MEPs and their clinical application
The generation of motor evoked potentials (MEPs) in the brain provides valuable information regarding the functionality of the central motor and sensory pathways, of particular interest when studying diseases of the motor system (Chiappa). Pioneered by Merton and Morton in 1980, the electric stimulation technique has been superseded in recent years, thanks to the development of TMS. Via TMS it is possible to generate MEPs without the discomfort associated with electrical stimulation and it is superior in the stimulation of deep and less accessible nerves.
With central nervous system responses, via a technique referred to as facilitation, it is possible to reduce the stimulation threshold by approximately 25%, increase the response amplitude 2-5 times and reduce response latency by some 1-3 ms through pre-activation of the target muscle (Rothwell at el. 1991).
Magnetically evoked motor potentials can be obtained by stimulating the motor cortex, spinal nerve roots and peripheral nerves. Specific patterns allow preferential stimulation of specific muscle groups. Responses can be measured using electromyographic (EMG) or evoked potential equipment.
Practical applications of TMS-generated MEPs include diagnosing multiple sclerosis and monitoring treatment response, stroke recovery and motor neuron disease diagnosis, amongst others.
- Chiappa KH, Evoked potentials in Clinical Medicine, 1982.
- Merton & Morton, Nature, 1980.
- Rothwell et al, Experimental Physiology, 1991.