Choroideraemia

Choroideraemia is an X-linked condition affecting the RPE, outer retina, and choriocapillaris. It is caused by mutations in the CHM gene. Due to this inheritance pattern, choroideraemia is much more common in males than in females.

The disease process is characterised by nyctalopia and peripheral vision loss in the second decade of life or before. In the early stages, peripheral pigmentary mottling may be noted and over time this develops into pigment clumping and atrophy of the RPE, photoreceptors, choriocapillaris and eventually, choroidal atrophy follows.

Over time, loss of the RPE means that the choroidal vasculature becomes more visible. As the disease progresses and further atrophy of the choroid occurs (usually in the mid-peripheral and peripapillary fundus first) and the bare sclera may become visible – seen clinically as areas of pale white areas on retinal imaging.

Pigment clumping occurs as a result of RPE degeneration. This pigment can be seen on OCT as hyper-reflective deposits. Also seen on OCT are interlaminar bridges (ILBs) which are hypo-reflective wedge-shaped structures that bridge the inner and outer retina in choroideraemia. Outer retinal tubulations are seen in late-stage disease on OCT. These appear as hyper-reflective round structures with a hypo-reflective lumen. The tubulations are composed of deteriorating photoreceptors and remnants of the external limiting membrane.

In recent studies, OCT angiography has been used to further our knowledge around choroideraemia. These studies showed that the choriocapillaris shows greater nonperfusion than the retina in choroideraemia.

Disease progression is relatively slow with central vision being fairly well preserved until late stage disease starts to affect the macula – typically around 50 to 70 years of age.

Being an x-linked condition, females are rarely diagnosed with choroideraemia, however the carriers of the defective gene may show a partial expression of the condition. Most carriers are asymptomatic and visual acuity is unaffected, however the fundus may show signs consistent with choroideraemia such as pigment mottling.

MacDonald, IM. Russell, L. Chan, CC (2009) Choroideremia: New Findings from Ocular Pathology and Review of Recent Literature, Survey of Ophthalmology, Volume 54, Issue 3, Pages 401-407.

Sun, LW, Johnson, RD, Williams, V. Summerfelt, P. Dubra, A. Weinberg, DV, Stepien, KE, Fishman, GA, Carroll, J. (2016). Multimodal Imaging of Photoreceptor Structure in Choroideremia. PLoS One, 11(12)

Zinkernagel, MS. & MacLaren, RE. (2015). Recent advances and future prospects in choroideremia. Clinical Ophthalmology, 9, 2195-2200.