Single-Fiber EMG

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Single-fiber electromyography (SFEMG) is a selective EMG recording technique that allows identification of action potentials (APs) from individual muscle fibers. The selectivity of the technique results from the small recording surface (25 µm in diameter), which is exposed at a port on the side of the electrode, which is 3 mm from the tip (see image below). The selectivity of the recording is further heightened by using a high pass filter of 500 Hz. Identification of APs from individual muscle fibers by SFEMG uniquely allows measurement of 2 features of the motor unit: fiber density and neuromuscular jitter. ^{[1, 2, 3, 4]}

Reference jitter values have been determined for many muscles. ^{[5, 6]}

Jitter increases slightly with age in normal subjects.

A study is abnormal if the mean (or median) jitter exceeds the upper limit for the muscle, or if more than 10% of pairs or endplates have increased jitter or blocking.

Jitter less than 5 µs is seen rarely in voluntarily activated SFEMG studies in normal muscles and more often in myopathies. ^[7]

These low values probably result from recordings that are made from split muscle fibers, both branches of which are activated by a single neuromuscular junction.

These values should not be included in assessments of neuromuscular transmission.

The MCD value that is measured during axonal stimulation is less than that measured during voluntary activation of the same muscle; the latter comes from only single endplates. ^[8]

Reference values for jitter during axonal stimulation have been determined for the extensor digitorum communis (EDC) and orbicularis oculi muscles. ^{[5, 6]}

For other muscles, the normative values for stimulation jitter can be obtained by multiplying the values for voluntary activation by a conversion factor of 0.8. ^[9]

MCD values less than 5 µs that are obtained during stimulation SFEMG occur when the muscle fiber is stimulated directly; these values should not be used for assessing neuromuscular transmission.

Single-fiber electromyography (SFEMG) is most valuable clinically in the patient with suspected myasthenia gravis (MG) in whom results of other tests of neuromuscular transmission and acetylcholine receptor (AChR) antibody measurements are normal. ^{[1, 10]}

The following technical items should be considered when performing single-fiber electromyography:

The electromyographer must have considerable experience with SFEMG to be able to perform adequate studies on most patients. Many EMG machines incorporate automated jitter analysis techniques that can greatly reduce analysis time.

Most adult patients can cooperate for adequate SFEMG studies. Patient discomfort rarely limits the use of this test, even when several muscles must be examined.

If the patient has a tremor, making adequate recordings from distal arm muscles during voluntary activation may be impossible. In such cases, recordings usually can be made from facial or more proximal arm muscles. Alternatively, recordings of jitter can be made during axonal stimulation.

Children older than 8 years usually can cooperate well enough for adequate studies. In uncooperative children, jitter studies can be performed with axonal stimulation under sedation.

The amplitude of the AP recorded with an SFEMG electrode from an average muscle fiber decreases to 200 microvolts (µV) when the electrode is approximately 300 µm from the muscle fiber. Thus, we can infer that APs greater than 200 µV arise from muscle fibers within 300 µm of the recording surface. By measuring in many sites within a muscle the mean number of time-locked APs that have an amplitude greater than 200 µV and rise time of less than 300 microseconds (µs), we can calculate the fiber density, which quantitates the local concentration of muscle fibers within the motor unit. This provides information that is analogous to type grouping in muscle biopsies. ^{[11, 12]}

Fiber density is a sensitive means of detecting and quantitating rearrangement of the muscle fiber topography in the motor unit; it is increased in neurogenic conditions but also in some myopathies. Fiber-density measurements are made by observing the signals on the oscilloscope screen. As the patient voluntarily activates the muscle, the electrode is positioned to record with maximum amplitude the AP from one muscle fiber. This AP triggers the oscilloscope sweep and is delayed for display so that the number of synchronized APs with amplitudes over 200 µV can be counted.

APs are recorded in 20 separate sites within a muscle, usually via 3 separate insertion sites. The fiber density (FD) is the mean number of APs, including the triggering AP, counted in these 20 sites (see image below). The normal FD is different among different muscles and increases in adults older than 60 years, especially in distal muscles.

When APs elicited by nerve stimulation are recorded with an SFEMG electrode, the latency from stimulus to response varies (see image below). This variation is the neuromuscular jitter, most of which is produced by fluctuations in the time for endplate potentials at the neuromuscular junction to reach the AP threshold. ^[13]

When the SFEMG electrode is positioned to record from 2 or more muscle fibers in one voluntarily activated motor unit, the neuromuscular jitter is seen as variations in the time intervals between pairs of APs from these fibers (see images below). This paired jitter represents the combined jitter in 2 endplates.

Jitter may be measured either as the nerve is stimulated or as the patient voluntarily activates the muscle.

Axonal stimulation

Stimulation jitter studies are particularly useful in the following cases:

In patients who have difficulty maintaining constant voluntary activation of the muscle

In patients who have a tremor

In children who are too young to cooperate

When controlling the firing rate precisely is desirable, as when assessing the effect of firing rate on jitter

The motor nerve may be stimulated proximally to its entry into the muscle, or individual motor nerve branches may be stimulated within the muscle (see image below).

