Insertional Activity:

Insertional activity is the electrical activity that occurs with mechanical depolarization of the muscle fibers due to needle insertion and movement through the muscle. Insertional activity is generated by single muscle fiber action potentials and is composed of combinations of positive and negative spikes, depending on the site of origin of the generated action potential.In a normal muscle, the burst of insertional activity reflects the number of muscle fibers that depolarize due to mechanical irritation; with larger needle movements the length of the bursts of insertional activity is longer, and with smaller needle movements the length is shorter. Regardless of the length of the insertional bursts, the activity ceases almost immediately following cessation of needle movement.

Insertional activity may be increased or reduced from the brief burst that occurs in normal subjects. Increased insertional activity may occur as two types of normal variants, as a result of denervated muscle, or in association with myotonic discharges. The normal variants are recognized by their widespread distribution. They occur most often in younger, muscular persons, especially in their calf muscles. One normal variant is composed of short trains of regularly firing positive waves. Some patients with this type of diffuse increased insertional activity have been found to have mutations in the CLCN1 gene associated with myotonia congenita.The second type is characterized by short recurrent bursts of irregularly firing potentials, sometimes termed “snap, crackle, pop.


Increased insertional activity may be the initial early sign, within the first 2–3 weeks, of denervation following an acute neurogenic disorder, such as an early radiculopathy or mononeuropathy. This often occurs prior to the development of more sustained fibrillation potentials. In addition, since needle movement often leads to the generation of fibrillation potentials in denervated muscle, most muscles that demonstrate more sustained fibrillation potentials have increased insertional activity.

In rare cases, decreased insertional activity may occur when the muscle fibers are unable to produce action potentials in response to membrane irritation. This most commonly occurs in severe or endstage neurogenic or myopathic disorders where the muscle is completely atrophic or has been replaced by connective tissue or fat. Additionally, disorders of muscle membrane dysfunction, such as periodic paralysis (during paralysis) or myophosphorylase deficiency myopathy (McArdle's) (during a contracture), may demonstrate decreased insertional activity or electrical silence during needle movement through the muscle.

Fibrillation Potentials:

Fibrillation potentials are the action potentials of single muscle fibers that are firing spontaneously in the absence of innervation. These potentials typically fire in a regular pattern at rates of 0.5–15 HZ . Infrequently, they may be intermittent or irregular, particularly early after a denervating process; in these cases the interspike interval is longer than 70 ms, distinguishing them from endplate spikes. Fibrillation potentials have one of two forms, either a brief spike or a positive wave. Fibrillation potentials that occur as brief spikes (spike form) may be triphasic or biphasic, 1–5 ms in duration, and 20–200 μV in amplitude, with an initial positivity or negativity (when recorded at the site of origin). Fibrillation potentials that occur as positive waves (positive wave form) are often of longer duration (10–30 ms) and biphasic, with an initial sharp positivity followed by a long-duration negative phase. The morphologic difference of the two forms reflect the site of the initiation of the fibrillation potential along the muscle fiber relative to the site of the needle electrode. The positive waveforms are muscle fiber action potentials recorded from an injured portion of the muscle fiber, when the action potential cannot propagate along the muscle fiber past the recording electrode. Rarely, fibrillation potentials are observed to transform from a spike to a positive waveform or vice versa; even less frequently, two fibrillation potentials are time-locked. The amplitude of a fibrillation potential is variable and is proportional to the muscle fiber diameter. In diseases with muscle fiber atrophy, fibrillation potentials may have low amplitude, whereas in hypertrophic muscle fibers the amplitude may be high. As a result of the range of sizes of fibrillation potentials, the configuration alone cannot be used to identify fibrillation potentials. Spike and positive wave form fibrillation potentials are both recognized as fibrillation potentials by their slow, regular firing pattern, which sounds like the “ticking or tocking of a clock.” Both forms have the same significance, indicating a denervated muscle fiber.

