Improved identification of dystonic cervical muscles via abnormal muscle activity during isometric contractions

The preferred treatment for cervical dystonia (CD) is injection of botulinum toxin in the dystonic muscles. Unfortunately, in the absence of reliable diagnostic methods it can be difficult to discriminate dystonic muscles from healthy muscles acting in compensation. We investigated if dystonic muscle activation patterns could be identified in cervical dystonia patients during a harmonized isometric contraction task. Furthermore, we investigated whether dystonia worsens at higher levels of voluntary contraction, which might further improve the identification of dystonic muscle activity.

An isometric device was used to investigate muscle activation during voluntary contraction tasks in 10 controls and 10 CD patients. Surface electromyography (EMG) of the sternocleidomastoidus, splenius capitis, and semispinalis capitis muscles was evaluated during a rest task and when performing submaximal (20%) and maximal voluntary contractions for eight head transversal force directions and for head twist. Two measures were developed to identify dystonic activation: 1) Muscle activity in the contraction direction in which the contribution of the muscle was lowest (Minimum EMG), and 2) the average muscle activity over all contraction directions (Total Mean EMG).

Patients showed increased dystonic activity in the rest task and during submaximal contractions relative to controls, but not during maximal contractions. Increases in Minimum EMG indicated an inability of patients to deactivate dystonic muscles counteracting the task. Increases in Total Mean EMG indicated dystonic activity in all task directions. During maximal contractions these effects were absent in dystonic muscles. Dystonia is therefore found not to worsen at higher levels of isometric voluntary contraction. The activity of dystonic muscles modulated with different loading directions similar to controls. Using Minimum EMG 54% of the muscles clinically diagnosed as dystonic and 91% of non-dystonic muscles were predicted correctly.

Dystonic muscle activity was found in cervical dystonia patients during submaximal contractions in all task directions using a harmonized isometric task, but no differences were found during maximal contractions. With some adaptation this method may prove useful to identify dystonic muscles.