Frequency analysis of EMG activity in patients with idiopathic torticollis

The pathophysiology of idiopathic dystonic torticollis is unclear and there is no simple test that confirms the diagnosis and excludes a psychogenic or voluntary torticollis in individual patients. We recorded EMG activity in the sternocleidomastoid (SCM) and splenius capitis (SPL) muscles of eight patients with rotational torticollis and eight age-matched controls, and analysed the signals in the frequency and time domains. All control subjects but one showed a significant peak in the autospectrum of the SPL EMG at 10-12 Hz, which was absent in all patients with torticollis. Conversely, patients with torticollis had evidence of a 4-7 Hz drive to the SPL and SCM that was absent in coherence spectra from controls. The pooled cumulant density estimates revealed a peak in both groups, and within the patient group there was a second narrow subpeak with a width of 13 ms. The activity in the SCM and SPL was in phase in the patients but not in the controls. The lack of any phase difference and the suggestion of short-term synchronization between SCM and SPL are consistent with an abnormal corticoreticular and corticospinal drive in dystonic torticollis. Clinically, the pattern of SPL EMG autospectra and of SCM-SPL coherence may provide a sensitive and specific feature distinguishing dystonic from psychogenic torticollis.