As with RP, the most common early symptom of choroideremia is difficultly seeing at night and in low-light conditions—this is called nyctalopia or “night blindness.” Symptoms usually manifest around the time of grade school, which is also the case with RP, though in choroideremia it is predominantly boys who are affected. Most men with choroideremia will experience legal blindness by the age of 40 and complete blindness by age 70-80. Though it is mostly males who are affected, approximately 30% of women who have the mutation will also develop vision loss, though it is rarely as severe as a man’s.
Since choroideremia is difficult to distinguish from RP and other retinal diseases, a family history is often important for a diagnosis. Genetic testing can help with this; in fact, the genetic test for choroideremia was developed in 1998 by Dr. Ian MacDonald at the University of Alberta with the support of the Foundation Fighting
Blindness. It involves testing a blood sample to confirm the disease and, in the case of women, determine if one is a carrier. You can read our Guide to Genetic Testing for more information.
There are also eye tests that can bring patients closer to a final diagnosis, though they may not be able to pinpoint the exact nature of the disease itself. These are the same tests used for RP and many other inherited retinal diseases:
- ERG (electroretinography): this is a test that measures the electrical responses of the retina to light, evaluating responses of both rod and cone photoreceptors. Although both rods and cones may be affected in people with RP, the most marked changes early in disease are in the rod cells; this characteristic pattern helps diagnose the condition. The ERG test involves staying in a darkened room for 30 minutes, with drops put into the eye or eyes being tested. A special contact lens or fine fibre electrode is then placed on the eye or lower eyelid, and the eye is exposed to flashes of light.
- OCT (optical coherence tomography): this is an imaging technique that involves taking digital images of the various layers of the retina. The process is uses light rather than sound or radio waves, which is why the images are in high resolution.
- Visual field test: this exam is designed to detect, measure, and monitor blind spots in vision. It involves looking into a device that emits flashes of light, with the patient asked to indicate which flashes can be seen. The flashes that are not seen are recorded. This gives a measure of how much vision is affected.