Subdural air limits the elicitation of compound muscle action potentials by high-frequency transcranial electrical stimulation

High-frequency transcranial electrical stimulation was performed in 8 patie nts undergoing surgery in the sitting position. Following the opening of th e dura of the posterior fossa changes in compound muscle action potentials were observed. These changes were not attributable to surgical manoeuvres a t the brain stem or spinal cord, or to anaesthetic changes. In all these ca ses intraoperative fluoroscopy of the skull revealed a subdural air collect ion underneath the stimulation electrodes. Such a subdural air collection, not infrequent in patients operated on in the sitting position, limits the application of high-frequency transcranial electrical stimulation as a moni toring technique. It remains unclear if this effect is due to the increasin g distance between scalp and cortex and the insulating effect of subdural a ir, or due to displacement of the motor cortex. The practical importance of this report is derived from the increasing application of intraoperative m otor pathway monitoring.