 Fibrillation potentials occur in any muscle fiber that is not innervated, either due to neurogenic or myopathic processes. These potentials may occur in muscle fibers that (1) have lost their innervation, (2) have been sectioned transversely or divided longitudinally, (3) are regenerating, or (4) have never been innervated. In neurogenic disorders, such as radiculopathies, mononeuropathies, or motor neuron disease, loss or degeneration of axons leads to denervated muscle fibers. In contrast, in myopathic diseases that produce pathologic changes of muscle fiber necrosis, fiber splitting, functional denervation of individual or segments of muscle fibers occurs as the fiber becomes separated from the endplate zone. In myopathies, fibrillation potentials are often of low amplitude and have a slow firing rate (e.g., 0.5 HZ). The density of fibrillation potentials is a rough estimate of the number of denervated muscle fibers and is commonly graded from 1+ (few fibrillation potentials in most areas of the muscle) to 4+ (profuse fibrillations filling the free-running baseline in all areas)

Other forms of electric activity could potentially be mistaken for fibrillation potentials. These include the spontaneous activity in the region of the endplate (endplate noise and endplate spikes), short-duration MUAPs, and MUAPs with a positive configuration. While the configuration of these waveforms may be identical to fibrillation potentials, all of them are distinguished from fibrillation potentials by their firing patterns, none of which fire in a regular pattern like a fibrillation potential.


Myotonic Discharges:

Myotonic discharges are the action potentials of single muscle fibers that are firing spontaneously in a prolonged fashion after external excitation. The potentials wax and wane in amplitude and frequency because of an abnormality in the membrane of the muscle fiber. Myotonic discharges are regular in rhythm, but the firing rates vary exponentially in frequency between 40 and 100 HZ, which makes them sound like a “dive-bomber.” Slowly firing myotonic discharges, which bear some resemblance to fibrillation potentials but demonstrate a more rapid rate of change in firing frequency and amplitude, may also occur

Myotonic discharges occur as brief spikes or positive waveforms, depending on the relation of the recording electrode to the muscle fiber. When initiated by insertion of the needle, myotonic potentials have the configuration of a positive wave, with an initial sharp positivity followed by a long-duration negative component. Both amplitude and frequency may increase or decrease as the discharge continues. Myotonic discharges that occur after a voluntary contraction are brief, biphasic or triphasic, initially positive spikes of 20–300 μV that resemble the spikes of fibrillation potentials. They wax and wane, similar to mechanically induced myotonic discharges. This afterdischarge corresponds to the clinically evident poor relaxation. The degree of waxing and waning has been shown to differ between different forms of myotonic dystrophy. In DM1, myotonic discharges typically wax and wane (increase and then decrease in firing rate), whereas in DM2 (previously known as proximal myotonic myopathy or PROMM), the discharges more commonly wane in frequency.

Myotonic discharges may occur in disorders with or without associated clinical myotonia. In those with clinical myotonia, the myotonic discharges are often prominent and frequent. Most commonly, these occur in myotonic dystrophy type 1 and 2 (DM1 and DM2) or myotonia congenita. The severity of myotonic discharges and the presence of waxing and waning discharges has been shown to be correlated with muscle weakness in DM1, but not DM2. In a study comparing the abundance of myotonic discharges in patients with sodium and chloride channelopathies, including myotonia congenita, paramyotonia congenita, and hyperkalemic periodic paralysis, no difference in the degree of myotonic discharges was found between the diseases.Rarely, briefer and less prominent myotonic discharges may occur with fibrillation potentials in chronic denervating disorders and with some medications. They are less readily elicited in a muscle that has just been active than a resting muscle, which is equivalent to the “warm-up phenomenon” that occurs in patients with myotonic myopathies.



Complex Repetitive Discharges:

Complex repetitive discharges (CRD), previously referred to as “bizarre repetitive potentials,” “high-frequency potentials,” or “pseudomyotonic discharges,” are the action potentials of groups of muscle fibers that discharge spontaneously in near synchrony in a regular, repetitive fashion. The groups of muscle fibers arise from several different neighboring motor units rather than from the same motor unit. Standard and single-fiber EMG recordings suggest that they are the result of ephaptic activation of groups of adjacent muscle fibers. A CRD is initiated by the spontaneous firing of a single muscle fiber action potential; however, that action potential ephaptically spreads and depolarizes a neighboring muscle fiber. Subsequently, a variable number of neighboring muscle fibers may be depolarized in sequence until the “circuit” is complete, whereby the initial muscle fiber discharges again. Therefore, each spike within a group in a CRD is composed of individual muscle fiber action potentials from fibers that may be part of a different motor unit, but lie adjacent to one another.

