Rare diseases: Retinitis pigmentosa

Dr Keith Barnard describes these inherited diseases of the retina.

Ophthalmoscope view showing dark spots and star-shaped blotches across the retina (Photograph: Paul Parker/SPL)
Ophthalmoscope view showing dark spots and star-shaped blotches across the retina (Photograph: Paul Parker/SPL)

What is it?

  • Retinitis pigmentosa is the name given to a group of inherited diseases of the retina that all lead to a progressive reduction in vision. It is the main cause of blindness associated with inherited retinal degeneration.
  • The name was first used by Dutch ophthalmologist Dr Franciscus Donders in 1857.
  • The defect of vision appears as an annular or ring scotoma. Central vision is the last to be lost.
  • Prevalence is about one in 4,000 and, because there are X-linked varieties, men may be affected more than women.

Pathology

  • Retinitis in this context is not accurate, because there is no inflammatory or infectious component. Many different genes can lead to the diagnosis, and patients with the same genetic mutation can present with different patterns of retinal damage.
  • Retinal examination usually reveals black or dark brown star-shaped concentrations of pigmentation. The patterns can vary, and may be limited to one quadrant of the retina, or appear to be radiating from the disc, or there may be significant vasculopathy and optic nerve atrophy.
  • The changes result in gradual deterioration of the light-sensitive cells of the retina affecting mainly the rods, but cones can also be affected, depending on the genetic variant.
  • More than 50 different genetic defects have been identified; 30-40% are autosomal dominant, up to 15% are X-linked (the most severe form causing blindness by the third decade) and the rest are autosomal recessive, tending to be milder, with vision preserved until the sixth decade.

Symptoms

  • Presentation may involve impaired night vision, a tendency to trip over things, or can follow a road accident.
  • Symptoms usually appear between 10 and 30 years. One severe type, Leber's amaurosis, can cause blindness by six months.
  • There may be associated myopia, cataract, glaucoma or keratoconus.
  • About one third of patients have problems outside the eye, such as deafness (Usher syndrome), or a combination of short stature, mental deficiency, renal impairment and polydactyly (Laurence-Moon-Biedl syndrome).

Diagnosis

  • Apart from a detailed assessment of the eye, specialist centres can perform an electroretinogram, a procedure that will show a reduction of rod and cone signals.

Management

  • There is no definite treatment. Low-vision specialists can help, and protection from UV light with sunglasses may slow down visual loss.
  • Genetic counselling is advised, and patients may need to be registered partially sighted or blind.
  • Treatments such as vitamin A, carbonic anhydrase inhibitors, steroids and other drugs are not supported by robust evidence.
  • Although the condition is relentlessly progressive, complete blindness is uncommon.
  • Dr Barnard is a former GP from Fareham, Hampshire.

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