Use of repetitive nerve stimulation in the evaluation of neuromuscular junction disorders

Neuromuscular junction (NMJ) disorders are characterized by fuctuating muscle weakness. Acquired myasthenia gravis is the most common NMJ disorder with an overall prevalence in United States estimated at 60,000. Depending on the site of neuromuscular transmission failure, NMJ disorders have been classified as: (1) presynaptic (e.g., Lambert-Eaton myasthenic syndrome), (2) synaptic (e.g., cholinesterase inhibitor toxicity), and (3) post-synaptic (e.g., myasthenia gravis). Electrodiagnostic techniques used for investigation of NMJ disorders include repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG). Recent literature widely explores the use of SFEMG in the diagnosis and monitoring of myasthenia gravis, but this technique has a lesser role in the daily clinical practice outside of academic institutions. RNS is not as sensitive as SFEMG, but it is the most widely used electrodiagnostic method in the evaluation of suspected neuromuscular transmission disorders. RNS is technically easier and does not require special technical training and skill as SFEMG. Repetitive nerve stimulation was utilized first by Jolly in 1895 using an electrical drum and faradic tetanization to demonstrate a "myasthenic reaction" (weakening muscle contractions). In 1941, decremental response following the repetitive nerve stimulation was described by Harvey and Masland. While the technology has improved tremendously since then, the RNS testing is still based on supramaximal repetitive nerve stimulation and the measurement of decremental (or incremental) responses.