Background: Patients with motor conversion disorders present an inability to execute movements voluntarily, although central and peripheral motor pathways are normal. We speculated that this phenomenon could be due to an abnormal loss of excitatory drive on central motor areas.
Methods: The effect of motor imagery on motor excitability was tested in a group of eight patients with a functional (psychogenic) hemiparesis and in an age-matched control group (n=8) by using transcranial magnetic stimulation (TMS). TMS pulses were applied at rest and while subjects imagined a tonic index finger adduction. Amplitudes of motor evoked potentials (MEP) recorded from the first dorsal interosseous muscle were used as an indicator of motor excitability.
Results: Motor thresholds and MEP amplitudes at rest were almost identical in the patient group and the control group. During motor imagery, MEP amplitudes increased by 200% in healthy subjects. In the patient group, motor imagery of the "paretic" index finger was associated with a mean MEP amplitude decrease of 37% compared with the MEP size at rest. Motor imagery of the unaffected index finger increased MEPs by 63% which was significantly different both from results in the control group and the "paretic" side.
Conclusions: We suggest that the paradoxical decrease of motor excitability during motor imagery is the electrophysiological correlate of a disturbed voluntary control in motor conversion disorder. The results further indicate that this abnormality is not restricted to the clinically affected body part.