Fixed, dilated pupils that do not respond to light are a medical emergency in most situations.
Ocular and neurological causes
Damage to the iris muscles or nerves prevents normal constriction. Brain injury can also interrupt pathways controlling pupil size.
- Severe eye trauma or surgery damaging the sphincter muscle
- Acute angle-closure glaucoma with very high pressure
- Compression of the third cranial nerve by aneurysm or tumour
- Global brain injury, cardiac arrest, or raised intracranial pressure
Drug-related causes
Some medications and recreational drugs can also produce large unreactive pupils. History and examination help distinguish them.
- Overdose of anticholinergic drugs or atropine-like agents
- Recreational stimulants such as MDMA or cocaine
- Topical dilating drops used in eye clinics
- Any unexpected fixed dilation should be treated as urgent
