Suspect herniation due to an intracranial mass lesion as a cause of fixed dilated pupil in an unconscious patient. A fixed dilated pupil in an awake patient is NOT due to herniation.
Pharmacologic blockade is the most common cause of a fixed dilated pupil in an otherwise normal healthy patient.
A single fixed dilated (mydriatic) pupil can be caused by:
- Pharmacological blockade– typically topical mydriatic drugs used to facilitate ophthalomological examinations.
- anticholinergic drugs: e.g. atropine, cyclopentolate and tropicamide
- alpha1-agonists: phenylephrine.
- Oculomotor nerve palsy (3rd cranial nerve)
- parasympathetic nerves are in the superficial parts of the nerve, so tend to be more vulnerable to compressivelesions and spared by vascular lesions (e.g. diabetes mellitus).
- If an acute third nerve palsy is accompanied by pupillary mydriasis an aneurysm arising from the posterior communicating artery must be excluded.
[See a Spider called Willis for an easy way to remember the components of the Circle of Willis and its relations.]
- Holmes-Adie pupil (tonic phase)
- post-traumatic iridocyclitis (e.g. direct facial trauma)
- acute closed-angle glaucoma
- ocular prosthesis – the normal pupil may be relatively constricted due to ambient light.