Electromyography and Evoked Potentials

Electrical Testing of Nerves and Muscles

All electrodiagnostic equipment can fundamentally be thought of as containing the following componentry:

• Electrodes: Electrodes allow for the recording of an evoked signal. The recording device may be surface or needle electrodes. The size (area) of the recording surface directly impacts the amplitude and duration characteristics of the recorded response. As such, the consistency of electrode size is paramount when comparing studies across time and between examiners and institutions.

• Stimulator: Stimulators can be constant voltage or constant amperage; typically, they are now constant current. An understanding of Ohms law (V=IR) quickly allows one to appreciate that If R (resistance) varies (skin impedance, sweat) over time, holding I (current) constant allows for the maintenance of a constant and stable electrical field (load despite variations in resistance). This results in improved tolerance for the test with less pain (stable intensity) and more stable recordings over time.

• High-gain differential amplifier: Differential amplifiers have both high frequency (low pass) and low frequency (high pass) characteristics. The frequencies that pass between the filters is the band path and is adjustable for environmental situations. Changing the amplifiers will impact on amplitude, duration, and latency of the recorded responses. It is critical to appreciate this phenomenon to avoid over or under interpretation of data accumulated in electrically challenging environments such as ICUs.

• Recording display or central processing device with A to D conversion capability: A to D conversion refers to the mathematical manipulation of the analog signal into a stable static waveform “seen” on the oscilloscope screen. The waveform “seen” is the digital representation of the analog wave resulting from multiple digitized (x, y coordinate) points. ^,

The EMG apparatus amplifies and displays biologic information derived from either surface or needle electrodes. Electrical information may be recorded from muscles, nerves, or other nervous system structures and is displayed on an oscilloscope. Besides the visual display on the oscilloscope, a permanent recording may be made, audio amplification may allow it to be heard over a loudspeaker, and analog-digital analysis of signals may be used. Nerves are electrically stimulated to measure conduction. ^,

For NCSs, skin surface electrodes are generally used for recording compound muscle or nerve action potentials. Rarely, needle electrodes are used.

For needle EMG, needle electrodes are used. They are typically monopolar or concentric. Each has unique characteristics and does impact on amplitude, duration, and phase complexity. Understanding these factors is important when comparing studies conducted by different examiners and different labs. ^{, ,}

For sensory testing, ring electrodes, self-adhering surface electrodes, and disc electrodes are used for measurement ( Fig. 19.1 ). Modern EMG equipment is manufactured by numerous companies and is generally standardized to allow reliable and reproducible testing by different laboratories, but normative data, including data for special populations such as geriatric, pediatric, diabetic, and even active workers, may differ among laboratories and require standardization by each laboratory.

NEE is an invasive procedure, but complications are rare. However, patients should be apprised of them. The most common is transient muscle soreness. Aseptic precautions should be observed. Safeguards for testing include extra care with patients who are taking warfarin or other anticoagulants or who have hemophilia or other blood dyscrasias, but since most muscles tested are superficial, they can easily be compressed, and the bleeding abated. Severe thrombocytopenia is a relative contraindication and should be considered carefully. Patients positive for human immunodeficiency virus (HIV) represent a transmission risk, but the use of disposable needles (which should be universal) protects against this hazard. One should carefully consider persons with a cardiac pacemaker or transcutaneous stimulator when doing stimulation for NCSs. Certain muscles, such as the rhomboids and abdominals, which are sometimes interrogated by NEE in pain patients, and the diaphragm (rarely), carry a risk for infectious peritonitis and pneumothorax, respectively. Beyond placing a needle through an infected site, there are probably no absolute but only relative contraindications to EMG. Extremely anxious patients and some children occasionally require some sedation. Aftereffects are negligible, with rare bruising, although occasionally a highly disturbed, suggestible, or litigious patient may complain of increased pain or disability vehemently.

Functional Physiology

When recording extracellularly, as with EMG, the electrode picks up the action potential as it is conducted through the medium that surrounds the active fiber. The impedance of the external medium is small compared to the impedance of the fiber interior, and hence the voltage of the extracellularly recorded potentials is maximally only 2%–10% of the intracellularly recorded potential changes. The functional unit ( Fig. 19.2 ) in reflex or voluntary activity is the motor unit; a motor unit is the group of muscle fibers innervated by a single anterior horn cell.

Conduction along the fine intramuscular branches of the anterior horn cell axon occurs so rapidly that all muscle fibers in a motor unit are activated nearly simultaneously. The number of muscle fibers per motor unit varies considerably from muscle to muscle; for example, in the gastrocnemius, the motor unit consists of about 1600 muscle fibers, whereas in the small muscles of the eye, there are only 5–10 fibers. The motor units in various muscles cover different areas of the muscle’s cross-section (e.g. brachial biceps, 55 mm; rectus femoris, anterior tibial, and opponens pollicis, 8–9 mm). The distribution of fibers is such that fibers from several different motor units are intermingled, which is why four to six motor units can be identified by EMG from the same intramuscular recording point.

