Ophthalmologist Dr Mark Llewellyn continues our series on diagnosing, managing and referring eye problems
A 25-year-old woman presents complaining of progressively worsening ‘tunnel vision' and poor night vision. Although she has never needed glasses, she says that since her early teens she has had trouble reading unless she is in bright light.
She never drives at night because she has never felt safe. She has also noticed she is bumping into walls and objects not in her centre of vision. Otherwise she has an unremarkable medical history. She reports no history of serious visual problems in her family. On examination, visual acuity is 6/15 in both eyes with no obvious pupil defects. But examination of the fundus revealed dark clumps of pigment. She is referred to ophthalmology where visual field testing finds large scotomas in the periphery. Electroretinography finds marked lack of rod and cone responses. She is diagnosed with retinitis pigmentosa.
The problem
• Retinitis pigmentosa is a group of inherited disorders characterised by progressive peripheral vision loss and night vision difficulties (nyctalopia) that can lead to central vision loss.
• Prevalence in the UK is around one in 4,000.
• Approximately 20% of cases are autosomal dominant, 20% are autosomal recessive and 10% X-linked, with no known family history in the remaining 50%. Because one in 10 cases are X-linked, there are slightly more men affected than women.
• Autosomal dominant forms are mildest with some central vision preserved until the fifth or sixth decade. Central vision is usually lost by the third decade in the X-linked form.
Features
• Hallmark symptom is poor night vision.
• Good central vision is usually retained until late in the disease, but can be affected by associated ocular conditions, such as open-angle glaucoma, myopia and drusen.
• Progressive visual field loss leads to disabling tunnel vision.
• Patients are often sensitive to glare.
• Fundus examination shows classic areas of spidery, bone-spicule pigment clumps – typically with perivascular disposition.
• Changes progress both peripherally and centrally.
• Optic disc later atrophies.
Differential diagnosis
Poor night vision also seen in:
• congenital stationary night blindness
• choroideremia
• gyrate atrophy of the choroid
• vitamin A deficiency.
Similar retinal appearance is caused by:
• drug toxicity – chloroquine, hydroxychloroquine
• trauma
• previous retinal detachment
• infectious diseases such as syphilis and congenital rubella.
Examination
• Check visual acuity with correction if necessary – patients may show some loss, though many maintain central macular vision late in disease.
• Check visual fields – affected individuals will have pronounced loss of peripheral vision.
• Fundoscopy will reveal characteristic pigment clumping.
Referral
• Referral is indicated for formal diagnosis.
• Detailed field testing will identify pattern of visual loss.
• Electroretinography provides a sensitive objective measure of rod and cone function and is the most critical test.
Management
• Retinitis pigmentosa is not treatable, but patients and their relatives should be offered genetic counselling.
• Rehabilitative services should be considered as the disease progresses.
• Cystoid macular oedema is a possible complication and patients may benefit from oral carbonic anhydrase inhibitors.
• Antioxidants like vitamin A may be useful, although the evidence is modest.2
Dr Mark Llewellyn is a consultant ophthalmologist at the Powys Teaching Health Board in Brecon
References
1 Bundey S and Crews SJ. A study of retinitis pigmentosa in the city of Birmingham: II clinical and genetic heterogeneity. J Med Genet 1984;21:421-8
2 Norton EW. A randomised trial of vitamin A and vitamin E supplementation for retinitis pigmentosa. Arch Ophthalmol 1993;111: 1460-3
