Mydriasis vs. Miosis

By Jaxson

  • Mydriasis

    Mydriasis () is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of drugs.

    Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A mydriatic pupil will remain excessively large even in a bright environment. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis. More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation.

    An informal term for mydriasis is blown pupil, and is used by medical providers. It is usually used to refer to a fixed, unilateral mydriasis, which could be a symptom of raised intracranial pressure.

    The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological. Anisocoria is the condition of one pupil being more dilated than the other.

  • Miosis

    Miosis is excessive constriction of the pupil. The term is from Ancient Greek μύειν, mūein, “to close the eyes”.

    The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.

  • Mydriasis (noun)

    The condition of having abnormally large and dilated pupils due to disease or drugs, particularly stimulants such as (meth)amphetamines, cocaine, etc.

  • Miosis (noun)

    Contraction of the pupil of the eye.

    “w|Horner’s syndrome consists of typically unilateral miosis and eyelid ptosis, and absent forehead sweating.”


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