Inadequacy of transcranial magnetic stimulation in the neurophysiologic assessment of Bell's palsy.

Transcranial magnetic stimulation is a non-invasive procedure which to stimulate the brain cortex and the peripheral nerve pathways. A new technique was recently introduced to record the muscle action potential of facial muscles by means of transcranial magnetic stimulation of the facial nerve. The experimental data that was obtained indicate that this technique allows to stimulate the facial nerve above the stylomastoid foramen: a greater tract of the nerve can therefore be explored than what was possible with the traditional electrical stimulation at the mastoid. Until now no comparison data was available on the clinical usefulness of the two methods. We decided to study 14 normal controls and 26 patients suffering from unilateral idiopathic facial palsy (Bell's palsy) and to submit these two groups to magnetic transcranial stimulation and electrical stimulation of the facial nerve in the mastoid region, to the purpose of observing where the nerve is stimulated by the magnetic impulse and which of the two techniques can be of accurate prognostic value in the study of the evolution of the clinical lesion. The electromyographic responses were elicited by the electrical stimulation at the mastoid and by transcranial stimulation after positioning the coil on the parieto-occipital scalp. A recording was taken from the ipsilateral orhicularis oculi muscle utilising two cupped electrodes. The latency and the amplitude of the compound muscle action potential were measured bilaterally in order to compare the results obtained on both the affected and the healthy sides. The patients were scheduled to two neurophysiological and clinical evaluations at about 30 days interval one from the other: the first test was not carried out before 20 days from the onset of the deficit; further clinical examination was carried out only 6 months later. The analysis of the results obtained in the normal controls submitted to transcranial magnetic stimulation indicate that the nerve is activated at the point where it originates from the brainstem. The study carried out showed that transcranial magnetic stimulation of the facial nerve, does not provide data which can be correlated to the clinical situation observed at the time of the study; furthermore, transcranial magnetic stimulation does not supply any prognostic data on the clinical evolution of the lesion.