From the Guidelines
Repetitive nerve stimulation (RNS) works by repeatedly stimulating a peripheral nerve and measuring the muscle's electrical response to diagnose neuromuscular junction disorders, such as myasthenia gravis, with a decremental response of more than 10% between the first and fourth or fifth responses indicating a positive result. The procedure involves placing electrodes on the skin over a nerve and delivering small electrical impulses at a rate of 2-5 Hz for several seconds, with the resulting compound muscle action potentials (CMAPs) recorded from the corresponding muscle 1. In normal individuals, these responses remain relatively stable throughout the stimulation, but in conditions like myasthenia gravis, the test typically shows a decremental response due to progressive failure of neuromuscular transmission with repeated stimulation 1. Some key points to consider when using RNS include:
- The test can be enhanced by performing it after exercise or with high-frequency stimulation
- RNS is particularly useful for diagnosing myasthenia gravis, Lambert-Eaton myasthenic syndrome, and botulism, though it has varying sensitivity depending on the specific condition and muscles tested
- The accuracy of RNS for diagnosing neuromuscular junction disorders, such as myasthenia gravis, has been established, but its application in certain muscles, like the laryngeal muscles, may be limited due to accessibility and technical challenges 1.
From the Research
How Repetitive Nerve Stimulation Works
- Repetitive nerve stimulation (RNS) is a technique used to evaluate neuromuscular junction (NMJ) disorders, such as myasthenia gravis and Lambert-Eaton myasthenic syndrome 2, 3.
- RNS involves stimulating a nerve repeatedly and measuring the response of the muscle to determine if there is a decrement or increment in the amplitude of the muscle action potential 3, 4.
- The technique is based on supramaximal repetitive nerve stimulation and the measurement of decremental (or incremental) responses 2.
- RNS is technically easier to perform than single fiber electromyography (SFEMG) and does not require special technical training and skill 2.
Diagnostic Criteria
- A 10% decrement in amplitude from the first to fourth or fifth intravolley waveform while stimulating at 2-5 Hz is valid for the diagnosis of myasthenia gravis (MG) 3.
- The degree of increment needed for the diagnosis of Lambert-Eaton myasthenic syndrome (LEMS) is at least 25% but most accurate when greater than 100% 3.
- Abnormal jitter or impulse blocking are the appropriate criteria for diagnosis of NMJ disorders when using SFEMG 3, 5.
Muscle Selection and Technical Performance
- The deltoid muscle has been found to have the highest sensitivity in all MG subtypes, with a mean of 77% sensitivity in all clinical subtypes 4.
- Technical issues with RNS are rare, and the technique can be optimized by tailoring the examination to clinical phenotype 4.
- RNS remains the preferred initial test for MG and LEMS, although SFEMG may be indicated in patients with negative RNS results 5, 6.