DELAYED HYPOGLOSSAL-FACIAL NERVE SUTURE AFTER PREDEGENERATION OF THE PERIPHERAL FACIAL-NERVE STUMP IMPROVES THE INNERVATION OF MIMETIC MUSCULATURE BY HYPOGLOSSAL MOTONEURONS

Surgical reconstruction of the facial nerve is common clinical practic e following destruction of the intracranial facial nerve. Delayed hypo glossal-facial anastomosis (HFA) is the procedure of choice, although the effect of delay on outcome remains unclear. To study the effect of delayed anastomosis on reinnervation, we sutured the proximal stump o f a freshly transected hypoglossal nerve of Wistar rats to the distal stump of the ipsilateral facial nerve, which had been transected 7-56 days earlier. Animals that had received HFA without delay served as th e control group. Forty days after HFA, horseradish peroxidase (HRP) wa s injected into the whisker pad; 2 days later, the animals were killed . Reinnervation was assessed by determining the proportion of labeled neuronal cell bodies in the brainstem. The control group had 68% reinn ervation of these muscles by hypoglossal neurons and had 32% reinnerva tion by facial neurons. When the distal facial nerve had been allowed to degenerate for 7 days before HFA, reinnervation of the hypoglossal nerve decreased to 54%, and reinnervation by the facial nerve increase d to 46%. However, after a delay of 10-56 days, the hypoglossal fracti on increased and stabilized at 77%, and the facial motoneuron fraction decreased to 23%. The presence of new neuromuscular junctions was con firmed by HRP labeling of motor end plates in vivo and by electromyogr aphy. We conclude that, under the conditions of hypoglossal-facial cro ssed nerve suture, the predegeneration of the distal stump of a transe cted facial nerve enhances the reinnervation of facial muscles by hypo glossal axonal sprouts. (C) 1997 Wiley-Liss, Inc.