Electrophysiological study in neuromuscular junction disorders

Abstract

This review is on ultrastructure and subcellular physiology at normal and abnormal neuromuscular junctions. The clinical and electrophysiological findings in myasthenia gravis, Lambert-Eaton myasthenic syndrome (LEMS), congenital myasthenic syndromes, and botulinum intoxication are discussed. Single fiber electromyography (SFEMG) helps to explain the basis of testing neuromuscular junction function by repetitive nerve stimulation (RNS). SFEMG requires skill and patience and its availability is limited to a few centers. For RNS supramaximal stimulation is essential and so is display of the whole waveform of each muscle response at maximum amplitude. The amplitudes of the negative phase of the first and fourth responses are measured from baseline to negative peak, and the percent change of the fourth response compared with the first represents the decrement or increment. A decrement greater than 10% is accepted as abnormal and smooth progression of response amplitude train and reproducibility form the crux. In suspected LEMS the effect of fast rates of stimulation should be determined after RNS response to slow rates of stimulation. Caution is required to avoid misinterpretation of potentiation and pseudofacilitation.

Keywords: Acetylcholine; Lambert–Eaton myasthenic syndrome; congenital myasthenic syndromes; myasthenia gravis; repetitive nerve stimulation; single-fiber electromyography.

Figure 1

Morphological and biochemical aspects of the neuromuscular junction are presented to either side of the vertical dashed line

Figure 2

In the diagram on the left of the figure, SFEMG electrode can record electrical activity from two muscle fibers of the same motor unit. In the figure on right two muscle fiber action potentials, and variability in the interval between potentials is seen as a variable position of the second potential with increased jitter and occasional blockings (arrows) in myasthenic muscle

Figure 3

Jitter measurements after single-fiber electromyography. IPI is the interpotential interval or sti mulus-to-response latency during axonal stimulation jitter analysis and calculation of the mean difference between consecutive discharges (MCD)

Figure 4

A prototype decrementing response to repetitive nerve stimulation in myasthenia gravis. The amplitude of the initial response is normal, and the decrement is maximal in the fourth response after which the responses may increase slightly, giving an envelope shape to the train of responses