The purpose of the assessment of pupillary size and symmetry and the pupillary light reaction is to provide information regarding the efferent pathways that constrict and dilate the pupils, as well as the afferent pathways through which light is transmitted in the optic nerves.

The purpose of examining the pupillary response to near is to assess for the presence of rare disorders that impair the ability of the pupil to constrict to light but spare the ability of the pupil to constrict when focusing on a close object.

The purpose of testing for an afferent pupillary defect is to look for any significant asymmetry of optic nerve function of one side compared to the other.

Observation of resting pupillary size and side-to-side symmetry should be performed on all patients as part of a standard neurologic examination.

Testing for the pupillary light reaction should be performed on all patients as part of a standard neurologic examination.

Testing for pupillary constriction to near needs to be performed only when there is an obviously absent or extremely slow pupillary light reaction, either unilaterally or bilaterally. There is no need to check for pupillary constriction to near if normal constriction to light is already demonstrated, because there is no clinically important condition that affects pupillary constriction to a near stimulus alone.

Testing for an afferent pupillary defect needs to be performed only when there is a clinical complaint (or evidence) of unilateral visual dysfunction or a history suggestive of a previous episode of significant visual dysfunction affecting one eye more than the other. Without a history of significant asymmetric vision loss, there is no need to test for an afferent pupillary defect.

Resting pupillary size is determined by the balance between the parasympathetic efferent pathways that constrict the pupils and the sympathetic efferent pathways that dilate the pupils. Symmetry of pupillary size occurs because of the reflex pathways that mediate the bilateral consensual pupillary constriction to light described in the following section, Pupillary Response to Light.

Pupillary constriction to light is a reflex mediated afferently by visual pathways that begin in each retina and travel in each optic nerve. After crossing in the optic chiasm, reflex fibers from each eye project to the midbrain and bilaterally innervate the Edinger-Westphal nuclei, components of the third nerve nuclei. Efferent fibers from each Edinger-Westphal nucleus travel with the third cranial nerve, synapse in the parasympathetic ciliary ganglion, and cause contraction of the muscles that constrict the pupil. Because of the bilateral reflex innervation to the Edinger-Westphal nuclei, light shined on one eye should cause constriction of that eye (the direct response) and also cause constriction of the opposite eye (the consensual response).

When attempting to focus on a close object, a reflex occurs that results in bilateral pupillary constriction mediated efferently by parasympathetic fibers from the ciliary ganglion. This pupillary constriction to near (which can be seen clinically) accompanies the reflex thickening of the lens, called accommodation (which cannot be assessed clinically), that occurs due to contraction of the ciliary muscles that are also innervated by the ciliary ganglion.

The neuroanatomy of an afferent pupillary defect is described in the section Abnormal Findings.