Choroideremia Health Article

Definition

Choroideremia is a rare genetic disorder causing progressive eyesight loss due to the wasting away of retinal layers. It first affects the choroid and the retinal pigmented epithelium (RPE) layers and finally the photoreceptor cell layer. Atrophy (wasting) of the optic nerve is also observed in choroideremia.

Description

Formerly called tapetochoroidal dystrophy, choroideremia is a chronic form of retinal disease characterized by degeneration of the layers of the retina, which is the light-sensitive part of the eye. There are four main retinal layers: the outer neural retina, consisting of nerve cells and blood vessels; the retinal pigment epithelium (RPE); the choroid layer that contains connective eye tissue and a capillary layer (chorio capillaris); and the photoreceptor (light-sensitive) layer that contain the rods and cones, which function as detectors to process light, color and shape signals to the brain. Choroideremia is a progressive disease, meaning that the layers become affected one after the other over time.

The pigmentary changes in the RPE begin with fine spotting and continue with areas of depigmentation and increasing loss of the chorio capillaris. Chorio capillaris loss and degeneration of the larger choroidal blood vessels causes areas of bare sclera, the tough white fibrous tissue that covers the "white" of the eye. The disease begins in midperiphery of the choroid but then progresses to include the entire choroid.

Choroidal vessels provide oxygen and nutrients to both the RPE and the retina's photoreceptor cells. The RPE, which lies directly beneath the retina, supports the function of photoreceptor cells. Photoreceptor cells (rods and cones) convert light into the electrical impulses that transfer messages to the brain where "seeing" actually occurs. In the early stages of choroideremia, the choroid and the RPE begin to deteriorate. Eventually, photoreceptor cells also degenerate, resulting in a loss of central vision.

The age at which choroideremia first appears varies; initial symptoms (usually night blindness) may occur as early as three years of age and as late as 40 years. However, occurrence peaks between the ages of ten and 40. The visual field becomes progressively constricted, and patients usually reach legal blindness by 25 years of age. Loss of central vision usually occurs after the age of 35. However, in nearly all patients with choroideremia, visual acuity (acuteness or sharpness of vision) is well maintained until the late stages of the disease.