Miosis: Definition, Causes, & Examples

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Miosis Definition

Miosis implies constriction of the pupil and is also called myosis. This medical term pertains to the action of small eye muscles that helps in closing or constriction of pupils. This can be a response to the altering light intensity of the environment that requires reducing the amount of light entering the eyes.

Various recreational and medicinal drugs can also cause miosis. It can serve as a diagnostic symptom for impaired driving and can be a reason for the arrest. The term can be confused with terms for cell division like meiosis and mitosis.

Miosis Causes

Miosis results due to the local actions of contraction and relaxation of 2 muscles present in the iris of the eyes. Iris seen under the protective cornea represents the colored part of the eyes. The size of the pupil is regulated by the iris sphincter and iris dilator muscles that cause miosis are the relaxation and contraction of two muscles in the iris of the eye.

Pupils regulate the amount of light entering the eyes that will pass through the lens and will be focused on the retina. The retina contains specialized cells that help to detect light signals that are then sent as signals to optic nerve and then finally to the brain.

The photosensitive ganglions are the specialized cells in the eyes that sense the intensity of light received by the eyes. They signal the brain and the ANS through the optic nerve.

The ANS then based on the input received will continually automatically adjust the pupil size. If the light intensity entering the eyes and hitting the retina is high then miosis will happen, resulting in constriction of pupils.

During this process, the iris sphincter muscle will contract, and simultaneously the iris dilator muscles relax. This will cause constriction of pupils and to reduce the amount of light entering the eyes.

If pupils are constricting too much then again the ganglion will deduce then signal the brain leading to dilation of muscles and is called mydriasis.

Miosis Features

There can be various factors affecting and causing miosis. There is a complex pathway involved in this pathway that engages different regions of the brain and many neurons and chemicals. This pathway can be influenced or impaired by drugs like antipsychotic medications, nicotine, eye drops, and opioids.

Some of these chemicals may result in anisocoria, a condition where mydriasis occurs in one eye and miosis in the other. The alterations in light intensity caused by various recreational drugs can contribute to altered perception.

Diseases or nerve tissue degeneration may also cause miosis. Miosis can be observed in other animals besides humans, though the mechanism to regulate the quantity of light entering the eyes may differ.

Only 2 groups of animals have developed complex eyes, the cephalopods, and the vertebrates. The structure of eyes may differ as they rose from convergent evolution due to similar selection pressures.

Miosis Citations
  • Posttraumatic headache with ptosis, miosis and chronic forehead hyperhidrosis. Headache . 1990 Jan;30(2):64-8.
  • Ptosis and Miosis Associated with Fibrosing Mediastinitis. Am J Case Rep . 2021 Jan 12;22:e927556.
  • Opioid-induced Miosis Is Unaltered by Obstructive Sleep Apnea: Comment. Anesthesiology . 2020 Feb;132(2):399-400.


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