Blown pupil is an informal medical term used by medical providers to refer to sudden pupil

Pupil

The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

lary dilation and loss of ability to constrict in response to light. It is an important clinical sign in physical diagnosis, especially in emergency department

Emergency department

An emergency department , also known as accident & emergency , emergency room , emergency ward , or casualty department is a medical treatment facility specialising in acute care of patients who present without prior appointment, either by their own means or by ambulance...

 or intensive care unit

Intensive Care Unit

thumb|220px|ICU roomAn intensive-care unit , critical-care unit , intensive-therapy unit/intensive-treatment unit is a specialized department in a hospital that provides intensive-care medicine...

 patients, where it can signal a stroke

Stroke

A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, impending brain herniation

Brain herniation

Brain herniation, also known as cistern obliteration, is a deadly side effect of very high intracranial pressure that occurs when the brain shifts across structures within the skull...

, or other brain catastrophe.

Pupillary dilation (mydriasis

Mydriasis

Mydriasis is a dilation of the pupil due to disease, trauma or the use of drugs. Normally, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day...

) indicates unopposed sympathetic activity due to impaired parasympathetic axons. This may reflect compression or distortion of the oculomotor nerve (CN III)

Oculomotor nerve

The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

 by either primary injury or herniation. Mydriasis also may be an effect of adrenergic stimuli such as epinephrine, anticholinergics, cocaine, PCP, and drug withdrawal. The classic fixed and dilated "blown pupil" is a unilateral phenomenon that may occur when a rapidly expanding intracranial mass, including blood from a hemorrhage, is compressing cranial nerve III

Oculomotor nerve

The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

. It may also represent herniation of the uncus of the temporal lobe.