Frequency of RP is reported to be 1/3,000 to 1/5,000. No ethnic specificities have been reported although founder effects are possible.
Retinitis pigmentosa is slowly progressive but relentless. There is however broad variability in age of onset, rate of progression and secondary clinical manifestations. Affected individuals generally first develop night blindness (nyctalopia) due to loss of rod function, often in adolescence or earlier. They then develop peripheral visual field impairment, and over time loss of central vision, usually at late stages, often around midlife. Central visual acuity loss may occur at any age as a result of cystoid macular edema or photoreceptor loss. Posterior subcapsular cataracts are common and severity is age dependent. Reduced color vision may also be found. Fundus examination reveals bone spicule pigment deposits, attenuated retinal vessels, retinal atrophy and waxy optic nerve pallor. Severity is partly correlated with the pattern of inheritance with X-linked cases having the most severe course, autosomal recessive and single occurrence cases having intermediate severity, and autosomal dominant the most favorable course.
More than 3,000 mutations in over 57 different genes or loci are currently known to cause non-syndromic RP.
The diagnosis of RP is based on peripheral visual field loss, pigment deposits in fundus, loss of photoreceptors at the optical coherence tomography (OCT) scan of the retina and decreased or abolished responses as measured by electroretinography (ERG). Molecular genetic testing using single-gene testing, an RP multi-gene panel or exome sequencing allows for genetic subtype classification.
Besides non syndromic forms, there are syndromic forms of RP of which the most frequent are Usher syndrome (RP and deafness) and BardetBiedl syndrome (RP and metabolic impairment). RP is to be distinguished from macular dystrophies (peripheral visual field is normal) and Leber congenital amaurosis (congenital retinal dystrophy) (see these terms).
Prenatal diagnosis for at-risk pregnancies is possible by DNA analysis following amniocentesis or chorionic villus sampling.
RP may be inherited in an autosomal dominant, autosomal recessive, or X-linked manner. X-linked RP mutations generally only affect men. Genetic counseling should be provided to affected individuals and their families once the mode of inheritance has been determined through family history or molecular testing. Identical mutations may however produce different clinical manifestations. In X-linked familial cases, carrier testing for female relatives can be performed.
Treatment is primarily aimed at slowing progression of the disease. Vitamin A palmitate and lutein-DHA may be provided as protecting antioxydants. Oral acetazolamide or topical dorzolamide are used to reduce cystoid macular edema. Lens extraction is required when cataracts reduce visual acuity. Sunglasses with short wavelength filtering improve visual performance and optical aids are recommended. Rehabilitation for reading and moving can be proposed in end-stage patients.
Except for mild cases or sectorial RP, most cases progress to legal blindness (visual acuity < 1/20 and visual field < 5 degrees).
Expert reviewer(s)
- Pr Christian HAMEL