CRDs fire in a regular pattern, characteristically with an abrupt onset and cessation. During the discharge, they may have sudden changes in their configuration or firing rates. The frequency is uniform, ranging from as slow as 3 HZ up to 40 HZ . Although their form is variable, it typically is polyphasic, with 3–10 spike components with amplitudes from 50–500 μV and durations of up to 50 ms. CRDs sound like “a motor boat that misfires” or a “jackhammer.”

CRDs are nonspecific in significance but occur in neurogenic and myopathic disorders that are chronic or longstanding in nature . Commonly, these include old or chronic radiculopathies, peripheral neuropathies, or slowly progressive myopathies. In rare cases of patients with chronic S1 radiculopathies associated with pain and calf hypertrophy, CRDs are seen in the gastrocnemius in ≈50%, raising the possibility that CRDs may contribute to neurogenic hypertrophy in these cases.Rarely, CRDs occur in otherwise normal muscles, such as the iliopsoas or biceps. CRDs may be confused with other repetitive discharges, such as myokymic discharges, cramps, neuromyotonia, tremor, and synkinesis. However, each of these has a characteristic pattern of firing best recognized by its sound and distinct from that of CRDs.


Fasciculation Potentials:

Fasciculation potentials are randomly discharging action potentials of a group of muscle fibers innervated by the same anterior horn cell (motor unit). These spontaneously firing MUAPs may be generated anywhere along the lower motor neuron, from the anterior horn cell to the nerve terminal, but usually from spontaneous firing of the nerve terminal. The rates of discharge of an individual potential may vary from a few per second to fewer than 1 per minute. The sum of all fasciculations in a muscle may reach 500 per minute. These potentials may be of any size and shape, depending on the character of the motor unit from which they arise and their relation with the recording electrode, and they may have the appearance of normal or abnormal MUAPs. They are identified by their irregular firing pattern and may sounds like “large raindrops on a tin roof.”

Fasciculation potentials may occur in normal persons and in many diseases. They are especially common in chronic neurogenic disorders but have been found in all neuromuscular disorders.  Fasciculations usually occur in an overworked muscle, especially if there is underlying neurogenic disease. Fasciculation potentials have not been shown to occur more often in patients with myopathy than in normal persons.

Electrodiagnostic testing, using surface EMG, detects fasciculations more frequently than clinical observation or muscle palpation, and therefore EMG is useful in assessing fasciculations and other changes in patients with suspected ALS. However, neither surface or needle EMG can reliably distinguish between benign fasciculations and those associated with specific diseases. In normal persons, fasciculations occur more rapidly, on the average, and are more stable. The presence of fasciculation potentials alone on EMG, without fibrillation potentials or changes in voluntary MUAPs, are not sufficient to make a diagnosis of progressive motor neuron disease, such as ALS.

Patients who have large motor unit potentials caused by chronic neurogenic diseases may have visible twitching during voluntary contractions. Such “contraction fasciculations” must be differentiated from true fasciculations by the pattern of firing.


Myokymic Discharges:

Myokymic discharges are groups of recurring spontaneously firing MUAPs that fire in a repetitive burst pattern. The individual potentials within each burst often have the appearance of normal MUAPs, although may also be of long duration and high amplitude. Each burst may be composed of few or many potentials (2–10), and the rate of firing of potentials within each burst is typically 40–60 HZ. Each burst fires with a regular or semirhythmic pattern at intervals of 0.1–10 s. The firing pattern is unaffected by voluntary activity, and simultaneously occurring myokyomic discharges may vary in burst duration or firing rates. Some myokymic discharges sound similar to groups of “marching soldiers.”

Although discharges that have regular patterns of recurrence but fire at different rates or with a regularly changing rate of discharge may have similar mechanisms, they are better classified with the broad group of “iterative discharges.” Some investigators consider iterative discharges and myokymic discharges to be forms of fasciculation because they arise in the lower motor neuron or axon. However, it is best to separate these discharges from fasciculation potentials because of their distinct patterns and different clinical significance.

