What does myosis mean?
Myosis is the medical term for contraction or narrowing of the pupil, the opening in the centre of the iris. The pupil becomes smaller when light increases or when the eye focuses on nearby objects, so myosis can describe a normal response as well as an abnormal state. Excessive or pathological myosis is especially associated with opioid exposure, certain drugs, and disorders affecting the nervous system or eyes. In clinical and toxicological writing, the word is usually treated as a technical term and is often contrasted with mydriasis, the dilation of the pupil. Because small pupils can be a significant diagnostic clue, accurate use of myosis matters in medicine, particularly when assessing an impaired or unconscious patient. Outside specialist contexts, the term is uncommon, but it is a precise and useful name for both a basic reflex of the eye and abnormal narrowing associated with several conditions.
nounConstriction or narrowing of the pupil of the eye. In clinical descriptions, the term may also refer to excessive or pathological narrowing of the pupil.
- The contraction or narrowing of the pupil of the eye.
- Excessive or pathological pupillary constriction, as may occur with opioid exposure, certain medications, or neurological and ocular disorders.
"The examination revealed marked myosis in both pupils."
"The examination revealed marked myosis in both pupils."
"Light entering the eye triggers myosis, reducing the amount of light that reaches the retina."
The plural is rare; medical writing generally uses cases of myosis instead.
"The report described two myoses and recommended specialist review."
When the pupil shrinks, it is myosis; when it expands, the opposite condition is mydriasis.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does myosis come from?
The English word comes from New Latin myōsis, based on Greek myōsis, a form associated with myein, “to close,” and the suffix -osis, “condition.” The Greek element refers to closing or making narrow. Words such as myopia and myopic belong to the same mye- family, although myopia developed the specialized meaning “short-sightedness.”
Related word forms
How myosis is actually used
This is primarily a formal medical term. It can describe normal pupillary constriction, but dictionaries and clinical discussions also use it specifically for excessive or abnormal narrowing.
Easily confused with myosis
Myosis is contraction or narrowing of the pupil, whereas mydriasis is dilation or widening of the pupil.