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Keratoconus - A General Overview of Vision Correction

 

  Methods in the Correction of Keratoconus

Keratoconus is usually discovered during routine examination at the opticians. It is a fairly uncommon condition that affects the cornea - the transparent window at the front of the eye. Due to its smoothly curved shape, the healthy cornea acts as a powerful lens bringing rays of light into focus on the retina at the back of the eye. Two thirds of the focusing power of the eye is provided by the cornea. However, in keratoconus the cornea becomes progressively steep and cone-shaped, causing myopia (short sight) and, if the steepening is uneven, also astigmatism (distortion of vision). It is not known what causes the disease but there is a strong genetic element, and it may run in a family sometimes.

Contact Lenses

In mild keratoconus it is often possible to correct the optical defects of myopia or astigmatism with spectacle lenses. However, if the pattern of corneal steepening is uneven, causing irregular astigmatism, spectacle lenses will only partly restore the vision. If perfect vision is required then contact lenses are necessary. When a Rigid or Gas permeable contact lens is placed onto the cornea, the front surface of the contact lens becomes the new optical surface of the eye, and irregularities of the cornea are filled in by the pool of tears that accumulate behind the contact lens.

Contact lenses will give perfect vision to the majority of keratoconus sufferers but only on a temporary/part-time basis, when compared with surgery when that becomes indicated. Although sometimes contact lenses may still be required after surgery to increase comfort and/or vision when that was difficult to be achieved before with contact lenses. Surgery may make it possible for a simpler/less complex contact lens to be worn. However contact lenses are the best at bringing the curvature of a Keratoconus cornea back to the correct curvature a cornea should be, and therefore should be explored as fully as possible in the first instance when vision correction is required to that level.

Unfortunately general soft contact lenses are not much use in keratoconus because they do not effectively cover over the irregularities of the corneal surface. In recent times technology advances with new types of custom fit soft lenses for Keratoconus is making an appearance in practices. Soft contact lenses for Keratoconus are more gentle on the cornea, creating less aggravation to the condition. For less advanced cases they do give both good vision and comfort for certain individuals. Gas permeable lenses do achieve this more often when contact lenses are required, but only at the price of occasional discomfort when wearing the lenses, particularly if the patient has not worn them before. For some patients the discomfort of Gas permeable lenses is so frequent that they can't be worn realistically for a length of time which they are needed to be used for, or being able to achieve vision which is functional, and vision that is stable to lead a normal life. This is when a good choice of fit, contact lens and other options are required to be evaluated for the individual.

The fitting of contact lenses for keratoconus can be carried out by an optician with experience of Keratoconus after diagnoses.

A skilled and experienced Contact Lens Fitter with all the options in contact lens designs should aim to:

  • Maintain Corneal Integrity
  • Provide Good Vision
  • Provide Comfortable Vision

The fit of the contact lens is very important especially when Gas permeable lenses are needed to be used on a sensitive/"at risk" Keratoconus cornea. This is so that the Gas permeable lenses does not move around too much once worn or the fit causing too much pressure to be placed on the compromised keratoconis cornea (and with every blink). This would help to avoid the Gas permeable lens from scarring the cornea, due to the mechanical friction caused by a bad fitting contact lens rubbing on the cornea. Wearing a soft daily disposable contact lens under a Gas permeable lens can increase comfort and act as a cushion for the Gas permeable lens to "Piggy-back" on. Soft-Perm contact lenses are Gas permeable lens with a soft "skirt" which sometimes proves useful to some patients. A Scleral lens is the only type of contact lenses which does not touch the cornea when worn, and is able to reduce the need for invasive surgery.

 

Corneal X Linking

Corneal Collagen Cross-Linking with Riboflavin (CXL or X Linking) is a new treatment used to stabilize keratoconus, and prevent it from progressing. Riboflavin (vitamin B 2) drops are applied to the cornea for 30 minutes, and the central part of the cornea exposed to UV light for a further 30 minutes. The riboflavin enhances the cross-linking effect of the UV light, as well as acting as a barrier to prevent the light from damaging other eye structures. The treatment increases the rigidity of the cornea and seems to prevent any further deterioration of the corneal condition. Once the keratoconus has been arrested by X Linking, further treatment to correct any residual optical defect may potentially be carried out.

To prevent damage to the inner corneal endothelial cell layer, it is sometimes necessary to build up the corneal thickness to a more normal level. This can be achieved with keratophakia surgery, Dextran or by instilling distilled water in to the cornea before carrying out X Linking.

 

Intacs

Intacs are a new surgical procedure for mild to moderate keratoconus. When inserted into the cornea, the Intacs segments make the central corneal profile flatter and more regular, and this reduces the optical defect. Intacs are 'C' shaped segments of Perspex (polymethyl-methacrylate or PMMA), that are inserted deep into the corneal stroma. Intacs Rings typically only partially correct the optical defect present in keratoconus (especially in advanced cases), therefore additional optical aids or surgical intervention may be required to obtain a full visual correction.

