Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.
2 min synopsisUK scopeSources checked 7 Sept 2026Clinical review pending
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An acute pupil abnormality can signal neurological disease
A newly enlarged poorly reactive pupil with ptosis, diplopia or severe headache raises concern for a compressive third-nerve lesion; a new small pupil with ptosis and head or neck pain can indicate carotid dissection.
Action: Arrange immediate emergency assessment and appropriate neurovascular investigation, recording pupils and eye movements before any dilating drops; do not wait for pharmacological confirmation or assume that vascular risk factors establish a benign cause.
Synopsis
Examine pupil size and reactions systematically, distinguish afferent asymmetry from unequal pupil size, and recognise pupil findings requiring emergency eye or neurological assessment.
Observe both pupils before drops, recording size, shape and asymmetry in bright and dim illumination with the patient fixing a distant target.
Anisocoria concerns unequal pupil size; a relative afferent pupillary defect concerns unequal sensory input and may occur with equal-sized pupils.
Compare direct and consensual constriction, then perform a swinging-light test using equally bright illumination at equal distances.
Key red flags
New anisocoria with severe headache, double vision, ptosis, eye-movement limitation or altered consciousness requires emergency assessment rather than routine optometry review.
Investigation priorities
01
Pupil size and shape in light and darkFirst step
Describe anisocoria and identify which response may be impaired.
Management branches
Baseline observationExamine before altering pupil function
A visual complaint or neurological presentation requires assessment of pupils.
Ask about symptom onset, eye surgery, trauma and topical or systemic medicines, then record size and shape before using drops.
Compare pupils in dim and bright light with distant fixation, inspect lids and alignment, and observe direct and consensual reactions.
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 7 Sept 2026; clinical approval remains outstanding.