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 practice following destruction of the intracranial facial nerve. Delayed hypoglossal-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 of 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 the control group. Forty days after HFA, horseradish peroxidase (HRP) was 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% reinnervation of these muscles by hypoglossal neurons and had 32% reinnervation 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 increased to 46%. However, after a delay of 10-56 days, the hypoglossal fraction increased and stabilized at 77%, and the facial motoneuron fraction decreased to 23%. The presence of new neuromuscular junctions was confirmed by HRP labeling of motor end plates in vivo and by electromyography. We conclude that, under the conditions of hypoglossal-facial crossed nerve suture, the predegeneration of the distal stump of a transected facial nerve enhances the reinnervation of facial muscles by hypoglossal axonal sprouts.