And, because of the crossing fibers, there is not only a direct pupillary reflex but also a consensual pupillary light reflex. This extensive pathway is being tested when a light is shined in the eyes. The contraction of the iris sphincter muscles leads to pupillary constriction (miosis) 6. From the Edinger-Westphal nucleus, efferent pupillary parasympathetic preganglionic fibers travel on the oculomotor nerve to synapse in the ciliary ganglion, which sends parasympathetic postganglionic axons in the short ciliary nerve to innervate the iris sphincter smooth muscle via M3 muscarinic receptors 2. Each pretectal area sends bilateral signals to the preganglionic parasympathetic nuclei in the midbrain called Edinger-Westphal nuclei of the oculomotor complex 5. The optic tracts join the brachium of the superior colliculus, and then signals travel to the pretectal area of the midbrain. Thus, the right optic tract will contain temporal retinal fibers from the right eye, as well as nasal retinal fibers from the left eye. At the optic chiasm, nasal retinal fibers will cross to the contralateral side of the optic tract, and the temporal retinal fibers continue on the ipsilateral side. The optic nerve then forms the optic chiasm, which diverges into a left and right optic tract. These are the first steps of the pupillary light reflex afferent pathway. The optic nerve sends impulses to the brain for further processing and image recognition 4. These signals are then relayed to the bipolar cells, which interact with ganglion cells, which in turn coalesce to form the optic disc and optic nerve (CN II). Photoreceptor cells in the outer layers of the retina, which are called rods and cones, convert light stimuli into neuronal impulses. Light travels through the cornea, anterior chamber, pupil, lens, and the posterior chamber, eventually reaching the retina.
Pupillary escape can occur on the side of a diseased optic nerve or retina, most often in patients with a central field defect. “Pupillary escape” is an abnormal pupillary response to a bright light, in which the pupil initially constricts to light and then slowly redilates to its original size 3.
A transient relative afferent pupillary defect can occur secondary to local anesthesia. An relative afferent pupillary defect can occur due to downstream lesions in the pupillary light reflex pathway (such as in the optic tract or pretectal nuclei) 3.
Alternatively, if the reactive pupil constricts more with the consensual response than with the direct response, then the relative afferent pupillary defect is in the reactive pupil. If the reactive pupil constricts more with the direct response than with the consensual response, then the relative afferent pupillary defect is in the unreactive pupil. Direct and consensual responses should be compared in the reactive pupil. Detection of an relative afferent pupillary defect requires two eyes but only one functioning pupil if the second pupil is unable to constrict, such as due to a third nerve palsy, a “reverse relative afferent pupillary defect” test can be performed using the swinging flashlight test 3. When the examiner swings the light to the unaffected eye, both pupils constrict. In patients with an relative afferent pupillary defect, when light is shined in the affected eye, there will be dilation of both pupils due to an abnormal afferent arm 3.
Testing of the pupillary light reflex is useful to identify a relative afferent pupillary defect due to asymmetric afferent output from a lesion anywhere along the afferent pupillary pathway as described above 2. Pupillary constriction occurs via innervation of the iris sphincter muscle, which is controlled by the parasympathetic system 1. The pupillary light reflex is an autonomic reflex that constricts the pupil in response to light, thereby adjusting the amount of light that reaches the retina 1. Accommodation pupillary reflex Pupillary light reflex
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