In MEP stimulation, where should the anode be placed?

In motor evoked potential (MEP) stimulation, the ideal placement of the anode is rostral to the cathode. This configuration is important because it helps to enhance the stimulation efficacy. The cathode, which delivers the electrical stimulus, is typically positioned over the target area, such as the motor cortex, while the anode's rostral placement allows it to create a favorable electrical field that maximizes the excitability of the neurons.

Having the anode positioned rostrally encourages a more effective depolarization of the axons in the descending motor pathways. This positioning helps ensure that the action potential can propagate down the motor pathways, leading to better recording of MEPs.

The other placements for the anode, such as being at the same position, caudal, or distal to the cathode, would not create the same optimal conditions for exciting the neural structures involved. These configurations could potentially diminish the effectiveness of the stimulation, as they may not facilitate the necessary depolarization and propagation needed for robust signal acquisition during monitoring.