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What is Miosis?

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  • Written By: T. Broderick
  • Edited By: Heather Bailey
  • Last Modified Date: 27 October 2016
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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.

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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.

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anon989313
Post 6

Wouldn't miosis be related to unimpeded parasympathetic activity?

It is a sympathetic response to dilate the eye in case of a fight or flight response; while parasympathetics allow constriction of the pupil under relaxed conditions.

My reference for this is Horner's syndrome. When the sympathetic trunk is damaged (say by carotid endarterectomy) there is partial ptosis, miosis and anhidrosis.

anon269822
Post 3

I have recently developed unexplained anisocoria. The MRI brain scan showed no abnormalities. I noticed the difference in pupil size for the first time about eight months after my last chemotherapy treatment.

I have not had any treatments since that time, but the anisocoria has continued. My neurologist doesn't think it could have anything to do with the chemo, but also can't tell me what is causing it. I still think that it may have been the chemotherapy.

Kat919
Post 2

@dfoster85 - There are actually a lot of causes of unequal pupils besides brain tumors. I get migraines and sometimes notice that one pupil gets small. But brain tumors and aneurysms can also cause it, so it's one of those things that may be worth checking out!

dfoster85
Post 1

I had never heard of a miotic pupil until my husband developed cluster headaches.

I had no idea what they were; neither of us did. All we knew was that all of a sudden, he was in so much pain that he was banging his head against the wall. And his pupils were different sizes!

I thought that different sized pupils could be a sign of a brain tumor, so you can imagine I was awfully worried.

But the treatment actually turned out to be really simple. The nasal sprays that treat migraines and cluster headaches once they've happened are terribly expensive, but he went on a calcium channel blocker and hasn't had another one since.

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