keratoconus

A clinical condition characterised by thinning and forward protrusion of the cornea resulting in conical shape. This disorder is usually bilateral, with onset at the age of 10 -14 years. Affects both sexes, with a slight female preponderance. Although it has been reported in families, the risk of inheritance is reasonably low.

Why does it occur ?

Etiology of this condition is still not clear. . Eye rubbing, hormonal variations , genetic predisposition have been proposed to be causative factors in the disease. It is possible that this condition could be secondary to increased breakdown of the stromal collagen lamellae by the lysosomal enzymes released by the basal epithelium in these eyes.

The substance of the cornea consists of hundreds of layers that are linked to each other by a substance called as collagen. if these collagen cross links between the layers are lost due to keratoconus, there is a progressive corneal thinning and stretching which progresses often in both eyes. The cornea bulges forwards into a irregular cone shape causing distortion of image.

The eye develops astigmatism and myopia and vision becomes severely blurred.

Clinical Features

The common conditions are:

• decreased vision
• astigmatism and myopia
• vision not improving with glasses and contact lenses

Keratoconus is characterized by central thinning and forward protrusion of the cornea. Angulation of the lower lid in downgaze known as Munson's sign, which is a nonspecific sign of advanced keratoconus

Management

Glasses can be prescribed to correct the induced astigmatism in early keratoconus. Once the cylindrical power increases beyond 4.0 diopters, patients find it difficult to tolerate such high power in glasses.

Semisoft or RGP or Rose K contact lens, can help in cases with mild to moderate keratoconus. The front surface of the contact lens now acts like the front surface of the cornea with a regular shape. This provides reasonably clear vision for normal daily activities. However, fitting of contact lens in eyes with keratoconus requires expertise in CL fitting and a special set of trial lenses. With increasing progression of keratoconus, patients may find it difficult to wear the contact lens, and at times the lens keeps falling out of the eye. In such situations, piggy back lens i.e. RGP lens mounted on a soft contact lens can be tried.

Collagen crosslinking or C3R or CXL

This is a new modality of treatment for keratoconus, where in Riboflavin eye drops are applied followed by exposure of the cornea to ultraviolet radiation for a duration of 30 minutes. This results in cross linking of the collagen fibres of the cornea, thereby increasing its physical strength by upto 300%. Thus further progression of keratoconus can be arrested, and in 15 -20% cases regression has also been noted. Longest follow-up duration following this treatment is for 10 years.

Intacs implants

This is a newer surgical treatment for mild to moderate keratoconus. In this surgery, fine plastic implants are placed in the peripheral cornea, which helps flatten the central cornea, thereby improving the unaided vision. Patients are able to return back to normal glass wear or comfortable contact lens wear post treatment. This procedure is painless and simple to perform. The surgery is performed under topical anesthesia using the fem to second laser, and takes only 5 -10 minutes per eye.

Surgical treatment

Surgical intervention is often necessary in advanced Keratoconus to restore corneal anatomy and thereby improve quality of vision. Traditionally full thickness corneal transplantation procedures have been commonly performed for advanced keratoconus.

Although this procedure has successful outcome in a large number of cases, it sacrifices the healthy endothelium of the host

cornea. The transplanted donor cornea is at risk for rejection, and requires steroid therapy for a long duration to prevent rejection. This predisposes the eye to steroid induced complications such as cataract and glaucoma.

Deep Anterior Lamellar Keratoplasty (DALK)

With advancement in corneal surgical techniques, it is now possible to selectively remove the anterior layers from the cornea and replace it with donor tissue to restore its anatomy and function. Deep anterior lamellar keratoplasty (DALK) is one such procedure wherein the host corneal endothelium is retained, and anterior corneal tissue is replaced with normal thickness donor tissue. As the host endothelium is retained there is no risk of rejection, and steroids have to be given only for a short duration of time. However DALK surgery requires more surgical expertise compared to the traditional full thickness keratoplasty, and hence performed by only well trained corneal surgeons all over the world.