title: Optimal intraoperative somatosensory evoked potential stimulus intensity can be determined by nerve action potential amplitude.
abstract: OBJECTIVE
  Muscle twitch threshold has been used to determine optimal stimulus intensity for somatosensory evoked potentials but neuromuscular blockade precludes the use of muscle twitch during surgery. Accordingly, nerve action potential (NAP) amplitude was investigated as a surrogate to muscle twitch.
  
  
  METHODS
  The ulnar and tibial nerves were stimulated at the wrist and ankle, respectively, in 27 patients undergoing spine and brain surgery. After neuromuscular blockade was gone, the stimulus intensity for just maximal NAP amplitude recorded from Erb's point and the popliteal fossa was compared with the stimulus intensity for hypothenar and plantar foot muscle twitch threshold (times two), respectively (Wilcoxon matched pairs test).
  
  
  RESULTS
  There was no significant difference between stimulus intensity for just maximal Erb's point and popliteal fossa NAP amplitude when compared with stimulus intensity for hypothenar and plantar foot twitch threshold (times two), respectively. Eight patients required more than twitch intensity (times two) to obtain maximum NAP.
  
  
  CONCLUSIONS
  The NAP amplitude may be used to determine optimal somatosensory evoked potential stimulus intensity when muscle twitch is not visible. This method should improve the success of intraoperative somatosensory evoked potential monitoring and decrease erroneous interpretation.
authors.affiliation: Neurophysiology Department, Medical Imaging, The Ottawa Hospital, Ottawa, Canada. dhoulden@toh.on.ca
authors.forename: David A
authors.initials: DA
authors.lastname: Houlden
authors.forename: Gino
authors.initials: G
authors.lastname: Ubriaco
authors.forename: Craig P
authors.initials: CP
authors.lastname: Stewart
authors.forename: Samantha L
authors.initials: SL
authors.lastname: Robertson
authors.forename: Meghan
authors.initials: M
authors.lastname: Cohen
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Potapova
authors.forename: Farhad
authors.initials: F
authors.lastname: Pirouzmand
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000328:Adult; D000368:Aged; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D019635:Neurosurgical Procedures / Q000379:methods*; D013979:Tibial Nerve / Q000502:physiology; D014459:Ulnar Nerve / Q000502:physiology