Anomalies in Size and Curvature of Cornea – Keratoconus, Keratoglobus, Megalocornea, Microcornea.
Keratoconus - usually a sporadic disorder but may occasionally be inherited. Thinning of the centre of the cornea leads to an ectatic, conical corneal shape, with marked myopia and an irregular astigmatism. The condition is due to a failure of adhesion between the collagen fibrils of the stroma, responsible for its mechanical strength. Thinning is due to an unravelling process.
Clinical features - cone-like bulging of the central cornea, presenting during the second or third decades of life with slowly progressive blurring of vision from irregular astigmatism. Both eyes are affected in almost all cases, though often asymmetrically.
- a. Early signs - abnormal ophthalmoscopy reflex, irregular ‘scissors’ retinoscopy reflex and fine vertical lines in the deep stroma.
- b. Late signs - iron deposits at the base of the cone (Fleischer ring), bulging of the lower lid in downgaze (Munson sign), central corneal oedema of sudden onset (acute hydrops) and scarring.
- c. Systemic associations - atopic dermatitis, osteogenesis imperfecta; syndromes – Down’s, Turner’s, Ehlers–Danlos, Marfan’s.
Treatment –
- Refractive correction with spectacles is initially adequate
- As progression occurs, correction with contact lenses Corneal grafting in advanced cases
- Supportive treatment for acute hydrops.
Keratoglobus - very rare congenital bilateral thinning and protrusion of the entire cornea. Acute hydrops may occur in some cases.
Megalocornea - very rare, X-linked recessive condition with corneal diameters over 13 mm. May develop lens subluxation. The cornea is usually clear with normal thickness and vision. It is often associated with Marfan’s syndrome.
Microcornea - corneal diameters of less than 10 mm. Hypermetropia and narrow angle glaucoma may be found in later years. The condition may occur as an isolated anomaly or in association with microphthalmos.