In normal muscle, these single motor unit potentials can be differentiated only during weak voluntary effort. Utilization of sub-maximal effort during the recruitment phase of needle EMG recording and evaluation is critical to allow for the objective assessment of motor unit morphology and recruitment parameters. The potentials from different motor units are recognized by their frequency of discharge, which varies for each motor unit (some are more or less excitable). The various potentials often differ in appearance because of the differential distance of the recording electrode from the individual fibers of the activated motor units and the differential distribution of the motor end plates in the several units within “range” of a concentric or single needle electrode in one position in the muscle.

An upward deflection on the oscilloscope is considered electrically negative, and a downward deflection is considered electrically positive. Motor unit morphology will vary as a direct consequence of distance from the recording electrodes. Only motor units with an initial negative deflection and a rapid rise time should be evaluated in terms of amplitude, duration, and presence of an abnormal number of turns or phases. These parameters are critical in evaluating potential neuropathic and myopathic processes and help determine the acuity versus chronicity of the suspected disorder. ^{, ,}

Physiology of Nerve Conduction

The cell membrane (axolemma) of a nerve axon separates the intracellular axoplasm from the extracellular fluid. The unequal distribution of ions between these fluids produces a difference in potential across the cell membrane. This resting potential is about 70 mV and is negative on the inside with respect to the outside of the cell membrane. When a nerve fiber is stimulated, it causes a change in the membrane potential; a rapid but brief flow of sodium ions occurs through ionic channels inward across the cell membrane and gives rise to an action potential. ^,

The way in which an action potential is conducted along an axon depends on whether the axon is myelinated or unmyelinated. In a myelinated fiber, the action potential is regenerated only at the nodes of Ranvier, resulting in action potentials that “jump” from node to node or saltatory conduction. The velocity of nerve conduction depends on the diameter of the myelinated fiber. Small myelinated fibers may conduct as slowly as 12 m/s, whereas large motor and sensory fibers conduct at a rate of 50–70 m/s in humans. In an unmyelinated fiber in humans, the conduction rate is about 2 m/s. ^,

Several factors affect conduction velocity other than whether the axon is myelinated:

• Temperature of the limb (low temperatures decrease conduction velocity).

• Age of the patient (infants have slow conduction velocities and older adults have increasingly slowed conduction velocities). ^,

• Height of an individual (increased height may increase the internodal distances of the nodes of Ranvier and potentially lead to erroneous interpretation of numeric data).

Basic Electromyography Examination

EMG must be combined with clinical examination of the patient by the electromyographer. This includes grading of muscle strength and elicitation of muscle stretch reflexes. It is of prime importance for the electromyographer to personally correlate the clinical data and that obtained by EMG. With the understanding that the EMG examination is an extension of the clinical examination, the patient must be evaluated fully, and the problem tentatively assigned to the portion of the anterior horn cell system that seems most likely to be involved. The electromyographer determines the segment or segments of the peripheral nervous system suspected to be involved, and the examination is planned to either substantiate or invalidate the presumptive clinical diagnoses. EDX data that are discordant with the clinical examination are not to be ignored, but rather an explanation and rationale for that data are required, and its significance, or lack thereof, needs to be addressed.

Needle Findings in Normal Muscle

Insertional Activity

When the needle is inserted into a normal muscle, it evokes a brief burst of electrical activity that lasts no more than 2–3 ms after the needle ceases to move. This point is critically important because persistent pressure on the needle can potentially lead to spurious determination of increased insertional activity and possible pathologic condition. ^{, ,} This activity is described as insertional activity and is generally 50–250 mV in amplitude ( Fig. 19.3A ). These insertional potentials are believed to represent discharges from muscle fibers produced by injury, mechanical stimulation, or irritation of the muscle fibers.

Voluntary Activity

The voluntary activity of the muscle is analyzed after the muscle is studied at rest ( Fig. 19.3D ). Electrical activity (termed a motor unit action potential) is noted. A motor unit refers to the number of muscle fibers supplied by one motor neuron and its axon. This number varies from muscle to muscle and maybe as few as 10 to more than 1000 muscle fibers. When a motor neuron discharges, it activates all the muscle fibers of the motor unit.

The force of contraction determines the number of motor units brought into play. ^{, ,} This begins with a single motor unit that fires and can be identified on the screen by its distinctive morphology. As effort is increased, other motor units come into play, which can still be individually discerned and have their own individual morphology and audio representation on the loudspeaker. As the contraction increases, the firing rate of each individual motor unit action potential increases, and the action potential is subsequently joined by other motor unit action potentials, whose firing rates also increase. This phenomenon is known as recruitment ( Fig. 19.4 ). In normal muscles, the strength of a voluntary muscle contraction is directly related to the number of individual motor units recruited and their firing rate. Analysis of motor units includes (1) waveform, (2) amplitude, and (3) interference patterns.

