Sternocleidomastoid muscle responses to transcranial magnetic stimulation in patients with cervical dystonia

Ten cervical dystonia (CD) patients, with involuntary head rotation to one side and contralateral sternocleidomastoid muscle (SCM) hypertrophy, were investigated with transcranial magnetic stimulation, and the results were compared to those of 10 healthy subjects. Monopolar needle electrodes with isolated shafts were used for bilateral electromyographic recordings in the SCMs of the motor evoked potentials (MEPs) elicited by the magnetic stimulator. The latencies of ipsilateral SCM MEPs were shorter in the CD patients than in the control subjects (P < 0.001). The latencies of SCM activity suppression by TMS were longer in the CD patients than in the control group when stimuli were given on the contralateral side (P < 0.05). Both the clinically dystonic and the contralateral SCM of the CD patients exhibited significantly abnormal latencies of the ipsilateral SCM MEPs (P < 0.01) and of the SCM suppression (P < 0.05). Three CD patients also had consistent activity in the SCM counteracting the direction of head rotation during the suppression experiments. The latencies of the suppression of this abnormal activation were shorter (P < 0.05), than the latencies of the suppression in the SCM during normal voluntary activation by these CD patients (i.e. rotation of the head in the contrary direction). The results suggest bilaterally enhanced motoneuronal excitability and disturbed inhibitory regulation in patients with CD.

Prognostication of Bell's palsy using transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) provides a method to noninvasive excitation of the facial nerve in its intracranial segment close to the internal acoustic meatus. Thus, the site of facial nerve activation with TMS is proximal to or within the site of the lesion in Bell's palsy. To evaluate the prognostic capability of TMS in unilateral Bell's palsy we examined 137 patients with this method, and compared the results with electroneuronography (ENoG). Within 0-4 days from the onset of palsy, the patients with elicitable TMS responses recovered better than those in whom TMS responses were not elicitable. If TMS was performed 5-9 days or 10-28 days after the onset of palsy, it did not provide any prognostic information. Based on amplitude side-to-side differences, ENoG did not contribute prognostic information during the first 9 days from the onset of palsy. Later on, 10-28 days after the onset of palsy, ENoG showed an increased capability to discriminate the patients with poor prognosis. Thus, elicitable facial motor response with TMS predicts good prognosis of Bell's palsy at an early stage whereas poor response with ENoG predicts less favorable prognosis at a later stage.

Activation and suppression of the sternocleidomastoid muscle induced by transcranial magnetic stimulation

Responses in the stemocleidomastoid muscle (SCM) induced by transcranial magnetic stimulation (TMS) were investigated in 10 healthy subjects. Stimuli were given with the Dantec MagLiteTM magnetic stimulator using a 12.5 cm circular coil with counter-clockwise current direction. Monopolar needle electrodes with isolated shafts were used for simultaneous bilateral electromyographic (EMG) recordings of the SCM. TMS given on either side invariably induced an ipsilateral motor evoked potential of the SCM (SCM-MEP), whereas a contralateral SCM-MEP was just seen in 25% of the performed stimulation series also when maximal intensity was used. The SCM-MEPs recorded ipsilaterally to the side of stimulation had significantly higher maximal amplitudes (P < 0.05) compared to the SCM-MEPs recorded contralaterally (mean +/- S.D.: 1.0 +/- 0.5 and 0.2 +/- 0.1 mV, respectively). These results support the concept of a predominantly ipsilateral control of SCM activation. The contralateral SCM-MEPs tended to have a shorter latency than the ipsilateral SCM-MEPs. The thresholds of the ipsilateral SCM-MEPs were significantly higher (P < 0.01) on the left side than on the right side (mean +/- S.D.: 78 +/- 18 and 60 +/- 16 A/microseconds, respectively), which could be due to the consistent use of counter-clockwise coil current direction. TMS given on either side induced suppression of voluntary SCM activity in all investigated subjects. Responses induced by TMS given ipsilaterally and contralaterally to the voluntary activated muscle did not differ significantly (P > or = 0.05) regarding threshold or latency of the suppression.

