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Miosis of a human eye
Constricted pupil in a domestic cat

Miosis ( ancient Greek μείωσις meiōsis , German 'reduction' ) or stenocoria (in ancient Greek στενός, stenos 'narrow' and κόρη, korē 'pupil') is the medical name for the constriction of the pupil on one or both sides . Based on the average diameter of the eye hole, it can vary in strength. The opposite of miosis, a widening of the pupil, is called mydriasis .


Miosis is caused by the contraction of the sphincter pupillae muscle in the iris of the eye or by failure or limitation of its counterpart ( antagonist ), the dilatator pupillae muscle ; both belong to the inner muscles of the eye and consist of smooth muscles .

Physiological miosis

The physiological miosis is mediated via parasympathetic nerve fibers and triggered as adaptation when light falls , and during near fixation together with the accommodation and the convergence movement as a so - called near adjustment trias as a result of a neurophysiological control circuit that controls these together.

Pathological miosis

Classification according to ICD-10
H57.0 Pupillary dysfunction
ICD-10 online (WHO version 2019)

Causes of pathological miosis can be damage to the sympathetic supply in Horner's syndrome ( miosis paralytica , usually unilateral), as well as bilateral reflex pupillary rigidity in Argyll-Robertson syndrome , usually caused by neurolues . An irritation of the parasympathetic nervous system in the oculomotor system can present itself as miosis spastica , which usually turns into a mydriasis paralytica and can be a preliminary stage of paralysis of the oculomotor nerve .


Miosis is specifically brought about with pharmacological agents, called miotics (e.g. pilocarpine ), for therapeutic purposes, for example in the case of glaucoma , or for differential diagnostic clarification in the pharmacodynamic examination of pupillotonia . In addition, the effects of opiates or opioids can cause miosis.

In contrast, mydriatics such as atropine or hyoscyamine can temporarily prevent miosis by paralyzing the sphincter pupillae muscle. When parasympatholytic drugs are administered, this is accompanied by a loss of accommodation , caused by their paralyzing effect on the parasympathetically innervated part of the ciliary muscle .

Miosis and visual acuity

A pronounced miosis (around 2.0 mm) can lead to a certain improvement in visual acuity , even in lensless ( aphakic ) people . Because the narrowing of the eye hole increases the depth of field , similar to the aperture effect when taking photos, the mode of action of a pinhole camera (camera obscura) or a Stenopean gap .


  • Theodor Axenfeld (founder), Hans Pau (ed.): Textbook and atlas of ophthalmology. 12th, completely revised edition. With the collaboration of Rudolf Sachsenweger and others Gustav Fischer, Stuttgart and others 1980, ISBN 3-437-00255-4 .
  • Herbert Kaufmann (Ed.): Strabismus. With the collaboration of Wilfried de Decker et al. Enke, Stuttgart 1986, ISBN 3-432-95391-7 .

Individual evidence

  1. Rudolf Sachsenweger (Ed.): Neuroophthalmology. 3rd, revised edition Thieme, Stuttgart et al. 1982, ISBN 3-13-531003-5 , p. 300.