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Corneal Dystrophy, Schnyder
Schnyder corneal dystrophy has its onset early in life as a haziness of the central cornea with some peripheral extension. The stroma gradually becomes more hazy and eventually in about 50% of patients yellow-white crystalline deposits can be seen in an annular pattern in the Bowman layer and the adjacent stroma just beneath. The remaining layers of the cornea are not involved. The needle-shaped crystals are often birefringent and composed of cholesterol and phospholipids. There is considerable variation in the progression of disease and in the symmetry of disease in the two eyes. Visual acuity may be relatively good in young people but older patients with denser central opacification eventually require corneal transplantation for better vision.
Some patients have hypercholesterolemia and hyperlipidemia. Skin fibroblast cultures in one patient have shown cytoplasmic deposits consistent with unesterified cholesterol but another study failed to find such deposits in skin or conjunctiva. Evidence points to a metabolic disorder of lipid metabolism in the cornea but the evidence for a more generalized systemic disorder is inconclusive. Genu valgum has been reported in some patients.
Schnyder crystalline dystrophy of the cornea results from a mutation in the UBIAD1 gene located on chromosome 1 (1p36.3). Multiple mutations have been identified. It is inherited in an autosomal dominant pattern.
Penetrating keratoplasty can be helpful in restoring vision but the corneal deposits and opacification often recur. PTK procedures can also be beneficial.