Assessment of axonal excitability properties in two branches of the human facial nerve.

Background: Axonal excitability methods have an established role in determining the biophysical properties of human axons in the clinical setting. The translation and refinement of these techniques for application to the facial nerve is important for advancing the pathophysiological understanding of facial nerve disorders. Facial nerve disorders are common and debilitating, yet in most cases diagnosis is based on clinical judgment alone. The pathophysiology of most causes of facial palsy remains unclear.

New Method: Novel techniques for the acquisition of facial nerve excitability properties were developed based on anatomical and surgical landmarks for two facial nerve branches. Zygomatic branch stimulation with nasalis recording and marginal mandibular branch stimulation with depressor angularis oris (DAO) recording were used. Comparisons were made between the two branches and with the median nerve, and the relationship between gender, age and nerve properties was explored through subgroup analysis.

Results: A full set of recordings were obtained in all participants across a wide age range. 27 nasalis recordings and 19 DAO recordings were completed and analysed. The studies were well tolerated in all participants. Excitability parameters were found to be similar for both branches of the facial nerve.

Comparison With Existing Method: Axonal excitability has proven to be of significant value in the study of motor and sensory neuropathy, however previous experience with facial nerve techniques has been limited. This study establishes normative data and a consistent technique for the application of axonal excitability testing to the study of facial nerve properties.