Synopsis
Interpret anisocoria and pupillary reactivity anatomically, identify complete and partial oculomotor palsies and escalate aneurysm, herniation, acute glaucoma and other pupil emergencies without false reassurance.
- Describe each pupil's size in bright and dim light, shape and direct and consensual reaction before attaching a syndrome label. Record previous eye surgery, trauma and topical or transdermal drug exposure.
- If anisocoria increases in bright light, the larger pupil is failing to constrict; if it increases in darkness, the smaller pupil is failing to dilate. Equal asymmetry across conditions may be physiological.
- The afferent pathway runs from retina through optic nerve and midbrain; the parasympathetic efferent pathway travels superficially in the third nerve to constrict the pupil. Relative afferent defects and anisocoria answer different questions.
Key red flags
Sudden painful ptosis and ophthalmoplegia with a dilated or sluggish pupil strongly suggests compression by a posterior communicating artery aneurysm and requires emergency CTA or equivalent neurovascular imaging.
Investigation priorities
Identify the abnormal pupil, light response, afferent asymmetry and muscles affected by a third-nerve or orbital lesion.
Management branches
New ptosis and impaired adduction, elevation or depression with headache, ocular pain, pupillary abnormality or uncertain completeness.
- Perform ABCDE, document exact onset, acuity, fields, pupils and all eye movements and examine the remaining cranial nerves and limbs for subarachnoid or midbrain features.
- Contact emergency neurovascular, stroke or neurosurgical services and obtain urgent CTA or the local definitive arterial imaging, not merely non-contrast CT or outpatient review.
Pupil asymmetry is noticed without reduced consciousness, acute severe pain, ophthalmoplegia or other focal neurology.