Supramaximal responses can be elicited in hand muscles by magnetic stimulation of the cervical motor roots

BackgroundThe amplitude of compound muscle action potentials (CMAPs) evoked in response to magnetic cervical motor root stimulation (MRS) has rarely been used as a diagnostic parameter because of the difficulty in obtaining supramaximal CMAPs.ObjectiveTo clarify whether supramaximal CMAPs could be elicited by MRS, and if so, whether their amplitude and area could be used to evaluate the conduction of proximal motor roots.MethodWith the use of a custom-made high-power magnetic stimulator, the CMAPs evoked in response to MRS of the first dorsal interosseous, abductor digiti minimi, and abductor pollicis brevis (APB) muscles were compared with those evoked by electrical stimulation at the wrist, brachial plexus, and cervical motor roots. The collision technique was also used to exclude volume conduction. The correlation between MRS-induced CMAP latency and body height was evaluated.ResultsIn 32 of 36 normal subjects, supramaximal CMAPs were obtained in response to MRS. The size of CMAPs occurring in response to MRS was the same as the size of those occurring in response to high-voltage electrical cervical motor root stimulation. The collision technique revealed that the APB muscle was highly contaminated by volume conduction from adjacent muscles. CMAP latency correlated significantly with body height.ConclusionsSupramaximal CMAPs can be obtained in most normal subjects. In subjects exhibiting confirmed supramaximal CMAPs in response to MRS, not only the latency of these CMAPs but also their amplitude and area can be clinically useful, excluding CMAPs in the APB muscle.