The pupil is the tiny dark hole in the center of the iris that dilates or constricts in response to the amount of light available and emotional states such as fear. The iris’s two muscles control the pupil’s size. Disorders that impact the iris and pupil include “aniridia," acute glaucoma and, “coloboma of the iris." In addition, reactions to medications can impact pupil size and pupil asymmetry can indicate a serious brain disease.
Miosis
“Miosis” is the technical term for pupil constriction. Gary H. Cassel, M.D., Michael D. Billig, O.D. and Harry G. Randall, M.D. authors of “The Eye Book” compare the muscles of the iris to a camera f-stop since the pupil enlarges in a darkened room to see better and constricts in a bright room. This is all normal. However, abnormal pupil constriction can result from medication toxicity. For example, the glaucoma medication—pilocarpine--prescribed to lower ocular pressure may cause miosis. Small pupils may impact vision because less light will be able to get into the eye. The condition called Horner’s Syndrome also results in decreased abilities of the pupil to dilate. According to The Mayo Clinic, Horner’s syndrome may be a sign of a medical problem that affects facial nerves. One diagnostic test involves placing medication drops in the eyes and evaluating how the pupils respond.
Iris Abnormalities
The condition where the iris is absent or not fully developed is called “aniridia”. According to Virginia E. Bishop contributor to the book “Foundations of Low Vision," aniridia is a rare condition that impacts visual acuity and causes light sensitivity since there is no iris to make the pupil constrict. Another condition called “coloboma of the iris” interferes with complete eye closure creating a pupil with a keyhole appearance and increased light sensitivity. This condition also causes acuity loss.
Mydriasis
The condition of pupil dilation is called “mydriasis." This naturally happens in a darkened room in order to allow more light to enter the eye and see better. According to the Family Practice Notebook website, medications and drugs that can cause bilateral mydriasis include antihistamines, antidepressants, anticholinergic medications, cocaine and amphetamines. Mydriasis to one eye may be seen in patients who are comatose, have cranial nerve injury, acute glaucoma or eye trauma.
Relative Afferent Pupillary Defects
According to New York Vision Associates, pupil asymmetry can be an early indication of a serious brain disorder such as an optic nerve tumor. The eyes are designed to move and focus together and react to light in the same way—unless a person was born with an abnormality. The condition called relative afferent pupillary defect (RAPD) may be a symptom of a life threatening situation such as blood clots released by an artery. The doctors at Richmond Eye Associates also report that conditions that can lead to RAPD include optic neuritis, glaucoma, ischemic optic neuropathies, retinal detachment, cerebral vascular disease and severe macular degeneration.
References
- Relative Afferent Pupillary Defect
- Mayo Clinic: Horner’s Syndrome
- Pupil Disorders Can Reveal Imminent Brain Disease
- Family Practice Notebook: Pupil
- “Foundations of Low Vision”; Anne L. Corn and Alan L. Koenig; 2004
