Workshop 6: Single-Fiber EMG WS6.1. Voluntary SFEMG: Principles and Parameters

Abstract

SFEMG is a selective recording technique that identifies action potentials (APs) from individual muscle fibers. Selectivity results from the 25 µm diameter recording surface that exits on the side of the electrode, 3 mm from the tip, and is further enhanced by using a 500Hz high pass filter. SFEMG measures fiber density and neuromuscular jitter. Fiber density is measured during slight voluntary muscle activation. Position the electrode to record with maximum amplitude the AP from one muscle fiber, which triggers the display, and count the number of time-locked acceptable APs (amplitude >200 μV, rise time <300 μs). The FD is the mean number of APs, including the triggering AP, counted at 20 sites throughout the muscle. The FD differs among different muscles and increases with age, especially after age 70 and in distal limb muscles. Jitter is measured either during axonal stimulation or voluntary activation. Stimulate facial nerve branches proximal to their entry into the muscle or anterior to the ear. In limb muscles, stimulate intramuscular axons via a monopolar needle electrode inserted near the endplate zone. Stimulate at 2 to 10 Hz (10 Hz is preferred) and adjust the stimulus intensity to produce a slight twitch. Measure jitter between the stimulus and individual APs. Analyze at least 50 stimuli for each AP and aim to measure at least 30 APs at different sites throughout the muscle. Jitter measured during voluntary activation is less subject to technical problems. Position the electrode to record at least two time-locked acceptable APs. Aim to measure jitter from 20 AP pairs from different sites throughout the muscle. Calculate jitter as the Mean value of Consecutive Differences (MCD) of successive interpotential intervals. A study is abnormal if the mean MCD exceeds the muscle upper limit, or if more than 10% of AP pairs or endplates have increased jitter. With more pronounced disturbances, individual muscle fibers fail to respond to nerve activation, blocking. Jitter can be measured with small concentric needle electrodes if careful attention is given to signal quality and appropriate reference values are used. Fiber density can only be measured with SFEMG electrodes.