photokeratoscopy view of early keratoconus. Note the pear-shaped pulling of the central keratoscopy mires. The close proximity of the rings (in the inferior-nasal portion of the eye) indicates corneal steepening and greater distance between the rings (in the superior-nasal portion of the eye) indicates flattening.

 

 
In the early stage of keratoconus as illustrated in Figure 7, the superior cornea (above the corneal midline) remains relatively unaffected and therefore “normal” in curvature, similar to that of a non-keratoconic eye. This combination of topographic findings—pear-shaped infero-temporal steepening and normal curvature of the superior half of the cornea, should be considered extremely suspect of early keratoconus. Additionally, the flatter, more “normal” superior corneal topography becomes the most important consideration in fitting rigid contact lenses for keratoconus, for it is at the flatter superior portion of the cornea that the rigid lens will be tightest. As previously noted, the earliest topographical change in keratoconus is a paracentral steepening most commonly located in the inferior-temporal quadrant. As the condition progresses, the steepening spreads nasally to include the inferior (6 o’clock) and inferior-nasal corneal areas. In advanced forms of the keratoconus, rotational steepening occurs at and above the midline along a path that eventually includes the temporal, superior-temporal, and superior (12 o’clock) cornea. The superior-nasal quadrant of the cornea is always the last to be affected, and thus an “island” of normal corneal topography often remains in this area even in the more advanced stages of the condition. (Figure 8.)

 

Keratoconus

Figure 8. The typical spiral pattern of keratoconus progression. The condition commonly begins in the inferior-temporal quadrant, with the last area of the cornea to be topographically affected in the superior-nasal quadrant. In color-coded topographic images, red represents steeper corneal curvature, and the spectrum of yellow, green, and blue represents progressively flatter curvatures.

 

Additionally, even in the advanced forms of keratoconus the superior 3 to 4 mm of the cornea often retains a relatively normal curvature. Although there are exceptions to this pattern, the vast majority of keratoconic eyes have been noted to follow this spiral progression. Many topographical changes occur well beyond the area evaluated by the conventional keratometer. Therefore, photo-keratoscopy and videokeratoscopy have become essential in documenting the complex topographic changes that occur in keratoconus. Furthermore, as will be discussed later, modern contact lens management of keratoconus depends almost entirely upon adequate documentation and understanding of topographic abnormalities that occur beyond the central apex of the cornea.

 

 

Corneal Topography in Advanced Keratoconus

Modern topographic techniques have demonstrated that in early keratoconus there is a characteristic steepening initially occurring mid-peripherally below the corneal midline. This is demonstrated by close proximity of the keratoscopy rings to one another. Above the midline, the superior cornea remains relatively normal in curvature. As the condition progresses, individual corneas can take on a wide range of topographical shapes that have been classified as “nipple,” “oval,” and “globus.” (Figure 9.)

 

Keratoconus

Figure 9. The three topographical shapes of advanced keratoconus: nipple, oval, and globus.

 

 

Nipple-Shaped Topography

The nipple form of keratoconus characteristically consists of a small, near central ectasia, less than 5.0 mm in cord diameter. (Figure 10.)

 

Keratoconus

Figure 10. The nipple-shaped form of keratoconus demonstrates a small central ectasis surrounded by 360 degrees of “normal” cornea.

 

The most striking features of the nipple topography are:
 

  • The often-high degree of with-the-rule corneal toricity, confined to the central 5.0mm of the cornea. (Figure 11.)

  • The nearly 360 degrees of “normal” mid-peripheral cornea that surrounds the base of the cone.

  • The occasional presence of an elevated fibroplastic nodule at the apex of the cornea, hence the name nipple keratoconus. (Figure 12.) The superficial nodules are frequently eroded by the presence of a rigid contact lens, often necessitating: rigid gas permeable/soft contact lens (RGP/SCL) piggyback designs, custom keratoconus SCL designs, or surgical removal of the nodule by manual superficial keratectomy or photo-therapeutic keratectomy techniques. (Figures 13 and 14.)

 


Keratoconus

Figure 11. Nipple-shaped keratoconus may also manifest as a small central ectasia with moderate to high with-the-rule corneal astigmatism

 

Keratoconus

Figure 12. An elevated fibroplastic nodule seen in a patient with moderate nipple shaped keratoconus. The superficial nodules frequently erode beneath the contact lens, resulting in an epithelial defect.

 

Keratoconus

Figure 13. Epithelial erosion and “spiral staining” of a fibroplastic nodule. Piggyback lenses are frequently used to manage this condition.

 

Keratoconus

Figure 14. Superficial keratectomy of a para-central nodule in keratoconus. Note the change in the central photokeratoscopy image pre- and post-keratectomy.

 

The overall topography of nipple-form keratoconus creates numerous fitting obstacles. The extremely rapid change in curvature from the steeper center to the more normal mid-peripheral cornea makes the nipple-form of keratoconus the most difficult to manage with rigid contact lenses alone. Although difficult to fit with contact lenses, the nipple-shaped topography may be ideally suited for keratoplasty surgery. The small-diameter central location of the cone permits complete removal of the diseased portion of the cornea within the area of trephination. The 360° of relatively normal recipient cornea creates an optimum bed for wound closure and helps to minimize postoperative corneal astigmatism.

 

Oval-Shaped Topography

The most common corneal shape noted in advanced keratoconus is oval topography. In oval-form keratoconus, the corneal apex is displaced well below the midline resulting in varying degrees of inferior mid-peripheral steepening. The result of this “pushing out” of the cornea inferiorly is an island of normal or flatter-than-normal superior cornea, located exactly 180 degrees away. (Figures 15 and 16.)

 

Keratoconus

Figure 15. Oval-shaped keratoconus is hallmarked by steepening of the inferior cornea.

 

Keratoconus


Figure 16. Photokeratoscopy of an oval shaped cone shows the inferior-nasal steepening as noted by the close proximity of the keratoscopy lines and the superior-nasal flattening 180 degrees away, resulting in an “island” of normal cornea above the corneal midline.

 

Globus-Shaped Topography

The globus-form of keratoconus affects the largest area of the cornea, often encompassing nearly three quarters of the corneal surface. Due to its size, nearly all of the keratoscopy rings will be encompassed within the area of the ectasia. (Figure 17.) Unlike the advanced forms of nipple or oval keratoconus, the globus cone has no island of “normal” mid-peripheral cornea above or below the midline.

 

Keratoconus


Figure 17. Due to the size of the globus-shaped keratoconus, all nine rings of the photokeratoscopy image are encompassed by the conical area and no “islands” of normal mid-peripheral cornea are seen.

 

The various corneal shapes (nipple, oval and globus) are most likely the result of simple apex location as well as yet unidentified stromal factors controlling mid-peripheral corneal shape. There is little doubt that the various central and mid-peripheral topographies will ultimately influence the final contact lens parameters. Therefore, it is imperative that we appreciate the diverse symmetrical and asymmetrical presentations seen in keratoconus. This is best accomplished through quantitative corneal surface analyses by photokeratoscopy and/or video keratography mapping techniques. Through this analysis the steepest and flattest portions of the cornea can be identified and this will help to clarify many of the peculiar fluorescein patterns noted during our diagnostic lens evaluation.Alternately, the three keratoconus topographies (nipple, oval, and globus) can be visualized through a technique called “photodiagnosis.” Using photodiagnosis, the red fundus reflex is viewed through a direct ophthalmoscope at a distance of approximately two feet (60 cm) and/or photographed using a fundus camera. (Figure 18.) The technique allows for direct visualization of the size, shape and location of the conical area for classification of nipple, oval, or globus keratoconus. (Figure 19.)

