Repetitive nerve stimulation is used to diagnose neuromuscular junction disorders, the most common of which is myasthenia gravis. A decremental response is abnormal and indicates NMJ dysfunction. This can be further confirmed if the response normalizes after administration of edrophonium or neostigmine.
Mechanism
Stimulation of a motor neuron causes it to release acetylcholine, which is stored up in vesicles at the axon terminal. The acetylcholine binds to nicotinic receptors on the muscle fiber, which open sodium channels and depolarizes the muscle cell. As nerve stimulation is rapidly repeated, the acetylcholine stored in the nerve terminal is gradually depleted, and there is a slight weakening of the acetylcholine signal sent to the muscle fiber, resulting in smaller endplate potentials. In normal muscle, although the EPPs become smaller with repetitive stimulation, they remain above the threshold needed to triggermuscle contraction. In myasthenia gravis, where many of the acetylcholine receptors are blocked, the EPP may exceed the threshold initially, but quickly falls below threshold with repetitive stimulation, resulting in the muscle fiber failing to contract. As one by one the muscle fibers fail to contract, the overall CMAP measured grows smaller and smaller, leading to the pathologic decremental response.
Preparation
As with all nerve conduction studies, the body part tested should be clean, free of lotions and conductive substances, with jewelry removed. It is best to advise patients to refrain from taking acetylcholinesterase inhibitors for 6–8 hours before the study, unless medically contraindicated. These agents make more ACH available to bind at the ACHRs and may potentially diminished CMAP decrement, resulting in a normal study.
Procedure
If the electrode is not properly immobilized, the result is a change in the CMAP amplitude which may lead to misinterpretation. So the recording electrodes should be secured well with tape, the stimulator secured with a Velcro strap and the entire hand with pad or board. The goal is to minimize the movement of the limb. Impulse transmission: Submaximal stimulation can give artifactual decrement or increment in the CMAP amplitude. So always check to ensure that the stimulus is supramaximal before beginning of RNS. A peripheral nerve is electrically stimulated, and the amplitude of the CMAP is recorded at rest and after a short voluntary activation. The stimulation frequency is 3 Hz, the number of stimuli 10. The result is reported as the difference in amplitude of the CMAP between stimulation one and four. The area value changes typically in parallel, but is not reported. If there is a major difference between amplitude and area decrement, technical factors should be considered.
