Retinitis pigmentosa

Definition:
Retinitis pigmentosa(RP) is an eye disease in which there is damage to the retinaretina. The retina is the layer of tissue at the back of the inner eye that converts light images to nerve signals and sends them to the brain.

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It is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some sight all their lives. Others go completely blind from RP, in some cases as early as childhood. Progression of RP is different in each case.

RP is a type of progressive retinal dystrophy, a group of inherited disorders in which abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium (RPE) of the retina lead to progressive visual loss. Affected individuals first experience defective dark adaptation or nyctalopia (night blindness), followed by reduction of the peripheral visual field (known as tunnel vision) and, sometimes, loss of central vision late in the course of the disease.

Symptoms:
Mottling of the retinal pigment epithelium with black bone-spicule pigmentation is typically indicative (or pathognomonic) of retinitis pigmentosa. Other ocular features include waxy pallor of the optic nerve head, attenuation (thinning) of the retinal vessels, cellophane maculopathy, cystic macular edema and posterior subcapsular cataract.

Symptoms often first appear in childhood, but severe vision problems do not usually develop until early adulthood.

•Decreased vision at night or in low light
•Loss of side (peripheral) vision, causing “tunnel vision”
•Loss of centralcentral vision (in advanced cases)

Causes:
It is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some sight all their lives. Others go completely blind from RP, in some cases as early as childhood. Progression of RP is different in each case.

RP is a type of progressive retinal dystrophy, a group of inherited disorders in which abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium (RPE) of the retina lead to progressive visual loss. Affected individuals first experience defective dark adaptation or nyctalopia (night blindness), followed by reduction of the peripheral visual field (known as tunnel vision) and, sometimes, loss of central vision late in the course of the disease.

Retinitis pigmentosa can run in families. The disorder can be caused by a number of genetic defects.

The cells controlling night vision (rods) are most likely to be affected. However, in some cases, retinal cone cells are damaged the most. The main sign of the disease is the presence of dark deposits in the retina.

The main risk factor is a family history of retinitis pigmentosa. It is an uncommon condition affecting about 1 in 4,000 people in the United States.

Diagnosis:-
The diagnosis of retinitis pigmentosa relies upon documentation of progressive loss in photoreceptor cell function by electroretinography (ERG) and visual field testing.

The mode of inheritance of RP is determined by family history. At least 35 different genes or loci are known to cause “nonsyndromic RP” (RP that is not the result of another disease or part of a wider syndrome).

DNA testing is available on a clinical basis for:

*RLBP1 (autosomal recessive, Bothnia type RP)
*RP1 (autosomal dominant, RP1)
*RHO (autosomal dominant, RP4)
*RDS (autosomal dominant, RP7)
*PRPF8 (autosomal dominant, RP13)
*PRPF3 (autosomal dominant, RP18)
*CRB1 (autosomal recessive, RP12)
*ABCA4 (autosomal recessive, RP19)
*RPE65 (autosomal recessive, RP20)

For all other genes, molecular genetic testing is available on a research basis only.

RP can be inherited in an autosomal dominant, autosomal recessive, or X-linked manner. X-linked RP can be either recessive, affecting primarily only males, or dominant, affecting both males and females, although males are usually more mildly affected. Some digenic (controlled by two genes) and mitochondrial forms have also been described.

Tests:-
Tests to evaluate the retina:

•Color vision
•Examination of the retina by ophthalmoscopyophthalmoscopy after the pupils have been dilated
•Fluorescein angiographyFluorescein angiography
•Intraocular pressureIntraocular pressure
•Measurement of the electrical activity in the retina (electroretinogramelectroretinogram)
•Pupil reflex response
•Refraction testRefraction test
•Retinal photographyRetinal photography
•Side vision test (visual field test)
•Slit lamp examinationSlit lamp examination
•Visual acuityVisual acuity

Genetic counseling depends on an accurate diagnosis, determination of the mode of inheritance in each family, and results of molecular genetic testing.

Associations:
Retinitis pigmentosa is seen in a variety of diseases, so the differential of this sign alone, is broad.

*RP combined with deafness (congenital or progressive) is called Usher syndrome.
*RP combined with opthalmoplegia, dysphagia, ataxia, and cardiac conduction defects is seen in the mitochondrial DNA disorder Kearns-Sayre syndrome (aka Ragged Red Fiber Myopathy)
*RP combined with retardation, peripheral neuropathy, acanthotic (spiked) RBCs, ataxia, steatorrhea, is absence of VLDL is seen in abetalipoproteinemia.

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Other conditions include neurosyphilis, toxoplasmosis(Emedicine “Retinitis Pigmentosa”), abetalipoproteinemia, and Refsum’s disease.

