Jitter measurement by axonal micro-stimulation. Guidelines and technical notes.

Single fiber EMG (SFEMG) with axonal micro-stimulation is a convenient method to study the neuromuscular jitter at the individual motor end-plates. Compared to the original method of jitter measurement in voluntarily activated muscle, it has the advantage of perfect control of the discharge rate, including pauses in activity, useful in quantitative estimation of the neuromuscular transmission defect. It obviates the need to search for muscle fiber pairs. It can be used in young children and in uncooperative patients, as well as those with impaired voluntary motor control. It is useful in animal experiments as well as veterinary medicine. The technique eliminates the possibility of overestimating the jitter due to unrecognized interdischarge interval dependent jitter, as well as that of underestimating it due to unrecognized low jitter in split muscle fibers. The technique has certain pitfalls causing under- or overestimation. The paper gives practical guidelines and hints as to how to avoid some of these, particularly errors due to overlooked threshold stimulation and to unrecognized direct muscle fiber stimulation.