Schnyder crystalline corneal dystrophy (SCCD) is a rare autosomal dominant stromal dystrophy that is characterized by bilateral corneal opacification, resulting from an abnormal accumulation of cholesterol and lipid. The causative gene for this disease is UBIAD1, which is present on 1p36. The gene is involved in cholesterol metabolism.
Please refer to the image below depicting the natural history of schnyder crystalline corneal dystrophy (SCCD) specific to age.
While the incidence in the general population is unknown, the world's largest pedigree (>200 patients with SCCD) has a Swede-Finn heritage and has been traced to the southwest coast of Finland on the Bay of Bothnia. However, the dystrophy has been reported in other ethnicities and in all racial groups.
The pathogenesis remains unknown, but it is postulated to result from a localized defect of lipid metabolism. It has been demonstrated in affected corneas versus normal corneas that the cholesterol content increases 10-fold and the phospholipid content increases 5-fold. Immunohistochemical analysis has revealed the preferential deposition of apolipoprotein components of high-density lipoprotein (HDL), that is, apoA I, apoA II, and apoC, but not of low-density lipoprotein (LDL), that is, apoB. This finding suggests an abnormal metabolism of HDL in the cornea with SCCD.
The recent discovery of the causative gene, UBIAD1, will be the link to further understanding of this disease. The gene produces a protein that contains a prenyltransferase domain that could play a role in cholesterol metabolism. In addition, UBIAD1 interacts with the C-terminal portion of apolipoprotein E (apoE), which is known to help mediate cholesterol removal from the cells. Further research will determine whether the excess cholesterol results from increased cholesterol production or decreased removal.
The dystrophy has been reported in the United States, although the incidence in the general population is unknown.
While the incidence is unknown, the dystrophy has been reported in eastern and western Europe, Taiwan, Japan, and Turkey.
A long-term study of 33 families over a period of 18 years reveals that most morbidity derives from progressive corneal clouding, leading to glare and decreased vision in daylight.
Mean Snellen uncorrected visual acuity (UCVA) was between 20/25 and 20/30 in patients younger than 40 years and between 20/30 and 20/40 in patients aged 40 years or older. Nevertheless, while scotopic vision remained relatively good, increasing corneal opacification with age resulted in decreased scotopic vision.
Studies of those affected reveal that 54% of patients aged 50 years and older and 77% of patients aged 70 years and older had corneal transplant surgery. Although study numbers are small, there is no evidence of increased mortality from cardiovascular disease in SCCD. Of note, however, 71% of patients who had corneal transplant surgery reported the use of cholesterol-lowering agents. This was not statistically different from those patients who had not undergone corneal transplant surgery.
SCCD can occur in whites, Asians, and African Americans.
Although rare sporadic cases have been reported, SCCD is primarily an autosomal dominant disease, affecting both sexes with equal probability.
The disease may appear as early as the first decade of life and slowly progresses with age. However, a diagnosis may be delayed until the fourth decade in patients with corneal opacification without crystalline deposits.