gms | German Medical Science

Objective: It is known from previous studies that intraoperative monitoring with motor evoked potentials (MEP) and somatosensory evoked potentials (SEP) during surgery in the semi-sitting position is susceptible to changes which are not related to neurological impairment. This fact reduces their reliability. These changes are presumably caused by the insulating effect of subdural air collection. If so, a negative correlation is expected so that the larger the subdural collection, the lower the final MEP and SEP amplitudes. This study sought to investigate the correlation of MEP and SEP amplitude changes to the volumetry of subdural air collection.

Methods: Fifty-five patients with posterior fossa and skull base tumors were prospectively enrolled between August 2006 and August 2007 and retrospectively evaluated. Median nerve SEP and hand MEP findings of 48 patients operated on in the semi-sitting position were compared with 7 patients operated on in the supine position. Cranial computed tomography (CT) was performed routinely on the first postoperative day in all patients and volumetry of the subdural air collection was calculated. Final-to-baseline MEP and SEP amplitude ratios were calculated and correlated to postoperative subdural volumetry.

Results: MEP and SEP amplitude reduction (greater than 50%) unrelated to postoperative neurological impairment were almost exclusively observed in patients operated on in the semi-sitting position. Postoperative subdural air collections were significantly different between groups (semi-sitting group, mean 31.95 cm³, range 0–145 cm³; supine group, mean 2 cm³, range 0–7.8 cm³) (p<0.0001). For the SEP ratios, a moderate negative correlation with subdural volumetry was found in the semi-sitting group (p=0.056), whereas in the supine group, the correlation was positive (p=0.015). Conversely, there was no correlation between MEP amplitude ratios and subdural volumetry in the groups (semi-sitting, p=0.890; supine, p=0.193).

Conclusions: Our results demonstrate that SEP and MEP recordings may have limited interpretation during surgery in the semi-sitting position. Although SEP amplitude reductions were associated with large subdural air collections, this was not observed for the MEP monitoring suggesting other pathophysiological mechanisms, such as brain shift, for the artificial amplitude reduction.