Facial-hypoglossal Nerve End-to-side Anastomosis:Basic Mechanisms And Applications

Objective: The anatomical and histological study of extracranial facial nerve and hypoglossal nerve of rat was aimed to provide an advisable anatomy reference for rat models of facial-hypoglossal anastomosis(FHA).Studies were also performed to compare the efficacies of end-to-end(ETE)neurorrhaphy and end-to-side(ETS)neurorrhaphy with different damage degrees on the donor hypoglossal nerve,and explore mechanisms underlying axon regeneration following ETS neurorrhaphy.Methods:(1)The extracranial facial nerve and hypoglossal nerve were dissected and observed under microscope,measurement of anatomical parameters and histomorphometric analysis were performed.(2)Rats were divided into three groups: intact control group,ETE,and ETS.Nerve regeneration was assessed with vibrissae motor performance,electrophysiological tests,retrograde labeling,and histomorphological analysis at 4 and 8 months postoperatively.(3)Rats were treated by ETE,ETE using the hemi-sectioned and longitudinally splitted donor nerve(ETEHemi),and ETS through a perineurial window(ETS-PW)or 30-40% partial neurotomy(ETS-PN),respectively.Four additional rats were left intact for reference.At 8 months postoperatively,behavioral,electrophysiological,and morphological studies were carried out to compare the groups.Results:(1)The extracranial length of facial nerve trunk(5.94 ± 0.15 mm)was significantly greater than the length from the facial nerve just prior its bifurcation to the ipsilateral hypoglossal nerve(5.20 ± 0.20 mm).Both the total cross-sectional area and number of myelinated nerve fibers of facial nerve were greater than those of hypoglossal nerve.(2)Significantly higher numbers of myelinated axons and labeled neurons giving regenerating fibers were found in ETE compared with ETS at both time points,consistent with the functional and electrophysiological recovery.ETS had a very slight effect on donor nerve,as determined electrophysiologically and histomorphologically.Sparsely distributed double-labeled and relatively large amounts of single-labeled neurons contributing to reinnervation were found via double retrograde neuronal labeling in ETS.Further quantitative analysis showed an obvious decline tendency in the percentage of double-labled neurons.(3)All techniques resulted in partial functional recovery,but complete restoration was not obtained.There were no significant differences between the experimental groups in axon diameter and myelin thickness.The facial nerve fiber count after ETS-PN,but not after ETE-Hemi or ETS-PW,was comparable to that after ETE,which agreed with the behavioral and electrophysiological results.The hypoglossal nerve fiber count after ETS-PN was slightly less that after ETS-PW,but markedly more than that after ETE-Hemi,corresponding to the electrophysiological examination.Quantitative measures of the cross-sectional area and connective tissue area density of the tongue demonstrated severe muscle atrophy on the operated side after ETE and ETE-Hemi when compared to ETS-PW and ETS-PN.Conclusion: FHA in a rat model has a defect in diameter matching of the recipient and donor nerves,but can well meet the requirement of tension-free nerve suturing.Successful reinnervation after ETS could be achieved through both collateral sprouting and terminal sprouting,with the latter seeming to be the principal origin of motor nerve sprouting.ETS neurorrhaphy with 30-40% partial donor neurotomy offers the best balance between motor reinnervation and donor deficits.The method of nerve spit for provision of hemi-hypoglossal nerve stump may not effectively preserve the tongue function.