Contributor: Gordon K. Klintworth
Schnyder crystalline corneal dystrophy (Schnyder dystrophy, central crystalline corneal dystrophy, MIM #121800) is an autosomal dominant corneal disorder caused by a mutation in the UBIAD1 gene. The condition usually begins at ~1 year of age. Both corneas are involved more or less symmetrically. Usually a round, ring-shaped opacity containing white-yellow or polychromatic crystals forms in the central corneal stroma. Peripheral corneal opacification is separated from the corneoscleral limbus by a clear line. Affected individuals may develop a diffuse stromal haze. Corneal sensation is normal. The clinical presentation is often diminished vision. Visual acuity is usually not severely affected, but occasionally penetrating keratoplasty becomes necessary to improve vision. Tissue examination reveals needle-like crystals of cholesterol and neutral fat. Some patients may have hyperlipidemia, xanthelasma, arcus lipoides, as well as genu valgum. Over time an initially unremarkable corneal stroma acquires small white opacities and a diffuse haze. Occasionally crystals are not evident clinically and only parts of the central corneal opacity may contain crystals. Typically a ring-shaped yellow-white opacity composed of innumerable fine needle-shaped crystals forms beneath the epithelium in Bowman layer and the adjacent anterior stroma of the central cornea. The crystals usually remain in the anterior third of the cornea. The remaining stroma is unremarkable initially, but with time it may acquire small white opacities and a diffuse haze. While sometimes appearing dull white, the crystals are frequently scintillating with variegated red and green hues. The epithelium, Descemet membrane and the endothelium are spared. In some cases crystals are only seen in parts of the central corneal opacity. The condition is usually bilateral, but one eye may become affected earlier than the other. The lipid deposits within the cornea are predominantly phospholipid and cholesterol (esterified and unesterified) probably reflect defective lipid metabolism. The predominant phospholipid is sphingomyelin. Most cases lack an apparent systemic disorder, but hypercholesterolemia is common, and so is an arcus lipoides, xanthelasma, familial hypercholesterolemia, familial dysbetalipoproteinemia or hypertriglyceridemia and other manifestations of hypercholesterolemia have been reported. An ultrastructural study of a skin biopsy and cultured fibroblasts from an affected person has disclosed lipid containing membrane-bound spherical vacuoles. Because the disorder usually stabilizes with time, only occasional patients with severe impairment require corneal grafting.