THE CRITICAL IMPORTANCE OF STIMULUS-INTENSITY IN INTRAOPERATIVE MONITORING FOR PARTIAL DORSAL RHIZOTOMY

During partial lumbosacral dorsal rhizotomy (PDR), intraoperative dors al rootlet stimulation (drs) evokes motor responses, presumed to be re flexes, which are used to select rootlets for section. However, dr sti muli may also costimulate ventral root (vr) and evoke an M rather than a reflex response, the two being distinguishable only by comparison o f response latencies after drs at two separate sites. In 15 consecutiv e spastic cerebral palsy patients undergoing PDR, we asked whether ref lex and M responses were distinguishable on the basis of stimulus inte nsity (SI). For soleus H reflexes evoked by percutaneous tibial nerve stimulation, the SI for reflex afferents was usually subthreshold for exciting motor fibers, Similarly, for nerve roots, reflexes were evoke d by drs at SIs generally less than that for M responses evoked by vr stimulation (vrs). In contrast, M responses evoked by drs required SIs that were on average 20 times greater. Finally, costimulation of cont ralateral vr after ipsilateral vrs occurred at SIs shown to evoke M re sponses after drs. We conclude that: (1) reflex and M responses evoked by drs are distinguishable on the basis of the required SI; and (2) d rs employing SIs greater than that required for vrs evokes M rather th an reflex responses due to costimulation of ipsilateral and contralate ral vr. (C) 1996 John Wiley & Sons, Inc.