42.1 Features
Choroideremia is X-linked chorioretinal dystrophy characterized by progressive degeneration and atrophy of the retinal pigment epithelium (RPE), choriocapillaris, and retina. First described by Mauthnuer in 1872, its prevalence is estimated to be between 1 in 50,000 and 1 in 100,000. Choroideremia is caused by mutations in the CHM gene, localized to the long arm of the X chromosome at Xq21.2, which encodes for Rab escort protein 1. This protein is involved in intracellular trafficking in both RPE cells and photoreceptors and is considered to play an important role in the removal of outer segment discs by the RPE. As intracellular trafficking is affected, the RPE and photoreceptor cells die prematurely. Given the X-linked nature of the disease, it is mostly seen in males.
42.1.1 Common Symptoms
Nyctalopia in the first or second decades of life; peripheral vision loss over the next three to five decades; central visual acuity is often maintained until the fifth decade of life. Once degeneration affects the macula, rapid deterioration of central vision occurs.
42.1.2 Fundus Findings
Findings on dilated fundus examination parallel the progression of symptoms (▶ Fig. 42.1). Early in the disease, the midperiphery photoreceptors, primarily the rods, are affected. This is followed by progressive atrophy with loss of peripheral field and night vision. Patchy areas of depigmentation are seen peripherally and spread centripetally. Well-defined regions of atrophy with scalloped borders develop where the underlying large choroidal vessels and sclera are visible. An island of preserved RPE around the macula is preserved until the late stages in the disease course.
Fig. 42.1 Fundus photographs of two choroideremia patients. (a) In the first image, the fundus shows widespread choroidal and retinal pigment epithelium (RPE) atrophy, with the underlying larger choroidal vessels visible. Characteristic pigment clumping at the level of RPE is also seen peripherally. (b) The fundus of this patient shows an advanced stage of the disease. An island of spared RPE is seen around the macula, but there is widespread choroidal and RPE atrophy on the periphery. On the center of the posterior pole, the atrophy has advanced to the stage where the underlying white sclera is visible.
42.2 Key Diagnostic Tests and Findings
42.2.1 Optical Coherence Tomography
Increased central retinal thickness early in the disease when visual acuity is unaffected. As the disease progresses and visual acuity declines, slow thinning of the central retina is observed. Cystoid macular edema is a common observable complication. Outer retinal tubulation signifies end-stage photoreceptor degeneration.
42.2.2 Fluorescein Angiography or Ultra-Widefield Fluorescein Angiography
Areas of atrophic RPE and choriocapillaris appear hypofluorescent, in contrast to the islands of preserved RPE and choriocapillaris, which appear hyperfluorescent as the dye is able to perfuse the choriocapillaris (▶ Fig. 42.2).
Fig. 42.2 Late-phase fluorescein angiogram in choroideremia. The areas of atrophic retinal pigment epithelium (RPE) and choriocapillaris appear as hypofluorescent with prominent visualization of the choroidal vessels. In contrast, the island of spared RPE around the macula and optic nerve appears as hyperfluorescent as the dye is able to penetrate through the choriocapillaris.
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