Retinitis Pigmentosa
Once signs of Retinitis Pigmentosa appear the condition will progress. The rate of visual decline varies depending on the genetic circumstances of each individual. Everyone has experienced it – you walk into a movie theatre after being in sunlight and you cannot see anything. You stumble around arms outstretched trying to find a seat. Finally you do and after about ten minutes or so your eyes adjust to the darkness. Now imagine your eyes never adjust. This is what it is like when you have retinitis pigmentosa (RP).
RP is the name given to a group of inherited eye diseases. Each of these diseases causes degeneration of the photoreceptor cells in the retina. The retina is the light-sensitive layer lining the back of the eye that receives visual images and it acts like the film in a camera. Millions of photoreceptor cells in the retina capture and process light and then convert it into electrical impulses that transmit to the brain. Generally they are the reason we see at all. There are two types of photoreceptors rods and cones. Rod cells are concentrated along the outer perimeter of the retina and they help us to see images that come into our peripheral or side vision. They also help us see in dark and dimly lit environments. Cone cells are concentrated in the macula the centre of the retina and allow us to see fine visual detail in the centre of our vision. Cone cells also allow us to perceive colours.
RP is a genetic disorder and is usually hereditary. Since retinal cells are among the most specialized in the body they depend on a lot of unique genes to create vision so any mutation on these genes is enough to lead to some form of vision loss. Signs of RP generally begin in childhood and then progress into adolescence and young adulthood (when most people are diagnosed) though it is possible for symptoms to appear anywhere from infancy all the way to the mid-30s and 50s. Once signs of RP appear the condition will progress though at a slow pace – usually over several decades. The rate of visual decline varies depending on the genetic circumstances of each individual.
Most forms of RP first cause the degeneration of rod cells (rod-cone dystrophy). In these cases symptoms start with difficulty seeing in dim illumination (night blindness) and later progress to a loss of peripheral vision. Eventually the eyes may become unable to adapt to darkness and tunnel vision may result – like looking through a long tube. It is not uncommon for people with RP to bump into things or have trouble seeing steps or curbs. A rarer form of RP is cone-rod dystrophy. The first symptom in this case is a loss of central vision. Since the cone cells are the first to degenerate there are also disturbances in colour perception. Soon afterwards there is a decrease in number of rod cells resulting in the symptoms described for rod-cone dystrophy. There is no pain or abnormal sensation in the eye in either of these cases. Patients with either form of RP often experience a ring of vision loss in their mid-periphery with small areas of vision in their very far periphery. Complete blindness resulting from RP is uncommon and many people retain a small degree of central vision throughout their life.
People with RP are also susceptible to cataract formation and swelling of the retina. Hearing loss can also occur in about 30 percent of RP cases. If one member of a family gets RP it is highly recommended that every other family member have a thorough eye examination to test for the condition. Visual field evaluations colour vision tests detailed retinal exams and possibly electrodiagnostic testing all help to detect RP. Genetic evaluation and counselling is also recommended to help identify the risk of passing it on to future generations.
Currently there are no treatment options for RP. Wearing sunglasses with special filters (usually dark amber) can help protect the retina from UV light and may help preserve some vision. Since RP sufferers also have difficulty with glare when exposed to bright lights sunglasses provide improved comfort. Recent studies have also indicated that treatment with Vitamin A may delay disease progression though this has not been necessarily proven. A variety of low vision aids including magnifiers telescopes and special lenses are very helpful in maintaining patient independence and rehabilitation. Future treatments being investigated include gene therapy anti-oxidant supplementation and medications that slow the retinal degeneration.