Take caution if you ever get tasked to a pre-hospital trauma at David Bowie’s house. His reduced level of consciousness and blown pupil might just be due to a copious amount of mind-bending drugs and the accident as a school-boy which left him with a unilateral pupillary defect.¹  In my mind he still gets a tube and the paramedic can do it, but he only gets “one shot“?!?²

Anisocoria is characterised by unequal pupil sizes, but not necessarily as a consequence of mydriasis; a dilated pupil. There are also many pre-hospital causes of a unilateral miosis; a constricted pupil.³

A “blown” pupil is the colloquial term used by medics to refer to a fixed unilateral mydriasis.⁴

Case: Our HEMS team was called to a rural trauma involving two motocross riders. The more seriously injured victim had obvious right-sided facial and thoracic injuries, had a blown right pupil and a GCS 3 (with no external signs of head trauma). He received an RSI and bilateral thoracostomies. His pupillary signs did not improve with a bolus of hypertonic saline bolus and hyperventilation in transit.

The head CT performed at the trauma centre revealed no gross intracranial pathology requiring neurosurgical intervention, but did show a right-sided retrobulbar haematoma, which was thought to account for his unilateral fixed dilated pupil.

Challenge: To understand the pathophysiology of pupillary signs and when a “blown” pupil might not be a consequence of uncal herniation.

Pathophysiology: Pupillary size is governed by the balance of actions of two opposing muscle groups of the iris: the dilator and sphincter pupillae controlled by the autonomic nervous system.

Constriction of the size of the pupil is mediated via parasympathetic (cholinergic) nerve fibers that travel superficially with the third cranial nerve. The pupil will respond to circulating catecholamines but dilation is controlled by sympathetic fibres originating from the superior cervical ganglia.^5,6

Each eye’s sensory pathway is linked with its counterpart by partial crossover of fibers in the Edinger-Westphal nuclei which accounts for the consensual response to light.⁷