The student's guide to diseases of the eye . cases of lasting occlu-sion of the retinal arteries. In a few cases where instantaneous blindness ofboth eyes has been associated with extremely di-minished arteries (ischcernia retina), iridectomyhas been followed by return of sight; lower tensioncausing re-establishment of circulation. These casesgenerally occur after whooping cough. (See alsoQuinine blindness.) Retinitis pigmentosa is a very slowly progressive symmetrical disease,leading to atrophy ofthe retina, with collec-tion of black pigmentin its layers and aroundits blood-vessels, andsecond
The student's guide to diseases of the eye . cases of lasting occlu-sion of the retinal arteries. In a few cases where instantaneous blindness ofboth eyes has been associated with extremely di-minished arteries (ischcernia retina), iridectomyhas been followed by return of sight; lower tensioncausing re-establishment of circulation. These casesgenerally occur after whooping cough. (See alsoQuinine blindness.) Retinitis pigmentosa is a very slowly progressive symmetrical disease,leading to atrophy ofthe retina, with collec-tion of black pigmentin its layers and aroundits blood-vessels, andsecondary atrophy ofthe disc. The earliest symptomis inability to see wellat night, or by artificiallight (night-blindness,nyctalopia). Concentriccontraction of the visualfield soon occurs (). These defects mayreach a high degreerwhilst central vision re-mains excellent in bright daylight. The symptomsare noticed at an earlier stage by patients in whomthe choroid is dark, and absorbs much light. Ophthalmoscopic examination, where these sym-. FlG-. 78.— Extreme concentriccontraction of field of vision(R.) in a case of advanced re-tinitis pigmentosa. The cen-tral dot shows the fixationpoint. The black shows thepart lost. DISEASES OF THE RETINA 199 ptonis have been present for some years, shows:—(1) at the equator or periphery a greater or lessquantity of pigment, arranged in a reticulated orlinear manner (Fig. 75), often with some small,separate dots; (2) in advanced cases, evidence ofremoval of the pigment epithelium, but never anypatches of choroidal atrophy; (3) that the pigmentis arranged in a belt, which is generally uniform,the pattern being most crowded at the centre, andthinning out towards the borders of the belt; (4)that the changes are always symmetrical, and thesymmetry very precise. These appearances are quitecharacteristic of true retinitis pigmentosa. In addi-tion, we find (5) diminution in size of the retinalblood-vessels, the arteries in advanced cases beingm
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