A . Romaniello et al. Brain Research 882 2000 120 –127
121
nial magnetic stimulation TMS. This technique is now EMG activity remained within the window for more than
widely used to assess motor pathways both in healthy 400 ms, the program automatically triggered the magnetic
subjects and in patients [3,6,31,40]. In the trigeminal stimulator MagLite-r25, Dantec, Denmark [37]. The
system TMS provides an indirect measure of the central subjects were asked to maintain the clenching level for
excitability of the masticatory system including motor about 2–3 s after each stimulus. The inter-stimulus interval
cortex and corticobulbar connections [8,25,27]. was 10–15 s. The duration of EMG activity recorded was
The aim of the present study was to investigate the 300 ms and included pre-stimulus 100 ms and post-
modulation of trigeminal motor pathways during tonic stimulus 200 ms periods.
muscle and skin pain. Moreover, since the recording of Transcranial magnetic stimulation was performed with
trigeminal MEPs requires voluntary activation of the the MagLite-r25 and a circular coil 140 mm diameter;
masticatory muscles the importance of clenching levels peak magnetic field: 1.9 T. The coil was placed over the
was determined in each experimental condition. Finally, midline, 3–4 cm anterior to the vertex with the current
different levels of motor cortical activation were examined flowing clockwise. After finding the optimal stimulation
using two different stimulus intensities 1.1 and 1.5 times site on the scalp, it was marked with a dark pen and the
the motor threshold. coil was fixed in a stable position, so that the same position
was kept during the whole experiment. The subject was asked to keep his head still and the position of the coil was
checked after each trial. The motor threshold Th was
2. Materials and methods
measured while the subject was clenching their teeth at about 30 of maximum. The threshold was determined by
2.1. Subjects descending and ascending methods and was defined as the
minimum stimulus intensity that produced five discrete A total of 17 healthy subjects 12 men and 5 women
MEPs in both muscles, with peak-to-peak amplitude of at aged 21–42 years mean age6S.E.M.: 26.261.7 partici-
least 0.10 mV, discernible visually on the monitor from ten pated. The subjects had no history of temporomandibular
consecutive stimuli. The mean Th, measured at 30 MVC, disorders or orofacial pain and they did not take any
was 48.161.4 of the maximum output of the magnetic medications. All the subjects fulfilled the inclusion criteria
stimulator. Low-intensity stimuli 1.13Th had a mean for magnetic stimulation of neural tissue [28]. Informed
intensity of 54.661.6; the high intensity 1.53Th had a consent was obtained prior to the study in accordance with
mean intensity of 70.261.8. the guidelines of the Helsinki Declaration. The Local
Ethics Committee had approved the study. 2.3. Experimental pain
2.2. Recording of MEPs Tonic muscle pain was induced by infusion of sterile
The subjects sat upright and relaxed in a dental chair 5.8 hypertonic saline into the deep mid portion of the left
with the head supported by a headrest. Electromyographic masseter muscle. The saline injection was given over a
activity EMG was bilaterally recorded with bipolar 20-s period followed by a steady infusion rate of 6 ml h
surface electrodes
Neuroline, Medicotest,
Denmark for the next 440 s and finally 9 ml h for the following 440
placed on the central part of the masseter muscles along s with the use a computer-controlled syringe pump IVAC,
the main direction of muscle fibers, with an inter-electrode Model 770, USA and a 10-ml plastic syringe [37]. A tube
distance of 2 cm. The disposable surface electrodes had an IVAC, G30303, USA connected the syringe to a dispos-
active recording area of 437 mm. The EMG signals were able needle 27G, 20 mm that was inserted into the left
amplified, filtered 20 Hz–1 kHz, sampled at 4 kHz masseter muscle. Tonic skin pain was induced by capsaicin
2
Counterpoint MK2, Dantec, Denmark and stored on disk 1 ml, 500 mg ml applied topically in a plaster of 4-cm
for off-line analysis. Initially, the EMG activity corre- area on the left cheek. This procedure has previously been
sponding to maximum voluntary contraction MVC of the shown to cause a steady, burning type of pain [30]. In both
masseter muscles was determined while the subjects were experiments subjects were instructed in the use of a 10-cm
biting in the intercuspal teeth position. The EMG level electronic visual analogue scale VAS with the lower
corresponding to the MVC calculated at the start of the extreme labeled ‘no pain’ and the upper extreme labeled
experiment was used to set a window 610 around ‘most pain imaginable’. The VAS score was sampled every
three different clenching levels: 15 13.5–16.5, 30 5 s and stored on a computer. The mean VAS score was
27–33 and 45 40.5–49.5 MVC. The subjects calculated in 500-s intervals in which the subject felt
received visual feedback from markers on the computer constant level of pain. After the infusion or the removal of
screen indicating clearly when the level was in the pre- the plaster, the subjects described the quality of pain using
defined interval. The EMG activity recorded from the left a Danish version of the McGill Pain Questionnaire MPQ
masseter muscle served as the feedback. When the ongoing [13].
