Central Transtentorial.

The central transtentorial herniation syndrome is demonstrated by rostrocaudal neurologic deterioration caused by an expanding lesion at the vertex or the frontal or occipital pole of the brain. It is less common than uncal transtentorial herniation. Clinical deterioration occurs as bilateral central pressure is exerted on the brain from above. The initial clinical manifestation may be a subtle change in mental status or decreased level of consciousness, bilateral motor weakness, and pinpoint pupils (<2 mm). Light reflexes are still present but often are difficult to detect. Muscle tone is increased bilaterally, and bilateral Babinski signs may be present. As central herniation progresses, both pupils become midpoint and lose light responsiveness. Respiratory patterns are affected and sustained hyperventilation may occur. Motor tone increases. Decorticate posturing, initially contralateral to the lesion, is elicited by noxious stimuli. This progresses to bilateral decorticate and then spontaneous decerebrate posturing. Respiratory patterns that may initially include yawns and sighs progress to sustained tachypnea, followed by shallow slow and irregular breaths immediately before respiratory arrest.

Cerebellotonsillar.

Cerebellotonsillar herniation occurs when the cerebellar tonsils herniate downward through the foramen magnum. This is usually caused by a cerebellar mass or a large central vertex mass causing the rapid displacement of the entire brain stem. Clinically, patients demonstrate sudden respiratory and cardiovascular collapse as the medulla is impinged. Pinpoint pupils are noted. Flaccid quadreplegia is the most common motor presentation because of bilateral compression of the corticospinal tracts. The mortality resulting from cerebellar herniation approaches 70%.

Upward Transtentorial.

Upward transtentorial herniation is occasionally seen as a result of an expanding posterior fossa lesion. A rapid decline in the level of consciousness occurs. These patients may have pinpoint pupils because of compression of the pons. A downward conjugate gaze with the absence of vertical eye movements is also observed.