gms | German Medical Science

61st Annual Meeting of the German Society of Neurosurgery (DGNC) as part of the Neurowoche 2010
Joint Meeting with the Brazilian Society of Neurosurgery on the 20 September 2010

German Society of Neurosurgery (DGNC)

21 - 25 September 2010, Mannheim

The impact of subdural air collection on intraoperative motor and somatosensory evoked potentials: fact or myth?

Meeting Abstract

  • Marcus A. Acioly - State University of Rio de Janeiro, Brazil; Klinik für Neurochirurgie, Eberhard-Karls-Universität Tübingen, Germany
  • Florian H. Ebner - Klinik für Neurochirurgie, Eberhard-Karls-Universität Tübingen, Germany
  • Till K. Hauser - Abteilung für Neuroradiologie, Eberhard-Karls-Universität Tübingen, Germany
  • Marina Liebsch - Klinik für Neurochirurgie, Eberhard-Karls-Universität Tübingen, Germany
  • Carlos Carvalho - Klinik für Neurochirurgie, Universität Ulm/Günzburg, Germany
  • Alireza Gharabaghi - Klinik für Neurochirurgie, Eberhard-Karls-Universität Tübingen, Germany
  • Marcos Tatagiba - Klinik für Neurochirurgie, Eberhard-Karls-Universität Tübingen, Germany

Deutsche Gesellschaft für Neurochirurgie. 61. Jahrestagung der Deutschen Gesellschaft für Neurochirurgie (DGNC) im Rahmen der Neurowoche 2010. Mannheim, 21.-25.09.2010. Düsseldorf: German Medical Science GMS Publishing House; 2010. DocP1742

doi: 10.3205/10dgnc213, urn:nbn:de:0183-10dgnc2137

Published: September 16, 2010

© 2010 Acioly et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



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.