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.
- A complete third-nerve palsy causes ptosis and impaired adduction, elevation and depression, leaving the eye typically down and out; partial fascicular or divisional palsies can be much less tidy.
- Pupillary involvement, severe headache, pain, progressive deficit or incomplete pattern must be assumed compressive until urgent arterial and structural imaging is reviewed by the appropriate specialist.
- Pupil sparing can occur in microvascular third-nerve palsy, but it does not guarantee a benign cause—particularly early, incomplete or in younger patients and when pain or other neurology is present.
- Third-nerve palsy with contralateral weakness, tremor, ataxia or altered consciousness localises to the midbrain or adjacent structures rather than an isolated peripheral diabetic neuropathy.
- A relative afferent pupillary defect reflects asymmetric retinal or optic-nerve input and usually does not cause anisocoria because both pupils can constrict when the healthier eye is stimulated.
- A tonic Adie pupil is typically large with poor light reaction, better near constriction and slow redilation, often with sectoral iris movement; confirmation is specialist-led after dangerous causes are excluded.
- Pharmacological mydriasis may produce a very large non-reactive pupil without ptosis or ophthalmoplegia; enquire about nebulised anticholinergics, scopolamine patches, plant exposure and accidental drop contact.
- A small irregular pupil, posterior synechiae, red painful eye or previous intraocular procedure points toward local ocular disease and needs slit-lamp assessment rather than purely neurological localisation.
- Do not perform or interpret dilute pilocarpine, cocaine or apraclonidine tests casually in an acute emergency; drug concentration, denervation timing and ocular contraindications can mislead.
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.
Acute diplopia and ptosis with complete external third-nerve weakness but normal pupillary responses in an older person with vascular risk can be microvascular, only after urgent dangerous alternatives and atypical features are assessed.
Third-nerve dysfunction with contralateral pyramidal weakness, tremor, sensory loss or ipsilateral ataxia suggests a fascicular midbrain lesion from stroke, demyelination, tumour or inflammation.
A newly enlarging poorly reactive pupil with falling consciousness, progressive hemiparesis or abnormal posturing indicates critical mass effect until proven otherwise and needs immediate airway and neurosurgical management.
Severe ocular pain, red eye, blurred vision or haloes, headache, nausea and a mid-dilated poorly reactive pupil suggests acute angle closure, an ophthalmic pressure emergency rather than an isolated cranial neuropathy.
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.
- Monitor consciousness, pupil size and focal findings while imaging is interpreted and follow specialist aneurysm, cavernous-sinus or alternative structural management.
Pupil asymmetry is noticed without reduced consciousness, acute severe pain, ophthalmoplegia or other focal neurology.
- Measure in bright and dim light, test direct, consensual and near responses and examine lids, acuity, fields, eye movements and the ocular surface; seek old photographs and exposure history.
- Route red or painful eye to ophthalmology, a small dark-abnormal pupil to Horner assessment and a large light-abnormal pupil to parasympathetic, tonic, traumatic or pharmacological evaluation.