2003 Volume 22 Issue 1-2 Pages 1-2_11-1-2_14
Light reflextest is an item of the ophthalmological examination, which is intended to evaluate the neurological function from photoreception, conduction of stimulus up to consequential miosis, differently from morphological observation of the eyes using a slit lamp and an ophthalmoscope. Although, evaluation of the light reflex is thought to be useful for better understanding of ophthalmological lesions observed in humans, light reflex abnormalities in rats have rarely been reported. In a 26-week toxicity study in rats which was conducted at our facility, we encountered Crj:CD(SD)ISG rats, one male and one female, exhibiting impaired light reflex in the ophthalmological examination. In the male, normal light reflex (bilateral miosis) was observed when the light beam was applied to the left eye, but both eyes failed to respond to the light beam to the right eye. A slit lamp and an indirect ophthalmoscope revealed no abnormalities in the eyes. At necropsy, the right optic nerve was thin and the left posterior hemisphere of the brain was depressed focally. Histopathology revealed a slight atrophy of the right optic nerve and focal defect of the left posterior cerebrum, the site where a part of afferent pathway for light reflex exists. From these, it was suspected that the cause of impaired light reflex in the male would be a focal defect of the posterior cerebrum containing afferent pathway for light reflex. In the female, similar to the case in male, normal light reflex was observed against the light stimulus to the left eye, but both eyes failed to respond to the light stimulus to the right eye. Ophthalmology revealed focal opacity of the lens and vitreous body, strong reflection of the light from the fundus and generalized thinning of the fundic vessels in the right eye. Necropsy revealed thinning of the right optic nerve and histopathology revealed a slight atrophy of the right optic nerve and severe atrophy of the right retina. From these, it was suspected that the cause of impaired light reflex in the female would be atrophy of the retina, a photoreceptor.
