Abstract
In order to establish a method of spinal cord monitoring during spinal column or spinal cord surgery, the compound muscle action potentials (CMAP) elicited by transcranial repetitive electric brain stimulation and the peripheral nerve evoked potentials (PNP) elicited by electric spinal cord stimulation were compared in 10 cats with a subacute compressive myelopathy. When compression of the spinal cord were gradually increased, disappearance of the CMAP preceded that of the PNP in all cases. The latency of the CMAP prolonged about 3.0ms, and the PNP prolonged about 1.0ms just before its disappearance. But, there were few cases in which the waveforms disappeared suddenly without the prolongation of the latency. When the latency of the PNP prolonged about 1.0ms, the CMAP disappeared simultaneously. Further increase of compression resulted in the PNP disappearance.
When the compression was released immediately after disappearance of the PNP waveform, the PNP could restore but not the CMAP any more. Three weeks after acute experiments, the hindlimb motor function was graded “motor useful” in all cats, in which compression was released after disappearance of the CMAP alone, but it was “flaccid paraparesis” in all cats, in which compression was released after disappearance of both CMAP and PNP.
The present study demonstrated that the impairment in the hindlimb motor function cannot be observed when the CMAP alone disappear. On the other hand, it is highly possible that irreversible spinal cord damage has occurred when the PNP disappeared.
The results indicate that disappearance of the CMAP or prolongation of the PNP latency not less than 1.0ms compared to the control is a sign of impairment in hindlimb motor function. Intraoperative monitoring of the spinal cord function with a combination of both the CMAP and the PNP constitutes a useful method of evaluating the impairment in motor function due to spinal cord compression.
