title: The impact of subdural air collection on intraoperative motor and somatosensory evoked potentials: fact or myth?
abstract: BACKGROUND
  Surgery in the semi-sitting position is susceptible to changes in motor (MEP) and somatosensory evoked potentials (SEPs), which are not related to neurological impairment. These changes have been suggested to be caused by the insulating effect of subdural air collection. This study sought to investigate the correlation of MEP and SEP final-to-baseline amplitude ratios to postoperative volumetry of frontoparietal subdural air collection.
  
  
  METHODS
  Median nerve SEP and hand MEP findings of 47 patients operated on in the semi-sitting position were compared with 7 patients operated on in the supine position. Computed tomography was routinely performed on the 1st postoperative day in all patients, and subdural air volumetry was calculated. Final-to-baseline MEP and SEP amplitude ratios were calculated and correlated to subdural air volumetry.
  
  
  RESULTS
  SEP changed in 12 patients, and MEP changed in 7 patients. Postoperative subdural air collections were significantly different between the groups (semi-sitting group, mean 31.2 cm(3); supine group, mean 2 cm3; p = 0.000). For the SEP ratios, a moderate negative correlation with subdural volumetry was found in the semi-sitting group (p = 0.044). Conversely, there was no correlation in the subset of patients with SEP attenuation (p = 0.846). As concerns the MEP ratios, no correlation was demonstrated in any group (semi-sitting, p = 0.967; supine, p = 0.193).
  
  
  CONCLUSIONS
  Although SEP amplitude reductions were associated with large subdural air collections, this was not observed in the subset of patients with SEP attenuation and for the MEP monitoring, suggesting other pathophysiological mechanisms, such as brain shift, for the artificial amplitude reduction.
authors.affiliation: Department of Neurosurgery, Eberhard Karls University Hospital, Hoppe-Seyler-Strasse 3, 72076, Tübingen, Germany. marcusacioly@yahoo.com.br
authors.forename: Marcus André
authors.initials: MA
authors.lastname: Acioly
authors.forename: Florian H
authors.initials: FH
authors.lastname: Ebner
authors.forename: Till K
authors.initials: TK
authors.lastname: Hauser
authors.forename: Marina
authors.initials: M
authors.lastname: Liebsch
authors.forename: Carlos Henrique
authors.initials: CH
authors.lastname: Carvalho
authors.forename: Alireza
authors.initials: A
authors.lastname: Gharabaghi
authors.forename: Marcos
authors.initials: M
authors.lastname: Tatagiba
mesh_terms: D000328:Adult; D000368:Aged; D002648:Child; D004618:Embolism, Air / Q000175:diagnosis* / Q000209:etiology / Q000503:physiopathology; D005071:Evoked Potentials / Q000502:physiology*; D005260:Female; D006801:Humans; D007431:Intraoperative Complications / Q000175:diagnosis* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D019635:Neurosurgical Procedures / Q000009:adverse effects* / Q000379:methods; D012189:Retrospective Studies; D013355:Subdural Space / Q000473:pathology* / Q000503:physiopathology; D055815:Young Adult