James D. Reynolds, M.D., Univ. of Arkansas School of Medicine
Retinitis pigmentosa or RP is a group of geneticallydetermined, progressive degenerations of the rods and cones of the retina. The term retinitis pigmentosa was coined in 1857 by Donders, and like many older terms, it is a misnomer. Despite the "itis," there is no inflammatory or infectious component to this group of diseases. RP classically occurs as an isolated, hereditary condition, but it can be associated with a variety of other diseases.
Physiology
The retina is the actual sense organ of the eye. The rest of the eye provides support, nourishment, and focusing power. The retina contains several layers of cells, but the group of cells that initially responds to light are the rods and cones. These extremely specialized and sensitive cells transform light energy into electrical energy which is processed in the retina and transmitted to the brain via he optic nerve. Compared to the cones, the rods are a more sensitive, less discriminating cell population. They function best in the dark or near dark. The cones are less sensitive, but more discriminating. They function in well-lighted conditions and are the cells which provide us with our optimal 20/20 vision. The cones are also the cells responsible for the perception of color. These two groups of cells form a complimentary dual system.
Classification
Generalized rod-cone degenerations are classified in a variable way and few authors agree completely on the classifications. The following grouping is somewhat simplified:
- Congenital RP or Leber's Congenital Amaurosis. 2) Autosomal Recessive RP. 3) Autosomal Dominant RP. 4) X-linked or Sex-linked Recessive RP. 5) Sporadic RP. 6) RP Associated with Systemic Diseases.
The first is manifested at birth or shortly thereafter. The next three are hereditary, follow different patterns of inheritance, and have a variable onset. The fifth occurs without a familial pattern, and the last is found in conjunction with other generalized or systemic illnesses.
Clinical Manifestations
The most classic symptom is night blindness or nyctalopia. This occurs because the rod system is affected long before the cone system. There is also a gradual constriction of the visual field until it begins to appear as though one is looking down the barrel of a gun. In other words, one's periperal or side vision progressively decreases. Finally, central vision or one's fine, detail reading vision may be affected. This is usually late in the course of the disease. Unfortunately, some individuals can progress to near or even total blindness.
The onset and progression and, to some degree, the order of the symptoms depends on what type is present. The congenital form presents with the full-blown disease at birth; these children have stable but persistent visual loss. Fortunately, this is quite rare. The recessively inherited types usually have their onset in childhood or the early teens and the outlook or prognosis is poor. The dominant form is likely to be a milder condition and the sporadic type is variable. The types associated with other illnesses often occur early, are severe, and have early central vision loss. The systemic illnesses are all rare, often life-threatening diseases.
Diagnosis
Although a complete ophthalmologic exam can be highly suggestive of RP, it is impossible to make a definitive diagnosis without further testing. This is especially true in early cases. The essential diagnostic test is an electroretinogram or ERG. A dynamic test of retinal function, 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 both light and dark conditions.
Treatment
Unfortunately, no therapy is presently available for the vast majority of people with RP. Attention is directed toward low vision aids, and proper social and governmental programs for the visually-impaired.
Genetic Counseling
Parents who have a child with a recessive disease who are normal themselves can expect that 25 % of their future children will be affected.
Unaffected parents who have a child with an X-linked disease have a 50 % risk of having affected male children. Females only carry the X-linked gene, while males suffer from the disease. A better known example of this pattern of inheritance is color blindness.
Dominantly inherited disease is manifested in all individuals who carry the gene. Thus one parent is usually affected as are 50 % of the children.
Sporadic diseases are new mutations and the particular inheritance is unclear until further generations exhibit the trait.
Conclusion
RP, or the more appropriately named rod-cone generalized dystrophy, is a varied group of disorders characterized by hereditary retinal rod and cone degeneration. There are many other hereditary retinal degenerations that are more rare than RP and have quite different clinical findings. Because much remains unknown about these conditions, research is very active in this area.
