Significance of Trigeminal Sensory Input on Regrowth of Hypoglossal and Facial Motoneurons after Hypoglossal Facial Anastomosis in Rats

Abstract

Hypoglossal facial anastomosis (HFA) is a standard surgical technique for restoration of facial movements in cases of intratemporal lesions of the facial nerve. Case reports provide evidence that an affected trigeminal system reduces functional outcome. In order to detect morphological changes in the hypoglossal nucleus responsible for this phenomenon, we used 18 Wistar rats and performed three different surgical combinations. In group 1, six animals received HFA only. In group 2, HFA was combined with resection of the contralateral infraorbital nerve. In group 3, HFA was combined with resection of the ipsilateral infraorbital nerve. Fifty-six days after the operation, horseradish peroxidase (HRP) was injected into the whisker pad. As shown in previous studies using HRP, retrograde-labelled motoneurons occurred in the hypoglossal and facial nuclei. Counts of the labelled motoneurons showed no change in the number of projecting hypoglossal motoneurons in group 2 when compared to HFA only, but a significantly smaller number in group 3 (-35%). Furthermore, the number of projecting facial motoneurons was significantly reduced in group 2 (-85%) and group 3 (-45%). These morphological findings indicate an absent or insufficient functional connection between the contralateral infraorbital nerve and the hypoglossal nucleus, and a strong influence of the infraorbital nerve to the ipsi- and contralateral facial nuclei. Additionally, our study provides morphological evidence that the integrity of the sensory trigeminal system is very important in reconstructive facial nerve surgery.