Myokymic discharges may or may not be associated with clinical myokymia, which appear as fine, worm-like quivering of the muscles. Although myokymic discharges are more commonly found in limb muscles, clinical myokymia is more often observed in facial muscles, probably due to the smaller degree of overlying subcutaneous tissue, than in limb muscles. Most commonly, myokymic discharges are found with radiation-induced nerve injury, chronic compressive neuropathies, or polyradiculopathies. The myokymic discharges seen in chronic compressive neuropathies, such as carpal tunnel syndrome, are often composed of a single or few potentials.


Neuromyotonic Discharges (Neuromyotonia):

Neuromyotonic discharges, or neuromyotonia, are rare, spontaneously firing MUAPs that are associated with some forms of continuous muscle fiber activity (Isaac's syndrome). Neuromyotonic discharges fire at very high frequencies of 100–300 HZ . These potentials may decrease in amplitude because of the inability of muscle fibers to maintain discharges at rates greater than 100 HZ. The discharges may be continuous for long intervals or recur in bursts. They are unaffected by voluntary activity.

Neuromyotonic discharges are seen in disorders of peripheral nerve hyperexcitability, such as Isaac's syndrome, and may occur as a result of a defect in potassium channels in the nerve membrane. Some forms of syndromes of peripheral nerve hyperexcitability are associated with bursts of doublet, triplet, or multiplet discharges, with intraburst frequencies often ranging from 40–350 HZ, which may appear similar to myokymic discharges. Neuromyotonia may also occur with tetany, where they may be precipitated by or augmented with ischemia, and Morvan's syndrome

A form of neuromyotonic discharges called neurotonic discharges occur intraoperatively with the mechanical irritation of cranial or peripheral nerves. These discharges are brief bursts of MUAPs discharging at very high rates, similar to the rates of spontaneously occurring neuromyotonic discharges. The identification of neurotonic discharges intraoperatively is valuable in alerting surgeons to possible nerve damage.


Cramp Potentials (Cramp Discharge):

Cramps are painful, involuntary contractions of muscle. The discharges associated with a muscle cramp (cramp discharges) are composed of MUAPs that fire in a unique firing pattern, which distinguishes them from other spontaneous activity and normal strong voluntary activation. The configuration of the individual potentials resembles MUAPs. However, in contrast to the pattern of activation that occurs with voluntary contraction, potentials in cramp discharges usually have an abrupt onset, rapid buildup, addition of subsequent potentials, and a rapid or “sputtering” cessation. The potentials fire rapidly (40–60 HZ), and during their discharge they may fire irregularly in a sputtering fashion, especially just before termination . Typically, an increasing number of potentials that fire at similar rates are recruited as the cramp develops and then stop firing as the cramp subsides

Cramps are a common phenomenon in normal persons, usually when a muscle is activated strongly in a shortened position. In addition, cramps may occur with any chronic neurogenic disorder, in metabolic or electrolyte disorders, or in disorders of peripheral nerve hyperexcitability (such as cramp fasciculation syndrome)


Synkinesis:

The aberrant regeneration of axons after nerve injury may result in two different muscles being innervated by the same axon, called synkinesis. In such cases, voluntary potentials may be mistaken for spontaneous activity. Groups of MUAPs fire in bursts in response to voluntary activation of a distant muscle. With synkinesis, MUAPs may be normal or abnormal and, when abnormal, they are typically of long duration due to reinnervation from a neurogenic lesion. A common example of this is facial synkinesis, in which facial muscles such as the orbicularis oris spontaneously fire MUAPs in association with blinking after facial reinnervation from facial neuropathy (Bells' palsy). Another, less common, example is arm-diaphragm synkinesis (also referred to as the breathing arm or hand) in which potentials in the shoulder girdle or hand muscles fire in association with respiration as a result of aberrant regeneration of the phrenic nerve



ABNORMAL ELECTRICAL ACTIVITY: VOLUNTARY MUAPS


The characteristic features of normal voluntary MUAPs have been discussed previously. The majority of normal MUAPs in limb muscles are triphasic with durations of 8–10 ms, stable appearing, and initially fire at rates of 6–8 HZ with an orderly increase in firing rate associated with the firing of additional units (normal recruitment). In neuromuscular diseases, MUAP firing rates and configurations may both be altered. The types of these alterations, in conjunction with the identification of spontaneous discharges, help to identify the underlying type, temporal profile of disease duration, and severity of neuromuscular disorder.


