Miosis is a condition where the pupil contracts due to a short circuit of the nerves that control the eye’s dilation and contraction. It can be caused by aging, diseases, or drug use, and treatment involves addressing the underlying disorder.
Miosis is a medical condition characterized by the contraction of the pupil for reasons other than increased light levels. The biological mechanism that produces miosis can be described as a short circuit of the nerves that control the eye’s 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. Treatment of miosis always involves treating the underlying disorder.
Regardless of the underlying cause, myosis 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, the so-called parasympathetic response causes the eye to dilate. The parasympathetic response is delayed or lost during miosis. With the pupils of one or both eyes constricted, vision is impaired because 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’s syndrome is the most common. A disorder of the sympathetic nervous system, an individual presents with a contracted pupil and a droopy eyelid in only one eye.
Cluster headache is another disease that causes miosis. The condition is characterized by intensely painful headaches that can last up to three hours. Cluster headaches affect about one in 1,000 people, more often women than men. A droopy eyelid usually occurs at the same time if one or both eyes experience abnormal contractions of the pupil.
A number of prescription and illegal drugs can cause miosis. Addict individuals in the opium family often experience pupil contraction during and after drug use. The same effect occurs in cancer patients undergoing chemotherapy. Constricted pupils are also a common side effect of antipsychotic drugs such as Thorazine. Although the drugs themselves can have other harmful effects on the body, constricted pupils rarely, if ever, cause permanent or long-term damage to the eye.
Treatment of unnatural pupil contraction always relies on treating the underlying disorder. If the cause is drug-related, the eyes return to normal after you finish medical treatment or stop using illegal drugs. Although some medications and treatments have shown promise in treating cluster headaches, the relief is always temporary as the condition is not well understood. An individual diagnosed with Horner’s syndrome must be patient, as a number of tests are needed to determine the cause of the syndrome.
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