The eye pupil is the hole in the middle of the iris, the colored part of the eye. Pupils dilate and shrink in response to light; two muscles control pupil size, according to "Fundamentals for Ophthalmic Personnel." Pupillary reactions are controlled by the autonomic nervous system, which control involuntary smooth muscles reactions.
Normal Pupil
Normal pupils are round, equal to one another in size, and react to light by becoming smaller, to limit the amount of light entering the eye. In the dark, a normal pupil dilates, to allow more light to enter the eye. People with light colored eyes and those with myopia (nearsightedness) have larger pupils than people with dark eyes who are farsighted. An adult pupil in normal light is around 3 millimeters; in elderly people, the pupil is smaller, about 2 millimeters on average, according to "Fundamentals for Ophthalmic Personnel."
Aniscoria
Aniscoria means that the two pupils are different sizes in the same light conditions; this occurs in approximately 10 percent of people, according to "Fundamentals for Ophthalmic Personnel." A difference of 1 millimeter or less is considered to be physiological, or within normal limits. A difference of greater than 1 millimeter is called essential aniscoria .
When aniscoria is present, it's necessary to determine which of the pupils is abnormal, the larger or the smaller. According to S. Kedar, M.D., the smaller pupil is the abnormal one if the difference is more pronounced in the dark than in the light. The larger pupil is abnormal if the difference is greater in the light.
Some types of aniscoria include Horner's Syndrome, an abnormally small pupil; Adie's pupil, also called tonic pupil, a pupil that initially constricts poorly in light but over time becomes permanently constricted and Reiger syndrome, a congenital syndrome which can cause pupil displacement and lack of centering, according to an article written by Jennifer Wilson, MD, in the Gale Encyclopedia of Genetic Disorders.
Marcus-Gunn Pupil
A Marcus Gunn pupil is also known as an afferent papillary defect; both pupils are of the same size until light is shone into them. During a "swinging flashlight" test, the room is darkened and the flashlight beam is shone into one pupil and then the other fairly rapidly. The Marcus Gunn pupil will constrict with the good eye when light shines into the good eye, but will dilate (and so will the good eye) when light shines into it, according to Thomas Walsh, M.D.
Argyll Robertson Pupil
An Argyll Robertson pupillary defect is associated with tertiary syphilis, late stage syphilis which affects internal organs. Argyll Robertson pupils are small, 2 millimeters or less, may be irregular in shape and react poorly to dilating eye drops, according to C. Dente and A. Gurwood of the Eye Institute of the Pennsylvania College of Optometry. The problem usually starts in one pupils and eventually affects both eyes.
