Repetitive Nerve Stimulation

Excerpt

The neuromuscular junction (NMJ) is the point of communication between the nervous system and skeletal muscle. This complex structure of the NMJ can be conceptualized as a one-way relay station with 3 main structural elements: the presynaptic terminal, the synaptic cleft, and the postsynaptic motor endplate. Signal transmission is impaired when one or more of these 3 elements are disrupted, resulting in an NMJ transmission or NMJ disorder.

Neuromuscular junction transmission disorders have various causes and manifestations and include myasthenia gravis, Lambert-Eaton syndrome, and botulism. Myasthenia gravis is an acquired autoimmune disorder and is the most commonly encountered NMJ disorder in clinical practice. Diagnosing NMJ disorders requires careful history taking, physical examination, and standard electrodiagnostic testing, including nerve conduction studies (NCS) and electromyography (EMG). Electrophysiological studies that test the NMJ specifically include repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG). These specific tests can confirm the diagnosis of an NMJ disorder and pinpoint which element, or elements, of signal transmission is impaired.

RNS is an electrodiagnostic technique that evaluates NMJ integrity. A modification of standard NCS, RNS utilizes an active electrode over a target muscle and a reference electrode over the distal target tendon. RNS repeatedly stimulates the target muscle at either a slow rate (2 to 5 Hz) or a fast rate (15 to 30 Hz or more) and measures the resulting compound muscle action potential (CMAP) amplitudes. The CMAP is the sum of action potentials from several muscle fibers. A decrement of >10% between the first and fourth CMAPs is abnormal.

Standard EMG records the action potentials of a group of muscle fibers within the same motor unit. In contrast, SFEMG utilizes a specialized electrode to selectively isolate and record the action potentials of a single muscle fiber. With the addition of a 500 to 1000 Hz high-pass filter, SFEMG can record the difference in action potential onset between two muscle fibers innervated by the same motor neuron (conventional SFEMG) or the same muscle fiber stimulated repetitively (stimulated SFEMG). This difference in these measured values is termed "jitter." Any increase in jitter beyond standard reference values based on age and the specific muscle tested is considered abnormal and suggests an NMJ disorder. Therefore, SFEMG serves as the more sensitive confirmatory test for NMJ disorders.