Figure D. Since a blue-eyed cat has no pigment in the nontapetal area, the large orange choroidal vessels can be seen. Retinal vessels are red.
The retinal vasculature in dogs usually consists of 3 or 4 major venules (dorsal, ventromedial, ventrolateral) and up to 20 art-erioles. While arterioles may be tortuous, venules are strai-ghter. Venules form an incomplete to complete anastomotic circle on the surface of the optic disc. Cats have 3 venules and 3 major arterioles (narrower and more tortuous than those of dogs) that leave the periphery of the optic disc.
Examining the Fundus
Complaints concerning visual capacity or ability present a major diagnostic problem for veterinarians. Except in cases of complete blindness, it is extremely difficult to define a specific level of visual capacity. Careful and systematic ophthalmic examination is the most effective and objective means available to determine the absolute presence or absence of significant visual disability in an animal with ocular pathology. While ophthalmic examination may define ocular lesions in animals, their significance in terms of the animal's ability to see is often uncertain. Vision tests in animals are necessarily empirical; however, there is at present no means of truly objectively and quantitatively assessing vision or visual disability in an animal that is not overtly blind. Maze testing in light and dark conditions, observation of dogs following a laser pointer, and evaluation of the behaviour of the dog or cat in an unfamiliar environment can provide some information about the animal's visual capability.
Step by Step: FUNDIC EXAMINATION: LIGHT RESPONSES
1. Pupillary Light Reflex
The fundic examination begins with evaluating the size of the pupils and testing the direct and con-sensual pupillary light reflexes (PLR). Pupillary reflexes to light are tested early in the examination so the pupils can be dilated for evaluation of the lens and fundus. The PLR (direct and indirect) evaluates the integrity of the retina, optic nerve, midbrain, oculomotor nerve, and iris sphincter muscle. Stimulation of one eye results in the constriction of both pupils. The PLR is valuable in testing potential retinal function in eyes with severe corneal opacity, but is not a true test of vision.
PLR testing should be done with a bright light (See Box) in a dimly lit room (A). The reflexes are affected by the psychic state of the animal, room illumination, age, many topical and systemic drugs, and the intensity of the light stimulus. If an animal is highly nervous or frightened, the pupils may be dilated and respond poorly to low intensity light (eg, bright penlight). However, with acclimation or a strong light source, this effect is minimized.