Miosis

Miosis is the constriction of the pupil, resulting in a smaller pupil size. It is part of the pupillary light reflex and the near response, and it contrasts with mydriasis, which is pupil dilation. The constriction is produced by the sphincter pupillae muscle of the iris and is mediated by the parasympathetic fibers of the oculomotor nerve (cranial nerve III). The retinal light signal travels to the pretectal area, then to the Edinger–Westphal nucleus, and via the oculomotor nerve to the ciliary ganglion. Postganglionic fibers release acetylcholine, which activates muscarinic receptors on the sphincter pupillae to produce constriction. In accommodation, focusing on a near object also triggers miosis.

Physiologic miosis occurs in bright light and during near work. Pharmacologic miosis can be produced by muscarinic agonists (such as pilocarpine) or by acetylcholinesterase inhibitors; opioids commonly cause miosis via central mechanisms. Conversely, sympathetic activity causes pupil dilation via the dilator pupillae, so loss of sympathetic input (as in Horner syndrome) produces miosis on the affected side along with ptosis and anhidrosis.

Assessment of miosis is often via the pupillary light reflex: shining light into each eye to observe direct and consensual responses. Unequal pupils (anisocoria) may reflect localized pathology affecting the afferent or efferent pathways or the iris musculature.

Etymology: the term derives from Greek roots meaning “to lessen” or “to close.”