Schnyder corneal dystrophy (SCD) is a hereditary corneal dystrophy caused by mutations in the UBIAD1 (UbiA prenyltransferase domain containing 1) gene, leading to abnormal lipid metabolism in the cornea and abnormal deposition of cholesterol and phospholipids in the corneal stroma. The inheritance pattern is autosomal dominant, and the gene locus is on 1p36.
It was first reported by Van Went and Winbaut in 1924. After the Swiss ophthalmologist Schnyder reported a family spanning three generations in 1929, the disease name became established. In the IC3D classification, it is classified as a stromal corneal dystrophy.
This disease is bilateral, with corneal opacification beginning in childhood but progressing slowly. A clinical feature is that visual acuity loss is mild relative to the degree of corneal opacification.
Despite slit-lamp findings, significant visual acuity loss is often not observed. However, opacification progresses with age, and visual dysfunction due to glare and light scattering may occur. Reports indicate that approximately 54% of patients over 50 years old and 77% over 70 years old eventually require corneal transplantation.
Corneal findings progress in a characteristic pattern with age.
| Age group | Main corneal findings |
|---|---|
| Childhood to teens | Crystalline opacity in the central cornea |
| 20s to 30s | Arcus-like limbal opacity appears |
| 40s and older | Central and limbal opacity progresses to full thickness |
Pathological findings: Oil red O staining shows fat droplets stained red. Electron microscopy reveals vacuole formation in the corneal stroma.
The cause of SCD is a mutation in the UBIAD1 gene (1p36). UBIAD1 encodes a prenyltransferase that synthesizes menaquinone-4 (MK-4, vitamin K2). This gene mutation leads to abnormal lipid metabolism in the cornea, resulting in cholesterol deposition in the corneal stroma. Genetic testing is useful for definitive diagnosis.
Hypercholesterolemia is known to be associated, and evaluation of the lipid profile is recommended. Additionally, genu valgum and rarely finger or spinal deformities may occur.
Ophthalmic Examination
Slit-lamp microscopy: This is the basic diagnostic method. It evaluates corneal crystals, central opacity, and arcus senilis-like changes.
Confocal microscopy: Can confirm the loss of corneal nerves. In advanced stages, highly reflective deposits are observed.
Anterior segment OCT: Shows diffuse hyperreflective findings in the epithelium, anterior, middle, and posterior stroma.
Definitive Diagnosis and Systemic Examination
Genetic testing: Identification of a mutation in the UBIAD1 gene provides a definitive diagnosis.
Blood tests: Evaluate the serum lipid profile (total cholesterol, HDL, LDL, triglycerides).
As a cause of corneal opacity, SCD is classified as a stromal corneal dystrophy in the IC3D classification1).
Despite slit-lamp microscope findings, visual acuity loss is often mild, and many cases do not require active treatment. Regular follow-up is the mainstay.
Currently, there is no drug therapy to halt progression.
Cholesterol deposition may recur in the graft after penetrating keratoplasty. The timing and extent of recurrence vary among individuals, but regular follow-up after transplantation is necessary.
SCD is caused by mutations in the UBIAD1 gene. UBIAD1 encodes a prenyltransferase that synthesizes menaquinone-4 (vitamin K2), and dysfunction of this enzyme leads to disruption of lipid metabolism in the cornea.
Normally, the corneal stroma contains very small amounts of cholesterol and phospholipids. However, UBIAD1 mutations impair the metabolism of HDL cholesterol in particular, causing excessive accumulation of cholesterol and phospholipids in the corneal stroma. The accumulated lipids precipitate as crystals, impairing corneal transparency.
Confocal microscopy has confirmed the disappearance of corneal nerves. However, despite nerve loss, no obvious neurotrophic keratopathy has been reported. The mechanism by which lipid deposition affects corneal nerves is not fully understood.
