Increased resistance to acute compression injury in chronically compressed spinal nerve roots. An experimental study.

STUDY DESIGN: Nerve conduction velocity was studied in the dog cauda equina subjected to chronic and, 1 week later, additional acute compression.

OBJECTIVES: To model the situation of spinal stenosis by first inducing a controlled, chronic compression injury and then to add compression at various pressure levels to the already compressed cauda equina.

SUMMARY OF BACKGROUND DATA: Previous models on nerve root and cauda equina compression have analyzed the effects of compression only on noninjured nerves. Clinically, the nerves that are compressed in spinal stenosis already are exposed to chronic compression.

METHODS: The cauda equine in the lumbar spine of 10 dogs was first compressed by inflating a plastic balloon, which was placed in the spinal canal, to 10 mm Hg with a viscous substance or leaving it noninflated for control. After 1 week, a second plastic balloon, which was welded together with the first balloon, which had been located in the spinal canal for 1 week, was inflated to 50 or 100 mm Hg for 2 hours with 1.5 hour of recovery, and changes in the nerve conduction velocity were analyzed. Before inflation, baseline data, reflecting the effects of the chronic compression per se, were obtained.

RESULTS: After 1 week, the nerve conduction velocity was significantly lower in the compressed cauda equina than in the control series, as an effect of the chronic compression. Additional compression demonstrated that the chronically compressed cauda equina was less susceptible to the compression-induced effects at 100 mm Hg compression.

CONCLUSIONS: The data of the present study indicate that chronically compressed nerve roots acquire a tolerance to acute compression, which may indicate that there has been adaptation processes present in the compressed nerve tissue.