Unequal pupil size, also known as anisocoria, naturally occurs in 20 percent of the population. However, it can also be a sign of a serious medical problem, according to About.com. This is why looking closely at the diameter of your pupils is a part of most medical and trauma assessments.
While a certain percentage of people naturally have slightly unequal pupils, it is often a symptom of trauma or disease and should be checked by a trained medical professional. Doctors typically check not only the relative size of the pupils but also the way they respond to reading fine print. The nerve that controls pupil size takes a very long path through the body, and any disruption along the way can manifest as anisocoria. Conditions that are known to cause anisocoria include glaucoma, lung cancer, aneurysm, head trauma, and stroke. Syphilis can cause Argyll-Robertson pupils which constrict for close-up vision but do not constrict when exposed to light. Certain antihistamines cause one pupil to appear larger than the other, and so do various recreational drugs. In order to arrive at a definitive diagnosis, a doctor must take unequal pupils into account along with whatever other symptoms a patient displays. People with naturally large pupils are typically not good candidates for LASIK surgery