Pupillary Light Reflexes
Following the menace responses, the clinician should evaluate pupil size and PLRs. Pupils should be assessed for symmetry in ambient light and a dark room. A bright light source should then be directed into each eye individually. Normal pets have rapid constriction of the pupil into which the light is directed (direct PLR) and the opposite pupil also should constrict (indirect or consensual PLR). The indirect response occurs because the majority of the optic nerve fibers cross at the optic chiasm and, again at the level of the pretectal nucleus, stimulating the parasympathetic oculomotor nuclei bilaterally.
In addition to optic and oculomotor nerve lesions, there are several possible localizations for PLR deficits. If a direct PLR is not elicited in one eye, the clinician should direct the light as close to that eye as possible and move the light to all aspects of the ocular fundus. If no response is present, the clinician should swing the light to the responsive pupil and the nonresponsive pupil should be assessed for constriction. If nonresponse is the result of an ocular or optic nerve lesion, it will constrict when light is directed into the contralateral eye (e.g., positive indirect PLR). Such testing may need to be repeated multiple times to differentiate between an ocular/optic and oculomotor nerve problem.