Effects of hydrocephalus and increased intracranial pressure on auditory and somatosensory evoked responses.

Recent studies in human and animal subjects have suggested a relationship between intracranial pressure (ICP) and ventricular dilatation and multimodality evoked responses which, if substantiated, would be of value to clinical practice as a noninvasive way of assessing the need for shunting in selected patients in whom computed tomography (CT) is not definitive. In an attempt to better define these changes, auditory evoked response (BAER) and somatosensory evoked response (SER) were performed on 16 cats as a base line, after which they were made hydrocephalic by the cisternal injection of kaolin. Nine cats survived, and CT or magnetic resonance scans were performed on them 4 to 6 weeks later. In those animals in which ventricular dilatation was noted, repeat evoked responses were recorded. In the 6 hydrocephalic cats, the ventricle was punctured to measure ICP, which in all cases was less then 5 mm Hg. The lumbar spinal dural sac was then ligated, which resulted in periodic plateau waves up to 75 to 100 mm Hg after 4 to 6 hours, lasting up to 10 minutes. In neither group of cats was any change in either BAER or SER observed until preterminally, when ICP was in the range of 75 to 100 mm HG and cerebral perfusion pressure was compromised. This suggests that the BAER and SER are not sensitive to either ventricular dilatation or intracranial hypertension.