Selective posterior rhizotomy requires that anesthesia be given in a manner that preserves the ability to elicit muscle contraction by stimulation of the sensory roots/rootlets. The preferred site for performing a selective posterior rhizotomy is currently the lower spine between the bottom of the rib cage and the top of the hips, because it affords a secure identification of the roots at the point of exit from the spinal canal. The evolution of this technique has been toward saving some of the sensory nerves, thus avoiding the side effects seen earlier in this century. The sensory nerve roots are separated from the motor ones and stimulated electrically; leg muscles are observed for contraction both by a clinical examiner and by electromyography (EMG; an electrical device that records muscle contraction). Roots that cause abnormal muscle contraction are separated into their component rootlets, which in turn are stimulated. Abnormally responding rootlets are cut.
It must then be decided how many may be safely cut, and how dense a lesion can be without creating a sensory deficit. Foerster noted that he could perform complete rhizotomies at two adjacent levels without creating a sensory deficit. We have observed sensory loss in only 2 percent of our patients; this is because we limit lesioning in roots adjacent to levels where the entire sensory root has been sectioned.