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Ophthalmology – Choroideremia
Choroideremia is a rare, progressive, X-linked inherited retinal disorder characterized by degeneration of the retinal pigment epithelium (RPE), choroid, and photoreceptors. It primarily affects males, while female carriers are usually asymptomatic or exhibit only mild findings. The disease leads to gradual vision loss over decades, eventually resulting in blindness.
Epidemiologically, choroideremia has an estimated prevalence of approximately 1 in 50,000 individuals. Because it is inherited in an X-linked recessive pattern, it predominantly affects males with a positive family history. Female carriers may occasionally show mild retinal changes but rarely develop significant visual impairment.
The underlying genetic defect involves mutations in the CHM gene located on the X chromosome (Xq21.2). This gene encodes the Rab escort protein (REP-1), which is essential for intracellular vesicle transport and normal RPE function. Loss of REP-1 disrupts cellular processes such as phagocytosis of photoreceptor outer segments, lysosomal activity, and intracellular trafficking. These abnormalities ultimately lead to progressive degeneration of the RPE, followed by secondary loss of photoreceptors and choroidal structures.
Clinically, affected males typically present with night blindness (nyctalopia), often beginning in childhood or adolescence. This is followed by progressive peripheral vision loss, initially manifesting as annular scotomas and eventually leading to concentric visual field constriction. By around 40 years of age, many patients have severe peripheral vision loss approaching legal blindness, although central visual acuity is often preserved until later in life.
On examination, early fundus findings include pigmentary changes and patchy areas of RPE and choroidal atrophy in the mid-periphery. As the disease progresses, these areas coalesce, exposing the underlying sclera and leaving only small islands of functioning retina, typically around the macula and optic disc. In advanced stages, the retina appears markedly thinned with extensive choroidal loss.
Diagnosis is primarily clinical but can be confirmed with genetic testing. Functional testing such as visual field analysis and electroretinography (ERG) is useful for assessing disease severity and progression. ERG initially shows a rod-cone degeneration pattern and eventually becomes non-recordable in advanced disease. Imaging modalities such as fluorescein angiography may demonstrate areas of capillary non-perfusion corresponding to atrophic regions.
There is currently no definitive treatment for choroideremia. Management focuses on monitoring disease progression and supportive care. Emerging therapies, particularly gene therapy aimed at restoring REP-1 function, show promise but remain under investigation. Patients may benefit from low vision rehabilitation as visual function declines. Protective measures such as UV-blocking sunglasses may help reduce additional retinal stress.
Genetic counseling is an essential component of care, given the X-linked inheritance pattern. Families should be educated about transmission risks, including the 50% chance of carrier mothers passing the mutation to offspring. Prenatal testing may be considered in affected families.
The prognosis involves progressive visual decline. Most affected males develop severe peripheral vision loss by midlife and eventual loss of central vision later in life, leading to blindness. Female carriers generally have a much milder course.
Complications primarily relate to progressive vision loss and its functional consequences, including reduced independence and quality of life. Posterior subcapsular cataracts may also develop in some patients and can be managed surgically if needed.
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