Gene mutations cause a large number of disorders that result in vision loss. These impact the retina lining the back of the eye which is the tissue that responds to light. The retina contains two types of cells called photoreceptors, the cones which enable us to see color, and the rods that respond to dimmer light and enable us to see at night. Some mutations impact primarily cone function and others primarily rod function. The former are known as cone-rod dystrophies, while the latter are often referred to as rod-cone dystrophies. However, while either type of cell dysfunction may be predominant, in most cases progression of disease eventually impacts both resulting in loss of color perception difficulty seeing at night. The cone-rod dystrophies are progressive disorders with symptoms that worsen with age.
Among the cone-rod dystrophies caused by gene changes, there are three that affect primarily males. These are due to mutations on the X chromosome. The first symptoms usually are extreme sensitivity to light beginning in the second and third decades of life followed by some decrease in vision. Loss of some color vision is experienced by most patients at about the same time. Many patients have normal or near normal vision early in their disease but after several decades they often become legally blind. Eventually all color vision is lost and many individuals notice difficulty seeing at night and experience loss of side vision. Early and extreme near-sightedness is often a feature as well.
There are no systemic symptoms in most cases.
The X-linked cone-rod dystrophies affect primarily males who have a single X chromosome but some females, who have two X chromosomes, can have some symptoms as well, such as mild vision loss, some light sensitivity, and difficulties with color perception. Thus this group of disorders has patterns of inheritance called X-linked recessive or X-linked dominant (see pedigrees).
The diagnosis of these dystrophies requires a complete eye examination. The symptoms combined with a visual field test, a color vision test, and examination of the retina are usually sufficient to detect the disease. An ERG (electroretinogram) test is done to determine the exact type of disorder.
Early in the disease, correction of the near-sightedness can significantly improve vision. Sensitivity to light requires the use of tinted lenses and those tinted red may even improve vision somewhat. Low vision devices and vocational counseling should be made available.