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Corneal Dystrophy, Thiel-Behnke
This type of anterior corneal dystrophy is genetically heterogeneous (caused by mutations in more than one locus). Recurrent corneal erosions are the main clinical feature and can begin in the first and second decades. The epithelium is irregularly thickened while the Bowman layer and basal lamina of the basement membrane have degenerative changes which lead to the clinically evident honeycomb pattern of opacities. Advanced changes in these tissues eventually leads to some vision loss.
The honeycomb pattern of degenerative changes in the corneal epithelium and Bowman membrane helps to distinguish this disorder from other anterior corneal dystrophies. These are more prominent centrally with relative sparring of the juxtalimbal areas. The epithelial basement membrane may be missing in some areas. Histology is required for a definitive diagnosis with electron microscopy revealing characteristic ‘curly’ collagen fibrils in the subepithelial and anterior stromal tissues. These degenerative changes tend to recur even after ablative procedures.
There is a great deal of clinical heterogeneity and the diagnosis is often unclear especially in younger individuals. No doubt much of this is due to the fact that mutations in the major gene (TGFBI) responsible are also responsible for at least 5 other heritable corneal dystrophies and the argument can be made that all are variants of the same condition (vida infra).
No systemic disease is associated with this corneal disease.
Thiel-Behnke dystrophy is an autosomal dominant disorder. However, it is genetically heterogeneous as mutations in at least two genes seem to produce the same phenotype. The majority of cases result from mutations in the TGFBI gene (5q31) but other corneal dystrophies (granular I or Groenouw type I, combined granular/lattice or Avellino type, Reis-B√ºcklers, epithelial basement membrane disease, and lattice type I) have mutations in the same gene. This is a classic example of the variable expressivity of a single gene mutation characteristic of autosomal dominant disease.
A second locus has been identified in a large 4 generation pedigree in which a presumed causative mutation was found on chromosome 10 (10q24). Some individuals in this family had evidence of two distinct types of dystrophies in the same cornea. The responsible gene has not been identified. Genotyping is necessary to distinguish between the two disorders.
Ablative treatments of the diseased cornea can be effective in reducing symptoms for extended periods but eventually the degenerative changes recur. Acute erosions can be treated with hyperosmotic agents with some improvement.