Most forms of NBIA involve eye disease. The most common problems are retinal degeneration and optic atrophy.
An estimated 20 percent to 30 percent of classic PKAN patients develop retinitis pigmentosa, which causes such problems as night blindness, loss of peripheral vision and even blindness in some patients.
To understand how RP affects patients, it’s helpful to know how the retina works. The retina lies at the back of the eye and acts like the film in a camera, receiving and processing everything a person sees. The retina is a delicate layer of cells. The cells that initially respond to light are the rods and cones.
Rod cells pick up movement out of the corner of the eye and also operate in poor light or at night. There are about 120 million rods in each eye and they are more numerous towards the outer edge of the retina. The cone cells are used in color vision and in close precision work, such as reading. They are more concentrated in the center of the retina.
The most classic symptom of RP is difficulty seeing at night or in poor illumination. With loss of rod function there is a narrowing of the field of vision, and as the degeneration progresses, patients feel as though they are looking into a tunnel. This is why RP is sometimes called “tunnel vision.”
Other symptoms include difficulty adapting to changes in lighting and differentiating between certain objects. For example, children in the classroom may have problems seeing writing on the blackboard, and an overhead projector may be difficult to read. Other patients may have problems negotiating concrete or carpeted stairs that lack edge markings.
Finally, central vision or one’s fine, detail reading vision may be affected. This is usually late in the course of the disease. Some individuals may go blind.
Although a complete ophthalmologic exam can suggest that the patient has RP, it is impossible to make a definitive diagnosis without further testing. The essential diagnostic test is an electroretinogram or ERG. The ERG is a measure of the retina’s electrical activity in response to light stimulation. The procedure requires a contact lens with electrodes attached. The electrodes monitor the retinal response to a series of light flashes under light and dark conditions.
There are probably more than 30 genetic conditions that have RP as a feature, including PKAN. In some people or families, RP occurs as an isolated condition, with no associated features. There is no cure. However, a daily dose of 15,000 International units of vitamin A Palmitate has been shown to slightly slow the progression of RP in some individuals. Patients who take the vitamin A treatment should be under a physician’s care. The use of vitamin A is probably only beneficial in certain forms of RP and can be toxic if not taken under the careful management of a physician.
An information packet on the research is available from The Foundation Fighting Blindness in Hunt Valley, Md. The Web address is http://www.blindness.org or call toll-free at (888) 394-3937.
Optic atrophy affects the optic nerve, which sends messages between the retina and the brain. The optic nerve is like a cable with thousands of tiny electrical wires that each carry some visual information to the brain. When the nerve is damaged or breaks down, vision can become blurry, side vision or color vision may be abnormal, the pupil may not work properly, or there may be decreased lightness in one eye compared to the other. Eventually, optic atrophy can cause blindness.