Motor Evoked Potentials / Transcranial Magnetic Stimulation
Core concepts
Motor evoked potentials (MEPs) assess the corticospinal (motor) pathway. Transcranial magnetic stimulation (TMS) over the motor cortex elicits a motor evoked potential recorded from a target muscle; stimulation over the cervical or lumbar spine can help distinguish central from peripheral conduction. Key measures include motor threshold, latency, amplitude, and central motor conduction time (CMCT), calculated by subtracting the peripheral conduction time (from spinal stimulation) from the total cortical latency.
Understand safety: TMS is contraindicated with implanted electronic devices (pacemakers, cochlear implants, deep brain stimulators), intracranial metal, and in patients with epilepsy risk. Coil orientation and stimulus intensity affect the response. MEPs are sensitive to anesthesia and neuromuscular blockade, unlike SSEPs.
Traps
Questions mix up MEP (motor, corticospinal) with SSEP (sensory, dorsal column). Another trap is forgetting that CMCT requires a spinal stimulation site; a single cortical latency cannot isolate central conduction. Also watch for safety contraindications.
Worked example / rule of thumb
If total cortical latency to a hand muscle is 21 ms and peripheral (cervical) conduction time is 14 ms, CMCT = 21−14 = 7 ms. Rule of thumb: central conduction time is always a subtraction of peripheral from total-never a single latency.