Retinitis pigmentosa, usually known as primary pigmentary retinal dystrophy is a hereditary disorder predominantly affecting the rods more than the cones. We all know what the rods and cones are right? Well, if you don’t know about it then here it is.
Rod Cells / Rods
Wiki says : Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 92 million rod cells in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.
Cone Cells / Cones
Wiki says : Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and are thus responsible for color vision, and function best in relatively bright light, as opposed to rod cells, which work better in dim light. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye and are most concentrated towards the macula.
Retinitis pigmentosa (RP) may occur as:
1. Sporadic disorder,
It is isolated without family history due to mutation of multiple gene (>50%) including rhodopsin gene (40%).
2. Inherited disorder as:
- Autosomal recessive (AR), most common (25%), intermediate severity
- Autosomal dominant (AD), next common (25%), least severe
- X-linked (XL), least common (10%), most severe.
It occurs in 1 person per 5,000 of the world population.
- Age. Well mostly, it is seen during the childhood and as child grows it progresses slowly which invites blindness in the advanced middle age.
- Race. It doesn’t matter what race your are to be safe from it.
- Sex. Males are more commonly affected than females in a ratio of 3:2.
- Laterality. Disease is almost invariably bilateral and both eyes are equally affected.
As a group majority of retinitis pigmentosa conditions are characterized by death of rod photoreceptors. The molecular mechanism by which the genetic mutation eventually causes rod cell death are unclear, although ample evidence indicates that apoptosis is involved in the final pathway of cell death. That the cone photoreceptors ultimately die from a disease that begins with rod-cell disease remains a puzzle.
Typical retinitis pigmentosa, i.e., rod-cone dystrophy, in which rods are degenerated early and cones are involved late, is characterized by following features:
A. Visual symptoms
- Night blindness: It is the characteristic and earliest feature and may present several years before the visible changes in the retina appear. It occurs due to degeneration of the rods.
- Dark adaptation: Light threshold of the peripheral retina is increased; though the process of dark adaptation itself is not affected until very late.
- Tubular vision, i.e., loss of peripheral vision with preservation of central vision occurs in advanced cases.
- Central vision is also lost ultimately after many years.
B. Fundus changes
- Retinal pigmentary changes. These are typically perivascular (around veins) and jet black spots resembling bone corpuscles in shape. Initially, these changes are found in the equatorial region only and later spread both anteriorly and posteriorly.
- Retinal arterioles are attenuated (narrowed) and may become thread-like in late stages.
- Thinning and atrophy of retinal pigment epithelium (RPE) is seen in mid and far peripheral retina with relative sparing of RPE at the macula.
- Optic disc becomes pale and waxy in later stages and ultimately consecutive optic atrophy occurs.
- Other associated changes which may be seen are colloid bodies, choroidal sclerosis, cystoid macular oedema, atrophic or cellophane maculopathy.
C. Visual field changes
Annular or ring-shaped scotoma is a typical feature which corresponds to the degenerated equatorial zone of retina. As the disease progresses, scotoma increases anteriorly and posteriorly and ultimately only central vision is left (tubular vision). Eventually, even this is also lost and the patient becomes blind.
D. Electrophysiological changes
Typical electrophysiological changes appear early in the disease before the subjective symptoms or the objective signs (fundus changes) appear.
- Electroretinogram (ERG) is initially subnormal (scotopic affected before photopic; b-wave affected before a wave) and eventually extinguished.
- Electro-oculogram (EOG) is subnormal with an absence of light peak.
Association of Retinitis Pigmentosa
I. Ocular associations: These include myopia, primary open-angle glaucoma, microphthalmos, conical come (keratoconus) and posterior subcapsular cataract.
II. Systemic associations: Most cases of retinitis pigmentosa (RP) are isolated (i.e., with no systemic features), but about 25% have associated systemic diseases. A number of specific syndromes are described:
- Laurence-Moon-Biedl syndrome. It is characterized by retinitis pigmentosa, obesity, hypogenitalism, polydactyly and mental deficiency.
- Cockayne’s syndrome. It comprises retinitis pigmentosa, progressive infantile deafness, dwarfism, mental retardation, nystagmus and. ataxia.
- Refsum’s syndrome. It is characterised by retinitis pigmentosa, peripheral neuropathy and cerebellar ataxia.
- Usher’s syndrome. It includes retinitis pigmentosa and labyrinthine deafness.
- Hallgren’s syndrome. It comprises retinitis pigmentosa, vestibulocerebellar ataxia, congenital deafness and mental deficiency.
- Other associated syndromes include Bussen Koranzweig syndrome (Abetalipoproteinaemia), Kearns-Sayer syndrome, Friedreich’s ataxia, Bardet-Biedle syndrome, NARP (neuropathy, ataxia, and retinitis pigmentosa), neuronal ceroid lipofuscinosis, and olivopontocerebellar degeneration.
There are different laboratory test that can be ruled out for the diagnosis of the RP conditions:
- In severe RP or Cancer Associated Retinopathy, to study the antiretinal antibodies, neoplasm is taken into consideration.
- Electro-oculogram (EOG): Not helpful in diagnosing RP, but essential to find changes in macular, normal findings of ERG and unusual EOG findings suggest Best vitelliform macular dystrophy.
- For RP the most critical diagnostic test Electroretinogram (ERG) is taken into consideration.
- Color testing: Commonly use to check, mild blue-yellow axis color defects, usually RP patients do not face difficulty with color perception.
- Formal visual field testing might be helpful to measure continuous follow-up care of RP patients.
- Dark adaptation study: Disproportionately decrease contrast sensitivity relative to visual acuity in RP; bright-light sensitivity.
Treatment of Retinitis Pigmentosa
Sad but true but there is no effective treatment for the disease. However, following stops can be taken as an option to stop from the condition to get worse.
- Measures to stop progression, which have been tried from time to time, without any breakthrough include: vasodilators, placental extracts, transplantation of rectus muscles intosuprachoroidal space, light exclusion therapy, ultrasonic therapy and acupuncture therapy. Recently vitamin A (15000 IU, PO, qd of palmitate form) has been recommended to check its progression.
- If there are any refractive error correct them by prescribing glasses.
- Systemic acetazolamide (500 mg po) for associated cystoid macular oedema.
- Low vision aids (LVA) in the form of ‘magnifying glasses’ and ‘night vision device’ may be of some help.
- Rehabilitation of the patient should be earned out as per his socioeconomic background.
- Prophylaxis. Genetic counselling for no consan guinous marriages may help to reduce the inci dence of disease. Further, affected individuals should be advised not to produce children.
What’s the latest research on RP?
NEI supports research to develop a variety of treatments to prevent vision loss and restore sight. Gene therapy for several different types of RP has shown promise in the laboratory. In a landmark clinical trial, gene therapy for a retinal disorder called Leber congenital amaurosis (LCA) led to improved vision for people with that disorder. This and other gene therapy clinical trials for LCA are ongoing to establish a maximally safe dosage and determine the long-term benefits of treatment. Stem cells have also shown promise in the lab. Thanks in part to basic research supported by NEI, the company Advanced Cell Technologies is conducting a clinical trial to test the safety of stem cell treatments for a type of retinal disorder called macular degeneration. Other researchers, including a team at NEI, are gearing up for similar trials. NEI researchers are also evaluating various drug and nutritional therapies in the lab.
Source : https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinitis-pigmentosa
Reference : Fortune Journals
Comprehensive Opthalmology : AK Khurana