This is a pupil abnormality where the eye loses its sympathetic nervous system innervation. The sympathetic system helps the body function during stressful situations … it is our “fight or flight” system. Sympathetic stimulation helps the heart beat faster and routes blood to the musculoskeletal system so we can run faster. In the eye, the sympathetic system makes the pupils dilate and the eyelids widen … presumably so that we can see better in the dark as we run away from attacking grizzly bears. If the sympathetic connections to the eye are blocked, the pupil doesn’t dilate well and the pupil looks small and constricted. Also, the eyelid droops a little (ptosis). The sympathetic nerves involved are very long, with a circuitous route from the brain, running down the neck, over the lung, and back up to the eyeball. Any obstruction in this pathway, such as a lung tumor, carotid artery dissection, or even a congenital malformation, can cause a Horner’s pupil. This diagnosis is made in the office by examining the pupils in light and dark conditions. The location of the sympathetic nerve ‘blockage’ can sometimes be localized using stimulant eye drops. More imaging, like a CAT scan or MRI may be required if the cause for the Horner’s is not immediately obvious.