Clinical signs and evoked response alterations associated with chronic experimental cord compression

Cortical and spinal evoked potentials were used to monitor the effect of experimental chronic cord compression in cats. An implantable compression screw made it possible to maintain the animals unrestrained. The mean compression period was 10 weeks (maximum 16.5 weeks). Compression was increased by stepwise tightening of the screw at intervals of 4 to 7 days under ketamine anesthesia. Evoked potentials were monitored before and after each compression step with repeated recordings, and were analyzed in terms of alterations in amplitude, latency, and waveform. Amplitude response curves were plotted where the amount of compression at each recording was expressed as a percentage of total compression. Changes in spinal evoked potentials occurred rather early (mean 36% of total compression), while obvious cortical evoked potential changes appeared rather late (mean 91% of total compression). Neurological alterations appeared later than alterations in spinal evoked response but prior to alterations in cortical evoked response. Thus, alterations in cortical somatosensory evoked potentials in the presence of chronic spinal compression indicate a severe degree of compression and do not seem to be of diagnostic value in the early detection of chronic spinal cord compression. It is suggested that the monitoring of spinal rather than cortical evoked responses would be more useful in locating and detecting chronic compression spinal cord damage.