AN EXPERIMENTAL-STUDY OF REFLEX VARIABILITY IN SELECTIVE DORSAL RHIZOTOMY

Significant problems regarding the measurement technique currently use d to choose nerve roots for sectioning in the selective dorsal rhizoto my procedure have recently been reported. To better understand the sou rce of these problems, a series of six experiments was performed in wh ich the selective rhizotomy technique was applied to cats that were ei ther intact, decerebrate, or spinalized. Measurements were made before and after partial rhizotomy. In decerebrate preparations, large, spon taneous changes in reflex threshold were observed over short periods o f time, especially after partial rhizotomy was performed, and threshol d changes greater than 1000% could be observed over 10-minute periods. Using constant-current stimulation of the dorsal root at threshold, t he response of each ipsilateral leg muscle demonstrated frequent chang es, and changes coincided with the variability in threshold estimation . In addition, very low thresholds were measured (0.1 to 0.25 mA) in h alf(3 of 6) of these experiments, these measurements being well below the currents customarily used for intraoperative decision making. Stim ulation at twice threshold was found to consistently increase the path ological quality of the responses observed. Although intact animals ne ver displayed contralateral responses when stimulated at threshold, su ch contralateral responses could readily be elicited at twice threshol d. These laboratory results raise further doubts regarding the reliabi lity of the measurement techniques now widely used for selective dorsa l rhizotomy.