The menace response is the most reliable test for vision assessment in animals. It is a learned response that may not be present until 10 to 12 weeks of age in puppies and kittens (tracking of moving objects may be helpful in such young animals). The menace response requires a functional optic nerve, optic tract (diencephalon), and optic radiation up to the occipital cortex, as well as the efferent pathway that includes the facial neurons. Although the exact mechanism is unknown, a functional cerebellum also is required for the menace response.1 The majority of the visual pathway caudal to the optic chiasm is contralateral to the eye being tested. With one pet eye covered, the clinician should make a menacing gesture toward the open eye (with careful attention not to touch or stimulate the pet with air currents!).
A normal menace response is manifested as complete eyelid closure. Eyelid closure is dependent on normal facial nerve innervation of the orbicularis oculi muscle. If the menace is absent or delayed, the eyelids must be assessed for their ability to close by eliciting the palpebral reflex. If facial paralysis is present, eyeball retraction, elevation of the third eyelid, and head retraction may help in the assessment of vision. Alternatively, the patient’s ability to navigate an obstacle course can be evaluated.