Herniation syndromes

When pressure gradients develop between compartments, brain tissue is displaced across the fixed dural and bony boundaries. Recognising the pattern localises the lesion:

Uncal (transtentorial): the medial temporal lobe compresses the third cranial nerve and midbrain — a fixed, dilated pupil on the side of the lesion, then contralateral weakness and falling consciousness.
Central: downward displacement of the diencephalon, producing a rostral-to-caudal deterioration in consciousness, pupils and breathing pattern.
Subfalcine: the cingulate gyrus passes under the falx, which may compress the anterior cerebral artery and cause leg-predominant weakness.
Tonsillar: the cerebellar tonsils descend through the foramen magnum onto the medulla — Cushing’s triad (hypertension, bradycardia, irregular respiration) and respiratory arrest.

Cushing’s triad is a late sign. It marks brainstem compression that is already underway, not an early warning. The deteriorating exam that preceded it was the warning.