Blown pupil is an informal medical term used by physicians and nurses to refer to sudden pupillary dilation and loss of ability to constrict in response to light. It is an important clinical sign in physical diagnosis, especially in emergency department or intensive care unit patients, where it can signal a stroke, impending brain herniation, or other brain catastrophe.

Pupillary dilation (mydriasis) indicates unopposed sympathetic activity due to impaired parasympathetic axons. This may reflect compression or distortion of the oculomotor nerve 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. It may also represent herniation of the uncus of the temporal lobe.

• Dilated pupil