title: Efficacy of transcranial motor evoked potentials, mechanically elicited electromyography, and evoked electromyography to assess nerve root function during sustained compression in a porcine model.
abstract: METHODS
  This is an animal experiment using transcranial motor evoked potentials (TcMEPs), mechanically elicited electromyographic (EMG) responses, and evoked EMG responses during nerve root compression in a pig model.
  
  
  OBJECTIVE
  To compare these 3 electrophysiological measures for compression applied to a lumbar nerve root.
  
  
  BACKGROUND
  Lumbar nerve root injury may result in motor weakness in up to 30% of spinal deformity cases. Compressive injury may occur during the surgical approach, decompression, and manipulation of the spine. Using an established porcine model, we examined the changes to TcMEPs, mechanically elicited EMG responses, and evoked EMG responses during varied compressive forces.
  
  
  METHODS
  TcMEPs, mechanically elicited EMG responses, and evoked EMG responses were recorded for the tibialis anterior muscle in 16 experiments. Precompression TcMEP and nerve root stimulation threshold (NRT) were obtained. The dominant root was compressed at 1 N (n = 8) or 2 N (n = 8) for 10 minutes. TcMEP was recorded every minute during compression, and TcMEP and NRT were recorded after both compression and 10 minutes of recovery.
  
  
  RESULTS
  After 10 minutes of 1-N compression, TcMEP amplitude of the tibialis anterior muscle decreased to 69% ± 13% of baseline (P < 0.02 vs. baseline). The mean NRT increased to 645% ± 433% (P < 0.02 vs. baseline NRT). After the recovery period, TcMEP in the 1-N group returned to 98% ± 11% of baseline (P = 0.36 vs. baseline). After 10 minutes of 2-N compression, TcMEPs from the tibialis anterior muscle decreased to 27% ± 15% of baseline (P < 0.02 vs. baseline). After the recovery period, TcMEP in the 2-N group returned to 30% ± 10% of baseline (P < 0.02 vs. baseline). Tonic EMG activity was observed in 3 nerve roots compressed at 2 N.
  
  
  CONCLUSIONS
  Compression at 1 and 2 N produced consistent changes in TcMEPs and EMG responses. TcMEP monitoring is sensitive to an increase in compressive force. TcMEP amplitude change was correlated to the force applied and the ability of the nerve root to recover. Mechanically elicited EMG responses were not sensitive to nerve root compression.
  
  
  METHODS
  N/A.
authors.affiliation: *Department of Orthopaedic Surgery, University of California, San Francisco †Division of Operating Rooms, Department of Clinical Neurophysiology, University of California, San Francisco; and ‡Department of Anesthesia and Perioperative Care, University of California, San Francisco.
authors.forename: Frank
authors.initials: F
authors.lastname: Valone
authors.forename: Russ
authors.initials: R
authors.lastname: Lyon
authors.forename: Jeremy
authors.initials: J
authors.lastname: Lieberman
authors.forename: Shane
authors.initials: S
authors.lastname: Burch
mesh_terms: D000818:Animals; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology*; D008159:Lumbar Vertebrae / Q000601:surgery*; D023421:Models, Animal; D016343:Monitoring, Intraoperative / Q000379:methods; D018482:Muscle, Skeletal / Q000294:innervation*; D013117:Spinal Cord Compression / Q000601:surgery*; D013126:Spinal Nerve Roots / Q000503:physiopathology*; D013552:Swine