Pupil size is controlled in the midbrain, from which efferent nerves travel to both pupils, resulting in symmetric pupils, even with a unilateral light stimulus. The perception of the light stimulus, however, may be decreased by disease within anterior visual structures such as the retina, optic nerve, chiasm, optic tract, and midbrain pathways. With diminished afferent stimulation, less light is “perceived,” and pupil contraction diminishes (ie, dilates) as a result. In this situation, the affected side is said to demonstrate an afferent pupillary defect (APD).
An APD is best appreciated by the swinging flashlight test, which discloses differences in afferent stimuli between the two eyes. A flashlight is directed onto one pupil and then the other. The normal response is a prompt constriction. (Generally, this is followed by a slight “release” dilation.) During the brief interval required to move the light from one to the other eye, both pupils begin to dilate. Thus, under normal circumstances, when the light reaches the second eye, reflex constriction again occurs. With a diseased eye or pathway, the diseased side perceives less light and the midbrain therefore sets the pupil size to be larger, and as a result, the pupils dilate (or continue to dilate following the “release” dilation).
Although sensitive, the swinging flashlight test is not specific, since the pathology may be anywhere in the visual pathway from the retina to the midbrain. The test cannot distinguish between unilateral afferent disease and asymmetric bilateral disease. Typically, a positive finding indicates optic nerve disease such as ischemic optic neuropathy, optic neuritis (including that seen with multiple sclerosis), retrobulbar optic neuritis, and glaucoma.
Afferent Pupillary Defect. Schematic representation of an afferent pupillary defect (APD) due to neurologic lesion in the anterior visual pathway.
Management and Disposition
The finding of an afferent pupillary defect is nonspecific. A fundoscopic exam may show occlusion of the central retinal vein or central retinal artery or a dense hemorrhage in the vitreous. Otherwise, a neurologic evaluation (history, physical examination, and computed tomography [CT] scan) is important to assess for treatable conditions. In the absence of gross ocular or neurologic disease, a clinically stable patient may be discharged from the ED with ophthalmology follow-up.
An APD does not cause anisocoria because any change in light input results in a bilaterally symmetric output from the midbrain.
In the swinging flashlight test, both pupils will dilate when the flashlight moves from the normal pupil to the affected pupil, since pupil size is centrally mediated in the midbrain.
Bilaterally symmetric disease produces no APD, since the basis for the swinging flashlight test is asymmetry of disease.
A dense vitreous hemorrhage is capable of producing a mild defect. Lesser ocular pathology such as branch retinal vein or artery ...