Genetics
Retinitis pigmentosa (RP) is one of the most common forms of inherited retinal degeneration. This disorder is characterized by the progressive loss of photoreceptor cells and may eventually lead to blindness.

There are multiple genes that, when mutated, can cause the Retinitis pigmentosa phenotype. In 1989, a mutation of the gene for rhodopsin, a pigment that plays an essential part in the visual transduction cascade enabling vision in low-light conditions, was identified. Since then, more than 100 mutations have been found in this gene, accounting for 15% of all types of retinal degeneration. Most of those mutations are missense mutations and inherited mostly in a dominant manner.

The rhodopsin gene encodes a principal protein of photoreceptor outer segments. Studies show that mutations in this gene are responsible for approximately 25% of autosomal dominant forms of RP.

Mutations in four pre-mRNA splicing factors are known to cause autosomal dominant retinitis pigmentosa. These are PRPF3, PRPF8, PRPF31 and PAP1. These factors are ubiquitously expressed and it is still a puzzle as to why defects in a ubiquitous factor should only cause disease in the retina.

Up to 150 mutations have been reported to date in the opsin gene associated with the RP since the Pro23His mutation in the intradiscal domain of the protein was first reported in 1990. These mutations are found throughout the opsin gene and are distributed along the three domains of the protein (the intradiscal, transmembrane, and cytoplasmic domains). One of the main biochemical causes of RP in the case of rhodopsin mutations is protein misfolding, and molecular chaperones have also been involved in RP. It was found that the mutation of codon 23 in the rhodopsin gene, in which proline is changed to histidine, accounts for the largest fraction of rhodopsin mutations in the United States. Several other studies have reported other mutations which also correlate with the disease. These mutations include Thr58Arg, Pro347Leu, Pro347Ser, as well as deletion of Ile-255. In 2000, a rare mutation in codon 23 was reported causing autosomal dominant retinitis pigmentosa, in which proline changed to alanine. However, this study showed that the retinal dystrophy associated with this mutation was characteristically mild in presentation and course. Furthermore, there was greater preservation in electroretinography amplitudes than the more prevalent Pro23His mutation.

Treatment:
Although incurable the progression of the disease can be reduced by taking some measures.
Wearing sunglasses to protect the retina from ultraviolet light may help preserve vision.

Some studies have suggested that treatment with antioxidants (such as high doses of vitamin A palmitate) may slow the disease.
Recent studies have shown that proper vitamin A supplementation can postpone blindness by up to 10 years (by reducing the 10% loss pa to 8.3% pa). However, taking high doses of vitamin A can cause serious liver problems. The benefit of treatment has to be weighed against risks to the liver.

Several clinical trials are in progress to investigate new treatments for retinitis pigmentosa, including the omega-3 fatty acid, DHA.

Microchip implants that go inside the retina and act like a microscopic video camera are in the early stages of development for treating blindness associated with this and other serious eye conditions.

It can help to see a low-vision specialist, who can help you adapt to vision loss. Make regular visits to an eye care specialist, who can detect cataractscataracts or retinal swelling — both of which can be treated.

Research on possible treatments:
Future treatments may involve retinal transplants, artificial retinal implants, gene therapy, stem cells, nutritional supplements, and/or drug therapies.

2006: Stem cells: UK Researchers working with mice, transplanted mouse stem cells which were at an advanced stage of development, and already programmed to develop into photoreceptor cells, into mice that had been genetically induced to mimic the human conditions of retinitis pigmentosa and age-related macular degeneration. These photoreceptors developed and made the necessary neural connections to the animal’s retinal nerve cells, a key step in the restoration of sight. Previously it was believed that the mature retina has no regenerative ability. This research may in the future lead to using transplants in humans to relieve blindness.

2008: Scientists at the Osaka Bioscience Institute have identified a protein, named Pikachurin, which they believe could lead to a treatment for retinitis pigmentosa.

2010: A possible gene therapy seems to work in mice.

2010:R-Tech Ueno(Japanese Medicine manufacture enterprise )Completes Phase II Clinical Study on Ophthalmic Solution UF-021 (Product Name Ocuseva (TM)) on Retinitis Pigmentosa.

Prognosis:
The disorder will continue to progress, although slowly. Complete blindness is uncommon.

Prevention:
Genetic counseling and testing may help determine whether your children are at risk for this disease.

Disclaimer: This information is not meant to be a substitute for professional medical advise or help. It is always best to consult with a Physician about serious health concerns. This information is in no way intended to diagnose or prescribe remedies.This is purely for educational purpose.

Resources:
http://en.wikipedia.org/wiki/Retinitis_pigmentosa
http://www.nlm.nih.gov/medlineplus/ency/article/001029.htm

http://www.rwjuh.edu/health_information/adult_eye_retin.html

http://www.nlm.nih.gov/medlineplus/ency/imagepages/1094.htm

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