 

Ferrara Rings 

These are smaller than Intacs and are positioned closer to the center of the cornea, they are more powerful than Intacs due to this and can reduce myopia up to -12.00 D. However night-time vision disturbance with glare has been reported in some individuals with larger pupils. Therefore, Intacs are preferred to be used for those with normal sized pupils and greater.

 

Phakic IOLs

The word "Phakic" refers to those who have not undergone cataract surgery and still have their eye's natural internal lens. IOL stands for "intra-ocular lens". In the Phakic IOL procedure, an intra-ocular lens is placed inside the eye. The patient's natural lens is not removed, as it would be in cataract surgery. There are three lens designs under development. The NuVita lens is placed in front of the iris. The Artisan, or iris claw lens is attached on the front of the iris. The Implantable Contact Lens, or ICL, is placed between the iris and crystalline lens.
  
Phakic IOL (Toric) procedures are being used on severely nearsighted and farsighted patients who may not be candidates for the more common laser procedures such as PRK , LASEK, and LASIK. However, unlike laser vision correction procedures that permanently change your vision, it is possible to remove Phakic IOLs (Toric). For Keratoconus patients who are candidates, a treatment first with Corneal X Linking before the implantation of a Phakic IOL (Toric) compliments this procedure.

 

Keratophakia

In keratophakia, a thin slice of donor corneal tissue (a 'lenticule') is surgically inserted into the cornea to build up the corneal thickness. Once the cornea has been restored to a normal thickness, further treatment such as X Linking or laser surgery may then be carried out.

 
Epikeratophakia

In epikeratophakia, the donor tissue lenticule is placed on top of the patients own cornea, to build up the corneal thickness.

 
Deep Anterior Lamellar Keratoplasty (DALK)


In more severe cases of keratoconus the steep area of the cornea becomes very thin and pliable. The stresses set up in the tissue can cause microscopical breaks which lead to scarring in the central part of the cornea, reducing the vision further. The scarring can be removed surgically by cutting away the outer layers of the cornea and replacing them with clear donor corneal tissue. The donor tissue used for this lamellar keratoplasty, or partial thickness graft , can be either fresh or freeze-dried. If the tissue is freeze-dried, no living cells are transplanted, and the patient's own corneal cells grow over the surface of the graft, with no risk of rejection. Lamellar keratoplasty will generally give substantial improvement of the unaided vision, but to obtain the best quality vision, spectacle or contact lens wear may be required.

 

Automated Lamellar Therapeutic Keratectomy (ALTK)

ALTK is a type of partial-thickness (lamellar) corneal graft in which the surface layers of the cornea are removed with a mechanical cutting device, and replaced with a graft of donor tissue that has been prepared in a similar way.

 

Laser Treatment

This is an option when keratoconus has been treated by X Linking, keratophakia, epikeratophakia, or lamellar keratoplasty, and the corneal thickness has been restored to or is of a more normal range. Excimer laser treatment can be carried out to correct some, or possibly all, of any remaining optical defect by vapourising a small area of the corneal tissue surface in a carefully controlled manner. Because of the risk of causing progressive ectasia, in this situation excimer laser treatment is usually performed on the corneal surface in a limited way (Topography-Guided) as photorefractive keratectomy (PRK), or Epi-LASIK (LASEK), rather than as laser in situ keratomileusis (LASIK).

 
Penetrating Keratoplasty (PK)


In severe keratoconus the cornea can become extremely unstable, and breaks in its inner layer (Descemet's membrane) can lead to the accumulation of fluid in the tissue. In this condition, known as acute hydrops , the cornea becomes waterlogged, the eye painful and inflamed, and the vision is severely reduced. Although these problems will settle down, it is almost always necessary to completely replace the central part of the cornea with a full-thickness graft of donor tissue. This type of graft, known as penetrating keratoplasty, must contain living donor cells if it is to remain clear. It can restore vision in a high proportion of cases and typically 80% - 90% of patients can expect to have a functioning graft 5 years after surgery. Full-thickness grafts may however fail or become rejected at any time, leading to severe visual loss. Although a full-thickness graft can be repeated, the chances of its survival are then not quite so good. Tissue-matching is sometimes recommended for re-grafts, but is generally undertaken only as a last resort, since waiting for matched tissue may delay the possibility of surgical treatment for an unacceptably long period.

Laser assisted cutting of the corneal tissue needed during the many forms of corneal surgery, in certain select locations, are being performed, with an aim to get a more favorable out-come than with traditional ways and methods.


AlphaCor - Artificial Cornea

The AlphaCor is a recently introduced device which is more flexible than the "Legeais" keratoprosthesis, and is made of a central clear hydrogel lens with an opaque surrounding skirt, which becomes bio-integrated into the cornea. Because the prosthesis is made from a hydrated hydrogel, the recipient eye must have a reasonable tear film, which is often not the case for many patients requiring a keratoprosthesis. Patients suitable for the AlphaCor are those for whom the chances of successful outcome of a conventional corneal graft are very poor, eg those who have had multiple previous graft failures from rejection.

 
All content is intended as an informational series and should not be used as a substitute for medical advice.

 

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