Repetitive Motor Nerve Stimulation

This technique is performed to specifically stress the safety factor by rapidly mobilizing and releasing multiple stores of ACh. With a 2-to 5-Hz RMNS, fewer stores of ACh are released, and less ACh is available to bind to the AchR with each stimulus, up to approximately 5 to 6 stimuli. By assessing the change in the recorded compound muscle action potential (CMAP) after depolarization of all of the axons, and therefore all of the muscle fibers within a muscle, defects of neuromuscular transmission can be identified.

When evaluating normal neuromuscular junctions (NMJs), the EPP is much larger than the threshold required to initiate an action potential along the muscle fiber. As a result, the reduction in ACh release after repetitive stimulation at slow rates does not reduce the EPP below the threshold for depolarization, and action potentials are initiated in all muscle fibers innervated by the nerve. The resulting CMAP amplitude and area after each stimulus is therefore identical, and no reduction (decrement) of the responses occur.

When patients have a presynaptic dysfunction, such as in Lambert-Eaton myasthenic syndrome (LEMS) or infantile botulism, the resting EPP is markedly reduced as a result of a reduction in release of ACh from the presynaptic nerve. The ACh release is diminished at the peripheral nerve terminal at the NMJ because of either an autoimmune disorder blocking presynaptic uptake of calcium (Ca²⁺) ions or a specific effect of the botulinum toxin having a similar effect. This EPP is often lower than the threshold for depolarization of the muscle fiber, and therefore a single stimulus will not produce a muscle fiber action potential in many fibers. With standard motor nerve conduction studies, the CMAP amplitude is often low as a result. With slow rates of stimulation during RNS, there is an additional reduction in the release of ACh stores with each stimulus and a decrement in the CMAP amplitude and area (similar to that occurring in a postsynaptic neuromuscular junction disorder) is seen. However, after brief isometric exercise for 10 seconds, the influx of Ca²⁺ and mobilization and release of additional ACh stores results in a significant increase (increment or facilitation) of the CMAP amplitude (Plate 11-5, upper panel). This facilitation is a characteristic and diagnostic finding in Lambert-Eaton myasthenic syndrome.

In patients with a much more common postsynaptic dysfunction, as is typical in myasthenia gravis (MG), there is an autoimmune disorder leading to accelerated breakdown of the ACh receptors at the muscle side of the NMJ as well as a blockade of the ACh at the postsynaptic end plate. This affects the ability of ACh to produce a normal EPP. Therefore the resting EPP may be lower than normal as the safety factor for neuromuscular transmission is reduced.

Therefore RMNS results in a reduction of both the recorded CMAP amplitude and area with each stimulus for the first 5 to 6 stimuli (Plate 11-5, lower panel). This decrement results from a loss of summated muscle fiber action potentials from those fibers in which the EPP does not reach the threshold for depolarization. Brief isometric exercise for 10 seconds leads to increased Ca²⁺ permeability in the presynaptic nerve terminal, causing mobilization and a release of additional stores of ACh. As a result, the degree of decrement immediately after brief exercise is less than at rest. However, with repeat testing between 1 to 4 minutes later, there is progressive NMJ fatigue, and the degree of deficit defined with RMNS increases up to a maximum at this time and then begins to improve with sequential testing.

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