Hemeralopia, also called day blindness, is an abnormal vision condition in which bright light causes blurring of vision. Hemeralopia develops as a side effect of certain anticonvulsant medications. Humans and animals which are day blind are called hemeralopic. The condition is thought to be inherited as autosomal recessive trait. Day blindness is a feature of a number of inherited retinal degeneration diseases: progressive retinal atrophy, retinal pigment epithelial dystrophy, congenital stationary night blindness and day blindness. It has been described in the Alaskan Malamute, Miniature Poodle, German Shorthaired Pointer, and Great Dane. Day blindness is apparent in the large majority of affected dogs by eight to ten weeks of age. Night vision is never impaired. Histologic and ultrastructural studies of the retina indicate that in the mature hemeralopic dog there are no identifiable cones, and that in the younger dog (six months of age), normal cones do not exist, although degenerate cones can be found. In all hemeralopic dogs, the rods and the inner retinal layers of the eye are normal.
Signs are usually apparent in dogs 8 to 10 weeks of age. Hemeralopic dogs show severe loss of vision both in daylight and in high levels of artificial illumination, but such dogs become less insecure when placed in dim lighting. Recovery of vision takes several minutes, though loss of vision on returning to bright light is immediate. As with progressive retinal atrophy, there is no effective treatment for hemeralopia.