[Experimental study on spinal cord monitoring--changes in spinal cord evoked potentials during vertical direction...

Experiments were carried out on 18 adult dogs ranging from 9.0 to 11.5 kg. The dogs were intubated and anesthetized with Halothene oxygen, and then mounted on a stereotaxic spinal apparatus. Facetectomies and a discectomy between L1 and L2 vertebrae were performed readily to give traction force to the spinal cord. Spinal cord function was monitored by the first and second negative deflections (I and II, respectively) of the descending spinal cord evoked potentials (descending SCEP) elicited at T7 and recorded at L4 through bipolar catheter electrodes. Both of them were inserted at the midline of the dorsal epidural space. The study was conducted in two parts. In the first part, the cyclic distraction-release program was carried out until motor function was impaired. Distraction was increased in increments of 5 mm, each time maintained for 10 minutes and then totally released for 10 minutes. If neurological deficits in the hind limbs were confirmed by the wake-up test, which was performed every 10 minutes after distraction and release, the 10 minutes' release period was extended for a total period of 30 minutes. The first change in the experimental protocol was transient augmentation of the amplitude of the II deflection which was always observed on a slight distraction, while the I deflection did not change in its amplitude and latency. Each distraction produced a reversible slight reduction of the amplitudes with delay of latencies of the I and II deflections before motor disturbance occurred. However, at a certain traction level paraparesis accompanied by irreversible decrease of amplitudes and delay of latencies was observed, which was confirmed by the wake-up test. At this point, which was designated as the critical point, SCEPs were so time dependent that only a slight amplitude reduction was noted immediately after distraction, but it decreased quickly in a short time during this traction level. Histopathology and microangiography did not show any hemorrhage in the spinal cord of any of the specimens, although there were formation of perivascular space, rupture of a part of nerve fibers in the white matter and findings of acute degeneration in the grey matter. In the second part of this study, a small amount of distraction was introduced until the amplitude of the II deflection depicted transient augmentation, which had a mean amplitude of 167.8 per cent as compared to the control. This enhancement returned approximately to the normal value within 10 minutes by total release.(ABSTRACT TRUNCATED AT 400 WORDS)