gms | German Medical Science

Objective: Intraoperative monitoring (IOM) and especially transcranial electrical motor-evoced potential monitoring (TcMEP) is an indispensible tool during complex operations of brain and spine. Whereas the influence of anaesthesiologic drugs is well examined since decades simple physical factors which may bias the recordings are not considered. Under the impression of strong TcMEP deviations during operations in sitting position the resistive effect of air ascending from the surgical field in the subarachnoid space is explored quantitatively.

Method: The TcMEP-recordings (train-of-fife applied at C3/C4 position) of 15 patients with brainstem tumors or lesions of the upper cervical spine were correlated with the volume of subarachnoid air detected in postoperative CT-scans (4 hours after leaving the OR). The air volume and its spatial distribution were determined with the help of a common computer system for medical navigation. The measurements were handicapped by the fact that the air raised up in the cerebrospinal fluid (CSF) and collected in the apex of the scull intraoperatively and moved ventrally towards the frontal sinus when the patient is lying in his bed again.

Results: Postoperatively none of the 15 patient suffered deficits of motor tracts controlled by TcMEP monitoring. Six of them showed mild (40%), four severe (26,7%) intraoperative variations of the motor potentials. The intracranial air volume ranged from 9.4 up to 121.2 ccm (mean: 42.8 ± 39.15) but we saw no correlation between the quantity of air and possible TcMEP variations. Searching for the real distribution of ascending air during operation we scrutinized the postOP-CT scans and identified small air bubbles trapped in different sulci. Residues in the central sulcus showed a strong dependence on severe intraoperative TcMEP-variation (4 patients). We created a computer program, which simulates the distortion of the electric field and the local reduction of the stimulating current in a CSF layer with air inclusions for a physical confirmation.

Conclusions: Deterioration of TcMEP-recordings need not be caused by impairment of motor pathways or wrong anesthesia. The pure existence of intracranial air cannot be linked up with temporal or permanent reduction of motor potentials. But air passage in the vicinity of the motor cortex might provoke a decrease of local conductivity and vary the outcome of IOM stimulation during operations in sitting position.