Bietti Crystalline Corneoretinal Dystrophy

Bietti crystalline corneoretinal dystrophy (BCD) is an autosomal recessive inherited retinal dystrophy characterised by the presence of multiple, refractive crystals in the retina with associated RPE and choriocapillaris atrophy. The crystalline deposits can also be found in the peripheral cornea. It is caused by mutations in the CYP4V2 gene, which is involved in fatty acid metabolism.

BCD is a rare condition but is more common in the East Asian population. The age of onset is usually in the second or third decade of life. Patients with BCD are typically asymptomatic during the earliest stage. As the disease progresses, they start to present with nyctalopia and restricted visual field. Toward the late stage of BCD, severe vision loss and legal blindness are common findings.

Three clinical stages of BCD have been described.
Stage 1 features the presence of refractive crystals in the retina and chorioretinal atrophy lesions limited to the posterior pole. OCT shows the drusen-like structure, loss of the interdigitation zone, and localised disruption of the ellipsoid zone in the macula.

Stage 2: Chorioretinal atrophy expand centrifugally beyond the posterior pole. OCT shows extensive loss of the outer retina and RPE with focal residual RPE and/or EZ in the macula. The crystals tend to disappear but are still been seen at the edge of the atrophic areas.

Stage 3: Diffuse RPE-choriocapillaris complex atrophy is observed throughout the posterior pole. OCT shows extensive loss of outer retina and RPE, and severe atrophy and thinning of the choroid, with the choroidal vessels virtually absent.

Unlike the retinal crystals, the corneal crystals persist in the course of the disease. They are found in the subepithelial and anterior stromal layers and more numerous towards the limbus and do not affect vision.

The diagnosis and staging of BCD are facilitated with multimodal imaging. Colour fundus photograph is useful to image the crystalline deposits, although infrared images are more superior if the crystals are in the intraretinal locations. OCT depicts the crystalline deposits as hyperreflective dots within the retinal layers and displays the disruptions of the retinal and choroidal architectures. Outer retinal tabulation is frequently identified, although it is not specific to BCD and can be found in other inherited retinal dystrophies. FAF is valuable in evaluating the RPE loss.

BCD can be complicated by choroidal neovascularisation and/or macular oedema.

Differential diagnosis of crystalline retinopathy can include cystinosis, hyperornithinemia, talc retinopathy, Alport syndrome, Sjögren-Larsson syndrome, macular telangiectasia, canthaxanthin toxicity and tamoxifen retinopathy

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