SUMMARY When the unaffected facial nerve was stimulated in 30 patients with facial palsy, evoked action potentials could be recorded from the contralateral (paralysed) perioral muscles. Similarly, in four normal subjects responses were evoked contralateral to the stimulated facial nerve. The latency of these responses in the patients remained unchanged over several months, and they were conducted at a rate compatible with conduction along muscle fibres. The contralateral responses are suggested to be due to conduction along muscle fibres crossing the midline rather than to cross-innervation.Reinnervation of perioral muscles from the contralateral facial nerve sufficient to produce visible movement, induced either voluntarily or by nerve stimulation, has been reported ( Lefebvre and Lerique, 1964;Wochnik-Dyjas, 1964;Passerini et al., 1968;Fisch et al., 1968;Gambi and Tonali, 1973;Kilimov, 1975). Although the observed activity disappeared after anaesthesia of the nonaffected facial nerve (Buchthal, 1957(Buchthal, , 1965Trojaborg and Siemssen, 1972) its basis has not been established with certainty.The purpose of this study was to determine whether the activity evoked either by voluntary effort or by stimulation of the unaffected nerve was due to conduction along nerve fibres or muscle fibres crossing the midline.
Patients and methodThirty patients with unilateral facial palsy, due in 12 to Bell's palsy, in 13 to removal of a malignant tumour of the parotid gland or an acoustic neuroma, and in five to trauma, were examined. Four subjects without history, signs, or symptoms of neuromuscular disorders were also included.The electromyogram during full effort was recorded photographically with a three channel electromyograph (DISA) The facial nerve was stimulated by needle electrodes with a bare tip of 3 mm. The near nerve electrode was placed at the stylomastoid foramen. Recording electrodes were placed symmetrically at either side of the midline. Muscle potentials were evoked by stimuli (0.2 ms duration) 4.5 times the threshold (Tf=0.9=40.1 mA, n=54*) delivered via a double-screened transformer (DISA ministim, type 14E01).An attempt was made to measure the propagation velocity of the muscle action potential along fibres of the perioral muscle. For this purpose muscle action potentials were recorded along the long axis of the orbicularis oris at different distances (10-60 mm) from the stimulating bifilar electrode placed near the corner of the mouth (see inset in Fig. 1). The action potentials recorded at threshold stimulation were mainly triphasic of 1.5-3 ms duration with a steep positive to negative deflection. With an increase in stimulus strength, the response increased in amplitude and new spikes appeared indicating activation of additional muscle fibres (Fig. 2, below, left).