Significance of trigeminal sensory input on regrowth of hypoglossal and facial motoneurons after hypoglossal facial anastomosis in rats

Hypoglossal facial anastomosis (HFA) is a standard surgical technique for r estoration of facial movements in cases of intratemporal lesions of the fac ial nerve. Case reports provide evidence that an affected trigeminal system reduces functional outcome. In order to detect morphological changes in th e hypoglossal nucleus responsible for this phenomenon, we used 18 Wistar ra ts and performed three different surgical combinations. In group 2, six ani mals received HFA only. In group 2, HFA was combined with resection of the contralateral infraorbital nerve. In group 3, HFA was combined with resecti on of the ipsilateral infraorbital nerve. Fifty-six days after the operatio n, 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 show ed 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 signi ficantly reduced in group 2 ( - 85%) and group 3 ( - 45%). These morphologi cal findings indicate an absent or insufficient functional connection betwe en the contralateral infraorbital nerve and the hypoglossal nucleus, and a strong influence of the infraorbital nerve to the ipsi-and contralateral fa cial nuclei. Additionally, our study provides morphological evidence that t he integrity of the sensory trigeminal system is very important in reconstr uctive facial nerve surgery.