The noise level in magnetic stimulation

The noise generated by stimulating coils may jeopardize the hearing of the patients as well as the hearing of the examiner. To evaluate the potential risk caused by the impulse noise of stimulating coils, we examined the A-weighted peak sound pressure levels from five different types of magnetic stimulator coils. At a distance of 10 cm, with 100% stimulation intensity, the coils with Dantec and Magstim stimulators created maximum peak sound pressure levels of 110 dB. Correspondingly, Cadwell MES-10 created maximum peak sound pressure levels of 132 dB. The decrease in the peak levels followed the distance rule quite closely. At a distance of 40 cm, the decrease in peak level was on average 14 dB (range -1-(+)1 dB). Based on American Conference of Governmental Industrial Hygienists (ACGIH) threshold limits of impact noise, the permitted maximum daily number of magnetic stimuli would be 1000 to 10,000. The permitted number of daily stimuli may be difficult to exceed in clinical practice. We consider the risk as small for the patients that are being examined and the operator using magnetic stimulation. The potential risk can be further diminished by even very light weighted hearing protectors providing proper attenuation to the coil impulses.

A comparison of transcranial magnetic stimulation with electroneuronography as a predictive test in patients with Bell's palsy

The aim of this study was to examine the neuronographic findings of electrical and transcranial magnetic stimulation of the facial nerve and to compare their ability to predict clinical recovery from idiopathic facial nerve palsy (Bell's palsy). Eighty-six patients were examined clinically and neurophysiologically immediately on presentation to Tampere University Hospital. Electroneuronography (ENoG) and transcranial magnetic stimulation (TMS) were performed 1-6 times for each patient. The time interval between each examination varied from 2 to 7 days. Seventy-eight patients were followed for a median period of 13 months after the onset of palsy. Facial nerve function was graded according to the House-Brackmann grading system. Relative amplitude differences of ENoG and TMS during the acute phase were then correlated with clinical outcome. Statistical analysis of the results showed that a TMS response elicitable during the first 5 days of the palsy was correlatable with a good prognosis. ENoG results correlated with clinical outcome at a later time from onset of symptoms. TMS was well tolerated and no adverse effects were seen. These results indicate that TMS is a useful method for the early prediction of outcome in patients with Bell's palsy.

Origin of the facial long latency responses elicited by magnetic stimulation

With magnetic stimulation (MS) it is possible to elicit bilateral long latency facial motor responses (LLRs). Due to a relatively wide magnetic field, the site of neural activation may take place in many different structures. The purpose of this study was to determine the site of origin of facial LLRs. The motor long latency responses were recorded bilaterally on the naso-labial folds (NLFs) with reference electrodes on the nose, and on some subjects also with reference electrodes on the chin. The stimulating coil was placed in the right parietal area. LLRs obtained with MS were compared to LLRs elicited electrically at the right stylomastoid foramen, supraorbital foramen, as well as cutaneous sensory area V1 of the trigeminal nerve. In addition, right sided high intensity electrical stimuli, paired magnetic stimulation and electrical stimulation with interstimulus intervals ranging from 0 to 80 msec were also applied for comparison. LLRs recorded with reference to the nose were always elicitable with MS as well as with the other stimulation procedures. The responses elicited with MS did not differ from those elicited electrically at various extracranial stimulation sites. With paired stimuli the second LLRs were inhibited by the preceding stimulation, whether given magnetically or electrically. In subjects with elicitable LLRs with chin references, the responses were always bilateral. Based on the similar characteristics with extracranial electrical stimuli, bilateral distribution of the responses, and inhibition of the second response with paired stimuli, it is concluded that the neural origin of LLRs to MS is in the extracranial trigeminal or facial nerve branches.

