EXPERIMENTAL DISC HERNIATION - EVALUATION OF THE NATURAL COURSE

Study Design. Changes in L7 nerve root conduction velocity and changes in appearance on magnetic resonance study of the L6-L7 intervertebral disc in the dog were assessed for 2-6 months after an experimental di sc herniation was performed. Objectives. To assess the time-related ch anges of nerve conduction velocity and magnetic resonance changes in t he invertebral discs. Summary and Background Data. It is known that nu cleus pulposus may induce nerve root morphologic and functional change s when applied epidurally. However, it is not known whether such chang es are reversible. Methods. The spinal canal was opened by laminotomy of the upper part of the L7 lamina and the lower part of the L6 lamina on the left side. The L7 nerve root was gently retracted (sham) or th e disc was punctured and injected with saline to produce herniation of the nucleus pulposus during the retraction time (herniation). After 1 day to 2 months, nerve root conduction velocity was determined by loc al electrical stimulation. Six dogs had the herniation or the sham pro cedure, and the L6-L7 disc was studied by magnetic resonance imaging a t various times up to 6 months after the procedure. Results. Decrease in nerve conduction velocity reached a maximum after 7 days and recove red to baseline level fully within 2 months. Although there was a clea r reduction-recovery patten, the difference in conduction velocity com pared with that of the sham group was statistically significant after only 7 days. Disc degeneration started in the herniated discs within 7 days after the herniation procedure. However, none of the experimenta lly induced disc herniations were visualized by magnetic resonance ima ging 7 days after the procedure. In no case was there subsequent nerve root compression. In one case, disc protrusion was visible 6 months a fter the herniation procedure. Conclusions. The results demonstrate fo r the first time that nucleus pulposus-induced nerve root injury rever ses in 2 months and that it may be present without simultaneous nerve root compression, as confirmed by findings in magnetic resonance imagi ng. The previously described nucleus pulposus-induced nerve root chang es may therefore be of clinical importance, and experimental studies o f these mechanisms will probably be relevant for expanded understandin g of the pathophysiologic mechanism behind sciatica that is caused by disc herniation.