Abnormal Recruitment:

As discussed earlier, in a normal muscle increasing voluntary effort causes an increase in the rate of firing of individual MUAPs and initiates the discharge of additional MUAPs. The relationship between the rate of firing of individual potentials to the number of potentials firing is constant for a particular muscle and is called the recruitment pattern. Normal and abnormal recruitment has been discussed previously.

In disorders in which there is a loss of MUAPs, the rate of firing of the remaining individual potentials will be disproportionately high compared to the number of potentials firing; this is referred to as reduced recruitment. Reduced recruitment may be found in any disease process that destroys or blocks conduction in the axons innervating the muscle or destroys a sufficient proportion of the muscle so that muscle fibers of entire motor units are lost. This pattern occurs in association with all neurogenic disorders associated with axonal loss and may be the only finding in a neurapraxic lesion in which the sole abnormality is a focal conduction block. Reduced recruitment may be the earliest finding in an acute axonal lesion in which fibrillation potentials or other MUAP changes have not yet developed. Although a hallmark of neurogenic disorders, reduced recruitment may also be seen in severe or endstage myopathies, where entire motor units are lost due to primary muscle fiber degeneration, such as in muscular dystrophies.

Rapid recruitment of MUAPs occurs in disorders in which the force that a single motor unit can generate is decreased due to loss of muscle fibers within the motor unit. As a result, more motor units are activated than would be expected for the force exerted by the patient. The recruitment frequency and rate of firing in relation to number are normal with rapid recruitment; however, the number of motor units that fire are increased relative to force. Rapid recruitment occurs primarily in myopathies. While in many cases abnormalities in MUAP configuration will occur along with abnormal recruitment, this is not always the case, and rapid recruitment may be the only abnormality identified on needle EMG, particularly in early or mild myopathies.


Long-Duration Motor Unit Action Potentials:

MUAP duration is measured as the time from the initial baseline deflection to the time of the return to baseline, and it reflects the density and area of fibers within a motor unit, as well as the synchrony of firing of those fibers. The size of MUAPs in a muscle is dependent on the level of activation and with larger MUAPs it becomes active at a stronger force. Normal values for MUAP duration have been published.

Individual MUAPs that are longer than the normal range for a particular muscle or groups of MUAPs that have a mean duration greater than the normal range for the same muscle in a patient of the same age are called long-duration MUAPs. Long-duration MUAPs occur in diseases in which there is increased fiber density in a motor unit, an increased number of fibers in a motor unit, or loss of synchronous firing of fibers in a motor unit, typically due to collateral sprouting and reinnervation of a motor unit. Long-duration MUAPs generally have high amplitude and show reduced recruitment, but since the spike amplitude reflects only the few muscle fibers closest to the needle recording tip, they may have normal or low amplitude. When assessing MUAP duration, those MUAPs recorded from damaged muscle fibers that are preponderantly positive with a long late negativity, which is a recording artifact, should not be measured or interpreted as long duration.

MUAP duration is an important parameter used to distinguish neurogenic disorders from primary muscle diseases. Long-duration MUAPs typically occur in chronic neurogenic disorders. Following an acute nerve injury, long-duration MUAPs may be seen within several weeks or months, after reinnervation has begun. Long-duration MUAPs may also be seen in conjunction with short-duration MUAPs in chronic myopathies, such as inclusion body myositis or long-standing polymyositis

Short-Duration MUAPs:

Single MUAPs that are shorter than the normal range or groups of MUAPs that have a mean duration less than the normal range for the same muscle in a patient of the same age are called short-duration MUAPs. Short-duration MUAPs occur in diseases in which there is (1) physiologic or anatomical loss of muscle fibers from the motor unit, or (2) atrophy of component muscle fibers. In these situations the number of innervated muscle fibers within the recording region of the electrode is decreased, thereby leading to a decrease in the area of that motor unit. Commonly, these potentials also have low amplitude and show rapid recruitment with minimal effort, but they may have normal or reduced recruitment and normal amplitudes. The actual duration that identifies a potential as short duration varies with the muscle and age of the patient. Some short duration MUAPs may be as short as 1–3 ms if only a single muscle fiber is in the recording area. This may appear identical to a fibrillation potential or endplate spike, and only the semirhythmic firing pattern may allow for correct identification.

