Bilateral small-hand-muscle motor evoked responses in a patient with congenital mirror movements

In a 56 year old patient with congenital mirror movements involving the muscles of the hands and forearms, transcranial electrical stimulation of either the left or right motor-cortex hand area induced bilateral surface electromyographic (EMG) responses in the abductor pollicis brevis (APB) muscles with normal latencies (21 msec). Slight volitional background contraction of the APB muscle contralateral to the stimulated motor cortex enhanced the motor evoked potential (MEP) and shortened the latency of the response by about 3 msec, while no definite facilitation was detected in the ipsilateral APB muscle. Volitional activation of the APB muscle ipsilateral to the stimulated motor cortex failed to enhance the response or shorten the latency of the response in either APB muscle. It is concluded that a fast conducting uncrossed or double crossed corticospinal system plays an important role in the execution of ipsilateral involuntary mirror movements and may be explained by aberrant reorganization of the pyramidal system.