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Physiology and abnormalities of the pupil

Last updated: February 24, 2021

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The pupil is an opening in the center of the iris through which light enters the eye. Pupillary size can vary in response to light intensity and neurologic stimuli. Increasing brightness causes pupillary constriction (miosis) while increasing darkness causes pupillary dilation (mydriasis). Pupillary abnormalities can be caused by a variety of conditions.

Pupillary control [2]

Pupillary control is mediated by both parasympathetic and sympathetic innervation.



Short ciliary nerves make the pupillary muscle fibers short (leading to miosis), while long ciliary nerves make the pupillary muscle fibers long.

Pupillary light reflex

Extraocular causes of nonphysiological anisocoria
Pupillary constriction Pupillary dilation

Anisocoria is not associated with afferent pupillary defects.

  1. Spector RH, Walker HK, Hall WD, Hurst JW. The Pupils. Clinical Methods: The History, Physical, and Laboratory. 1990 .
  2. Furlan JC, Sundaram AN. Sudden-onset anisocoria in a patient with upper respiratory tract infection.. CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne. 2014; 186 (1): p.57-61. doi: 10.1503/cmaj.130581 . | Open in Read by QxMD
  3. Pupillary Responses. https://stanfordmedicine25.stanford.edu/the25/pupillary.html. Updated: January 1, 2018. Accessed: June 28, 2018.
  4. Adie Tonic Pupil. https://www.aao.org/bcscsnippetdetail.aspx?id=1af235eb-71a5-497f-8fac-308c9ea3a0eb. Updated: January 1, 2018. Accessed: June 28, 2018.