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Keratoconus
   

 

 

Introduction

 
Keratoconus (kehr-uh-toh-KOH-nus) is the most degenerative condition of the front of the eye. Keratoconus can lead to variable, complex, unstable and severe visual impairment. Its an unpredictable progressive eye condition with an unclear etiology and pathogenesis. Keratoconus is generally first diagnosed in young people, resulting in a disproportionate negative effect on their quality of life. Keratoconus is an ectasia of the cornea with an incidence in the general population of 1 to 430/2000. The cornea is the clear front window of the eye, responsible for focusing most of the light coming into the eye.

Keratoconus is not a commonly known eye disorder, although it's by no means rare. Keratoconus is becoming more common due to the increasing use of modern diagnostic equipment at refractive clinics and at optometry practices where patients who seek vision correction are often unaware of their condition.

A late or missed diagnosis and care of Keratoconus is typically the problem encountered by innocent patients. Keratoconus patients want as many options as possible to be able to live as close to a normal life as possible. This is normally achievable with early diagnosis and an unbiased presentation and discussion of all the treatment options.

Corneal transplants are the most commonly performed transplant surgery. Keratoconus is the most common degenerative condition of the front of the eye which leads to corneal transplantation in the Western world.

Keratoconus (KC) is a condition in which the normally round, dome-shaped cornea (front window of the eye) thins and becomes distorted and irregular. A cone-like bulge develops, resulting in significant visual distortion. The apex of the cone is usually displaced outwards and downwards and in the line of sight, creating irregular astigmatism.

The known prevalence of KC (1 to 400/2000) clusters at optical centers, but it does not include those who have been misdiagnosed or those who are made aware too late that their symptoms were actually due to keratoconus. These patients deserve nothing less than the earliest awareness of those treatment options that can make the most crucial difference in their lives before it is too late.  Keratoconus, after all, does not just diminish visual acuity, it has lasting negative impact on all aspects of a patient's life.

Until recently, Keratoconus was managed by fitting rigid gas-permeable contact lenses, and penetrating keratoplasty when contact lenses could not be tolerated any more. With one in five needing a penetrating keratoplasty (the traditional corneal transplant), our mission was to reduce the numbers having this invasive surgery, and which is now happening. 

Advanced treatment modalities enable earlier intervention to improve the prognosis and visual rehabilitation, are now available.

 

Keratoconus
 
An eye with Keratoconus
 

Etiology

  • The underlying reason for its development remains obscure; currently the topic of much research
  • Usually an inherited corneal disorder, often in an autosomal recessive fashion 
  • Family members may have it, but it also can be random with no other family members affected (sporadic)
  • Most likely that the cause is due to more than one factor

The path that leads to the thinning of the cornea may start with a "trigger", followed by a cascade of events in the eye tissues

The primary trigger is not known

 Possible options include hormonal changes in the patient, genetic predisposition to altered cytokines and/or their receptors in the patient's cornea, or a pattern of eye-rubbing or poor fitting RGP lenses that could release factors

Abnormal levels of degradative protease activity may then lead to a slow, progressive dissolution of Bowman's layer and the epithelial basement membrane

The epithelium then comes into contact with the stroma, cytokines/growth factors are released, and as a result, the cells begin to produce scar tissue and proteases

  • A disorder with local micro-environmental changes rather than a situation where the entire cornea is involved

Within a single keratoconus cornea there are areas of increased protease activity (thinning) and other areas of ongoing wound healing (scar tissue buildup)

  • Recent advances in molecular techniques make it possible to examine the molecular components in wounded or diseased corneas

CD45 (leukocyte common antigen or T200), a transmembrane phosphotyrosine phosphatase (normally found associated with blood cells), has been found to be present in more cells within keratoconus corneas than normal

 These cells may represent the source of some of the proteolytic enzymes reported to be associated with keratoconus

A gene product referred to as glucose regulated protein (GRP78) has been found in greater quantity in normal corneas

 A protein that aids or "chaperones" the folding and secretion of other proteins produced in cells.The reduction in GRP78 could affect the secretion and folding of the major structural proteins of the cornea which are reduced in the keratoconus cornea.

The cause of Keratoconus is not fully known
 

Keratoconus 
 The map above shows the distribution of the general population world-wide

 

Incidence and Carrier Frequency 

  • Some cases have a hereditary component and studies indicate that about 10% of patients have affected relatives
  • Most cases appear to be sporadic
  • The actual incidence is uncertain, large studies estimate 50 to 230 per 100,000
  • According to one study, the estimated prevalence in first-degree relatives is 3.34%, which is 15 to 67 times higher than that in the general population (0.23-0.05%). (Am J Med Genet 2000 Aug 28;93(5):403-9 )
  • Occurs in all ethnic groups, with certain ethnic groups effected more
  • Has a slight or raised male/female preponderance depending on the study
  • Is more common in contact lens wearers and nearsighted eyes
  • Some researchers believe allergy may play a role

Keratoconus
  
Shown above is the contour of a normal cornea 
 
 



Keratoconus
 
Shown above is the contour of a Keratoconus cornea

 

