What causes one pupil not to react to light properly?

When you indicate that one pupil is not reacting to light properly, I interpret that to mean that the pupil does not constrict (get smaller) when a light is directed into the eye. There can be a number of problems that can cause a pupil not to constrict when exposed to light and many of these problems are potentially serious. Problems that can cause a pupil not to constrict to light exposure include traumatic injury to the muscles of the iris that control the pupil, inflammation inside the eye that causes the iris to become sticky and to adhere to the lens, and problems that result in severe vision loss in an eye such as a retinal detachment or a problem with the optic nerve. A trip to your ophthalmologist is definitely in order to have this problem checked out.

This question was originally answered on Jan. 12, 2013.