What does MEP stand for?

MEP stands for the “motor-evoked potentials” testing procedure, which is frequently used in the diagnosis of ALS. In ALS, nerve cells in the spinal cord (second motor neuron) and in the motor cortex of the brain (first motor neuron) are damaged. The MEP test is used to detect damage to the first motor neuron.

The examination is performed using a magnetic coil (which resembles a conductor’s baton) that the examiner places on a specific area of the head where the motor areas of the brain are located. The coil is connected to a stimulation device that generates an electromagnetic pulse inside the coil and transmits a minimal pulse to the nervous tissue (through the skin and the skull). This impulse is transmitted through the brain’s nerve pathways to the arms and legs. There, the stimulation signal is recorded using special electrodes. Damage to the first motor neuron can weaken or slow the transmission of the electromagnetic pulse from the head to the measurement point on the arm or leg, and this can be detected using the MEP method.

MEP testing is used to supplement the clinical diagnosis and to objectively confirm involvement of the upper motor neuron. MEP findings do not allow for a prognosis. Furthermore, there are known forms of ALS in which the MEP findings are normal, even though a diagnosis of ALS has been made (ALS with predominant involvement of the second motor neuron).