 

Keratoconus

Figure18. The three forms of keratoconus (nipple, oval, and globus) can be visualized directly by observing the red fundus reflex through a dilated pupil with a direct ophthalmoscope.

 

Keratoconus

Figure 19. The photodiagnosis technique demonstrates the size, shape, and location of the conical area.

 

Keratoconus Detection Software

Today, many of the modern corneal topography systems incorporate specific keratoconus software to aid in the detection and subsequent clinical diagnosis of keratoconus. One of the more popular versions of this software is the Humphrey Atlas Pathfinder Corneal Analysis System (http://www.meditec.zeiss.com). The software performs an analysis of three indices of the individual patient’s corneal topography:
 

  • Corneal irregularity measurement (CIM),

  • Shape factor (SF), and

  • Mean toric corneal measurement (TKM). 

 
The patient’s topographical analysis is then compared to known normal and abnormal (keratoconic) eyes, and the software classifies the patient’s topography as "normal," "corneal distortion," or "keratoconus suspect." The Pathfinder software displays the control (normal) indices on three color bars: 

  • The red areas of the bars indicate the abnormal range in the control population.

  • The yellow indicates borderline measurements.

  • The green indicates measurements within normal limits.

The black arrow indicates the individual patient’s topographic values. (Figure 20.)

 

Keratoconus

Figure 20. Output from the Humphrey Pathfinder keratoconus detection software. The black arrows indicate the patient’s individual topographic values as compared to a large cohort of normal and abnormal eyes.

 

The Corneal Irregularity Measurement (CIM) is a number or index assigned to represent the irregularity of the corneal surface. The higher the number, the more uneven or irregular the corneal surface. A high CIM value would tend to indicate a more advanced form of keratoconus with greater amounts of irregular astigmatism present in the central cornea. CIM uses thousands of data points within the first 10 rings of the corneal topography image to determine the difference in “height” or elevation between the patient’s cornea and a perfect toric model cornea. The difference between the perfect model and the actual cornea is measured in microns with "normal," "borderline," and "abnormal" values as shown below. 

  • Normal CIM: 0.3 to 0.60 microns

  • Borderline CIM: 0.61 to 1.0 microns

  • Abnormal CIM: 1.1 to 5.0 microns

The Shape Factor (SF) represents the degree of corneal asphericity or eccentricity. Shape factor can be used to determine whether the cornea is more oval or elliptical by assigning a factor or index to represent the shape of the surface. The higher the shape factor, the more keratoconus-like the cornea will be. The ranges for "normal," "borderline," and "abnormal" shape factors are shown below.

  • Normal Shape Factor: 0.13 to 0.35

  • Borderline Shape Factor: 0.02 to 0.12 and 0.36 to 0.46

  • Abnormal Shape Factor: 0.47 to 1.0

The Toric Corneal Measurement (TKM) is a value derived using elevation data from a best-fit toric reference surface as compared to the actual cornea. Two values are calculated at the apex of the flattest meridian and their mean determined. This is described as the mean value of apical curvature. By fitting the topography of the patient’s cornea to that of a best fit toric (using elevation data), all of the correctable sphere and cylinder can be accounted for in the topographical data. The ranges for "normal," "borderline," and "abnormal" TKM values are shown below.

  • Normal TKM: 43.12 to 45.87D

  • Borderline TKM: 41.12 to 43.00 D. and 46.00 to 47.25 D.

  • Abnormal TKM: 36.00 to 41.75 D. and 47.37 to 60.00

Figures 21 and 22 illustrate a case of unilateral keratoconus. Note the differences in Pathfinder analysis of the keratoconic right eye (Figure 21) versus the non-keratoconic left eye (Figure 22).

 

Keratoconus

Figure 21. The Pathfinder analysis of a patient with unilateral keratoconus. The right eye values (black arrows) are all in the red, indicating abnormal topographic findings characteristic of keratoconus.

 

Keratoconus

Figure 22. The left eye of the same patient shows normal with-the-rule astigmatism with all three topographic indices in the normal yellow and green.

 

The diagnosis of keratoconus can be extremely difficult and one in which a number of anatomical and optical findings must be considered together. It is, therefore, important to remember that despite the sophistication and accuracy of modern keratoconus detection modules, they can serve only as another tool to aid in the diagnosis. This is because a number of conditions can topographically mimic keratoconus. These include corneal trauma, post herpes simplex infection, and contact lens wear. Therefore, many clinicians agree that the diagnosis of keratoconus should not be made solely based on the topographic findings, but instead should be based on combined clinical findings that include positive topographic data as well as optical and slit lamp findings.

  

Tissue Changes in Keratoconus

In 1931, Professor Alfred Vogt at the University of Zurich described in detail many of the classic biomicroscopic findings in keratoconus. Von der Heydt and Appelbaum classified the corneal changes into seven distinct types of tissue alterations. These changes may appear at varying times throughout the progression of keratoconus but may not be present in all cases.

 

Apical Thinning

When an eye with advanced keratoconus is viewed in optic section, the apex of the cornea can be reduced in thickness to one third that of the periphery. (Figure 23.) (Due to the excessive corneal curvature, it may be difficult to keep the entire section in exact focus at one time.)

 

Keratoconus

Figure 23. Optic section of a keratoconic eye showing significant apical thinning.

In the later stages of keratoconus, this can lead to Munson’s sign in which an angular curve is present at the lower lid margin when the patient looks down. (Figure 24.) In the early stages of keratconus, apical thinning is often difficult to detect with the slit lamp. Other positive slit lamp findings often precede apical thinning and therefore may be more helpful in early diagnosis.

 

Keratoconus

Figure 24. Munson’s sign in advanced keratoconus.

  

Fleischer's Ring

Fleischer's ring is a yellow-brown or olive-green pigmented line that partially or completely encircles the base of the cone. (Figure 25.) It is the result of deposition and collection of iron (haemosiderin) anterior to Bowman's layer in the adjacent epithelium. The broken or interrupted ring occurs in approximately 50 percent of keratoconus cases and the ring is often best viewed using blue slit lamp illumination. (Figure 26.)

 

Keratoconus

Figure 25. Yellow-brown or olive-green pigmented iron line (Fleischer's ring) in moderate keratoconus.

 

Keratoconus

Figure 26. A Fleischer's ring is often best visualized under the blue light illumination of a slit lamp.

 

Ruptures in Bowman’s Layer

These opacities form at or near the apex of the cone and represent structural breaks in Bowman's layer resulting in irregular superficial opacities and scars. (Figure 27.) The opacities begin as grayish dots located at the level of Bowman's layer. Later the spaces between the opacities become opaque and an irregular superficial opacity forms. (Figure 28.) These changes are caused by ruptures in Bowman’s, which are followed by filling in of the spaces with fibrillar connective tissue. In advanced cases, this can account for a considerable loss of visual acuity secondary to the induced irregular astigmatism and loss of corneal clarity. (Figure 29.)

 

Keratoconus

Figure 27. In this case of unilateral keratoconus, corneal opacities seen on the right eye (left image) represent structural breaks in Bowman’s layer. Note the normal thickness and optical clarity of the left eye (right image).

 

Keratoconus

Figure 28. Corneal histology shows breaks in Bowman’s layer and subsequent accumulation of fibrillar connective tissue within the spaces.

