Double innervation occurs in the facial mimetic muscles after facial-hypoglossal end-to-side neural repair: rat model for neural supercharge concept

Abstract

Double innervation of facial mimetic muscles by both facial and hypoglossal nerves after end-to-side neurorrhaphy has not been proven, although facial -hypoglossal end-to-side neurorrhaphy has been used in persistent incomplete facial palsy recently, and has achieved clinical evidences of recovery with rare synkinesis. We established a rat model to compare synkinesis after end-to-end and end-to-side neurorrhaphy techniques between facial and hypoglossal nerves, and confirmed double innervation using retrograde tracers. Rats were divided into three groups (each consisting of six rats), a facial palsy group (Group A), a facial-hypoglossal end-to-end neurorrhaphy group (Group B), and a facial-hypoglossal end-to-side neurorrhaphy group (Group C). Eight weeks after surgery, synkinesis of the facial mimetic muscles was observed and recorded via video camera. In Group C, post operative, intramuscular injections of retrograde neural tracers (Fast Blue, Diamidino Yellow and DiI) into the facial mimetic muscles were performed to prove double innervation by both the facial and hypoglossal nerves. In Group B, all rats showed facial palsy. However while eating and drinking, their half of the face showed mass movements (strong contraction of whisker pad muscles, curved nose and eye-closure). In Group C, four rats showed no significant changes however, two rats showed synkinesis of the eyelid while eating and drinking (frequent eye-closure distinguishable from the contralateral normal side). In Group C, retrograde tracers injected in the mimetic muscles were detected in both the facial and hypoglossal motor nuclei in situ of all the rats' brain stem. This study proved that double innervation of mimetic muscles by both facial and hypoglossal nerves occurs after the end-to-side neurorrhaphy. Double-innervated mimetic muscles around the mouth after hypoglossal-facial end-to-side neurorrhaphy showed less synkinesis than the end-to-end neurorrhaphy.