Persistence of auditory nerve response and absence of brain-stem response in severe cerebral ischaemia.

Cerebral ischaemia, in which the brain-stem components of the ABP were isoelectric, was accompanied by the paradoxical persistence of the compound action potential of the auditory nerve (wave 1). This ischaemia was induced in cats by reducing mean arterial blood pressure and elevating intracranial pressure, resulting in decreased cerebral perfusion pressure (CPP). This is unexpected since the inner ear is supplied by a branch of an intracranial artery. To study this phenomenon, CPP was manipulated and when average CPP was 13.5 mm Hg, only wave 1 remained. In 7 of 9 experiments, clamping of both common carotid arteries did not abolish wave 1. Experiments with radioactive tracers demonstrated a remaining residual blood flow through the inner ear. This remaining auditory nerve response is probably not due to a very low metabolism of the inner ear or to the cochlea being supplied by anastomoses from the middle ear, supplied by the external carotid artery. The residual cochlear blood flow and the persistent wave 1 can probably be explained in the following way: at low CPPs the smaller intracranial blood vessels collapse so that the brain tissue is not perfused, leading to loss of the brain-stem components of the ABP. However, flow still persists in the larger intracranial arteries. This blood preferentially flows to the cochlea since the intracochlear pressure is slightly below the intracranial pressure due to the presence of the oval and round windows. Thus a sufficient blood flow to the cochlea is maintained, with sparing of wave 1.