Abnormal pain processing in chronic tension-type headache: a high-density EEG brain mapping study.

Central sensitization caused by prolonged nociceptive input from muscles is considered to play an important role for chronification of tension-type headache. In the present study we used a new high-density EEG brain mapping technique to investigate spatiotemporal aspects of brain activity in response to muscle pain in 19 patients with chronic tension-type headache (CTTH) and 19 healthy, age- and sex-matched controls. Intramuscular electrical stimuli (single and train of five pulses delivered at 2 Hz) were applied to the trapezius muscle and somatosensory evoked potentials were recorded with 128-channel EEG both in- and outside a condition with induced tonic neck/shoulder muscle pain (glutamate injection into the trapezius muscle). Significant reduction in magnitude during and after induced tonic muscle pain was found in controls at the P200 dipole in response to both the first (baseline versus tonic muscle pain: P = 0.001; baseline versus post-tonic muscle pain: P = 0.002) and fifth (baseline versus tonic muscle pain: P = 0.04; baseline versus post-tonic muscle pain: P = 0.04) stimulus in the train. In contrast, there were no differences between the conditions in patients. No consistent difference was found in localization or peak latency of the dipoles. The reduction in magnitude during and after induced tonic muscle pain in controls but not in patients with CTTH may be explained by impaired inhibition of the nociceptive input in these patients. This may be the first evidence that the supraspinal response to muscle pain is abnormal in patients with CTTH.