Most forms of NBIA involve eye disease. The most common problems are retinal degeneration and optic atrophy.
The most classic symptom of retinal degeneration 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 it is sometimes called "tunnel vision."
The retina is a thin membrane that lines the back of the eyeball; it 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.
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. Loss of peripheral vision may contribute to falling and gait problems.
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.
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.
Evaluation by electroretinogram often detects retinal changes that are asymptomatic. Individuals with a normal eye examination at the time of diagnosis generally do not develop retinopathy.
In the early stages of PKAN the retinal degeneration follows a typical clinical course, with nyctalopia (night blindness) followed by progressive loss of peripheral visual fields and sometimes eventual blindness.
About two out of three individuals with PKAN develop retinal degeneration; it is more common in classic PKAN.
Retinal degeneration was also found in 93 percent of Japanese individuals with aceruloplasminemia.
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, each carrying 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.
Optic atrophy is only found in 3% of PKAN patients and has not been observed in atypical PKAN. For infantile neuroaxonal dystrophy (INAD), a form of PLAN, optic atrophy is common later on and can cause poor vision and eventual blindness. MPAN and FAHN individuals also may have optic atrophy.