The former technique is ideal for activating facial muscles, since individual branches of the facial nerve can be stimulated with a monopolar needle electrode, which is inserted through the skin anterior to the ear.

If a surface electrode is used for stimulation, many motor units usually are activated, making identifying the responses of single muscle fibers difficult.

Some artifactual jitter may be introduced by variations in the intensity of the stimulus that reaches the individual motor nerve fibers, especially when surface stimulation is used.

Voluntary activation

Jitter measurements performed during voluntary activation of the muscle are less subject to technical problems that can lead to misinterpretation of the results. However, this technique requires more patient cooperation than stimulation jitter studies.

As the patient slightly contracts the muscle, the SFEMG electrode is inserted into the muscle near the endplate zone; it is positioned to record 2 or more time-locked APs from the same motor unit (see images below).

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).

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).

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.

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. ^[14]

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).

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.

SFEMG demonstrates increased jitter in virtually all patients with myasthenia gravis (MG). SFEMG is most valuable clinically in the patient with suspected MG in whom results of other tests of neuromuscular transmission and acetylcholine receptor (AChR) antibody measurements are normal. ^{[1, 3]} When abnormal neuromuscular transmission has been demonstrated by RNS, the finding of abnormal jitter does not add to the diagnosis, although baseline jitter values may be useful for comparison with subsequent studies (see Assessment of Neuromuscular Transmission).

Although no one muscle is more abnormal in every patient with MG, the extensor digitorum communis (EDC) is abnormal in most.

If the symptoms or weakness is limited to the ocular muscles, the orbicularis oculi or frontalis may be examined first. In most cases, the EDC is examined first.

If the EDC is examined first and is normal, the frontalis or orbicularis oculi muscles usually are examined next. If the first of these facial muscles is normal, the other should be examined before excluding the diagnosis of MG.

If any limb muscle is weak and jitter is normal in all other muscles tested, a weak limb muscle also should be examined. If jitter is normal in a muscle with definite weakness, the weakness is not due to MG.

Jitter is less abnormal in patients with ocular myasthenia than in those with generalized disease. Jitter is increased in a limb muscle in more than half the patients with ocular myasthenia, demonstrating that the physiologic abnormality is more widespread than the clinical findings would suggest.

In patients with antibodies to muscle-specific tyrosine kinase (MuSK), the electrodiagnostic abnormalities are more focal and single-fiber studies should be performed in clinically weak muscles. ^[15]

Jitter usually is increased even when patients are taking cholinesterase inhibitors. In patients with mild or purely ocular weakness, however, jitter may be normal unless these medications have been discontinued for at least 24 hours. Comparisons among studies are valid only if they were made at the same time after the same dose of cholinesterase inhibitors. Jitter also is increased in diseases of nerve and muscle; these must be excluded by other electrophysiologic and clinical examinations before concluding that the patient has MG.

Several recent studies have examined the quantitative evaluation of neuromuscular transmission using disposable concentric needle electrodes (CNE). ^[1]

A larger recording area results in potentials that are easier to acquire.

This larger area is more likely to produce recordings with overlapping pairs.

The overlapping pairs, along with contributions from more distant motor units within the recording area, probably results in an underestimation of the true jitter value.

The several studies available suggest that the specificity may be similar to recordings performed with a single-fiber electrode.

CNE with the smallest possible recording surface should be used.

A high-pass filter setting of 1-2 Hz is recommended.

Normative data studies now exist for CNE jitter studies. The mean MCD is lower for CNE jitter recording compared to classic SFEMG.

CNE studies cannot measure fiber density.

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David E Stickler, MD Assistant Professor, Department of Neurosciences, Director of Electromyography Laboratory, Director of MDA Clinic, Director of Neuromuscular Service, Director of ALS Clinic, Medical University of South Carolina

David E Stickler, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Nicholas Lorenzo, MD, MHA, CPE Co-Founder and Former Chief Publishing Officer, eMedicine and eMedicine Health, Founding Editor-in-Chief, eMedicine Neurology; Founder and Former Chairman and CEO, Pearlsreview; Founder and CEO/CMO, PHLT Consultants; Chief Medical Officer, MeMD Inc

Nicholas Lorenzo, MD, MHA, CPE is a member of the following medical societies: Alpha Omega Alpha, American Academy of Neurology, American Association for Physician Leadership

Disclosure: Nothing to disclose.

Aashit K Shah, MD, FAAN, FANA Professor and Associate Chair of Neurology, Director, Comprehensive Epilepsy Program, Program Director, Clinical Neurophysiology Fellowship, Detroit Medical Center, Wayne State University School of Medicine

Aashit K Shah, MD, FAAN, FANA is a member of the following medical societies: American Academy of Neurology, American Clinical Neurophysiology Society, American Epilepsy Society, American Neurological Association

Disclosure: Received research grant from: Lundebck pharma.

The authors and editors of eMedicine gratefully acknowledge the contributions of previous author Donald B Sanders, MD, to the development and writing of this article.

Single-Fiber EMG

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