RETINITIS PIGMENTOSA

 

 

Retinitis Pigmentosa (RP) refers to a group of inherited diseases that damage the light sensitive rods and cones in the retina. The rods are affected more than the cones. The rods provide peripheral vision and night vision. The cones provide colour vision and clear central vision. RP patients initially have night blindness (nyctalopia) and peripheral vision loss that may subsequently lean onto central vision loss.

 

 

Inheritance

 

·        Autosomal recessive – most common type of RP. The chance of having this condition is higher if the patients are related (eg:cousins).

·        Autosomal dominant - in this form of RP, only one parent has the gene and is usually affected by the disease as well. Each child has a 50 percent chance of inheriting this gene and developing RP.

·       
X linked – if the father is affected, all sons will be unaffected and all daughters will be carriers. If the mother is the carrier, 1 in 2 sons will be affected and in 2 daughters will be carriers.

 

 

Prevalence and Demography

 

·        Prevalence – RP is considered a rare disorder. It is generally estimated that the disorder affects roughly 1 in 4000 people.

·        Age – It appears in the childhood and progress slowly, often resulting in blindness in advanced middle age.

·        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.

 

Pathogenesis

            

RP is an inherited disorder that results from harmful changes in any one of more than 50 genes. These genes carry the instructions for making proteins that are needed in cells in the retina, called photoreceptors. Some of the changes, or mutations with in genes are so severe that the gene can’t make the required protein, limiting the cells functions. Other mutations produce a protein that is toxic to the cell. Still other mutations lead to an abnormal protein in that doesn’t function properly. In all these cases, the result is damage to the photoreceptors.

 

Clinical Features

 

Typically RP, in which rods are degenerated early and cones are involved late, is characterized by following features:

 

1.       Visual changes –

·        Night blindness- It is the characteristic and earliest features 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 untill very late.

·        Tubular vision, ie) loss of peripheral vision with preservation of central vision occurs in advanced cases.

·        Central vision is also lost ultimately after many years.

 

2.       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 narrowed and may become thread- like in late stages.

·        Thinning and atrophy of retinal pigment epithelium is seen in peripheral retina.

·        Optic disc becomes pale and waxy in later stages and ultimately consecutive optic atrophy occurs.

 

3.      Visual field changes –

·        Annular or ring shaped scotoma is a typical feature which corresponds to the degenerated equatorial zone of retina.

 

Associations of Retinitis pigmentosa

 

  1. Ocular associations

·        Myopia

·        Primary open angle glaucoma

·        Microphthalmos

·        Conical cornea

·        Posterior subcapsular cataract

2. Systemic associations

·        Laurence Moon Biedl syndrome- It is characterized by retinitis pigmentosa, obesity, hypogenitalism, polydactyly and mental deficiency.

·        Cockayne's syndrome- It is characterized 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.

 

Diagnosis

 

·        Acuity test – measures the accuracy of central vision.

·        Colour vision testing – helps determine the status of cone cells.

·        Visual field test – to determine the extent of vision loss.

·        Dark adaptation – measures how well eyes adjust to change in lighting.

·        ERG test (electrophysiological test)- records the electrical currents produced by the retina due to light stimulus.

Electroretinogram (ERG) is the most critical diagnostic test for RP because it provides an objective measure of rod and cone function across the retina.

 

Treatment

 

There is no effective treatment for retinitis pigmentosa. There are some general measures for supportive management.

 

·        Vitamin A Palmitate- High doses of this compound may slow RP.

·        Systemic acetazolamide for associated cystoid macular edema.

·        Sunglasses- These make eyes less sensitive to light and protect eyes from harmful UV rays that may spread vision loss.

·        Low vision aids in the form of 'magnifying glasses' and ' night vision device' may be of some help.

·        Replacement of damaged cells or tissues with healthy ones under investigation.

·        Gene therapy to put healthy genes into the retina under trial.

·        Bionic Eye System, a visual prothesis designed to restore functional vision to the blind suffering from RP.

·        Rehabilitation of the patient should be earned out as per his socioeconomic background.

·        Prophylaxis- Genetic counselling for no consanguineous marriages may help to reduce the incidence of disease.

 

 

 

Dr. Devin Prahbakar MS, FRCS

www.DivyaPrabha.in