When light enters one eye, it is detected by retinal photoreceptors and transmitted via the optic nerve to the pretectal nucleus in the midbrain. From there, signals are sent bilaterally to the Edinger-Westphal nuclei, which send parasympathetic fibers via the oculomotor nerves to the ciliary ganglia. These fibers innervate the sphincter pupillae muscles in both eyes, causing both pupils to constrict—the direct response in the stimulated eye and the consensual response in the other eye. This bilateral response ensures equal pupillary adjustment to light.
For an adequate test, vision must not be entirely lost. In dim room light, the examiner notes the size of the pupils. The patient is asked to gaze into the distance, and the examiner swings the beam of a penlight back and forth from one pupil to the other, and observes the size of pupils and reaction in the eye that is lit.
Interpretation
Normally, each illuminated pupil promptly becomes constricted. The opposite pupil also constricts consensually.
When ocular disease, such as cataract, impairs vision, the pupils respond normally.
When the optic nerve is damaged, the sensory (afferent) stimulus sent to the midbrain is reduced. The pupil, responding less vigorously, dilates from its prior constricted state when the light is moved away from the unaffected eye and towards the affected eye. This response is a relative afferent pupillary defect (or Marcus Gunn pupil).[1]
^ abNeuroLogic Examination Videos and Descriptions: Cranial Nerve > Normal - Cranial Nerves 2 & 3 - Pupillary Light Reflex (46" video). The University of Utah. May 2020. The swinging flashlight test is used to test for a relative afferent pupillary defect or a Marcus Gunn pupil. Swinging the flashlight back and forth between the two eyes identifies if one pupil has less light perception than the other. Shine the flashlight at one eye noting the size of both pupils. Then swing the flashlight to the other eye. If both pupils now dilate then that eye has perceived less light stimulus (a defect in the sensory or afferent pathway) than the opposite eye.
