Cornea is the clear structure in the front part of the eye, which together with the crystalline lens inside the human eye, helps focus light onto the retina. Any abnormality in the shape of the cornea has a direct impact on the quality of vision.
Keratoconus refers to a clinical condition characterised by thinning of the cornea and its forward protrusion, giving it a conical shape. This condition is thought to be due to an inherent defect in the structure of the cornea. One of the proposed hypotheses is, that frequent eye rubbing associated with eye allergy results in micro-trauma to the corneal surface, which releases enzymes leading to breakdown of collagen and thinning of the cornea. The thin cornea is structurally weak and is no longer able to maintain its normal curvature, and normal eye pressure causes it to bulge outwards into a conical shape.
The disease usually manifests in children and young adults, often affecting both eyes, although the severity may be different. Progression of disease is variable, often associated with worsening vision.
The earliest symptom of keratoconus is usually inability to read small letters at a distance, which is difficult to correct even with glasses. Later on, symptoms such as ghosting of letters, doubling i.e. double vision, or blurred vision result in further drop in vision. Glasses do not help at this stage, and vision can only be improved by wearing special contact lenses such as soft toric contact lenses (Kerasoft), or rigid gas permeable contact lens (Rose K contact lens). Other options of lenses include hybrid lenses with a rigid central part and a peripheral soft skirt such as synergy lens. In very advanced stage of the disease, Boston Ocular Surface Prosthesis (BOSP), a large diameter contact lens, works well to improve eyesight, although it takes a lot of motivation to wear the large contact lens on the eye.
Corneal topography is a diagnostic test which maps the entire surface of the cornea, and presents a colour coded map of the curvature of the cornea and its thickness. This test is very sensitive and can diagnose early keratoconus even before the changes have become clinically apparent. It can also be used to monitor progression of keratoconus by serial measurements over time.
Pachymetry is a diagnostic test which allows measurement of corneal thickness. As there is corneal thinning in keratoconus, serial pachymetry is also helpful in monitoring progression. It can also help in patient selection for various interventional procedures for keratoconus.
- Collagen crosslinking is a new procedure, which helps improve physical strength of corneal collagen fibres, thereby preventing progression of keratoconus. This procedure involves application of riboflavin phosphate drops on the cornea, followed by exposure to UVA light.
- Intacs are implants made of plastic, which help flatten the cornea once they are placed within the cornea via a minor surgical procedure. This can reduce severity of keratoconus and thus take away the need for contact lens wear.
- Toric ICL (STAAR Surgicals) is a special implant, which can correct refractive error (glass power) due to keratoconus in select patients. This lens is urgically implanted in front of the natural lens and visual improvement is seen after surgery.
- Deep Anterior Lamellar Keratoplasty (DALK) is a special type of corneal replacement surgery, wherein only the abnormal outer layers of the cornea are replaced with healthy donor tissue to build up corneal thickness and normal curvature. Since the inner layer is not replaced, the body does not recognise the donor tissue, hence has minimal risk of rejection by the body.
- Penetrating Keratoplasty (PK) is a corneal replacement surgery, wherein all the layers of cornea are replaced with donor tissue. This is required in the last stage of keratoconus, wherein there is scarring of the cornea with involvement of the inner layer.
Keratoconus can be associated with other systemic conditions like atopic dermatitis, asthma, Marfan’s syndrome etc., and also with other eye conditions such as allergic conjunctivitis, retinitis pigmentosa etc. Hence, it is important to be examined for any associated disorders.
Prognosis for keratoconus is usually good,and near normal eyesight can be restored by one or more procedures by a corneal specialist.