 

Keratoconus

Figure 29. In the advanced stage of keratoconus, ruptures in Bowman’s can account for significant visual loss due to corneal opacification and induced irregular astigmatism.

 

Vertical Striae

Vertical striae are a series of parallel whitish lines seen in the deep stroma. They are most likely tension lines caused by apical stretching of the corneal lamellae. The striae are most often orientated vertically, but they can sometimes be aligned in the meridian of the greatest corneal curvature. (Figure 30.)

 

Keratoconus

Figure 30. Vertical striae represent tension lines caused by apical stretching.

 

Most often the striae are seen in the region of the corneal apex before it becomes densely scarred. Crossing systems of striae may produce a lattice-like design. (Figure 31.) As a rule, the lines do not cross at the same level. Vertical or Vogt's striae can often be the earliest slit lamp finding noted in keratoconus. Some clinicians believe that the diagnosis of keratoconus cannot be made without the presence of vertical striae. 

 

Keratoconus


Figure 31. The presence of vertical striae is often the first positive slit lamp finding noted in keratoconus.

 

Increased Visibility of the Corneal Nerve Fibers

The corneal nerve fibers may become more visible in certain cases of keratoconus. The nerves can be seen as a network of grayish lines with corpuscle-like nodes at the point of branching. (Figure 32.)
 
 
Keratoconus


Figure 32. The corneal nerve fibers in keratoconus may be more easily visualized.

 

It is not likely that the nerve fibers are more numerous in keratoconic patients, but only that they are more easily seen due to changes in density. Because a similar clinical picture is often seen in both normal corneas and in keratitis, an increased visibility of the corneal nerve fibers cannot be considered a singular diagnostic sign of keratoconus. (Figure 33.)

 

Keratoconus

Figure 33. The increase visualization of the nerves is made possible by an increase in fibril density of the superficial nerve. In some cases the fibers can create localized elevations in the epithelium that will result in a negative staining pattern with fluorescein.

 

Ruptures in Descemet’s Membrane

Spontaneous ruptures of Descemet's membrane occur in approximately 5 percent of patients with keratoconus. The ruptures are characterized by a crescent-shaped tear in Descemet's and the endothelium near the apex of the cone. (Figure 34.)

Keratoconus

Figure 34. Ruptures or tears in Descemet's permit aqueous to pass into the stroma resulting in significant corneal edema and opacification.

 

Aqueous from the anterior chamber passes through the tear resulting in corneal edema and opacification (hydrops). (Figure 35.)

 

Keratoconus

Figure 35. Corneal histology of a rupture in Descemet's.

 

The extent of the opacification varies with rupture size. In most cases, the endothelium recovers and within days begins a slow but steady deturgescence of the corneal opacification. This process can take 3 to 4 weeks. (Figure 36.)

 

Keratoconus

Figure 36. In most cases the endothelium recovers and tear closes. The cornea then begins the slow 6 to 10 week process of deturgescence.

 

Following resolution of the hydrops, the rolled edges of the tear in Descemet's can be noted. Corneas that do not regain transparency may require keratoplasty surgery. (Figure 37.)

 

Keratoconus

Figure 37. In some cases the corneal fails to clear and surgical intervention, in the form of a corneal transplant, is required.

 

Endothelial Cup Reflex

This brilliant reflex is seen at the apex of the cone and accounts for the characteristic "dew-drop" or crystalline appearance of the cornea. The intensified reflective properties are related to the increased curvature of the posterior corneal surface that can appear as a convex mirror. (Figure 38.)

 

Keratoconus

Figure 38. A bright reflex can be seen at the apex of the cone called the endothelial cup reflex. This is related to the increased curvature of the posterior corneal surface.

 

Use of the Slit Lamp

Throughout the years, the slit lamp has allowed practitioners to examine the multitude of corneal changes occurring in keratoconus. However, it is important to remember that different slit lamp findings may appear at various stages throughout the course of the disease. In addition, not all the classic slit lamp findings may be seen in every patient.Modern video keratoscopy techniques have also furthered our knowledge of the many topographical changes occurring across the corneal surface but corneal mapping cannot replace the slit lamp in making the definitive diagnosis of keratoconus, especially in the condition's incipient stages. The topographical changes of inferior steepening and superior flattening are common findings in the normal non-keratoconic population and are simply related to corneal apex position. Therefore, we can state that keratoconus may be suggested by corneal mapping techniques, but the actual diagnosis can only be confirmed through positive slit lamp findings. Theses findings include vertical striae, ruptures in Bowman’s layer, Fleischer's ring, corneal thinning, ruptures in Descemet's membrane, and increased visibility of the corneal nerve fibers.

 

Clinical Management of Keratoconus

 

Spectacle Correction in Keratoconus

In the early stages of keratoconus, the patient's refractive error can often be successfully managed with spectacle lenses. It is important to communicate to the patient that there is no evidence to support the theory that early contact lens intervention is of therapeutic benefit in preventing or lessoning the progression of the disease. However, wearing contact lenses typically provides the patient with better visual acuity than can be obtained with glasses by neutralizing the regular and irregular refractive errors induced by the condition. As keratoconus progresses, spectacle lenses often fail to provide adequate visual acuity, especially at night. This can be further complicated by the fact that the patient's glasses prescription may change frequently and can be limited by the degree of myopia and astigmatism that must be corrected. Also, keratoconus is often asymmetric therefore full spectacle correction may be intolerable because of anisometropia and anisokonia. However, despite these limitations, spectacles can often provide surprisingly good visual results in the early stages of the condition.  

  

Wave Contact Lenses

Wave Contact Lenses are Contact Lenses which are custom designed from your corneal “fingerprint”, its a new technology which is being used to aid for a better fit and visual accuracy. Corneal Topography equipment digitally maps your cornea with thousands of numbers. The doctor uses the map data and Wave-front special design software to create a custom lens that follows almost every tiny shape on your cornea. Custom designed lenses differ greatly from plain “mass produced” lenses. The doctor using Wavefront technology for contact lenses will specify the lens diameter and thickness for comfort and adjust the position of the optics for best visual performance. In all there are 26 different adjustments, offering infinite design possibilities.

Keratoconus Post Refractive Patient's are the most challenging to fit with off-the shelve contact lenses and most in need of  Wave-front contact lenses custom “match the cornea” design philosophy. 

 

Contact Lens Designs for Early Keratoconus

The successful fitting of contact lenses for keratoconus requires that three objectives be met:

  • The lens-to-cornea fitting relationship should create the least possible physical trauma to the cornea.

  • The lens should provide stable visual acuity throughout the patients entire wearing schedule.

  • The lens should provide all day wearing comfort.

Although it may be impossible to meet all of these objectives for every patient, we should remain focused on utilizing all of the modern lens designs and fitting techniques at our disposal to achieve the best possible outcomes.

 

Spherical Lens Designs

Traditional 3 to 4 curve spherical lens designs are generally used only in the early stages of keratoconus when there is minimal topographic difference between the central and mid-peripheral corneal topographies. These lenses can be designed to provide a superior alignment fitting relationship or a more intra-palpebral, three point touch relationship.

 

Superior Alignment Fitting Technique for Early Keratoconus

Superior alignment fitting lenses for keratoconus commonly consist of aspherical designs that incorporate a larger 9.5 mm overall diameter with an 8.3 mm posterior optical zone. These lenses are designed to provide a superior alignment fitting relationship across the more “normal” portion of the keratoconic cornea. (Figure 39.)

