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What is Retinitis Pigmentosa?

Retinitis pigmentosa (RP) is a group of inherited disorders characterized by progressive peripheral vision loss and night vision difficulties (nyctalopia) that can lead to central vision loss.

With advances in molecular research, it is now known that RP constitutes many retinal dystrophies and retinal pigment epithelium (RPE) dystrophies caused by molecular defects in more than 100 different genes. Not only is the genotype heterogeneous, but patients with the same mutation can phenotypically have different disease manifestations. In this article, the clinical manifestations for diagnosis, the new molecular understandings of the pathogenesis, and the latest therapeutic options for patients are reviewed.

RP can be passed on by all types of inheritance: 20-25% is autosomal dominant, 15-20% is autosomal recessive, and 5-10% is X linked, while the remaining 45-50% is found in patients without any known affected relatives. RP is most commonly found in isolation, but it can be associated with systemic disease. The most common systemic association is hearing loss (up to 30% of patients). Many of these patients are diagnosed with Usher syndrome. Other systemic conditions also demonstrate retinal changes identical to RP.

RP is a misnomer, as the word retinitis implies an inflammatory response, which has not been found to be a predominant feature of this condition. As molecular understanding increases, RP will be further characterized by the specific protein/genetic defect. This characterization will have increasing importance in the determination of a prognosis and will likely allow clinicians to use gene-targeted therapies.

Pathophysiology

RP is typically thought of as a rod-cone dystrophy in which the genetic defects cause cell death (apoptosis), predominantly in the rod photoreceptors; less commonly, the genetic defects affect the RPE and cone photoreceptors. The phenotypic variation is very significant because over 100 genes can cause RP.

Histopathologic changes in RP have been well documented, and, more recently, specific histologic changes associated with certain gene mutations are being reported. The final common pathway remains photoreceptor cell death by apoptosis. The first histologic change found in the photoreceptors is shortening of the rod outer segments. The outer segments progressively shorten, followed by loss of the rod photoreceptor. This occurs most significantly in the mid periphery of the retina. These regions of the retina reflect the cell apoptosis by having decreased nuclei in the outer nuclear layer. In many cases, the degeneration tends to be worse in the inferior retina, thereby suggesting a role for light exposure.

The final common pathway in RP is typically death of the rod photoreceptors that leads to vision loss. As rods are most densely found in the midperipheral retina, cell loss in this area tends to lead to peripheral vision loss and night vision loss. How a gene mutation leads to slow progressive rod photoreceptor death can occur by many paths, as illustrated by the fact that over 100 gene mutations can lead to a similar clinical picture.
Cone photoreceptor death occurs in a similar manner to rod apoptosis with shortening of the outer segments followed by cell loss. This can occur early or late in the various forms of RP.

Frequency

The prevalence of typical RP is reported to be approximately 1 in 4000 in the United States. The carrier state is believed to be approximately 1 in 100. The highest reported frequency of occurrence for RP is among the Navajo Indians at 1 in 1878.
International

Worldwide prevalence of RP is approximately 1 in 5000. The frequency of occurrence for RP has been reported to be as low as 1 in 7000 in Switzerland.

Mortality/Morbidity

A multicenter population study by Grover et al of patients with RP who were at least 45 years or older found the following findings: 52% had 20/40 or better vision in at least one eye, 25% had 20/200 or worse vision, and 0.5% had no light perception.41

Sex

Usually, no sexual predilection exists. X-linked RP is expressed only in males; therefore, because of these X-linked varieties, men may be affected slightly more than women.

Age

The age of onset can vary. RP usually is diagnosed in young adulthood, although it can present anywhere from infancy to the mid 30s to 50s.


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