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Other namesBlown pupil[1]
Dilated pupils 2006 (cropped).jpg
Pupil dilated for examination by ophthalmologist
SpecialtyOphthalmology, neurology

Mydriasis is the dilation of the pupil, usually having a non-physiological cause,[2] or sometimes a physiological pupillary response.[3] Non-physiological causes of mydriasis include disease, trauma, or the use of drugs.


Fixed, unilateral mydriasis 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.


There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is innervated by (signaled by nerves of) the parasympathetic nervous system, and the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil.


Pupil diameter also increases in reaction to cognitive tasks requiring memory and attention, and this phenomenon is used as an indicator of mental activation (‘arousal’) in psychophysiological experiments.[4]


A mydriatic is an agent that induces dilation of the pupil. Drugs such as tropicamide are used in medicine to permit examination of the retina and other deep structures of the eye, and also to reduce painful ciliary muscle spasm (see cycloplegia). One effect of administration of a mydriatic is intolerance to bright light (photophobia). Purposefully-induced mydriasis via mydriatics is also used as a diagnostic test for Horner's syndrome.

Mydriasis can be induced via modulation of adrenergic or cholinergic signalling.

Drugs that can cause mydriasis include:

Natural release of the hormone oxytocin can cause mild to moderate mydriasis.[citation needed]

Long term effects of drugs can also cause mydriasis, for example opioid withdrawal.

Autonomic neuropathy[edit]

Parasympathetic fibers travel with cranial nerve III, the oculomotor nerve, to innervate the circular layer of muscle of the eye (sphincter pupillae). Damage to this nerve typically manifests itself as mydriasis, because the sympathetic supply to the pupil, which causes mydriasis, remains unaffected, and therefore unopposed.





    • The American Heritage Medical Dictionary (2007)
    • Mosby's Dental Dictionary, 2nd edition.
  4. ^ Koss MC. Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation. J Pharmacol Methods. 1986;15:1–19. [PubMed]...Peavler WS. Pupil size, information overload, and performance differences. Psychophysiology. 1974;11:559–566. [PubMed]...Aston-Jones G, Rajkowski J, Kubiak P, Alexinsky T. Locus coeruleus neurons in monkey are selectively activated by attended cues in a vigilance task. J Neurosci. 1994;14:4467–4480. [PubMed]...Laeng B, Sirois S, Gredebäck G. Pupillometry: a window to the preconscious? Perspect Psychol Sci. 2012;7:18–27. [PubMed]...Kloosterman NA, Meindertsma T, van Loon AM, Lamme VA, Bonneh YS, Donner TH. Pupil size tracks perceptual content and surprise. Eur J Neurosci. 2015;41:1068–1078. [PubMed]
  5. ^ Saenz-de-Viteri, Manuel; Gonzalez-Salinas, Roberto; Guarnieri, Adriano; Guiaro-Navarro, María Concepción (2016). "Patient considerations in cataract surgery – the role of combined therapy using phenylephrine and ketorolac". Patient Preference and Adherence. 10: 1795–1801. doi:10.2147/PPA.S90468. ISSN 1177-889X. PMC 5029911.
  6. ^ "Cyclomydril - FDA prescribing information, side effects and uses".

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