Keratoconus Doctor in Houston
The cause of keratoconus is not known. It may not be a single disorder, but rather a phenotypic expression of perhaps many causes, both genetic and environmental.
The inheritance pattern of keratoconus is incompletely defined. In the past it was believed that more than 90% of cases were sporadic. With the advent of videokeratography to assess family members, however, pedigrees have been analysed. These studies show corneal changes consistent with keratoconus in some family members, which suggest an autosomal dominant pattern of inheritance.
Keratoconus may be associated with wide variety of systemic and ocular conditions.
- Atopy (a genetic predisposition to develop an allergic reaction): One of the most common associations of keratoconus is atopy and is seen in condition like vernal keratoconjunctivitis. Eye rubbing seen in systemic atopy may play a role in the development of keratoconus.
- Down syndrome (Trisomy 21): In Down syndrome (Trisomy 21), frequency of acute hydrops is higher, perhaps because of eye rubbing and/or these patients are treated infrequently with keratoplasty and their disease is allowed to progress further.
- Ehlers-Danlos syndrome.
- Marfan syndrome.
- Crouzon syndrome.
- Apert syndrome.
- Leber’s congenital amaurosis.
- Retinitis pigmentosa.
- Retinopathy of prematurity.
- Fuchs’ corneal endothelial dystrophy.
- Posterior polymorphous dystrophy.
Given the relatively high prevalence of keratoconus in general population, some of the associations may be coincidental.
Studies suggest various contributory factors such as:
- Enzyme abnormalities in corneal epithelium: Enzyme abnormalities such as increased expression of lysosomal enzymes (catalase and cathepsin) and decreased levels of inhibitors of proteolytic enzymes (tissue inhibitor matrix metalloproteinases), may play a role in corneal stromal degradation.
- Differentially expressed corneal epithelium: Differentially expressed corneal epithelium between keratoconus and myopes (as controls) in both genetic expression and protein expression.
- Molecular defect: Molecular defect producing unusual absence of water channel protein aquaporin 5 in keratoconus as compared to normal corneal epithelium.
- Gelatinolytic activity: Gelatinolytic activity in stroma has been described, which may be due to decreased function of enzyme inhibitors.
- Abnormalities in corneal collagen and its cross-linking: Abnormalities in corneal collagen and its cross-linking may be the cause of keratoconus.
- Eye rubbing: Cytokine has been suggested as a mediator of eye rubbing and stromal degradation. Keratoconus in patients with atopic dermatitis, Down syndrome, and Leber congenital amaurosis, a congenital photoreceptor degeneration, could be related to forceful eye rubbing or the oculodigital reflex. The latter is a behavioural pattern seen in visually and mentally handicapped children who repeatedly strike their eyes with hands.
- Hard contact lens wear.
All layers of the cornea are believed to be affected by keratoconus, although the most notable features are the thinning of the corneal stroma, the fragmentation of the Bowman layer and the deposition of iron in the basal epithelial cells, forming the Fleischer ring. Folds and breaks in the Descemet’s membrane result in acute hydrops and striae, which produces variable amount of diffuse scarring.
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