An Afferent Pupillary Defect (APD) is caused by damage to the neuropathway connecting the eyes and the brain. Your doctor or technician will check for this by shining a handheld light into your eyes and looking for an equal response. They are looking to see the pupils are equal, round, reactive to light, and accommodating.
An APD could be caused by optic nerve pathology, a macular scar, retinal detachment or retinal artery or vein occlusion.
If you are concerned about pupil differences please see your eye doctor and have them evaluated.