 

Keratoconus

Figure 39. The superior alignment fitting technique for early keratoconus. Note the lens touch along the horizontal meridian at 3 and 9 o’clock and the inferior edge clearance across the steeper portion of the cornea.

 

When fitting these lenses, the central keratometry (“K”) readings are of little value. Instead, knowledge of the more normal nasal temporal and superior mid-peripheral cornea (which lies beyond the area measured by the keratometer) becomes the most important fitting consideration.For example, a patient with early keratoconus presents with topographical findings that demonstrate the characteristic inferior steepening and superior flattening. The central keratometric readings are 46.25 D/ 49.75 D. Inferiorly, the cornea steepens to 51.25 D and superiorly it flattens, to 42.00 D. (Figure 40.)

 

Keratoconus

Figure 40. In early keratoconus, there is a characteristic steepening of the inferior cornea with a subsequent flattening of the superior cornea.

 

If a standard spherical contact lens is fitted on flat “K” (46.25 D) or steeper than “K,” the lens will be forced downward due to impingement of the steep curve on the more normal superior cornea. (Figure 41.) This would prevent the lens from freely traversing along the vertical meridian with each blink. It would also prevent the lens from achieving the desired superior resting position, which will inevitably result in superior compression of the epithelium and an inferiorly-fixed lens position. This compression is best noted by the broken lines of the keratoscopy mires, as well as by the broadening and flattening of the reflected rings. (Figure 42.)

 

Keratoconus


Figure 41. A contact lens with a base curve radius fitted “on-flat-K", (46.25 D.) will impinge on the flatter superior cornea preventing the lens from freely moving along the vertical meridian.

 

Keratoconus

Figure 42. The relationship of a large optical zone, steeply fitted lens along the superior cornea results in an area of epithelial compression that can be easily visualized, following lens removal, by the broken and brooding of the keratoscopy reflex.

 

Initially, patients may be comfortable with the steeply fitted, spherical lenses. However, with time they often experience increased lens awareness and a decrease in wearing time. Therefore, adequate superior clearance of a standard spherical lens design can only be accomplished by fitting the lens with a base curve radius flatter than that of the cornea. Clinical experience has shown that the initial diagnostic lens should have a base curve radius equal to the radius of curvature 4.0 mm to the temporal side of the cornea. This lens is placed on the cornea and evaluated with fluorescein. The ideal lens-to-cornea fitting relationship should be one in which the following are present:

  • The base curve is flat enough to provide lens alignment across the flatter superior cornea.

  • However, the base curve of the lens should be steep enough to provide slight touch mid peripherally at 3 and 9 o’clock along the horizontal meridian. This will prevent the lens from rocking and pivoting on the cornea. (Figure 43.)

  • The lens will most likely exhibit slight bearing at the apex of the cornea and slight edge lift inferiorly across the steeper portion of the cornea.

It is clear that use of the superior alignment fitting technique is possible only in the early stages of keratoconus because as the central ectasia progresses, the superior alignment fit can result in greater localized apical bearing. This can lead to excessive lens rocking and instability, resulting in symptoms of lens awareness.
 

Keratoconus
.
Figure 43. In fitting a superior alignment lens for early keratoconus, a base curve is selected that is steep enough to land the lens at 3 and 9 o’clock along the horizontal meridian yet flat enough to avoid excessive impingement on the flatter superior cornea. 

 

The Intra-Palpebral Three Point Touch Fitting Technique for Early Keratoconus

Using the intra-palpebral or three-point touch fitting technique, a spherical lens design is selected that has an overall lens diameter of 8.0 to 8.5 mm with a posterior optical zone diameter of 6.4 to 6.9 mm. The initial diagnostic lens is selected with a base curve radius equal to the flat keratometric reading. This lens is placed on the cornea and evaluated with fluorescein. The ideal lens-to-cornea fitting relationship should be one in which the following are present:

  • The base curve should be steep enough to provide a slight central touch, shown by thinning of fluorescein, at the corneal apex and slight touch mid-peripherally at 3 and 9 o’clock along the horizontal meridian. This creates three points of lens touch along the horizontal meridian. (Figure 44.)

  • The lens will most likely position centrally or slight low on the cornea due to slight impingement of the lens across the flatter superior cornea.

 


Keratoconus

 Figure 44. Intra-palpebral, three-point touch fitting technique for early keratoconus.

 

Aspheric Lens Designs for Early Keratoconus

Often in keratoconus, the steepness of the corneal apex and the radical flattening of the mid-peripheral and peripheral cornea limit the effective use of spherical lens designs. Therefore, today many practitioners advocate fitting designs that incorporate aspheric radii. These designs accomplish three important fitting objectives:They allow the lens to be fitted with apical clearance without the periphery impinging on the flatter mid-peripheral cornea. (Figure 45.)The amount of posterior lens asphericity can be independently varied to allow the lens to clear the flatter mid-peripheral cornea.The highly aspheric posterior designs provide better alignment and weight distribution over a larger area of the cornea. This often provides improved lens centration and comfort. 

 

Keratoconus

Figure 45. Large diameter, aspheric lens designs for keratoconus allow the lens to be fitted with apical clearance centrally and greater alignment mid-peripherally.

 

Aspheric lens designs for keratoconus are available in three different configurations:

  • Lenses with small spherical optical zones and aspheric peripheries.

  • Lenses that incorporate two or more aspheric radii (bi-aspheric designs) with a low eccentricity in the center and a higher asphericity in the periphery.

  • Lenses that incorporate a specific asphericity (total aspheric) throughout the posterior surface of the lens.

 

RGP Fitting Approach

The following is our step-by-step approach to fitting aspheric RGP lenses for early keratoconus. The diagnostic lens fitting begins by performing corneal mapping and viewing the topography as an axial map display. The topography is quantitatively viewed to identify the size, shape, and location of the red zone (steepest area of the cornea) as well as the blue zone (flattest area of the cornea). The dioptric curvature of the corneal apex is also identified for reference. (Figure 46.)

 

Keratoconus


Figure 46. The fitting of an aspheric lens for keratoconus begins by performing corneal mapping in straight ahead gaze, (left image). This is followed by instructing the patient to look up and mapping the cornea at its apex (right image).

 

A diagnostic lens is then selected with a base curve radius equal to the dioptric curvature at the corneal apex. The lens is placed on the eye and its position and relationship to the cornea evaluated with fluorescein.An optimum lens-to-cornea fitting relationship is accomplish when three fitting criteria are present:

  • The base curve radius should provide slight clearance across the corneal apex with no fixed, mid-peripheral bubbles.

  • The mid-periphery of the lens should “land” (touch) the mid-peripheral cornea at 3 and 9 o’clock to prevent nasal/temporal lens decentration.

The lens should create minimal impingement across the flatter superior cornea. (Figure 47.)

 

Keratoconus
.
Figure 47. The computer simulated and actual fluorescein pattern of the optimum aspheric lens for the patient's eye. Note the apical clearance and the mid-peripheral landing zones at 2 and 8 o’clock.

 

The lower edge of the lens will often lift slightly away from the cornea due to inferior corneal steepening. Additionally, intermittent bubbles or frothing may be noted inferiorly. This is not only acceptable but may be essential to ensure both adequate superior alignment and vertical lens movement with the blink. Any attempt to decrease the inferior edge lift (i.e., steepening the base or peripheral lens design), may result in a tight superior lens-to-cornea fitting relationship. Furthermore, with a steep fitting relationship, the lens edge may cause further mechanical distortion of the cornea that may exacerbate the condition.Having achieved the desired fit, a sphero-cylinder over-refraction is performed to determine the final contact lens power. A compensating anterior surface aspheric surface is often incorporated to neutralize the radial astigmatism and coma induced by the highly aspheric posterior lens surface. The final lens is then ordered in a moderate to high Dk rigid gas permeable material. Each lens manufacturing laboratory will incorporate a slightly different aspheric central and/or peripheral lens design, so it is imperative that the diagnostic lens design match the final lens ordered.

