Objective: To quantify the percentage of motor units of a foot muscle that can be activated by transcranial magnetic stimulation (TMS) in normal subjects and patients.
Methods: We adapted the recently described triple stimulation technique (TST) for recordings from abductor hallucis (AH). Conventional motor evoked potentials (MEPs) of this muscle are usually small and variable in shape, because of an important temporal desynchronization of the TMS induced spinal motor neuron discharges. The TST allows 'resynchronization' of these discharges and thereby a quantification of the proportion of motor units activated by TMS. The lower limb (LL-) TST was applied to 33 sides of 18 normal subjects and 51 sides of 46 patients with multiple sclerosis, amyotrophic lateral sclerosis, or spinal cord disorders.
Results: In healthy subjects, the LL-TST demonstrated that TMS achieves activation of virtually all motor neurons supplying the AH. In 33 of 51 patient sides, abnormal LL-TST responses suggested corticospinal conduction failures of various degrees. The LL-TST was 2.54 times more sensitive to detect central conduction failures than the conventional LL-MEPs. Combining the LL-TST with TST of the upper limbs further increased the sensitivity to detect a conduction failure by 1.50 times.
Conclusion: The LL-TST markedly improves the examination of corticospinal pathways.