Retinitis pigmentosa

Retinitis pigmentosa, or RP, is a group of genetic eye conditions. 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 life. Others go completely blind from RP, in some cases as early as childhood. Progression of RP is different in each case.

RP is 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 constriction of the peripheral visual field and, eventually, loss of central vision late in the course of the disease.

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Signs

Mottling of the retinal pigment epithelium with bone-spicule pigmentation is typically pathognomonic for retinitis pigmentosa. Other ocular features include waxy pallor of the optic nerve head, attenuated retinal vessels, cellophane maculopathy, cystic macular edema, and posterior subcapsular cataract..

Diagnosis

The diagnosis of retinitis pigmentosa relies upon documentation of progressive loss in photoreceptor 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. 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 females are usually more mildly affected. Some digenic and mitochondrial forms have also been described.

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

RP combined with progressive deafness is called Usher syndrome.

Genetics

In 1989, a mutation of the gene for rhodopsin, a pigment that plays an essential part in the visual transduction cascade, 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 mutation and inherited mostly in a dominant manner.

There are multiple genes that, when mutated, can cause the Retinitis pigmentosa phenotype. ^[1]

Causes

A few causes for RP are:

• Fahr disease
• Bardet-Biedl syndrome
• Lowe syndrome
• Usher's syndrome
• Subacute necrotising encephalomyelopathy
• Pyruvate carboxylase deficiency
• MELAS
• Hereditary sensory-motor neuropathy type 7
• Rud's syndrome
• Refsum's disease
• Kearns-Sayre syndrome
• Carbohydrate deficient glycoprotein syndrome type 1a
• Loken Senior syndrome
• Hallervorden-Spatz disease
• Abetalipoproteinaemia
• Homocarnosinase deficiency
• Mirhosseini-Holmes-Walton syndrome
• Shwachman-Diamond syndrome
• HARP syndrome
• Alström syndrome
• Medullary cystic renal disease
• Stargardt's disease
• Sjogren-Larsson syndrome
• Tapetochoroidal dystrophy

Treatment

There is currently no medical treatment that can completely cure retinitis pigmentosa, although the progression of the disease can be reduced by the daily intake of 15000 IU of vitamin A palmitate.^[2] Recent studies have shown that proper vitamin A supplementation can postpone blindness by up to 10 years.^[3] Scientists continue to investigate possible treatments. Future treatments may involve retinal transplants, artificial retinal implants,^[4] gene therapy, stem cells, nutritional supplements, and/or drug therapies.

In a study published in the journal Nature, researchers working with mice at the University College London Institutes of Ophthalmology and Child Health and Moorfields Eye Hospital, transplanted mouse stem cells which were at an advanced stage of development, and already programmed to develop into photoreceptors, 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.^[5]

See also

• Visual prosthetic
• List of eye diseases and disorders
• Progressive retinal atrophy for the condition in dogs

References