Miosis
Summary
Miosis is constriction of the pupil (<2 mm). It results from parasympathetic activation (CN III, sphincter pupillae) or sympathetic loss, and is a component of Horner syndrome and opioid toxicity.
Detail
Pupil size is balanced by parasympathetic constrictor (Edinger-Westphal -> CN III -> sphincter pupillae via M3 receptors) and sympathetic dilator (T1-T2 -> superior cervical ganglion -> long ciliary nerves -> dilator pupillae via alpha-1) tone. Causes of miosis include: Horner syndrome (sympathetic lesion: miosis + ptosis + anhidrosis), opioid intoxication ('pinpoint pupils'), organophosphate poisoning (cholinergic excess), pontine hemorrhage ('pinpoint pupils' from bilateral sympathetic disruption), pilocarpine and other cholinergic eye drops, and Argyll Robertson pupil (neurosyphilis: 'accommodates but doesn't react' to light). Senile miosis is normal aging. Boards classic: comatose patient with pinpoint pupils responsive to naloxone = opioid overdose.
Sources
- First Aid for USMLE Step 1 2024
- Robbins Basic Pathology 10th ed
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