Exteroceptive Suppression Periods in Jaw-Closing Muscles. Variability and Relation to Experimental Pain and Sustained Muscle Contraction

Abstract

The duration of the late exteroceptive suppression period (ES2) of temporal muscle EMG activity has been reported to be reduced in patients suffering from chronic tension-type headache. Methods of recording and analysing ES2 have varied between centers and reproducibility of results within subjects, although insufficiently studied, has generally been poor. ES2 was investigated in 30 healthy subjects, using a computerized technique of recording, rectifying and averaging the EMG signals. Hour to hour and week to week variations of ES2 durations were calculated, and the influence of pain during a cold pressor test and of sustained muscle contraction on ES2 durations was investigated. The intra-individual variation of ES2 durations was 16.0% from hour to hour and 20.7% from week to week. The inter-individual variation was 36.7%. The present method for analysis of ES2 periods proved to be reliable, as the intra-observer variation was 4.2% and the inter-observer variation 4.6%. ES2 periods were significantly shorter on the first compared to the second day of examination (p = 0.006) and during experimental pain (p = 0.0005). We recommend the use of the computerized average technique in future studies and caution against the dependence of results upon factors such as conditioning and pain.

A computer program for automated scoring of exteroceptive suppression periods

“…For example, the scoring and suppression parameters that best discriminates between tension-type headache disorders and other headache disorders. Gobel and Schoenen (1993) have suggested that an 80% reduction in EMG activity is the preferred criteria for identification ofthe onset ofa suppression period, whereas Bendtsen et al (1993) have suggested that a 50% reduction in EMG activity is likely to be a better criterion. To date, however, no effort has been made to empirically compare different suppression criteria or different indices of suppression for their ability to identify diagnostic groups.…”

“…Although some have adopted an 80% reduction in EMG activity criterion for measuring suppression, baseline level of EMG activity has been calculated using both prestimulation (Wang, De Pasqua, Gerard, & Schoenen, 1995) and poststimulation muscle activity (Gobel et al, 1994). Finally, Bendtsen et al (1993), in an investigation ofES variability, found greater inter-and intrasubject ES variability among headache-free subjects when they defined exteroceptive suppression as a reduction below 80% baseline EMG activity, as compared with a 50% suppression criterion. These authors also reported a large number of absent ES2 responses in their "healthy" sample, suggesting that an 80% suppression criterion may be too stringent for identifying abnormal responses.…”

“…Attempts to calculate ES parameters, using automated and semi-automated scoring procedures, have been reported previously (Bendtsen et al, 1993(Bendtsen et al, , 1996van der Glas & van Steenberghe, 1981). …”

“…Furthermore, levels ofsuppression were not automatically expressed in percent change from prestimulus EMG levels, which has since become the recommended methodology (Gobel & Schoenen, 1993) to facilitate comparisons across research sites. Bendtsen et al (1993Bendtsen et al ( , 1996 describe a semi-automated scoring program that generates an ES waveform from raw EMG data and superimposes horizontal suppression threshold lines representing 50% and 80% of mean prestimulation EMG activity. In order to calculate onset and duration of ES periods using this method, the experimenter must manually place vertical cursors at points on the tracing where the waveform crosses the threshold lines.…”

“…Manual scoring may be required for poorly recorded or otherwise unusual recordings. This procedure essentially duplicates the program described by Bendtsen et al (1993).…”