The site of impulse generation in transcranial magnetic stimulation of the facial nerve

The facial nerve can be stimulated in its intracranial course through transcranial magnetic stimulation (TMS). We studied the site of impulse generation produced by TMS by comparing the latencies of the muscle evoked potentials (MEPs) elicited with TMS and intracranial electrical stimulation (IES) of the facial nerve during neurosurgical posterior fossa procedures. In a series of 25 patients, the mean latency of the TMS elicited MEPs, recorded in the orbicularis oris muscle, was 5.0 ms (SD 0.58). Also IES of the distal part of the facial nerve in the internal acoustic meatus showed a mean latency of 5.0 ms (SD 0.68). Proximal IES in the root entry zone of the facial nerve, and intermediate IES between root entry zone and meatus, produced MEPs with significantly longer latencies compared to TMS and distal IES (p < 0.05). The findings suggest that the TMS induced facial nerve activation, leading to a MEP response, takes place within the internal acoustic meatus.

Magnetic facial nerve stimulation in Bell's palsy

The transcranial magnetic stimulation (TMS) technique makes it possible to stimulate the intracranial part of the facial nerve. In a total of 51 patients with acute Bell's palsy, TMS was performed, and the responses were compared with those elicited by conventional extracranial electric stimulation (EES). Clinical recovery was evaluated at 258-539, mean 410, days from the beginning of the palsy. With both techniques the motor evoked potentials (MEPs) could always be elicited on the healthy side, the mean latency being 4.7 ms with TMS and 3.7 ms with EES. In the acute phase, TMS elicited MEPs on the paralyzed side in 47% of the patients, and EES in 98%. The patients with TMS elicitable MEPs during the first 4 days of the palsy had significantly better recovery than those without response (p less than 0.05). The difference in recovery between patients with or without elicitable TMS responses on days 5-8 and 9-14 was not significant. In EES, the amplitude difference between the two sides within the first 4 days was not significantly (p greater than 0.05) different. On days 9-14 the patients with a less than 80% difference between the two sides recovered significantly (p less than 0.05) better than those with a difference of greater than or equal to 80%, So, TMS may be of help in the early prognosis of Bell's palsy.

Magnetic facial nerve stimulation in normal subjects. Three groups of responses

Magnetic stimulation provides a method to stimulate the facial nerve transcranially. With this method, the stimulation can be directed to the intracranial part of the facial nerve, whereas conventional electric stimuli are delivered to a more peripheral part of the nerve. In 40 healthy subjects, ipsilateral responses with latencies of 4.5 +/- 0.4 ms were recorded on the nasolabial folds. The latencies were 1.1 ms longer than those elicited at the stylomastoid foramen by electric stimulation. Furthermore, a response with a mean latency of 12 ms (range 10-16 ms) appeared in 6 out of 10 healthy subjects and a polyphasic response with a mean latency of 32 ms in 9 out of 10 of these subjects. Transcranial magnetic stimulation seems to allow the examination of motor conduction through the proximal part of the facial nerve. In addition, the method may give further information concerning the facial activation mechanisms possibly by other central pathways.

Transcranial facial nerve stimulation by magnetic stimulator in normal subjects

Magnetic stimulation provides a new method to stimulate facial nerve transcranially. Stimulation can be directed to the intracranial part of the facial nerve, whereas the conventional electric stimuli are delivered extracranially to a more peripheral part of the nerve. Fourty healthy volunteers were examined to determine the normal responses for transcranial facial nerve stimulation. The center of the inducing coil ring was located so that its center was 3 cm posterior and 6 cm lateral to the vertex. Responses were recorded on the nasolabial fold. Latencies were 4.5 +/- 0.4 ms on both sides, being 1.1 ms longer than those elicited by electric stimulation of the nerve at the stylomastoid foramen. Amplitudes with magnetic stimuli were equal to those obtained with electric stimuli. The transcranial magnetic stimulation seems to be an accurate and promising method to examine the facial nerve.