Observations on intraoperative somatosensory evoked potential (SEP) monitoring in the semi-sitting position

Background: Former case reports suggest that monitoring of median nerve somatosensory evoked potentials (M-SEP) is unreliable in patients operated in the semi-sitting position due to the occurrence of evoked potential changes unrelated to neurological damage. This study was designed to analyze these changes in greater detail and confirm that these changes are not caused by neurological damage. Methods: M-SEP monitoring findings of 50 patients with surgery in the semi-sitting position were analyzed and compared with a group of 50 patients who underwent surgery in the supine position. M-SEP amplitudes and latencies at distinct steps of the monitoring procedure were used for further analysis. In 10 of the 50 semi-sitting patients, M-SEP were recorded additionally after surgery with the anesthetized patient in the supine position. Results: Significant M-SEP changes occurred in the semi-sitting patients only. An amplitude loss of greater than 50% on at least one side was observed in 24 patients. The magnitude and the time course of the amplitude loss was considerably variable. A complete loss of the evoked potential was not observed in any case. In all 10 patients, M-SEP recovered completely when recorded in the supine position. Conclusions: In about half of the patients with M-SEP monitoring in the semi-sitting position, a significant amplitude loss occurs which is unrelated to neurological damage and presumably caused by subdural gas collections. There is no characteristic pattern of M-SEP changes which enables a differentiation of these ‘artificial’ alterations from true events. The only appropriate criterion to indicate an impending neurological damage in these patients seems to be a complete loss of the M-SEP potential.