Retinitis pigmentosa

Retinitis pigmentosa

35 Retinitis pigmentosa This interesting retinal condition is a primary degene­ ration of the neuro-epithelium, accompanied by a widespread pigment ...

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Retinitis pigmentosa

This interesting retinal condition is a primary degene­ ration of the neuro-epithelium, accompanied by a widespread pigment disturbance (35.1), narrowing of the vessels and optic atrophy. The aetiology is un­ known. The condition is hereditary, although the type of transmission varies and sporadic cases occur in which no family history can be obtained. It is important to attempt to determine the type of transmission in each case so that advice can be given on the likelihood of offspring being affected. If the disease occurs as a dominant characteristic, half the children of an affected person are likely to exhibit the condition. Consanguinity is an important factor in the incidence of the recessive type. Sexlinkage is common, males exhibiting the fully deve­ loped condition, whereas the female carriers who are symptomless may nevertheless show a curious increase in reflection from the fundus, somewhat reminiscent of the tapetal reflex of the cat. Gene probing can identify some carriers of retinitis pigmentosa. Retinitis pigmentosa is the final phenotypic expression of probably a great number of varied inherited defects of rhodopsin metabolism. Other defects of a degenerative nature may be encountered; thus the association of pigmentary de­ generation of the retina with obesity, polydactyly, hypogenitalism and mental retardation constitutes the Laurence-Moon-Biedl syndrome. The earliest symptom is relative night blindness, which may be first noticed in childhood and is due to the depression of function of the rods, which subserve scotopic vision. The degeneration is bilateral, first affecting the equatorial region and producing a ring scotoma in the visual fields. The scotoma gradually spreads out towards the periphery and in towards fixa­

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tion, until only a small central field remains. Although the acuity of central vision may be quite good the patient is very severely handicapped by the gross field loss. The remaining visual field typically disappears in middle age, and cataract formation is common. 35.5 is a composite chart of the visual fields showing a typical ring scotoma above, which in the chart below has progressed to leave only a small central area, some­ times described as a 'tubular' field. The fundus picture is striking in an advanced case. The optic disc is pale and Waxy in appearance, the retinal vessels are markedly narrowed and may appear as mere threads. The typical spidery pigment deposits, which have been likened to the Haversian systems in bone, are scattered widely over the peripheral retina. The pigment is frequently found lying along the wall of the veins, as is clearly shown in the fundus photo­ graph (35.2). This migration of pigment from the retinal pigmentary epithelium allows the choroid to be seen more clearly, so that a tessellated pattern and individual choroidal vessels can be seen (35.3). In an early case the pigment is only present in the equatorial region, while the extreme periphery and central areas appear normal (35.4). Patients and their families are investigated to confirm the diagnosis and identify the pattern of inheritance so that prognostic and genetic advice can be offered if desired. Electroretinography (ERG), visual fields, family pedigree and ophthalmoscopy aid the diagnosis. Treatment is very disappointing. The disease is subject to remissions which may coin­ cide with some particular therapy, giving rise to false hopes of improvement, but prevention on eugenic lines gives the best hope of eradicating the condition.

35.1

35.2

Pigmentary deposits on vessels 35.3

35.4

Typical pigmentary ' changes

Localised distribution of pigment

Increased visibility of choroidal vessels

35.5

Typical field defects 92