A new technique to improve cortical-evoked potentials in spinal cord monitoring: A ratio method of analysis

Robert R. Madigan, A. E. Linton, S. L. Wallace, J. H. Dougherty, M. L. Eisenstadt, L. B. Powers, A. W. Zimmerman

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


Systemic effects such as anesthesia, hypotension, hypothermia, and hypoxia affect the cortical evoked responses. We propose, that by sequential stimulation of the median and posterior tibial nerves, and the construction of a ratio from the value of their amplitudes, the systemic effects can be eliminated and thus improve the reliability of the cortical evoked responses. Two groups of scoliosis patients who underwent spinal surgery with instrumentation were analyzed retrospectively. Both groups had spinal cord monitoring using peripheral nerve stimulation and cortical recordings of the somatosensory-evoked response (SER). In Group 1,50 patients were analyzed for changes in posterior tibial nerve response before and after distraction. Wide variability in the response suggested this method to be less reliable in predicting spinal cord conduction deficits. Thirty-eight patients in Group 2 were analyzed using both the median and posterior tibial nerve amplitudes. A ratio of the posterior tibial to median nerve wave amplitude was constructed, thus eliminating any systemic variables. A critical value, alerting the surgeons to possible decreases in spinal cord conduction, was calculated by subtracting one standard deviation from the mean of the postdistraction ratios of the posterior tibial to median nerves (1.20 -.633 =.567).

Original languageEnglish (US)
Pages (from-to)330-335
Number of pages6
Issue number4
StatePublished - 1987
Externally publishedYes


  • Median nerve
  • Posterior tibial nerve
  • Scoliosis surgery
  • Spinal cord monitoring
  • Systemic effects

ASJC Scopus subject areas

  • Clinical Neurology
  • Orthopedics and Sports Medicine
  • Physiology


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