Mydriasis

Mydriasis (from the Greek amydros — dark, unclear) is a persistent pathological dilation of the pupil of the eye, not reacting or weakly reacting to bright light. In neurotraumatology, it is one of the most important local symptoms of brain compression.

Etiology and pathophysiology

The pupil size is regulated by two muscles: the constricting muscle (sphincter) and the dilating muscle (dilator). The sphincter’s activity is controlled by parasympathetic nerve fibers, which pass as part of the oculomotor nerve (third pair of cranial nerves).

The anatomical peculiarity is that these delicate fibers are located on the very surface of the nerve trunk. When the shifted brain tissue begins to press on the oculomotor nerve during temporal-tentorial herniation, these superficial fibers are the first to suffer. The signal to constrict the pupil is blocked, allowing the dilator muscle to freely enlarge the pupil.

Clinical significance

The appearance of unilateral persistent mydriasis in a patient with cranial trauma is a classic symptom, which with more than 80% likelihood indicates the presence of a large intracranial hematoma.

A crucial diagnostic rule states that the dilated pupil almost always occurs on the side of the hemorrhage. This enables the neurosurgeon to accurately determine which side the hematoma should be accessed from, even if a CT scanner is unavailable. Bilateral mydriasis (wide pupils in both eyes unresponsive to light) is an extremely poor prognostic indication, suggesting total brainstem destruction or deep hypoxia.