KERATOCONUS , KERATOGLOBUS , KERATOCONUS POSTERIOR

KERATOCONUS
Keratoconus (conical cornea) (Fig. 1) is a noninflammatory bilateral (85%) ectatic condition of cornea in its axial part. It usually starts at puberty and progresses slowly.

Etiopathogenesis. It is still not clear. Various theoriesproposed so far label it as developmental condition,degenerative condition, hereditary dystrophy and

endocrine anomaly. Essential pathological changes are thinning and ectasia which occur as a result of defective synthesis of mucopolysaccharide and
collagen tissue.

Clinical features. 

Symptoms. Patient presents with a defective vision due to progressive myopia and irregular astigmatism, which does not improve fully despite full correction with glasses.

Signs. Following signs may be elicited:
1. Window reflex is distorted.
2. Placido disc examination shows irregularity of the circles (Fig. 1B).
3. Keratometry depicts extreme malalignment of mires.
4. Photokeratoscopy reveals distortion of circles.
5. Slit lamp examination (Fig. 1C) may show thinning and ectasia of central cornea, opacity at the apex and Fleischer's ring at the base of cone, folds in Descemet's and Bowman's membranes. Very fine, vertical, deep stromal striae (Vogt lines) which disappear with external pressure on the globe are peculiar feature.
6. On retinoscopy a yawning reflex (scissor reflex) and high oblique or irregular astigmatism is obtained.
7. On distant direct ophthalmoscopy an annular dark shadow (due to total internal reflection of light) is seen which separates the central and peripheral areas of cornea (oil droplet reflex).
8. Munson's sign, i.e. localised bulging of lower lid when patient looks down is positive in late stages.
  

Fig. 1. Keratoconus showing: A, configuration of
cone-shaped cornea; B, irregular circles on Placido disc
examination; C, clinical photograph.

Morphological classification. 

Depending upon the size and shape of the cone. the keratoconus is of three types:

• Nipple cone has a small size (<5mm) and steepcurvature.
• Oval cone is larger (5-6 mm) and ellipsoid in shape.
•  Globus cone is very large (>6 mm) and globe like.

Complications. Keratoconus may be complicated by development of acute hydrops due to rupture of Descemet's membrane. The condition is characterised by sudden development of corneal oedema associated with marked defective vision, pain, photophobia and lacrimation.

Associations. Keratoconus may be associated with :

• Ocular conditions e.g. ectopia lentis, congenital cataract, aniridia, retinitis pigmentosa, and vernal keratoconjunctivitis (VKC). 
• Systemic conditions e.g., Marfan's sysndrome, atopy, Down's syndrome, Ehlers-Danlos syndrome, osteogenesis imperfecta and mitral valve prolapse.

Treatment. Falling vision may not be corrected by glasses due to irregular astigmatism.

• Contact lenses (rigid gas permiable) usually improve the vision in early cases.
• In later stages penetrating keratoplasty may be required.
• Intacs, the intracorneal ring segments, are reported to be useful in early cases.

KERATOGLOBUS
It is a familial and hereditary bilateral congenital disorder characterised by thinning and hemispherical protrusion of the entire cornea. It is non-progressive and inherited as an autosomal recessive trait. It must be differentiated from congenital buphthalmos, where increased corneal size is associated with raised intraocular pressure, angle anomaly, and/or cupping of optic disc.

KERATOCONUS POSTERIOR
In this extremely rare condition there is slight conelike bulging of the posterior surface of the cornea. It is non-progressive.

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