Blown pupil

Mydriasis or a blown pupil refers to the dilation of the pupils and is the opposite of miosis (pupil constriction).

Definition

Mydriasis is commonly defined as the dilation of the pupil to a diameter of more than 5 millimeters[^1], although the definition may vary depending on the literature.

Illustration

Neurosurgical Relevance

Mydriatic pupils can be a sign of increased supratentorial intracranial pressure and is usually a result of the compression of the oculomotor nerve. This nerve carries, among others, parasympathetic fibers responsible for miosis through the sphincter muscle of the pupil. When there is an increase in intracranial pressure, for example, due to an intracerebral hemorrhage, a tumor, or a cerebral edema, it can lead to a shift of brain tissue. A typical scenario is transtentorial herniation, where parts of the temporal lobe are pressed through the tentorial notch. The oculomotor nerve, which runs along the tentorium, is pressed against bony or dural structures, such as the edge of the clivus.

This pressure initially damages the sensitive, superficially located parasympathetic fibers, leading to a loss of their function. As a result, the influence of the sympathetically innervated dilator muscle of the pupil predominates, and the pupil on the affected side dilates and no longer responds, or responds sluggishly, to light exposure (light rigidity).