Change in facial nerve innervation following hypoglossal-facial anastomosis. An animal study.

Hypoglossal-facial nerve anastomosis was carried out in 20 adult guinea pigs. Horseradish peroxidase (HRP) was applied to the buccal branch of the facial nerve at intervals of 2, 4 and 6 months after the anastomosis operation. HRP labeled neurons and the number of regenerated axons in the buccal branch were counted to determine postoperative nerve regeneration. The number of surviving neurons in the facial and hypoglossal nuclei was also counted to determine postoperative change of these nuclei. Following anastomosis, 97% of the hypoglossal neurons remained surviving, while the facial neurons underwent pronounced degeneration of 65% survival ratio at 2 months and 37% at 6 months after the anastomosis operation. In 80% of the animals, a new nerve bundle regenerated from the proximal stump of the facial nerve to the anastomotic trunk. A linear increase of HRP-labeled neurons in the facial and hypoglossal nuclei paralleled the increase of the axons in the buccal branch. The HRP-labeled neurons in the facial nucleus were demonstrated to have direct connection with the newly formed bundle and the facial mimetic muscles were dually innervated by both the hypoglossal and facial nerves. Although the present study design might not fully represent the clinical situation, possible advantages of hypoglossal-facial nerve anastomosis are discussed from the view of nerve regeneration.