122 A
2.4. Protocol 3. Results
The experiment was performed in two separate sessions. 3.1. Experimental pain
In the first session the effect of muscle pain was examined and in the second session skin pain was studied. Each
The mean amount of hypertonic saline infused into the experiment was performed in ten subjects; three subjects
left masseter muscle was 2.260.3 ml. The infusion caused participated in both sessions. The recording procedure was
a local sensation of ‘intense’ nine subjects out of ten pain the same on both occasions: MEPs were recorded at three
from the masseter muscle, with a spread toward the upper clenching levels 15, 30 and 45 MVC and for each
or lower molar teeth and the temporomandibular joint clenching level two stimulus intensities were used: 1.1
3 10. The mean pain intensity on the VAS was 5.460.3 low intensity5L and 1.5 high intensity5H times the
cm. Topical application of capsaicin produced a local motor threshold. With this paradigm, MEPs were measured
painful sensation described as ‘burning’ 7 10. Sponta- prior to the application of experimental pain baseline,
neous pain was located in the capsaicin-treated area only. during pain and 20 min after pain had disappeared post-
The mean pain intensity on the VAS was 2.960.2 cm. baseline. MEPs were recorded when the pain intensity
was constant. In three subjects, attempts were made to 3.2. Motor evoked responses
obtain MEPs in the three conditions with the jaw-closing muscles at rest 0 MVC. The sequence of clenching
During clenching, MEPs were obtained bilaterally in the levels and stimulus intensities was randomized. A total of
masseter, in all subjects. No responses were detected with 16 EMG sweeps were recorded in each trial and averaged
the jaw muscles at rest. The MEPs appeared as biphasic off-line. The onset-latency and peak-to-peak amplitude
and reproducible responses within individuals Fig. 1. were measured on the non-rectified, averaged MEPs. The
Furthermore, the MEPs were symmetrical and the overall root-mean-square RMS amplitude of the 100 ms preced-
four-way ANOVA showed no significant differences in ing the magnetic stimulus RMS pre was measured from
latency or amplitude between the two sides. The central rectified averaged signals.
position of the coil prevented stimulation of the trigeminal root and no early responses were observed on either side
[8]. 2.5. Statistical analysis
3.3. Effect of tonic pain Mean values6S.E.M. are given in the text, tables and
figures. The onset latencies, the amplitudes of the MEPs Neither muscle pain nor skin pain affected the MEPs
and the pre-stimulus EMG activity of both left and right Figs. 2 and 3. For all clenching levels and stimulus
masseter were compared using analysis of variance intensities, latency and amplitude did not change with
ANOVA with four repeated factors: muscles two levels: muscle pain F 2,950.19; P.0.50 F 2,950.19; P.
painful and non-painful side; clenching level three levels: 0.50 or skin pain F 2,950.08; P.0.50 F 2,950.30;
15, 30 and 45 of MVC; stimulus intensity two levels: P.0.50. Pre-stimulus EMG activity remained constant
low and high intensity; conditions three levels: baseline, with skin pain F 2,950.05; P50.93. During and after
pain, post-baseline. Post-hoc Tukey tests were performed muscle pain a significant asymmetry of the pre-stimulus
to adjust for multiple pair-wise comparisons. Significance EMG activity was observed due to a relative increase in
was set at P,0.05 for all the analyses. the pre-stimulus EMG activity in the right masseter non-
Fig. 1. Example of MEP recordings in one subject in the baseline condition from the left MAL and right masseter muscles MAR at different clenching levels 15, 30, 45 and rest, during low 1.1 and high intensity 1.5 stimulation. The increase in the clenching level and the stimulus intensity produced
a clear increase in amplitude and tended to shorten the onset-latency. No MEPs could be recorded at rest.
A . Romaniello et al. Brain Research 882 2000 120 –127
123
Fig. 2. Effects of different conditions j Baseline, m Pain and d Post-Baseline on the MEP latency at different clenching levels 15, 30 and 45 and stimulus intensity ——— Low and — — — High. Experimental tonic pain had no statistically significant effects on the onset-latency muscle pain:
P.0.50; skin pain: P.0.50. The increase of the clenching level and the stimulus intensity tended to shorten the onset latency P.0.05 and P50.11, respectively.
painful side F 2,9518.3; P,0.01, at 30 and 45 latency shortened by less than 1 ms; this change, however,
MVC Fig. 4. Since the aim of the study was to record the did not reach statistical significance F 2,953.4; P5
masseter MEPs in a standardized condition, the left 0.11. The stimulus intensity did not influence the pre-
masseter painful side was used to provide feedback for stimulus EMG activity F 2,951.4; P.0.50 Fig. 4.
the optimal control of the clenching level. For this reason no modulation of the pre-stimulus EMG activity in the left
masseter could be detected.
4. Discussion