 

Fitting Advanced Keratoconus

Because of the varying peripheral corneal topographies found in advanced keratoconus, no single lens design or fitting philosophy will consistently result in an optimal fit. For this reason, a variety of different fitting approaches must be employed, each based on the status of the central as well as the mid-peripheral corneal topography. The following describes our fitting approach for advanced keratoconus and is based on the nipple, oval, and globus photokeratoscopy and videokeratoscopy classification of corneal topography

 

Fitting the Nipple Cone

Because of the rapid change in corneal shape from center to mid-periphery, the nipple-shaped topography creates numerous fitting obstacles. As previously discussed, the nipple cone consists of central ectasia measuring 5 mm or less surrounded by 360 degrees of essentially normal cornea. When fitting a nipple-shaped cornea with a spherical multicurve lens design, it is important to use a diagnostic lens fitting set that incorporates a small overall diameter of 8.1 mm and a small posterior spherical optical zone of 5.5 mm. (Table 1.)

 

Table 1. Nipple Cone Fitting Set

 

Keratoconus

 

The peripheral lens design should consist of a series of 3 or more spherical peripheral curves that gradually fatten the lens periphery. (Figure 48.) Because of the multiple junctions present on the posterior lens, it is necessary for the laboratory to incorporate multiple blending curves. The resulting lens design is a non-definable aspheric surface.

 

Keratoconus

Figure 48.The fitting of a spherical multi-curve lens design for nipple shaped keratoconus is best accomplished with a small overall lens diameter of 8.1 mm and a small posterior optical zone of 5.5 mm. It is important that the peripheral lens design be flat enough to clear the flat mid-peripheral cornea that extends 360 degrees around the base of the cone.

 

The nipple-shaped topography is often best managed through the use of designs that incorporate aspheric radii. The aspheric lens fitting technique is identical to that described for the fitting of early keratoconus, but, due to rapid topographical flattening from center to periphery, it is often necessary to increase the amount of posterior lens asphericity. This increase in asphericity flattens the periphery of the lens to provide an improved peripheral lens alignment.

 

Fitting the Oval Cone

The oval shaped form of keratoconus is hallmarked by an inferior steepening with varying degrees of normal corneal topography superiorly. When fitting the oval-shaped cone, careful attention must be directed to the status of the superior and horizontal i.e., (along the 180 degree meridian) corneal topography. A superior alignment fitting technique similar to that described in early keratoconus can be considered only if the superior topography and the horizontal topography are relatively normal. In this situation, the normal cornea at 9, 12, and 3 o'clock provides a sufficient base on which to support the superior alignment lens. (Figure 49.)

 

Keratoconus

Figure 49. When fitting the more advance form of oval-shaped keratoconus, a superior alignment fitting technique can be use as long as the lens can adequately land on the cornea at 3, 9, and 12 o'clock.

 

As the oval cone progresses, the normal corneal topography in the temporal horizontal meridian is lost. The superior alignment fitting philosophy is less likely to provide an adequate fit due to the lack of a horizontal fulcrum (contact point) temporally. This loss of normal cornea temporally is followed by involvement of the superior cornea resulting in the last island of normal cornea being in the superior-nasal quadrant. A more contoured fitting philosophy can be used with a spherical multicurve lens design that incorporates an overall diameter of 8.6 mm and a posterior optical zone of 6.0 mm. (Table 2.) (Figure 50.) Alternately, the oval-shaped cornea can be fit with an aspheric lens design having adequate peripheral eccentricity to provide the needed peripheral clearance and lens stability. Currently, our aspheric lens of choice for keratoconus is the I-Kone lens design manufactured by Valley Contax in Springfield, Oregon, USA. (http://www.valleycontax.com)

 

Table 2. Oval Cone Fitting Set

Keratoconus

 

Keratoconus

Figure 50. The fitting a spherical multi-curve lens design for oval shaped keratoconus can be accomplished with an overall lens diameter of 8.6 mm and a posterior optical zone of 6.0 mm. It is important that the peripheral lens design be flat enough to clear the flat superior mid-peripheral cornea that extends almost 180 degrees above the base of the cone.

 

Fitting the Globus Cone

In nipple keratoconus, the ectatic central cornea is surrounded by 360 degrees of normal cornea. In the oval-form, 180 degrees or less of normal cornea remains above the midline. The globus-form is hallmarked by a nearly total involvement and ectasia of the cornea. In this advanced stage, the only normal unaffected portion of the cornea may be located in the superior limbal area. Because of the size and extensive involvement of the peripheral cornea, contact lens fitting for globus cones requires large lens diameters of 9.1 mm and a 6.5 mm posterior optical zone. (Table 3.) (Figure 51.) Alternately, an aspheric lens design can be used, however it might be necessary to incorporate less peripheral asphericity than that of the diagnostic lens to accommodate the steeper peripheral cornea associated with the globus shaped topography.

 

Table 3. Globus Cone Fitting Set

 

Keratoconus

 

 

Keratoconus


Figure 51. The fitting a spherical multi-curve lens design for globus shaped keratoconus can be accomplished with an overall lens diameter of 9.1 mm and a posterior optical zone of 6.5 mm. In globus-shaped topography, there is no normal mid-peripheral cornea topography, therefore peripheral lens impingement is less common.

 

Refraction Over A Trial Lens

Over-refraction is an integral part of diagnostic fitting. It may be important to over-refract using large diopter steps (e.g., 0.50 D or even 1.00 D) because keratoconus patients often find it difficult to distinguish small changes in power due to the high degree of myopia and irregular astigmatism caused by the disease. It is not uncommon for keratoconus patients wearing rigid contact lenses to manifest moderate to high amounts of residual astigmatism. This can be easily evaluated by placing a plus/minus 0.50 D cross-cylinder at 90, 180, 45, and 135 degrees, and asking the patient if there is any subjective difference in acuity. Correcting the residual astigmatism is best accomplished with glasses, which often improves visual acuity three to four lines. Front surface toric rigid lens designs have been successfully fitted in keratoconus patients resistant to wearing spectacles over their contact lenses.

 

Lens Materials

All keratoconus contact lenses should be ordered in a moderate to high Dk rigid gas permeable material to avoid epithelial hypoxia and corneal erosion during the long wearing schedule of keratoconus patients. All lenses must be manufactured under the strictest controls possible. Not only must the laboratory be capable of manufacturing one perfect lens for the patient, but they must also be able to duplicate that lens should the patient need a replacement. Every aspect of the lens design (base curve, optical zone, diameter, edge, etc.) plays an integral role in the overall success of the lens. Therefore, the practitioner should carefully evaluate the lens prior to dispensing and communicate any design concerns to the manufacturer.

 

Large Diameter Semi-Scleral Gas Permeable (GP) Lenses

In certain cases, traditional spherical and aspherical GP lens designs may not provide the desired centration, optics or comfort required by the patient. In these situations, the patient may benefit from a large diameter (13.5 to 16.0 mm) semi-scleral lens design, (Figure 52.) These lenses can be manufactured in a wide range of parameters (base curves, powers, diameters, flange radii, etc.) in high Dk (100+) materials.

