Motor evoked potentials (MEP)

Motor evoked potentials (MEP) explore the integrity of the corticospinal motor pathway (the pyramidal tract) using transcranial magnetic stimulation (TMS). A magnetic coil placed on the scalp generates a brief and powerful magnetic field that induces an electrical current in the motor cortex, activating the pyramidal neurons. The resulting muscle response is recorded by surface electrodes. The examination measures the central motor conduction time (CMCT), that is, the time needed for the nerve impulse to travel along the pyramidal tract from the motor cortex to the motor neurons of the spinal cord. A lengthening of the CMCT or an absence of response indicates a condition of the central motor pathway. MEP are complementary to SEP: while SEP explore the ascending sensory pathways, MEP assess the descending motor pathways. Together, they provide a complete functional mapping of the spinal cord. MEP are particularly useful in the diagnosis and follow-up of multiple sclerosis, myelopathies and motor neuron diseases. The Clinique Pasteur in Tunis has a latest-generation transcranial magnetic stimulator, allowing focal and reproducible stimulation. The examination is non-invasive, painless and without side effects in the vast majority of cases.