How is single-fiber electromyography (SFEMG) performed?

Updated: Mar 13, 2019
  • Author: Pradeep C Bollu, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Answer

The amplitudes of the APs are optimized by slightly adjusting the electrode position; in the best recording position for jitter measurements, all APs of interest should have sharply rising phases and adequate amplitudes.

APs should be measured from 20 potential pairs and recorded from different portions of the muscle, using 3–4 skin insertions.

The jitter is expressed as the mean value of consecutive differences of successive interpotential intervals (MCD) (see image below).

Single-fiber electromyography. Calculation of the Single-fiber electromyography. Calculation of the mean difference between consecutive discharges (MCD). IPI is the interpotential interval or stimulus-to-response latency during axonal stimulation jitter analysis.

Jitter is increased whenever the ratio between the AP threshold and the endplate potential is greater than normal; thus, it is a sensitive measure of the safety factor of neuromuscular transmission (see image below).

Single-fiber electromyography. A pair of action po Single-fiber electromyography. A pair of action potentials recorded from a patient with myasthenia gravis during voluntary activation of the muscle. Consecutive discharges are superimposed. The sweep is triggered on the first potential, and increased jitter is seen in the second potential.

The normal mean MCD value varies from 10–50 µs among different muscles.

With more pronounced disturbances, impulses to individual muscle fibers intermittently fail to occur, producing neuromuscular blocking (see image below). Only when blocking occurs is clinical weakness or a decrement on repetitive nerve stimulation tests noted. In diseases of abnormal neuromuscular transmission, jitter may be increased in muscles that are clinically normal, showing no decrement to repetitive nerve stimulation. In certain situations the interpotential interval (IPI) may be influenced by the preceding interdischarge interval (IDI), which may introduce an additional variability due to changes in the velocity of AP propagation in the muscle fibers.

Single-fiber electromyography. A pair of action po Single-fiber electromyography. A pair of action potentials recorded from a patient with myasthenia gravis during voluntary activation of the muscle. Ten consecutive discharges are shown. The oscilloscope is triggered on the first potential, and increased jitter is seen in the second potential. In 3 discharges (arrows), the second potential does not occur (ie, blocking).

This is not a problem with stimulation jitter studies using a constant stimulus rate, if the first 10 intervals of each train are excluded from the jitter calculation.

The effect of preceding depolarizations becomes constant at that point, provided no impulse blocking is present to produce an irregular discharge rate. [16]

The effect of variable firing rates (when jitter is measured during voluntary activation) can be minimized by sorting the IPIs according to the length of the preceding IDI, then calculating the mean of the consecutive IPI differences in the new sequence. The result is called the mean sorted-data difference (MSD).

If the MCD:MSD ratio exceeds 1.25, then variations in the firing rate have contributed to the jitter; the MSD should be used to represent the neuromuscular jitter. The MCD is used to express the jitter if the MCD:MSD ratio is less than 1.25. Most EMG machines make these calculations automatically.

Presenting the results of jitter measurements in each muscle in all 3 of the following ways is useful:

  • The mean or median value of the MCD values in all the pairs or endplates that are measured (mean or median MCD)

  • The percentage of paired potentials or endplates in which blocking was seen (percent blocking)

  • The percentage of pairs or endplates in which jitter exceeded the normal limit for that muscle (percent abnormal pairs or endplates) (see images below).

    Graphical display of the results of a normal singl Graphical display of the results of a normal single-fiber electromyography (SFEMG) study. This is a nonsequential histogram of the jitter values that were recorded from 20 potential pairs during voluntary activation. The upper limit of normal for individual-paired jitter is shown. Summary statistics are listed at right.
    Graphical display of the results of a normal singl Graphical display of the results of a normal single-fiber electromyography (SFEMG) study. These are the results of a jitter study from a patient with myasthenia gravis (MG). The asterisk indicates a potential pair in which blocking was seen.

The mean MCD may exceed normal limits when a few individual jitter values are extremely high.

To avoid this, jitter values greater than 150 µs may be excluded from the mean MCD calculation, or the median MCD may be used to express the central tendency of the data.

In normal muscle, the mean and median MCD values are the same.


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