 

Keratoconus

Figure 52. Large diameter semi-scleral RGP lens.

 

Semi-scleral lenses are best fitted through the use of a diagnostic set. The fitting procedure begins by selecting a diagnostic lens with a base curve radius equal to the steepest keratometric reading. The lens is placed on the eye and evaluated with fluorescein. The ideal fitting relationship is one in which the corneal portion of the lens exhibits apical clearance across the central cornea. (Figure 53.)

 

Keratoconus

Figure 53. In fitting semi-scleral lens designs, the ideal lens-to-cornea fitting relationship is one in which there is clearance of the corneal portion of the lens.

 

The lens should demonstrate a 1.0 mm band of pooling adjacent to the limbus and alignment in the area of the scleral curve. These lenses often incorporate a large limbal fenestration to reduce lens adhesion and facilitate lens removal. Clinical experience has shown that central corneal alignment (or a flatter than alignment fitting relationship) will often result in excessive lens adhesion and subsequent tightening of the lens onto the eye. Therefore, adequate corneal vaulting is essential to prevent this binding phenomenon. Semi-scleral lenses have proven to be extremely beneficial for patients with highly irregular and/or asymmetric keratoconic corneas. The lens design can dramatically decrease many of the comfort and centration complications associated with more traditional rigid lens designs.

 

Soft Contact Lenses For Keratoconus

For the past 100 years, rigid corneal contact lenses have offered the best visual correction for keratoconus, however with all of their benefits modern GP lenses may not meet the needs of many patients. The inability to achieve all-day wear, and the discomfort experienced by some patients wearing GP lenses has created a need to investigate the use of soft lenses for keratoconus. Clinically, it would seem unlikely that soft contact lenses could compensate for the refractive error associated with keratoconus, but new soft lens designs and manufacturing techniques have made it possible to correct the complex optics created by keratoconus. The Kera-soft contact lens is one such lens which is a high performance soft contact lens for Keratoconus and is comparable to RGP’s. Kera-soft lenses can be toric and can have wave-front technology built in to it, so moderate to advanced stages can use them, but results can vary.

 

Corneal Sensitivity in Early Keratoconus

Clinical experience has shown that the corneal apex in early keratoconus can go through a period of hypersensitivity. This hypersensitivity may be due, in part, to a dilatation or stretching of the corneal nerve fibers, as seen by biomicroscopy. In addition, the minute ruptures in Bowman’s layer may be a contributing factor to patient hypersensitivity. It has been the authors’ experience that some patients with early keratoconus find it difficult or impossible to tolerate even extremely well-fitted GP lenses. This intolerance may not be psychological, but may be due in large part to hypersensitivity of the corneal apex during the early disease. In later stages of keratoconus, this hypersensitivity decreases and the cornea can become hyposensitive (Axenfeld’s sign).The use of soft contact lenses in keratoconus encompasses a variety of different modalities each of which have proven successful in providing functional optics and comfort for selected patients. These modalities include piggyback lens designs, soft toric lenses, custom spherical and aspherical lens designs, and hybrid designs.

 

Piggyback Soft Lenses

The technique of placing a rigid contact lens on top of a soft lens (piggyback) was first described in the mid 1970’s. Early piggyback systems consisted of thick, low Dk, soft lenses in combination with low Dk silicone/acrylate rigid lenses. It was not surprising that this combination of frequently resulted in corneal hypoxia and neovascularization, which limited its usefulness. However, with the recent introduction of high Dk silicone hydrogel lenses and stable high Dk GP materials, the dual lens system is now enjoying a rebirth, particularly for keratoconus patients experiencing comfort or lens position issues.

 

The Traditional Piggyback Lens System

The traditional piggyback system consists of a high Dk silicone hydrogel soft lens over which a high Dk RGP lens is fitted. The fitting procedure begins with the diagnostic fitting of the soft lens to optimize lens movement and position. This is followed by keratometry or videokeratography over the anterior surface of the soft lens to determine the radii of the “new” corneal surface. A GP lens is selected with a base curve radius equal to the flat K and a diameter of approximately 9.0 to 9.5 mm. The base curve is adjusted until an appropriate lens-to-lens fitting relationship is established. The GP lens should accomplish three fitting objectives:

  • Apical clearance to prevent the lens from rocking and pivoting over the corneal apex.

  • Lens contact or “landing zone” along the horizontal meridian at 3 and 9 o’clock to center the lens along the horizontal meridian.

  • Unobstructed lens movement along the vertical meridian. (Figure 54.)

 


Keratoconus
Figure 54. A piggyback lens system for keratoconus consisting of a high Dk soft lens and an RGP lens combination.

 

An over-refraction is performed to determine the final power of the RGP lens, which can be manufactured in a spherical or aspherical design in a high Dk material with customary peripheral lens and edge configurations.

 

Custom Piggyback Lens System

The custom piggyback soft lens differs from the traditional system in that it incorporates a circular, recessed depression in the center of the soft lens. Within the boundaries of the cutout, a high Dk RGP lens is fitted. (Figure 55.) The system provides optimal performance by virtue of the now centered RGP optics and enhanced comfort through the presence of the soft lens.

 

Keratoconus

Figure 55. The Flexlens custom piggyback lens design. The soft lens incorporates a grove cut into its anterior surface allowing the RGP lens to be recessed away from the upper lid.

 

In the United States, the custom piggyback soft lens is manufactured by X-Cel Laboratories in Duluth, Georgia (http://www.visionslens.com/contact.html). The soft lens is available in a wide range of parameters, including base curve radii from 6.00 to 11.00 mm and lens diameters from 12.5 to 16.5 mm. The recessed cutout can be manufactured in diameters of 7.5 to 11.5 mm. The fitting criteria for the soft lens is identical to that of any lens, with movement and centration the primary fitting objectives. Diagnostic fitting of the soft lens is enhanced by inserting any rigid lens into the recessed cutout to mimic final lens weight and lid/lens interaction. Once the appropriate soft lens fit has been determined, the rigid lens can be removed and K readings can be obtained over the central portion of the soft lens. A diagnostic GP lens with a base curve radius equal to flat K is then inserted into the cutout and its fitting relationship evaluated and adjusted. It is important to select an GP lens with an overall lens diameter 1.0 mm smaller than the cut out diameter to allow for some movement and tear exchange within the soft lens cut-out boundaries. For example, if the cut-out diameter is 9.5 mm, the RGP lens diameter should be 8.5 mm.

 

Soft Toric Lens Designs

Toric soft contact lenses have been successfully used in the early stages of keratoconus or in form fruste keratoconus. The fitting procedure begins by placing a soft lens with powers equal to the manifest refraction vertexed to the plane of the cornea on the eye. Final lens power is best calculated by performing a sphero-cylinder refraction over a well equilibrated lens. Modern production techniques allow the final lens to be manufactured in a wide range of custom parameters that include, base curve, power, diameter, and thickness.

 

Custom Spherical and Aspherical Soft Lenses Designs

Most soft lenses designed specifically for keratoconus utilize a tri-curve posterior lens design with increased central thickness to mask much of the regular and irregular astigmatism. (Figure 56.) The base curve is designed to fit the apex of the cone, the intermediate curve parallels the peripheral cornea, and the scleral curve rests approximately 1.0 mm beyond the limbus. These lenses can be custom-designed and manufactured in a broad range of base curves, powers, and diameters. Other parameters that can be controlled include the diameter of the anterior and posterior optical zones, center thickness, and the width and radius of the intermediate curve and scleral curves.