Clinical Features 
  
  • Non-inflammatory eye condition in which the normally round dome-shaped cornea progressively thins causing a cone-like bulge to develop

 Results in significant visual impairment

  • A degenerative condition where the cornea thins in affected areas and scar tissue develops

 Can lead to astigmatism - often regular at first but becoming increasingly irregular as the disease progresses

  • Surface thinning can create several optical zones that individually focus the same image to different areas of the retina, thus creating multiple perceived images
  • Has been associated with other medical disorders including atopic disease, Down's syndrome, Ehlers-Danlos syndrome, Marfan's syndrome, craniofacial dysostosis and osteogenesis imperfecta
  • Not a blinding disorder in the classical sense, but does result in increasing near-sightedness (things are out of focus, varying to counting fingers distance or worse) and irregular astigmatism (things look distorted)
  • Is almost always bilateral (90% of cases)

Natural History 

  • In the earliest stages, slight blurring, distortion of vision, and increased sensitivity to glare and light occur

 Symptoms usually first appear in the late teens and early twenties

 Mean age of onset is 16; onset as young as 6 yrs has been recorded

 Rarely develops after age 40

  • May progress for 10-20 years and then slow in its progression

 May halt at any stage from mild to severe

  • Each eye may be affected differently

 Disease develops asymmetrically: diagnosis of the disease in the second eye generally occurs about five years after diagnosis in the first

  • Patients may endure discomfort and reduced vision over a long period of time.



Keratoconus 
   
Testing
  
  • Can usually be diagnosed with slit lamp examination of the cornea
  • Early cases may require corneal topography, a test that makes a stereo image which gives a topographic map of the corneal curvature
  • When advanced, the cornea will be thinner in areas and this can be measured by pachymetry
  • The biomicroscope is the only tool which allows a clinician to observe many classical signs of keratoconus: Fleischer's ring, stress lines of Vogt, corneal thinning and scarring, various types of staining with and without lens wear, increased visibility of corneal nerves, and corneal hydrops

 

Keratoconus

 Ghosting

Keratoconus
 
Glare
 
 Keratoconus
 
Normal Vision

 

 Symptoms

As Keratoconus progress’s the abnormalities of the cornea severely affect the way we see the world, making simple tasks like driving, watching TV or reading a book, difficult. This distortion has been compared to viewing a street sign through your car windshield during a driving rainstorm.

  • Decreased visual acuity due to irregular astigmatism and/or corneal scarring (substandard vision or frequently changing prescriptions with glasses or contact lenses)
  • Sensitivity or intolerance to contact lenses
  • Distorted and blurred vision (depending on light levels, changing/patchy/fluctuating vision can also occur
  • Varying contrast, ghosting, streaking of light, multiple images
  • Glare and light sensitivity
  • Eye strain, fatigue and headaches
  • An acute corneal hydrops may induce pain


Keratoconus
 

Signs

  • Bilateral, progressive thinning of the corneal stroma, most often paracentral temporal inferior
  • Distortion of the corneal contour with keratometry and computerized corneal topography, retinoscopy, and keratoscopy
  • Vertical Vogt's striae may be evident in Descemet membrane at the apex of the cone
  • Munson's sign: indentation of the lower lid by the conic cornea in downgaze
  • Fleischer's ring: ring shaped iron deposition at the level of the epithelial basement membrane
  • Descemet membrane rupture may result in acute hydrops with stromal edema and subepithelial central scarring

 Corneal curvature
Mild keratoconus    < 45D
Moderate keratoconus    45 - 52D
Advanced keratoconus    52 - 65D
Severe keratoconus    > 62D

 

Corneal thickness in microns
Normal cornea     543 Microns
Early keratoconus    506 Microns
Moderate keratoconus    473 Microns
Advanced keratoconus    446 Microns

  

Keratoconus

 

      Management
      • Glasses can be prescribed to correct the induced astigmatism in early Keratoconus. Once the cylindrical power increases beyond 4.0 diopters visual intolerance may occur at which time prescribed contact lenses are needed
      • UV - Crosslinking to treat the progression of KC, which in differential use (please ask your ophthalmologist) can be accompanied in accordance to the remit of various other vision correction methods (as seen below
      • Fitting of high performance soft lenses, soft-perm hybrid lenses, piggybacking lens systems, and rigid gas-permeable Prescribed Contact Lenses
      • Scleral Lens Prosthetic Device

Correct fitting and continued aftercare is essential with all medically necessary Contact Lenses.

Selective cases (and by selecting your surgeon carefully) may be suitable for the following surgical operations :

      • Intacs
      • Ferrara Rings
      • Implantable Contact Lenses (Phakic / Toric)
      • Deep anterior lamellar keratoplasty (DALK)

      At select locations, selective cases may be eligible for (at the same time as UV - Crosslinking) limited customized topography guided surface ablation, keraflex, microscopic surgery, wedge resection, epikeratoplasty or an ALTK. A Penetrating Keratoplasty is considered as an absolute last option.


Prognosis 
With advanced treatment modalities, complete visual rehabilitation may be achieved with an excellent long-term prognosis.
 
 
 


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