Reflex excitability of facial motoneurons at onset of muscle reinnervationafter facial nerve palsy

We studied 18 patients with complete unilateral denervation of the facial m uscles after idiopathic facial nerve palsy to determine whether motoneurona l excitability is enhanced in the few motor units that are active at onset of muscle reinnervation, The study was carried out between 75 and 90 days a fter the facial nerve lesion. We used two needle electrodes to record simul taneously the spontaneous and voluntary activity of the orbicularis oris (O Oris) and orbicularis oculi (OOculi) muscles, as well as the responses to i psilateral and contralateral facial and supraorbital nerve stimuli. All pat ients showed involuntary firing of motor unit action potentials (MUAPs) in at least one of the muscles. Synkinetic activation of motor units in the OO ris was induced by spontaneous blinking in all patients, and by inhalation and swallowing in some. Electrical stimulation of the ipsilateral facial ne rve induced a direct M response in only 4 patients. In contrast, long-laten cy reflex responses were induced in both muscles by electrical stimulation of ipsilateral and contralateral facial and supraorbital nerves in all pati ents, at latencies ranging between 44 and 132 ms. The shape of such MUAP re flex responses was the same as that of the MUAPs seen to fire at rest. Thes e findings provide evidence of enhanced excitability of facial motoneurons in our patients. Such hyperexcitability may be partly responsible for the p ostparalytic motor dysfunction syndrome that occurs after facial palsy with severe axonal damage. (C) 1999 John Wiley & Sons, Inc.