When the expression “blown pupil” is used by doctors it indicates that the eye pupil is fixed and dilated. This is an indication that a person has sustained head trauma. An open head injury or trauma results when a bullet or some other object backed by force enters the head and damages brain tissues or membranes or fractures the skull.
Pupils Don't Respond to Light
One or both of the pupils can become fixed and dilated following trauma to the head. The pupils will not respond to light or become smaller or constrict as they would normally do.
Lack of Brain Activity
Blown pupils can indicate lack of brain activity meaning the individual is brain-dead.
After head trauma, if a pupil is blown this indicates that an intracranial mass is expanding, which can include hemorrhaging in the brain and the resulting compression on cranial nerve 3, according to Edmedmag.com. The blown pupil may also represent herniation of the temporal lobe in the brain.
When cerebral herniation occurs, the medial temporal lobe or uncus is forced down and squeezes the brainstem. A dilated pupil is an indication that this has occurred, according to Neurosurgery, University of Mississippi Medical Center.
Brain Function Disrupted
A traumatic brain injury or TBI can result in dilated pupils because trauma to the head disrupts the regular function of the brain, according to Neurosurgerytoday.org.