Acute intracranial hypertension and auditory brain-stem responses. Part 3: The effects of posterior fossa mass lesions on brain-stem function.

The auditory brain-stem responses (BER's), infratentorial intracranial pressure (ICP), systemic blood pressure (BP), and heart rate were recorded before, during and after expansion of an infratentorial epidural mass in anesthetized cats. Two types of BER's to increasing posterior fossa pressure were noted. In Type 1, there was predominantly suppression of the electrical activity of the auditory nuclei of the upper brain stem (Waves V and IV) and upward transtentorial herniation of the midbrain. In Type 2, the neural activity of the lower brain-stem nuclei (Waves III and II) was affected as well as that of the upper brain stem. There was upward and foraminal impaction of the brain stem and cerebellum which was confirmed by the postmortem brain sections. The change in the amplitudes of BER Waves V and III proved useful in detecting upward transtentorial herniation of the midbrain and foraminal herniation of the cerebellum in acute expanding lesions of the posterior fossa. Medullary paralysis was also detected by observing Wave III.