Miosis is a medical condition characterized by contraction of the pupil for reasons other than increased light levels. The biological mechanism that produces miosis can be described as the short circuiting of the nerves that control eye dilation and contraction. The condition has a variety of causes from normal aging to illegal drug use. The condition is also an early symptom of a handful of diseases. Treating miosis always involves treating the underlying disorder.
No matter the underlying cause, miosis occurs due to the same biological mechanism in every patient. In healthy individuals, increased light hitting the eye causes a sympathetic response that causes the pupil to contract. When light is removed, what is known as a parasympathetic response causes the eye to dilate. The parasympathetic response is delayed or lost during miosis. With the pupils of one or both eyes contracted, vision becomes impaired as too little light enters the eye.
Miosis is a common but not universal sign of aging. Even if an older individual presents with the condition, other diseases are possible causes. Horner syndrome is the most common. A disorder of the sympathetic nervous system, an individual presents with both a contracted pupil and a drooping eyelid in only one eye.
Cluster headaches is another disease that causes miosis. The condition is characterized by intensely painful headaches that can last up to three hours. Cluster headaches affect roughly every one out of 1,000 people, more often women than men. A sagging eyelid will generally occur simultaneously if one or both eyes experience abnormal pupil contractions.
A number of prescription and illegal drugs can cause miosis. Individuals addicted to drugs in the opium family often experience contracted pupils during and after drug use. The same effect occurs in cancer patients on chemotherapy. Constricted pupils is also a common side effect of antipsychotic drugs such as thorazine. Though the drugs themselves may have other harmful effects on the body, constricted pupils rarely if ever cause any permanent or long-term damage to the eyes.
Treating unnatural pupil contraction always relies on treating the underlying disorder. If the cause is drug related, the eyes return to normal after finishing medical treatment or stopping illegal drug use. Though some medications and treatments have shown promise in treating cluster headaches, relief is always temporary as the condition is not well understood. A individual diagnosed with Horner syndrome must be patient, as a variety of tests are necessary to determine the syndrome's cause.