Waveform

Typical motor units are biphasic or triphasic. The number of phases is determined by the “baseline crossings.” Motor units that cross the baseline more than four times are called polyphasic motor units. Polyphasic motor units are determined by the number of baseline crossings plus one. Four baseline crossings plus one equals five, and by definition, that is a polyphasic motor unit potential. Changes in phase not associated with baseline crossings are referred to as turns and can be seen as a consequence of a reinnervated maturing polyphasic motor unit potential after axonal injury in the past. Though occasionally seen in healthy muscle, they do not exceed 15% of the total number of motor units. In some muscles, polyphasic motor units are more prevalent. Polyphasic potentials are a measure of fiber synchrony. ^,

Amplitude

The amplitude depends on the number of fibers in the motor unit, the synchronization of the firing of the fibers in the motor unit, and the type of EMG needle used. Monopolar needles are associated with higher amplitude potentials than bipolar or coaxial needles are. Normal amplitude ranges from 1–5 mV. Because the motor unit is the sum of the action potentials of each muscle fiber of the unit, a large motor unit has a larger amplitude; conversely, a smaller motor unit has a smaller amplitude ( Table 19.1 ). ^{, ,}

Table 19.1

Normal Values for Commonly Tested Sensory and Motor Nerves

Nerves	Amplitude (Avg.)	Distal Latency in msec (Avg.)∗	Conduction Velocity in m/sec (Avg.)
Median (sensory)	10–85 µV (20)	2.0–3.7 (3.2)
Ulnar (sensory)	5–70 µV (15)	1.6–3.2 (2.8)
Radial (sensory)	10–60 µV (18)	1.7–2.8 (2.4)
Median (motor)	5–25 mV (8)	2.0–4.0 (3.3)	48–69 (54)
Ulnar (motor)	5.5–20 mV (8)	1.6–3.1 (2.6)	50–69 (55)
Sural (sensory)	3–38 mV (8)	2.3–4.6 (4.1)	41–61 (46)
Peroneal (motor)	2.5–18 mV (4)	2.3–6.0 (4.1)	41–58 (45)
Posterior tibial (motor)	4–38 mV (11)	2.1–6.0 (4.3)

*Distal latencies are based on standard distance: 13 cm for the median (S), 11 cm for the ulnar (S), 10 cm for the radial (S), 14 cm for the sural (S), 4–6 cm for the median (M) and ulnar (M), 6–8 cm for the peroneal (M), and 8–12 cm for the posterior tibial (M) nerves.

Interference Patterns

With maximum voluntary effort, a large number of motor units are brought into play, and their firing rate increases. They tend to “interfere” with each other and are not recognized further as individual units. This gives rise to a situation called an interference pattern ( Fig. 19.4 ). A normal muscle has a “full” interference pattern. With improvement in EMG equipment and the development of A to D conversion, interference pattern analysis is of limited utility. Analysis of sub-maximal contraction allows for evaluation of individual motor units, which in turn offers much richer detail and data in the evaluation of suspected neuropathic or myopathic processes.

Needle Findings in Abnormal Muscle

Various abnormalities may occur that indicate the presence of muscle dysfunction, which can be neurogenic or myogenic in etiology. The appreciation for the dynamic interplay and synchrony of abnormal EDX findings with a detailed, organized physical exam, buttressed by patient specific historical data, will ease the delineation of radiculopathy, generalized neuropathy, focal neuropathy, or mononeuropathy, or plexopathy.

The following are needle abnormalities in abnormal muscle:

• Insertional activity (decreased or increased).

• Spontaneous activity (fibrillations, positive sharp waves, CRDs [complex repetitive discharges] or fasciculations) ( Fig. 19.5 ).

  

• Abnormalities in voluntary motor unit activity, especially recruitment ( Fig. 19.4 ).

• Abnormal motor unit morphology (e.g. excessive or extreme polyphasia).

NCSs are of value in the following cases:

• Determining whether a disease of the nerve is present.

• Determining the distribution of neuropathy (e.g. mononeuritis, polyneuropathy, mononeuritis multiplex; this may be a valuable point in the differential diagnosis of the cause of a neuropathy).

• Determining at what point in a nerve a conduction block is present and locating an entrapment site.

• Studying the progress of disease of a peripheral nerve (e.g. Is it getting better? Worse? Staying the same?).

• Seeing whether reinnervation of a previously sectioned nerve has taken place.

• Establishing in a disease of the myoneural junction (e.g. myasthenia gravis) the fact that conduction along the nerve is adequate or normal. ^,

There are a few important limitations to the use of NEE in diseases of muscle and nerve, but particularly muscle. Some disorders may not result in abnormal findings on EMG, including certain congenital conditions and endocrine disorders such as steroid myopathy and polymyalgia rheumatica, an important cause of muscle pain and weakness in an older person. Rarely are the findings specific for a particular type of myopathy within the family of primary muscle disorders. ^,

Conduction velocity studies are carried out by inserting a needle electrode into a muscle innervated by the nerve under study or by the use of surface electrodes over that muscle ( Fig. 19.6 ). For example, the first dorsal interosseous muscle may be examined to determine the function of the ulnar nerve ( Fig. 19.7 ). The nerve is stimulated at the elbow in the case of the ulnar nerve, and the latency of the response is determined. The response is generally a spike-like large motor unit action potential. The ulnar nerve is then stimulated in the wrist or the axillary region, or both. The difference in latencies between the two points of stimulation and the distance between the two points of stimulation provide the basis for the calculation of conduction velocity.

F-Wave