These are a heterogeneous group of conditions usually characterized by an early, bilateral and slowly progressive loss of central vision. Although frequently sporadic in nature, when inherited it is usually as an autosomal dominant trait. Autosomal recessive and X-linked recessive forms have been described. Autosomal dominant cone dystrophy has been mapped to chromosome 17. In some cases of cone dystrophy, rod dysfunction will also develop over time (cone-rod degeneration), whereas, in others, rod dysfunction may precede that of the cones (rod-cone degeneration).
The two most common initial symptoms in cases of cone dystrophy are blurred vision and photophobia. Once vision has declined to a level of 6/12-6/18, color vision may be severely affected. Redgreen color vision defects are most commonly seen. Vision loss may well commence in the first decade of life, declining slowly to around 6/60 by the third or fourth decade. Some patients are quite sensitive to glare and will complain of difficulty adjusting to changing light levels. Some difficulty with night vision is usually present in cases of rod-cone degeneration, with this subtype carrying the worse overall prognosis for vision.
Fundus changes in these conditions are very variable. The fundus may appear normal in the early stages of the disease, even in the presence of blurred visual acuity. Later in the disease process, a bull’s eye or central pattern of retinal pigment epithelium (RPE) atrophy may be observed. Other potential fundus signs include pigment clumping in the macular region, a tapetal sheen at the macula, peripheral pigmentary changes that may lead to confusion with retinitis pigmentosa and temporal pallor of the optic disc. Nystagmus is commonly seen.
Examination of the visual fields typically shows central, paracentral and pericentral ring scotomata. Central scotomata are likely to deepen and enlarge with time. Peripheral visual fields are usually normal to begin with, but, in patients where rod involvement ensues, peripheral sensitivity becomes depressed.
Other conditions that may lead to bull’s eye or geographic RPE changes at the macula, and pigmentary changes in the fundus such as chloroquine maculopathy; Stargardt’s disease; age-related macular degeneration; central areolar choroidal dystrophy; retinitis pigmentosa; optic atrophy.
Additional investigations Electrophysiological studies are important in establishing a definitive diagnosis. Typically with the electroretinogram (ERG) in cone dystrophy, the photopic ‘B’ wave is depressed and the 30 Hz flicker shows a reduced amplitude. Rod involvement, either early or late, leads to a reduction in rod-mediated ERG responses and reduced dark adaptation. Electro-oculography is unlikely to provide additional information. Fluorescein angiography may reveal macular RPE window defects at an early stage in cases of cone dystrophy, and may also aid in the more accurate delineation of bull’s eye macular changes early in their development.
Refractive correction and low vision assistance There is no treatment for cone dystrophy. Low vision services are
Granular pattern of pigmentary change at the macula.
Bull’s eye pattern of retinal pigment epithelial atrophy.
appropriate. Tinted spectactes or contact lenses have been suggested to reduce photophobia and thereby improve vision,
Topical medication Weak miotics may be used during the day such as 0.5 per cent pilocarpine.
Genetic counselling Patients and their relatives may be offered genetic counselling.