What is a Horner’s pupil?

A Horner’s pupil occurs when the sympathetic innervation to the eye is cut off. This results in a loss of sympathetic dilation. The pupil will look constricted compared to the other eye – this difference will be greater in dim lighting. In dim lighting, the eyes normally receive lots of sympathetic stimulus to dilate.

The sympathetic pathway is a long one, and a lesion along its course can occur from a few potentially deadly causes … such as apical lung tumors, and carotid dissection. A new onset Horner’s pupil warrants pharmacologic testing with topical cocaine and paradrine to determine the location of the lesion.