Diagnosis of ascending transtentorial herniation by cranial computed tomography.

The secondary effects of large infratentorial masses may include ascending transtentorial herniation. Rostral displacement of the superior vermis through the tentorial incisura can be accurately detected by cranial computed tomography. Signs of early or impending upward herniation are compression and slight posterior flattening of the quadrigeminal plate cistern. Progressively more severe herniation produces amputation of the peritcetal cerebrospinal fluid diamond, leading to a triangle or "squared off" appearance of the confluent quadrigeminal and superior cerebellar cisterns. When the disorder is severe, the herniated vermis plugs the incisura, completely effacing these cisterns and flattening the posterior third ventricle. Obstructive hydrocephalus may also occur with moderate or severe herniation.