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Abstract
Intravitreal administration of 0.5 mg. per 0.1 c.c. of chloroquine hydrochloride produces an irreversible toxic effect to the cat photoreceptors within 40 hours. Of 24 cats that received 0.5 mg. per 0.1 c.c, 19 showed an average reduction of 50 per cent in the amplitude of the electroretinogram (ERG), whereas doses less than 0.5 mg. produced no ERG changes. When the near-threshold dose of 0.5 mg. per 0.1 c.c. is used, the inner retinal layers, examined with the light microscope, appear preserved even though the drug must have passed through these layers to reach the photoreceptors. Simultaneous reduction of a-wave and b-wave amplitude in the ERG as well as light microscopic studies indicate that this near-threshold dose initially damaged the photoreceptors, although an additional primary toxicity to the pigment epithelium cannot be excluded. Comparable destruction was present in retina overlying the pigmented (nontapetal) and nonpigmented (tapetal) pigment epithelium in the same eye. Twenty times the near-threshold dose of chloroquine required for acute photoreceptor damage by the intravitreal route results in no ERG changes when administered by the long posterior ciliary artery (LPCA). Results of LPCA injections of chloroquine hydrochloride and sodium iodoacetate are compared.