The iris is the colored muscle, called the sphincter muscle, of the eye that expands and contracts the pupil to control the amount of light entering the eye. In an eye that functions properly, the pupil will appear larger in low light and look smaller in a well lit area. Though light constantly affects dilation of the pupil, other conditions might also cause the iris to change size.
Eye Drops
Eye doctors will usually use an eye drop that dilates the pupil in order to perform a complete examination of the back of the eye. The duration of the effect of the drops varies, depending on the type of medication used, says The Pediatric Glaucoma & Cataract Family Association. For many people, the pupils remain dilated for a few hours.
Medical Conditions
Some medical conditions may cause changes in the size of the pupils, and some people may have unequal pupil sizes. Doctors call this anisocoria. The unequal pupils may result from the presence of an aneurysm, head injury or a brain tumor, notes MedlinePlus, a health information service provided by the U.S. National Library of Medicine and the National Institutes of Health. Other conditions affecting the pupils may include glaucoma or migraine headaches. Some people may not even be aware they have different pupil sizes until a family member or friend points out the condition. A person experiencing any changes in pupil size or vision should contact a doctor immediately.
Organic Matter
Certain plants can affect the eyes. Any part of the honeysuckle plant, for example, might cause pupil dilation, warns the University of Georgia. This may occur with ingestion of the organic matter or with physical contact. In many cases, a piece of matter from the plant will land on the eye, and before the person can remove the foreign matter, the pupil dilates in reaction to the chemical properties of the plant matter. A person with such a reaction should contact an eye care professional immediately. The doctor may recommend flushing the eye with water to lessen or shorten the duration of the effects. For most people, once the active agents from the plant work their way out of the system, the pupil will return to normal function.
