title: Somatosensory evoked potential monitoring during positioning of the patient for posterior fossa surgery in the semisitting position.
abstract: OBJECTIVE
  Midcervical flexion myelopathy is a rare but well-known complication of posterior fossa surgery. To reduce the risk, we routinely used somatosensory evoked potential (SSEP) monitoring during positioning of the patient.
  
  
  METHODS
  Fifty-five consecutive patients were operated on for posterior fossa lesions in the semisitting position via a median (5 patients) or a lateral (50 patients) suboccipital approach. During positioning, monitoring of SSEPs by stimulation of the tibial nerve (T-SSEP) as well as by stimulation of the median nerve (M-SSEP) was established. In the case of pronounced SSEP changes, the head was repositioned. Surgery was started after SSEP recordings were unchanged as compared to the baseline investigation.
  
  
  RESULTS
  Effective monitoring was possible in all cases. Whereas M-SSEP recordings showed no changes while placing patients in the sitting position, T-SSEP recordings were altered in 14 cases (25%). In cases using the midline approach, SSEP changes were never so pronounced to require repositioning of the head. Head flexion and rotation resulted in significant changes of T-SSEP recordings in eight patients (14.5%), requiring repositioning. In two cases, an amplitude loss was noted. In only two of these eight patients were M-SSEP recordings markedly changed. SSEP recordings after repositioning disclosed recovery of spinal cord function. In no patient were clinical signs of myelopathy observed postoperatively.
  
  
  CONCLUSIONS
  We observed a high incidence of pronounced changes of T-SSEP recordings when the patient's head was flexed and rotated for lateral suboccipital craniotomy in the semisitting position. Despite the low specificity monitoring of T-SSEPs during positioning of the patient for posterior fossa surgery, the semisitting position is strongly recommended.
authors.affiliation: Neurosurgical Clinic, Justus-Liebig-University Giessen, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Deinsberger
authors.forename: P
authors.initials: P
authors.lastname: Christophis
authors.forename: A
authors.initials: A
authors.lastname: Jödicke
authors.forename: M
authors.initials: M
authors.lastname: Heesen
authors.forename: D K
authors.initials: DK
authors.lastname: Böker
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001927:Brain Diseases / Q000503:physiopathology / Q000601:surgery*; D001932:Brain Neoplasms / Q000503:physiopathology / Q000601:surgery*; D002648:Child; D002675:Child, Preschool; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D007431:Intraoperative Complications / Q000175:diagnosis* / Q000503:physiopathology / Q000517:prevention & control; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D011187:Posture / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012016:Reference Values; D012307:Risk Factors; D013116:Spinal Cord / Q000503:physiopathology; D013117:Spinal Cord Compression / Q000175:diagnosis* / Q000503:physiopathology / Q000517:prevention & control