by Noelle La Croix, DVM, Dip. ACVO
The Ophthalmic Examination – Part 1: Menace response, pupillary light and dazzle reflexes
Veterinary ophthalmic examinations are performed in a defined anatomical order while evaluating visual responses and reflexes (Table 1) with the objective of establishing a primary diagnosis. Full physical examinations are also routinely performed to identify underlying systemic disease. Diagnoses are not made by simply comparing ocular findings to pictures within an ophthalmic atlas or online. Unfortunately, ophthalmic diseases display a broad spectrum of presentations that will rarely match these images.
Ophthalmic patients are evaluated from multiple points of view and at different levels of magnification. Initially, an animal’s behavior in reponse to a novel visual environment (e.g. the exam room) is observed at a distance. An animal’s attempt to establish eye contact (with the examiner) is also noted. The animal’s face is then viewed from approximately 2 meters to reveal subtle asymmetry, unilateral ptosis, buphthalmos, lagophthalmos, facial paralysis, orbital swelling, and/or anisocoria. During this gross examination the examiner’s and patient’s eye levels should match. The patient is similarly viewed from the top, front, and sides.
Ocular examination then proceeds with the determination of the menace response under lighted conditions. A menace response tests the continuity of a neurologic pathway initiating at the medial retina (optic nerve); continuing through the contralateral geniculate nucleus, motor cortex, and pontine nucleus; to the cerebellum; and terminating at both facial nerves. A menace reponse is not well elicited from the lateral retina, as a menace gesture originating on the medial side of the eye is typically blocked by the patient’s nose.
A menace response is elicited by a threatening hand gesture heading towards the eye. An exclusively visual menace reponse requires minimal airflow during the gesture, and all physical contact with the patient (e.g. whiskers and hand) must also be avoided. The test should be performed while covering the contralateral eye to prevent its contribution to the menace response. A blink is considered a positive menace response. If a reponse is not elicited the intactness of palpebral responses are tested by palpation of the face. It is important to note that the menace response is a learned response that is not consistently present in dogs and cats under 3 to 4 months in age. The feline menace response is also more subtle and incomplete than its canine counterpart. If ambiguous menace results are obtained cortical vision can be further evaluated with mazes, the tracking of objects, and/or visual placing reactions.
An ophthalmic examination then proceeds in the dark to optimize visualization of internal ocular structures. Examination room lighting reflects and diffuses light across the cornea and lens surface producing Purkinje images that preclude visualization of internal ocular anatomy. Darkened rooms also promote pupillary dilation increasing visualization of the lens and posterior eye.
The pupillary light reflex (PLR) is performed in the dark utilizing a strong light source (i.e. the Finnoff transilluminator). A penlight will not produce the focused intense light necessary for these tests and cannot be used. Canine eyes will also typically dilate in sympathetic reponse to agitation within an examination room. Under these conditions, intense illumination is also required for significant pupillary constriction. Both the direct and indirect PLR are evaluated. An indirect PLR is reported as from the stimulated (illuminated) eye to the contralateral responding (constricting) eye. Indirect responses can aid in the evaluation of retinal function as a positive response indicates the continuity of a neurological pathway connecting the eyes (e.g. from the left retinal nerve through to the right pupillary sphincter muscle). An indirect or direct PLR can be consistent with, but does not establish, cortical vision.
The dazzle reflex is an involuntary aversion response (blinking, globe retraction, third eyelid protrusion, and/or head movement) to intense illumination of the eye. This response involves the retina, cranial nerve II, the rostral colliculus, and cranial nerve VII. Dazzle reflexes are stronger in eyes acclimated to the dark. This test is particularly useful when the internal structures of the eye are obscured by opaque media (e.g. blood, cataracts, corneal edema) and when a menace response and/or PLR cannot be determined.
Ophthalmic examinations are challenging but proper diagnoses can be determined through a systemic approach. A thorough ocular examination continues with several important tests that will be the subject of my next article. If you have any questions about the menace response, the PLR, or the dazzle reflex, please consult your veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006