Descending spinal cord evoked potentials in cervical spondylotic myelopathy: Characteristic waveform changes seen at the lesion site

•We characterized the type of D-wave changes seen at the lesion site in cervical spondylotic myelopathy with MRI evidence of single-level cord compression.•An abrupt reduction of the negative peak accompanied by an enhancement of the initial-positive peak helps identify the site of conduction block.•The enhancement of the positive peak tended to diminish with a more caudal compression, which may be explained by progressive loss of the descending motor volleys at the synapses in the cervical enlargement.

ObjectivesTo characterize waveform changes of descending spinal cord evoked potentials (D-SCEPs) seen in cervical spondylotic myelopathy (CSM).MethodsIntraoperative D-SCEP recording from serial intervertebral discs after transcranial electrical stimulation in 19 CSM patients with cord compression at a single level.ResultsCompared to the baseline (100%) obtained one level rostrally, the D-SCEP recorded at the compression site showed a significantly (p < 0.001) decreased amplitude (48%) and area (48%) of negative peak and increased amplitude (171%) and area (279%) of initial-positive peak. The degree in reduction of negative peak remained the same irrespective of the cord level involved, whereas enhancement of the positive peak tended to diminish with a more caudal compression.ConclusionsIn intraoperative electrophysiological studies of CSM with D-SCEP, an abrupt reduction of the negative peak accompanied by an enhancement of the initial-positive peak helps identify the site of conduction block. We speculate that progressive loss of the descending motor volleys at the synapses in the cervical enlargement may account for limited or absent enhancement of positive peak seen caudally.SignificanceThe current finding helps us understand the pros and cons of various electrophysiologic techniques for intraoperative localization of maximal cord involvement in CSM.