Differential diagnosis of Schnyder corneal dystrophy

Schnyder corneal dystrophy (SCD) is a rare corneal dystrophy characterized by abnormally increased deposition of cholesterol and phospholipids in the cornea leading to progressive vision loss. SCD is inherited as an autosomal dominant trait with high penetrance and has been mapped to the UBIAD1 gene on chromosome 1p36.3. Although 2/3 of SCD patients also have systemic hypercholesterolemia, the incidence of hypercholesterolemia is also increased in unaffected members of SCD pedigrees. Consequently, SCD is thought to result from a local metabolic defect in the cornea. The corneal findings in SCD are very predictable depending on the age of the individual, with initial central corneal haze and/or crystals, subsequent appearance of arcus lipoides in the third decade and formation of midperipheral haze in the late fourth decade. Because only 50% of affected patients have corneal crystals, the International Committee for Classification of Corneal Dystrophies recently changed the original name of this dystrophy from Schnyder crystalline corneal dystrophy to Schnyder corneal dystrophy. Diagnosis of affected individuals without crystalline deposits is often delayed and these individuals are frequently misdiagnosed. The differential diagnosis of the SCD patient includes other diseases with crystalline deposits such as cystinosis, tyrosinemia, Bietti crystalline dystrophy, hyperuricemia/gout, multiple myeloma, monoclonal gammopathy, infectious crystalline keratopathy, and Dieffenbachia keratitis. Depositions from drugs such as gold in chrysiasis, chlorpromazine, chloroquine, and clofazamine can also result in corneal deposits and are different from SCD. Diseases of systemic lipid metabolism that cause corneal opacification, such as lecithin-cholesterol acyltransferase deficiency, fish eye disease and Tangier disease, should also be considered although these are autosomal recessive disorders.