A-z of posterior eye conditions C

47 Cone dystrophies Progressive cone degeneration Cone-rod dystrophy Figure 1. Granular Rod-cone dystrophy pattern of pigmentary change at the DESCRIPTION macula These are a heterogenous group of conditions usually characterised 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 chromo- some 17. In some cases of cone dystrophy, rod dysfunction will also develop over time Figure 2. Bull’s eye (cone-rod degeneration), whereas in others, pattern of retinal rod dysfunction may precede that of the pigment epithelial cones (rod-cone degeneration). atrophy

PREVALENCE Low vision services are appropriate. SYMPTOMS Tinted spectacles or contact lenses have The two most common initial symptoms in Rare. been suggested to reduce cases of cone dystrophy are blurred vision and thereby improve vision. and photophobia. Once vision has declined to a level of 6/12 to 6/18, colour vision may be DIFFERENTIAL DIAGNOSIS severely affected. Red-green colour vision Other conditions that may lead to Topical medication defects are most commonly seen. Vision loss bull’s-eye or geographic RPE changes Weak miotics may be used during the day may well commence in the first decade of at the maculae, and pigmentary changes such as 0.5 per cent pilocarpine. life, declining slowly to around 6/60 by the in the fundus such as: Chloroquine third or fourth decade. Some patients are maculopathy; Stargardt’s disease; Age- quite sensitive to glare and will complain of related macular degeneration; Central Genetic counselling difficulty adjusting to changing light levels. areolar choroidal dystrophy; Retinitis Patients and their relatives may be offered Some difficulty with night vision is usually pigmentosa; Optic atrophy. genetic counselling. present in cases of rod-cone degeneration, with this sub-type carrying the worse overall prognosis for vision. MANAGEMENT Additional investigations SIGNS The full series of these articles will be Electrophysiological studies are available in the book Posterior Fundus changes in these conditions are very important in establishing a definitive and Glaucoma A-Z by Bruce AS, O’Day J, variable. The fundus may appear normal in diagnosis. Typically with the ERG in McKay D and Swann P. £39.99 For further the early stages of the disease, even in the cone dystrophy, the photopic ‘B’ wave information click on the Bookstore at presence of blurred . Later in is depressed and the 30Hz flicker shows www.opticianonline.net. the disease process, a bull’s eye or central a reduced amplitude. Rod involvement, pattern of RPE atrophy may be observed. either early or late, leads to a reduction ◆ Adrian Bruce is a Chief Optometrist at Other potential fundus signs include pigment in rod-mediated ERG responses and the Victorian College of Optometry and a clumping in the macular region, a tapetal reduced dark adaptation. Electro- Senior Fellow, Department of Optometry sheen at the maculae, peripheral pigmen- oculography is unlikely to provide and Vision Sciences, The University of tary changes that may lead to confusion with further information. Fluorescein angiog- Melbourne. ◆ and temporal pallor of the raphy may reveal macular RPE window Justin O’Day is an Associate Professor in the Department of , optic disc. Nystagmus is commonly seen. defects at an early stage in cases of cone The University of Melbourne and Head Examination of the visual fields typically dystrophy, and may also aid in the more Of Neuro-Ophthalmology Clinic, Royal shows central, paracentral and pericentral accurate delineation of bull’s eye macular Victorian Eye and Ear Hospital. ring . Central scotomas are likely changes early in development. ◆ Daniel McKay is a Medical Officer at to deepen and enlarge with time. Periph- the Royal Victorian Eye & Ear Hospital. eral visual fields are usually normal to begin ◆ Peter Swann is Associate Professor with, but in patients where rod involvement Refractive correction and low vision assistance in the School of Optometry, Queensland ensues, peripheral sensitivity becomes University of Technology. depressed. There is no treatment for cone dystrophy. www.opticianonline.net September 29, 2006 No 6071 Vol 232 Optician