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At a glance

A genetic disorder characterized by small crystals deposits in the retina resulting in retinal degeneration and in visual impairment.

Synonym

Bietti Tapetoretinal Degeneration with Marginal Corneal Dystrophy.

History

First described by the Italian physician Giambattista Bietti (1907-1977) in 1937.

Incidence

Globally, the overall estimated prevalence of BCCD is around 1 in 67,000. It is more common in East Asia (particularly Japanese and Chinese population).

Genetic inheritance

Autosomal recessive disorder. Parental consanguinity is a risk factor. The responsible mutations affect the CYP4V2 (Cytochrome P450, Family 4, Subfamily V, Polypeptide 2) gene, which has been mapped to chromosome 4q35.1-q35.2 and is encoding a protein belonging to the cytochrome P450 family. This protein is expressed in human retinal pigment epithelium and cornea, but also in many other organs/tissues such as heart, kidney, liver, lung, and lymphocytes.

Diagnosis

Based on the clinical findings. Refractive deposits found in the paracentral and peripapillary retina and marginal cornea are the most common features of this disease. Electrodiagnostic testing may show pathologic changes in the electrooculogram and decreased scotopic and photopic responses with variable degree of cone and rod dysfunction in the electroretinogram. However, it should be kept in mind that a significant number of patients have standard ERG responses in earlier stages of the disease and it seems that multifocal ERG (mfERG) is more accurate than standard ERG to detect early retinal changes. Skin biopsies and histologic examination of tissue from corneal and conjunctival biopsies may show crystal-like cholesterol (or cholesterol ester) and complex lipid inclusions in fibroblasts. Similar inclusions are present in skin fibroblasts and circulating lymphocytes, which could imply that BCCD is a systemic anomaly of lipid metabolism. In fact, one study reported mild to moderate hypercholesterolemia in almost half of BCCD patients. Molecular genetic testing with proof of CYP4V2 mutations will confirm the diagnosis.

Clinical aspects

Clinically, patients with BCCD typically become symptomatic in their 2nd to 3rd decade of life presenting with reduced visual acuity, nyctalopia (night blindness), paracentral scotomata, and occasionally impaired color vision. The progressive nature of the disease results in loss of peripheral vision and eventually legal blindness in the 5th or 6th decade of life in the vast majority of patients. Eye examination shows degeneration and atrophy of the retinal pigment epithelium at the posterior pole of the retina with pigment clumping and choroidal vessel sclerosis in combination with numerous glistening, yellow-white intraretinal crystal deposits, and/or complex lipid deposits. Some patients also have corneal dystrophy and similar crystals at the corneoscleral limbus. The pathologic process is often asymmetric. In some patients, central vision is spared until the later stages of the disease. Choroidal neovascularization has occasionally been reported, which seems to respond well to intravitreal bevacizumab injections. ...

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