Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous group of inherited retinal disorders characterized by diffuse progressive dysfunction of predominantly rod photoreceptors, with subsequent degeneration of cone photoreceptors, and retinal pigment epithelium (RPE).
RP is inherited as autosomal dominant, autosomal recessive or X-linked. Many cases are due to mutation of the rhodopsin gene. It is causes a dystrophy or genetically determined degeneration and not an inflammatory disorder. This finally leads to photoreceptor cell death by apoptosis (rods followed by cones).
Visual impairment usually manifest as:
1) Night blindness, and
2) Progressive visual field loss (Usually bilateral).
Its prevelance is 1:3000 to 1:5000.
- Usually bilateral involvement (can be asymmetric);
- Impairment of night vision and loss of peripheral vision;
- Rod dysfunction evidenced by elevated rod final threshold on dark adaptation and/or rod responses on ERG testing that are either reduced in b-wave amplitude and prolonged in implicit time or are essentially non-detectable;
- progressive loss in photoreceptor function (rods > cones).
Signs & Examination Findings
The classic clinical triad of RP is arteriolar attenuation, retinal pigmentary changes could be either hypopigmentation and/or hyperpigmentation in form of bone-spicule and pigment clumpings, and waxy disc pallor. Other common signs include vitreous cells, depigmentation and atrophy of the RPE, posterior subcapsular cataracts, cystic macular lesions, and refractive errors including myopia and astigmatism.
Other genetic disorders that cause retinal degeneration include gyrate atrophy, choroideremia, cone-rod dystrophy, cone dystrophy, and Leber congenital amaurosis.
Drug toxicity from thioridazine hydrochloride (Mellaril) can lead to diffuse pigmentary clumping and RPE atrophy, ring scotoma on visual field testing, and markedly abnormality on ERG testing in later stages.
Infectious causes including syphilis, rubella, and diffuse unilateral subacute neuroretinitis (DUSN) can cause similar pigmentary retinopathy seen in patients with RP.
These include various vitamins and minerals, vasodilators, tissue therapy with placental extract, cortisone, cervical sympathectomy, injections of a hydrolysate of yeast RNA, ultrasound, transfer factor, dimethyl sulfoxide, ozone, muscle transplants, and subretinal injections of fetal retinal cells.
The use of high dose vitamin A, docosahexaenoic acid (DHA), and lutein to slow the progression of RP. It has been speculated that vitamin A rescues remaining cones, thereby explaining how one supplement may help a group of patients with different rod-specific gene defects. Vitamin E may lead to an adverse effect on the course of RP by inhibiting the absorption or transport of vitamin A.
Patients who develop cystic macular lesions (about 30%) may benefit from oral acetazolamide, topical dorzolamide drops, and intravitreal steroids in some cases.
Future Treatment Modalities
1. Gene therapy
2. Ciliary neurotrophic factor (CNTF)
3. Retinal prosthesis or phototransducing chip