Effect of experimental pain from trigeminal muscle and skin on motor cortex excitability in humans

The pathophysiology of many orofacial pain syndromes is still unclear. We i nvestigated the effect of tonic muscle and skin pain on the excitability of the trigeminal motor pathways using transcranial magnetic stimulation (TMS ). Motor evoked potentials (MEPs) were recorded in the masseter surface ele ctromyogram (EMG). Magnetic pulses were delivered with a large coil at inte nsities 1.1 and 1.5 times the motor threshold, and for each intensity, MEPs were recorded at three different clenching levels: 15, 30 and 45% of maxim um voluntary contraction (MVC). Baseline, pain and post-baseline recordings were compared in two sessions. Firstly, muscle pain was induced by infusio n of hypertonic saline (5.8%) into the left masseter. Secondly, skin pain w as induced by topical application of capsaicin (5%) on the left cheek. Musc le and skin pain did not induce significant effects on the amplitude or lat ency of the MEPs (ANOVAs: P>0.50). In both sessions, the amplitude of the M EPs increased with the increase of the clenching level and stimulus intensi ty (P<0.0001; P<0.005) whereas the latency was not significantly changed (P >0.05; P=0.11). Muscle pain was associated with an increase in the pre-stim ulus EMG activity on the non-painful side compared with baseline (P<0.01), which could be due to compensatory changes in the activation of the painful muscle. The need for voluntary contraction to evoke MEPs in the masseter m uscles and compensatory mechanisms both at the brainstem and cortical level might explain the lack of detectable modulation of MEPs. Nonetheless, the present findings did not support the so-called 'vicious cycle' between pain - central hyperexcitability - muscle hyperactivity. (C) 2000 Elsevier Scie nce B.V. All rights reserved.