BASICS
DESCRIPTION
Schnyder’s corneal dystrophy (SCD) is a rare bilateral condition characterized with slowly progressive abnormal deposition of lipids in the cornea.
EPIDEMIOLOGY
Incidence
• Unknown
• Worldwide largest pedigree (>200 patients with SCD) has a Swede-Finn heritage.
RISK FACTORS
Genetics
• Defective gene localized to short arm of chromosome 1, at 1p34–36 (1)[A].
– Mutations in the UbiA prenyltransferase domain containing protein 1 (UBIAD1) gene have been identified in 28 unrelated families with SCD to date (2,3) [A].
PATHOPHYSIOLOGY
• Related to a primary corneal lipid metabolism disorder
• Deposition of phospholipid, unesterified cholesterol, and cholesterol esters in Bowman’s membrane and corneal stroma
ETIOLOGY
Rare autosomal dominant condition
COMMONLY ASSOCIATED CONDITIONS
• Hypercholesterolemia
• Hypertriglyceridemia
• Xanthelasma
• Genu valgum (rare)
DIAGNOSIS
HISTORY
• Symptoms of glare from late teenage years
• Visual acuity only affected in severe cases and worse in photopic conditions
PHYSICAL EXAM
• Central ring-shaped or disciform gray opacities in the anterior stroma, which may consist of fine polychromatic crystals. A crystalline variety in up to 46% of affected individuals (4)[A].
• Diffuse progressive stromal haze can be seen in patients older than 40 years old.
– Corneal arcus develops later with age.
– Progressive decrease in corneal sensation may occur over lesions.
DIAGNOSTIC TESTS & INTERPRETATION
Lab
Initial lab tests
Perform fasting cholesterol and triglyceride tests as they may be abnormal.
Follow-up & special considerations
Genetic testing will reveal mutations in UBIAD1 gene in affected individuals.
Imaging
Initial approach
None
Follow-up & special considerations
Confocal microscopy
Diagnostic Procedures/Other
Characteristic histopathologic features in host corneal button after penetrating keratoplasty
Pathological Findings
• Histopathology: Oil-red-O positive lipid material in stroma, unesterified and esterified cholesterol in Bowman’s layer and stroma (5)[A]
• Confocal microscopy: Needle-shaped or rectangular crystals may be seen in the anterior stroma.
DIFFERENTIAL DIAGNOSIS
• Cystinosis
• Bietti’s peripheral crystalline dystrophy
• Infectious crystalline keratopathy
• Gout
• Multiple myeloma
• Hodgkin’s disease
• Waldenström’s macroglobulinemia
• Benign monoclonal gammopathy
• Plant sap injury
• Tangier disease
• Lecithin–cholesterol acyltransferase deficiency
TREATMENT
MEDICATION
Lipid lowering agents if abnormal lipid profile
SURGERY/OTHER PROCEDURES
• If cornea opacity is severely debilitating vision then corneal transplantation is needed (lamellar or full thickness).
• Excimer laser phototherapeutic keratectomy (PTK) for removal of superficial pathology causing glare
ONGOING CARE
FOLLOW-UP RECOMMENDATIONS
Every 6 months
Patient Monitoring
Visual acuity, glare symptoms
DIET
Control diet to maintain normal lipid profile.
PATIENT EDUCATION
Condition is slowly progressive and hereditary.
PROGNOSIS
• SCD can be diagnosed easily during the first decade, but patients with SCD sine crystals may be harder to diagnose and so be delayed up to the fourth decade.
• Visual acuity remains good usually and may not necessitate corneal transplantation.
• Haze may progress as well as reduced corneal sensation.
COMPLICATIONS
Recurrence of dystrophy may occur in corneal graft or after PTK, but rare.
REFERENCES
1. Orr A, Dube MP, Marcadier J, et al. Mutations in the UBIAD1 gene, encoding a potential prenyltransferase, are causal for Schnyder crystalline corneal dystrophy. PLoS ONE 2007;2(1):e685.
2. Weiss JS, Kruth HS, Kuivaniemi H, et al. Mutations in the UBIAD1 gene on chromosome short arm 1, region 36, cause Schnyder crystalline corneal dystrophy. Invest Ophthalmol Vis Sci 2007;48(11):5007–5012.
3. Kobayashi A, Fujiki K, Murakami A, et al. In vivo laser confocal Microscopy findings and mutational analysis of Schnyder’s crystalline corneal dystrophy. Ophthalmology 2009;116:1029–1037.
4. Weiss JS. Visual morbidity in thirty three families with Schnyder’s crystalline corneal dystrophy. Trans Am Ophthalmol Soc 2007;105:616–648.
5. Rodrigues MM, Kruth HS, Krachmer JH, et al. Unesterified cholesterol in Schnyder’s corneal crystalline dystrophy. Am J Ophthalmol 1987;104(2):157–163.
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