Short-duration MUAPs are most characteristic and are often seen in primary muscle diseases in which loss of muscle fibers from necrosis or degeneration occurs. Some myopathies, such as metabolic and endocrine disorders, show no or few short-duration MUAPs. In rare circumstances, short-duration MUAPs can occur due to technical problems, such as incorrect filter settings (e.g., low-frequency filter increased from 20 HZ to 500 HZ) or an electrical short in the recording electrode or connecting cables. When short-duration MUAPs occur when not expected, these technical problems should be considered and checked.

In addition to myopathies, short-duration MUAPs may occur in severe neuromuscular junction disorders or in newly reinnervated motor units following severe nerve injury. These nascent MUAPs are composed of only a few muscle fiber action potentials. They are typically polyphasic, and fire at a very high rate with reduced recruitment.

Polyphasic MUAPs:

A phase of an MUAP is defined as the area of a potential on either side of the baseline and is equal to the number of baseline crossings plus one. Most normal MUAPs contain three or four phases, and less than 15% will have over four phases. When an MUAP consists of five or more phases, it is called a polyphasic MUAP . The individual components of a polyphasic potential are action potentials recorded from a single or a few muscle fibers. The degree of phases reflects the synchrony of firing of the action potentials of muscle fibers within the MUAP, and when the fibers fire asynchronously, the number of phases (or turns) increases. This may occur as a result of collateral sprouting, reinnervation, or an increase in fiber density (in neurogenic disorders), or due to relative asynchrony from drop-out of muscle fibers in the motor unit (in myopathies), potentials become polyphasic.

Polyphasic potentials may be of any duration—normal, long, or short. Some may have late, satellite components, sometimes called linked potentials or satellite potentials, that give the total unit a long duration. However, isolated satellite potentials should not be included in the duration measurement of the MUAPs when comparing normative data. Polyphasic MUAPs may occur in any of the myopathies or neurogenic disorders and are graded by the percentage of MUAPs in the muscle that are polyphasic.


Mixed Patterns: Long-Duration and Short-Duration MUAPs:

Occasionally, patients have a combination of the abnormalities described for short, long, and polyphasic MUAPs, but instead of having the usual pattern of an excess of either long-duration or short-duration potentials, both types occur. The quantitative distribution becomes broad rather than shifting to long or short. Rarely, the distribution of durations may be bimodal. These combinations commonly occur in chronic myositis or rapidly progressing motor neuron disease.


Varying or Unstable MUAPs:

MUAPs fire repetitively under voluntary control, and they normally have the same amplitude, duration, and configuration each time they fire. Fluctuation of any of these variables during repeated discharge of an MUAP is abnormal and produces varying or unstable MUAPs. Varying MUAPs are caused by blocking of the discharge of action potentials of one or a few of the individual muscle fibers comprising the motor unit. The disorders in which MUAPs fluctuate from moment to moment  are listed in Table . Varying MUAPs are classically seen in disorders of neuromuscular transmission, such as myasthenia gravis or Lambert–Eaton myasthenic syndrome, but they may also be seen in reinnervating neurogenic disorders and occasionally in myopathies. In disorders of muscle membrane, such as myotonia, there may be a slower progressive decrease or increase in an MUAP . In myasthenia gravis or in cases of active reinnervation, the amplitude initially may decline, but in the myasthenic syndrome it may increase

Doublets (Multiplets):

Motor units under voluntary control normally discharge as single potentials in a semirhythmic fashion. In some disorders or occasionally in otherwise normal individuals, they fire two or more times at short intervals of 10–30 ms . These are called doublets, triplets, or multiplets. The bursts of two or more potentials recur in a semirhythmic pattern under voluntary control. They are often increased by hyperventilation, hypocalcemia, or ischemia. Additionally, doublets or multiplets may be seen in patients with disorders of peripheral nerve hyperexciteability, often associated with voltage-gated potassium channel antibodies.  In these patients the doublets and multiplets have been reported to occur more commonly in distal muscles, and the intraburst frequency ranges from 40–350 HZ.
 
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