Carefully evaluating the pupillary light reflex (PLR), both direct and indirect (consensual), is critical when assessing the prognosis for vision, particularly in patients with glaucoma. So properly performing this quick and easy evaluation is essential.
The PLR is a subcortical reflex requiring relatively few intact photoreceptors, thus a patient can have intact PLRs without functional vision. Likewise, a patient can have normal vision and an efferent PLR deficit due to a pharmacologic parasympathetic blockade (e.g. atropine, tropicamide), atrophy of the pupillary sphincter muscle, or an abnormality of the ciliary ganglion or long posterior ciliary nerves.
Evaluating PLRs requires the control of ambient light and a bright, focal light source. A pocket penlight is inadequate; a Finoff transilluminator with halogen light is ideal. The room lights should be dim, barely allowing you to see the iris and pupil. Direct the focal light into one eye; that eye's reflex is the direct PLR. Withdraw the light for several seconds and then stimulate the same eye again but this time observe the indirect, or consensual, PLR in the opposite eye. It may be helpful to have someone else control the light stimulus while you observe the unstimulated eye.
The PLR is a relative response, with interpretation requiring assessment of the speed of the response and the endpoint (pupil size). Observation of asymmetry of either the direct or indirect response between the two eyes may reveal valuable information. Both the direct and consensual PLR should be brisk and result in an equal degree of miosis. Normally, the consensual PLR is slightly slower than the direct.
In patients with glaucoma, the pupils are often midsize to dilated and poorly responsive or unresponsive relative to a normotensive eye. Elevated intraocular pressure may limit pupil constriction in the affected eye (direct PLR) presumably because of ischemia of the iris sphincter muscle. However, the reflex of the opposite eye (consensual PLR) should still be intact if the retina has not been irreversibly damaged. Likewise, in cases of anterior lens luxation in which the pupil is mechanically obstructed or corneal or anterior chamber opacity prevents observation of the direct PLR, evaluating the consensual PLR will help establish whether intact retinal function exists and thus assist with a prognosis for vision.
Cynthia S. Cook, DVM, PhD, DACVO
Veterinary Vision Animal Eye Specialists
219 N. Amphlett Blvd.
San Mateo, CA 94401