Bietti crystalline dystrophy (BCD) is a chorioretinal degeneration characterized by the presence of yellow-white crystals and/or complex lipid deposits in the retina. Progressive atrophy and degeneration of the retinal pigment epithelium (RPE)/choroid lead to symptoms similar to those of other forms of retinal degeneration that fall under the category of retinitis pigmentosa and allied disorders, namely: reduced visual acuity, poor night vision, abnormal retinal electrophysiology, visual field loss, and often impaired color vision. Marked asymmetry between eyes is common. Onset is typically during the second to third decade of life, but ranges from the early teenage years to beyond the third decade. With time, loss of peripheral visual field, central acuity, or both result in legal blindness in most, if not all, affected individuals.
The diagnosis is based on the finding of numerous small, glistening yellow-white retinal crystals associated with atrophy of the RPE, pigment clumps, and sclerosis of the choroidal vessels; variable crystalline deposits in the corneal limbus; varying degrees of rod and cone dysfunction on electroretinography (ERG); visual field defects; and reflective dots visualized by spectral domain optical coherence tomography (sdOCT). Molecular genetic testing of CYP4V2, the only gene in which mutations are known to cause BCD, is available clinically.
BCD is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the disease-causing mutations in the family are known.