Show Explanatory Sources

This patient has anisocoria (ie, pupillary asymmetry), which indicates a unilateral defect in input from either the ocular sympathetic (pupil dilation) or parasympathetic (pupil constriction) pathway.  In response to dim light, input from the oculosympathetic pathway initiates pupillary dilation, allowing more light to reach the retina.  Under bright light, parasympathetic input from the ipsilateral oculomotor nerve (CN III) initiates pupillary constriction, limiting the amount of light received by the retina.

Determining which pathway is affected in anisocoria can be accomplished by observing the change in pupillary discrepancy in both bright and dim light:

• Asymmetry that increases in dim light indicates that the smaller pupil is unable to dilate due to loss of sympathetic nerve input.  Under bright light, the asymmetry will decrease because parasympathetic input (pupillary constriction) is unaffected.

• Asymmetry that increases under bright light indicates that the larger pupil is unable to constrict due to loss of parasympathetic nerve input.  In dim light, the asymmetry will decrease because sympathetic input (pupillary dilation) is unaffected.

This patient has increased asymmetry in a dim room, indicating that the smaller (right) pupil is unable to dilate due to loss of sympathetic nerve input secondary to a lesion in the right oculosympathetic pathway.

(Choices A and D)  An oculomotor nerve lesion would disrupt parasympathetic innervation of the ipsilateral pupil, resulting in anisocoria that increases under bright light because the abnormal larger pupil is unable to constrict in response to light.

(Choice B)  Damage to the left oculosympathetic pathway would cause loss of sympathetic input to the left eye, resulting in a fixed, constricted pupil in the left eye (with increased asymmetry in dim light) rather than in the right.

(Choices C and F)  The optic nerves carry the afferent portions of the pupillary reflex from each eye.  A unilateral optic nerve lesion would not cause pupillary asymmetry, but instead monocular vision loss and decreased bilateral pupillary constriction in response to light entering the affected eye (ie, relative afferent pupillary defect).

Educational objective:
Pupillary asymmetry (ie, anisocoria) is caused by a lesion in the ocular parasympathetic (pupillary constriction) or sympathetic (pupillary dilation) pathways.  In this patient, the pupillary asymmetry increases in a dim room, indicating that the smaller right pupil is unable to dilate due to a lesion in the right oculosympathetic pathway.