Repetitive nerve stimulation

Repetitive nerve stimulation is a variant of the nerve conduction study where electrical stimulation is delivered to a motor nerve repeatedly several times per second. By observing the change in the muscle electrical response (CMAP) after several stimulations, a physician can assess for the presence of a neuromuscular junction disease, and differentiate between presynaptic and postsynaptic conditions. The test was first described by German neurologist Friedrich Jolly in 1895,^[1] and is also known as Jolly's test.^[2]

Medical uses

Repetitive nerve stimulation is used to diagnose neuromuscular junction (NMJ) disorders, the most common of which is myasthenia gravis. A decremental response (a smaller and smaller muscle response with each repetitive stimulus) is abnormal and indicates NMJ dysfunction. This can be further confirmed if the response normalizes after administration of edrophonium or neostigmine.^[1]

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 (EPPs). In normal muscle, although the EPPs become smaller with repetitive stimulation, they remain above the threshold needed to trigger muscle 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 acetylcolineasterase inhibitors (e.g., Pyridostigmine Mestinon) 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 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. Supramaximal stimulation: Sub maximal stimulation can give art factual decrement or increment in the CMAP amplitude. So always check to ensure that the stimulus is supra maximal 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 (in %). 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.

Interpretation of results

Recording protocol:

Recording at rest
Immediately after activation ( 20 s activation default, 10 s in severe cases)
1 minute after activation
3 minutes after activation

References

1 2 Ropper, Allan H.; Samuels, Martin A. (2009). Adams and Victor's Principles of Neurology (9th ed.). McGraw Hill. pp. 1241–2, 1410. ISBN 978-0-07-149992-7.
↑ Shin J. Oh. Clinical electromyography: Nerve Conduction Studies, page 20. Lippincott Williams & Wilkins, 2002. ISBN 978-0-7817-3681-7. Google books

External links

Jolly's myasthenic reaction at Who Named It?

Medical test: Electrodiagnosis

Electrocardiography	Vectorcardiography Magnetocardiography

Central nervous system	Electroencephalography (Intracranial EEG) Magnetoencephalography

Peripheral nervous system	Electromyography (Facial electromyography) Nerve conduction study

Eyes	Electronystagmography Electrooculography Electroretinography

Digestive system	Electrogastrogram Magnetogastrography

Eponymous medical signs for nervous system

CNS

Head, general	Battle's sign Broca aphasia Kernig's sign Macewen's sign Myerson's sign Stroop test Wada test Dix-Hallpike test Hirano body

Multiple sclerosis	Dawson's fingers Lhermitte's sign Charcot's neurologic triad Uhthoff's phenomenon

Other	Romberg's test increased intracranial pressure Cushing's triad Miller Fisher test

PNS

Reflexes of extremities/
corticospinal tract

Combination	Jendrassik maneuver

Lower limb	Babinski sign Chaddock reflex Oppenheim's sign Westphal's sign

Upper limb	Hoffmann's sign

Other torso/limbs

Upper limb	Froment's sign carpal tunnel syndrome Tinel sign Phalen maneuver

Lower limb	Abadie's sign of tabes dorsalis Gowers' sign Hoover's sign Lasègue's sign Trendelenburg's sign

Torso	Beevor's sign

General	Todd's paresis Jolly's test

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