 

Keratoconus

Figure 56. Custom soft lens designed for keratoconus.
 

 

Patient Selection for Soft Lenses Designs

The corneal topography of an individual keratoconus patient can play an important role in the successful use of a soft lens. Corneas that seem to do best are the nipple- and globus-types in which there is a relatively concentric 360 degree peripheral topography. (Figure 57.)

 

Keratoconus


Figure 57. Advanced globus-shaped corneal topography successfully fit with custom soft contact lenses.

 

Patients with large diameter sagging cones may be poor candidates due to corneal asymmetry in which the steepening of the inferior cornea causes the lens to lift inferiorly. It is important to remember that the base curve, as well as the intermediate and scleral curves of the lenses, can be custom-designed to correspond to the topography of the individual cornea. Also, if the patient is being refitted from rigid contact lenses, it is best to fit one eye at a time (the most needy eye first).

 

Fitting Custom Spherical and Aspherical Soft Lenses Designs

Keratometric readings are taken (as best as possible) to estimate the steepness of the central cornea, as well as the amount and location of corneal astigmatism. A diagnostic lens is selected with a base curve 1.00 mm flatter than the mean K. For example, if the central keratometric readings are 52.00 at 45 / 62.00 at 135, the mean K equals 57.00 D (5.90 mm). A diagnostic lens can be selected that is 1.00 mm flatter than 5.90 mm and the 6.90 mm diagnostic lens curves with a peripheral radius of 8.60 mm and placed on the cornea to be evaluated with the slit lamp. (Table 4.) (Figure 58.)

 

Table 4. Keratoconus Soft Lens Diagnostic Set

 

Keratoconus

 

 

Keratoconus

Figure 58. In fitting a custom soft lens for keratoconus, a diagnostic lens is selected with a base curve radius that is 1.00 mm flatter than the mean keratometric reading.

 

The lens-to-cornea fitting relationship is evaluated with traditional soft lens fitting techniques, the lens should exhibit 1.0 to 1.5 mm of limbal clearance, and the lens edges should rest 360 degrees onto the sclera and the lens should exhibit 0.25 mm of movement with a blink. The thicker soft lens design will be more rigid than a traditional soft lens, therefore a true tear lens will form between the posterior surface of the lens and the cornea. It is the presence of this lacrimal lens that aids in the correction of the regular and irregular astigmatism. (Figure 59.) It is important to note, however, that the tear lens may change due to settling of the lens during the first week of lens wear.

 

Keratoconus


Figure 59. The masking of regular and irregular corneal astigmatism is made possible by the increased center thickness of the custom soft lens. Note the change in the central keratoscopy mires without the lens (left image) and with the lens (right image).

 

Following evaluation of the trial lens, a spherical and/or sphero-cylinder over-refraction is performed. The power is only an estimated because changes in the tear lens should be anticipated during the first week of lens wear. Due to lens rigidity, it is not uncommon for patients to need only small cylindrical over-corrections or, in some cases, none at all.

 

Hybrid (Soft-Perm) Lens Designs

In 1977, Precision Cosmet in Minneapolis Minnesota USA began work on a hybrid combination GP and soft lens design. Their work culminated in introduction of the Saturn lens in 1985. This lens incorporated a 6.5 mm rigid styrene-based material with a Dk of 14. This was surrounded by a 13.5 mm diameter, 25% water content skirt. The Saturn lens was replaced by the Sola-Barns Hind, Softperm lens in 1989. The new lens incorporated a larger 8.0 mm styrene center in a bi-curve lens design and a 14.3 mm diameter, 25% water content skirt. (Figure 60.)

 

Keratoconus

Figure 60. The SoftPerm II lens design for keratoconus.

 

The Softperm hybrid design enjoyed only limited success due to physiologic problems secondary to minimal oxygen permeability, frequent loss of adhesion between the GP lens and the soft lens skirt, and limitations in lens design and parameter availability. (Figure 61.)

 

Keratoconus


Figure 61. Peripheral neovascularization associated with the early generation low Dk hybrid lens designs.

 

In September 2001, a California based research group called Quarter Lambda Technologies began development of a new high Dk hybrid lens called SynergEyes (http://www.synergeyes.com ). The lens incorporates an 8.2 mm high Dk rigid center, (Paragon HDS 100, Dk 100) and a 28% water content non-ionic soft lens skirt. The overall diameter of the lens is 14.5 mm. (Figure 62.)

 

Keratoconus

Figure 62. The new high Dk SynergEyes hybrid lens design. The lens incorporates an 8.2 mm RGP portion and a 14.5 mm overall diameter.

 

We have successfully used this lens for many keratoconus patients, especially those with irregular astigmatism or those who had comfort and centration issues with traditional GP lens designs. The SynergEyes is available in two designs for keratoconus: the SynergEyes A is the standard aspherical design ideal for patients with early keratoconus and the SynergEyes KC has been designed specifically for advanced keratoconus patients.The fitting procedure begins by selecting a diagnostic lens with a base curve radius equal to steep K. High molecular weight fluorescein is placed into the bowl of the lens and the lens is placed on the eye and allowed to equilibrate. The RGP portion of the lens should exhibit central apical clearance and mid-peripheral lens bearing. The soft lens skirt should exhibit 0.25 mm of blink-induced movement. (Figure 63.)

 

Keratoconus

Figure 63.The SynergEyes lens design for management of keratoconus.

 

Surgical Treatments for Corneal Ectasia

Since the 1950’s, penetrating keratoplasty has been the primary surgical procedure for treatment of keratoconus. (Figure 64.)

 

Keratoconus

Figure 64. Penetrating keratoplasty for treatment of keratoconus.

 

The decision to pursue keratoplasty is one that must be made by the individual surgeon and the patient. The factors that can influence the decision include:

  • The patient can no longer tolerate or be fitted adequately with contact lenses.

  • The contact lenses fail to provide adequate visual acuity due to corneal scarring.

  • Sharp bilateral visual acuity is required for recreational or professional requirements.

 

It is important to recognize that certain patients are perfectly content with 20/50 to 20/60 (6/15 to 6/18) visual acuity. Over the years, these individuals have developed blur interpretation skills that allow them to function surprisingly well in their daily endeavors. The major limiting factor for these individuals is difficulty driving at night. For most surgeons, keratoplasty is not considered until contact lens options have been exhausted.

 

Surgery involves risks that include:

  • Graft rejection,
  • Infection,
  • Glaucoma,
  • High post operative regular and/or irregular astigmatism, and
  • Steep or flat grafts.

 

Patients should be informed that keratoplasty for keratoconus has a success rate of approximately 92% for a clear transplant on the first surgery and that occasionally a second or third transplant might be needed secondary to post-operative great rejection, infection, or wound healing complications. Patients should also be advised that following keratoplasty, approximately 85% of them will find it necessary to wear a contact lens to correct the surgically-induced regular and irregular astigmatism and anisometropia. Eye bank data from 2004 shows that in the United States 30,668 penetrating keratoplasties were performed with 4,744 of the transplants performed to treat corneal ectasias. (Table 5.)  

