Gentle dorsal root retraction and dissection can cause areflexia: Implications for intraoperative monitoring during "selective" partial dorsal rhizotomy

During partial dorsal rhizotomy (PDR), intraoperative dorsal rootlet stimulation often evokes nonreflex, rather than reflex, motor responses that are due to costimulation of adjacent ventral roots. Intraoperative areflexia typically predicts that motor responses evoked by dorsal rootlet stimulation are nonreflexive. The cause of areflexia during PDR is in part due to anesthesia, but other mechanisms are likely to play a role as well. In this study of three consecutive patients undergoing lumbosacral neurosurgery, soleus H-reflexes evoked by tibial nerve stimulation at the popliteal fossa were found to suddenly decline in amplitude following retraction and gentle dissection of the S-1 dorsal root. In one areflexic patient, dorsal rootlet stimulation proximal to the main site of dissection evoked soleus H-reflexes, although they could not be evoked by tibial nerve stimulation. We conclude that the gentle retraction and dissection of dorsal rootlets that occurs during PDR can induce conduction block of reflex afferents. High-intensity dorsal rootlet stimulation distal to the site of conduction block may then evoke not reflex responses, but rather nonreflex motor responses, due to the costimulation of adjacent ventral roots. (C) 2001 John Wiley & Sons, Inc.