 

Table 5. 2004 Corneal Transplant Diagnoses

 

DiagnosisNumber of TransplantsPercentage of Transplants
Pseudophakia Corneal Edema5,56918.4%
Endothelial Corneal Dystrophies4,77115.8%
Ectasias/Thinning Disorders4,74415.7%
Regraft Non Rejection2,4118.0%
Regraft Secondary to Rejection1,5045.0%
Noninfectious Ulcerative Keratitis1,2574.2%
Aphakic Corneal Edema9163.0%
Corneal Degenerations7832.6%
Mechanical Trauma4901.6%
Congenital Opacities4841.6%
Stromal Corneal Dystrophies4491.5%
Post Viral Keratitis4071.3%
Post Microbial Keratitis2951.0%
Chemical Injuries970.3%
Post Syphilitic Keratitis820.3%

  

Lamellar Graft

A lamellar graft (also known as DALK or a partial graft) replaces part, rather than all, of the thickness of the cornea. It can be used instead of a penetrating graft if the deeper layers of the cornea are healthy.   

 

Intrastromal Rings/Intacs-Ferrara Rings

Intacs inserts are 150 degree PMMA ring segments that are placed in the peripheral stroma of the cornea, typically to correct myopia. However, the technique has also been recently US FDA approved for the treatment of keratoconus. (Figure 65.)

 

Keratoconus


Figure 65. Intacs intra-stromal rings for treatment of keratoconus.

 

The inserts are designed to be placed at a depth of approximately two-thirds the corneal thickness and are surgically inserted through a small radial incision into a trough created within the corneal stroma. The inserts shorten the corneal arc length and have a net effect of flattening the central cornea. The amount of flattening is determined by the insert's thickness. Rings are available in thicknesses of 0.250, 0.275, 0.300, 0.325 and 0.350 mm and are oriented horizontally in the cornea at 12 and 6 o'clock. Intecs are indicated for contact lens intolerant patients with early keratoconus who have minimal central stromal scarring. (Figure 66.)

 

Keratoconus

Figure 66. Intacs rings flatten the central cornea in keratoconus by an amount proportional to the thickness of the two 150 degree arc segments.

 

In theory, the inserts are designed to flatten the central cornea and provide reinforcement for the peripheral cornea. The procedure may also decrease the cornea surface irregularity and improves the patient's best-corrected visual acuity. Its been reported that sometimes using one segament is sufficient and favorable.

Ferrara Rings works in a similar way to Intacs, but there are some differences in when they are used, depending on the the size of the flattening that needed and also the pupil size of the patient.

Ferrrara Ring, which like Intacs have ring segments 150º of arc but have a smaller inner and outer diameter (4.4mm and 5.6mm) and are triangular in cross-section. They range in thickness from 0.2mm to 0.35mm in 0.05mm increments. Ferrara Rings are approved in Europe for Keratoconus.

 

Laser Surgery for Keratoconus

Clinicians typically rule out LASIK surgery for patients with keratoconus because of a greater risk for scarring and excessive thinning leading to possible post-LASIK ectasia. However, some surgeons have reported success in performing a smoothing procedure for patients over the age of 40 years with stable keratoconus. Candidates for the procedure are most often individuals with mild keratoconus who are contact lens intolerant and reluctant to pursue corneal transplant surgery.

 

Management of Corneal Problems that can Mimic Keratoconus

 

Pellucid Marginal Degeneration

Pellucid Marginal Degeneration (PMD) is a bilateral corneal disorder hallmarked by a thinning of the inferior, peripheral cornea. The corneal thinning begins approximately 1.0 to 2.0 mm above the inferior limbus and is separated by a region of uninvolved, normal cornea between the thinned zone and the limbus. (Figure 67.) Ruptures in Descemet’s membrane or acute hydrops may be seen in the area of inferior thinning. (Figure 68.)

 

Keratoconus


Figure 67. Pellucid marginal degeneration.

 

Keratoconus


Figure 68. Acute hydrops in pellucid marginal degeneration. Note the dramatic reduction in against-the-rule astigmatism OS following the hydrops.

 

PMD commonly manifests between the ages of 20 and 40 years with no apparent hereditary transmission and equal gender distribution. Subjective symptoms are visual secondary to a dramatic increase in against-the-rule corneal astigmatism.In this condition, the inferior cornea is free of vascularization or lipid infiltration, which differentiates PMD from other peripheral thinning disorders such as Terrien’s Marginal Degeneration and Mooren’s ulceration. (Figure 69.)

 

Keratoconus

Figure 69. Pellucid marginal degeneration.

 

PMD can manifest many of the same features as keratoconus however in PMD the central cornea retains a normal thickness and also manifests unique topographic features. Corneal mapping in PMD demonstrates inferior mid-peripheral zones of corneal steepening at 4 and 8 o’clock. This produces the “butterfly wing-like” or “kissing pigeons” pattern, diagnostic of PMD. (Figure 70.)

 

Keratoconus

 Figure 70. The topographical findings in PDM include high against-the-rule corneal astigmatism and inferior mid-peripheral steepening at 4 and 8 o’clock. This pattern in creates a “kissing pigeon” pattern diagnostic of PMD.
 

 

Management of PMD

The management of PMD involves a wide range of modalities including spectacles, contact lenses, and surgery. Spectacle correction is often satisfactory in the early stages of PMD due to the minimal degree of induced irregular astigmatism however in the more advanced cases, contact lenses are the suggested mode of treatment. Contact lens management of PMD can be difficult due to the high degree of against-the-rule and asymmetrical astigmatism.Management of the PMD has recently been enhanced by the wide range of available lens materials and designs that include:

  • spheric and aspheric GP lenses,
  • bi-toric GP lenses,
  • semi-scleral and scleral GP lenses,
  • toric soft lenses,
  • hybrid lenses, and
  • piggyback lenses. (Figure 71.)

 


Keratoconus

 Figure 71. A wide range of contact lens designs provide the primary treatment for PMD.

 

Surgical intervention for PMD often involves penetrating keratoplasty. Alternative treatment includes a kidney-shaped penetrating keratoplasty or an inferior lamellar patch graft to more directly manage the localized inferior corneal thinning. (Figure 72.)

 

Keratoconus

Figure 72. A kidney-shaped penetrating keratoplasty for treatment of PMD.
 

 

Summary

The progression of Keratoconus can be treated and the deformation made more regular with the early use of the crosslinking treatment to avoid advanced Keratoconus which can be problematic to correct. Perhaps no other group of patients has benefited more from the science of contact lenses than those with keratoconus, with now even high performance soft lenses available for Keratoconus patients. However as these modulaties become more available, in the mean-time, all too often the fitting process can be a frustrating experience for both patient and practitioner. To better manage the condition in it’s advanced stages, practitioners must first be aware that no one lens design or fitting technique will provide adequate results in all cases. Additionally, it is important to avoid designing the lenses based on information provided by central keratometry readings. Instead, central and peripheral information obtained from corneal mapping and detailed fluorescein evaluation of diagnostic lenses should be used.For many practitioners, the use of steeper, apical clearance fitting philosophies will be a major departure from their traditional approach to managing keratoconus patients.The successful management of the keratoconus patient with contact lenses requires a complete rethinking of corneal topography; it must be recognized that the disease affects the cornea far beyond the range of the keratometer. Therefore, regardless of the fitting technique used, it is important for the contact lens practitioner to envision a three-dimensional picture of the corneal shape during the fitting process so that steep peripheral fitting relationships (which will eventually lead to superior corneal compression, increased lens awareness, and decreased wearing time) can be avoided. It is anticipated that as new and more accurate computer algorithms are developed for the quantitative assessment of corneal topography, our ability to successfully fit even advanced stages of keratoconus will be greatly enhanced.

 

 

All content is intended as an informational series and should not be used as a substitute for medical advice. Up-dated info-sys 2 060606 (Wiki) FDL KC data.
 
 

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