Electromyography and nerve conduction studies are the primary electrodiagnostic studies employed in the evaluation of patients with weakness and suspected myopathy. This article discusses the physiologic principles that serve as a framework for understanding the purpose, limitations, and interpretation of these tests. In the process the authors also review the differential diagnosis of myopathy.
Purpose and limitations
The purpose of electromyography (EMG) is to localize a lesion within the peripheral nervous system. Peripheral causes of weakness can be divided into neuropathic processes, myopathic processes, and diseases affecting the neuromuscular junction. A neuropathic process is one which affects the anterior horn cell or its axon as it passes through the nerve root, plexus, or peripheral nerve. A myopathic process affects the muscle fibers. Specialized techniques can be performed, if necessary, to evaluate the neuromuscular junction. Weakness caused by disorders of the central nervous system such as multiple sclerosis or stroke is not usually diagnosed with EMG. This review focuses on physiologic principles involved in differentiating lesions of the lower motor neuron from primary muscle diseases.
Electrodiagnostic testing has some important limitations. A few muscle diseases do not cause abnormalities on EMG. Any muscle disease affecting the contractile apparatus of muscle fibers without affecting their electrical properties would not be detectable on EMG (eg, some congenital or endocrine myopathies). A normal EMG can also be seen in the setting of steroid myopathy. This condition causes atrophy preferentially of type II muscle fibers, but EMG detects primarily type I fibers during the weaker levels of contraction at which individual motor units can be seen. Thus, a normal EMG would not rule out these conditions.
Even an abnormal EMG by itself does not often produce a specific diagnosis, because there are no findings that are specific to any given disease. For example, electrical myotonia is almost always observed in myotonic dystrophy, but can also be seen in other conditions such as paramyotonia congenita, acid maltase deficiency, or rarely in neuropathic conditions. However, sometimes clinical history and examination findings supplement the electrophyisologic results to produce a specific diagnosis. Suppose EMG reveals myotonia in a 45-year-old man with weakness of the distal hand muscles, ankle dorsiflexors, and face. He also has frontal balding, cataracts, and diabetes mellitus. There is a family history of weakness in a similar pattern showing genetic anticipation. This patient can be diagnosed without further testing as having myotonic dystrophy because of the characteristic clinical history and EMG myotonia. At other times EMG aids diagnosis by localizing the pathology to muscle, directing further diagnostic testing such as muscle biopsy or genetic testing, which can be helpful in making a specific diagnosis. EMG can also be helpful in selecting an affected muscle for biopsy. Typically a biopsy site contralateral to the limb examined with EMG would be chosen, because damage from the EMG needle may affect biopsy results.
Principles of muscle physiology
A brief review of basic nerve and muscle physiology serves as a foundation on which to build understanding of common EMG findings. Muscles are organized into motor units. A motor unit consists of a single alpha motor neuron and all of the muscle fibers it innervates, as shown in Fig. 1 A. One motor neuron may innervate several muscle fibers (in muscles requiring high precision such as extraocular muscles) or hundreds of muscle fibers (in muscles requiring steady power such as the gastrocnemius). The muscle fibers of a single motor unit are not situated adjacent to each other.
When a motor neuron fires, an action potential propagates down its axon and all of the axon twigs. When the action potential reaches the nerve terminal, the depolarization triggers opening of voltage-gated calcium channels. The influx of calcium then triggers the release of acetylcholine into the extracellular space by fusion of the storage vesicles with the muscle membrane. Acetylcholine diffuses across the neuromuscular junction and activates nicotinic receptors, which are ligand-gated sodium channels. The influx of sodium into the muscle cell triggers an action potential in the muscle fiber similar to that in the nerve. However, muscle fibers contain a system of tunnels (called transverse or “T” tubules), which are invaginations of the cell membrane into the depths of the muscle cell, allowing the action potential to propagate rapidly throughout the muscle cell. The lumen of the T tubule is contiguous with the extracellular space. T tubules lie adjacent to an intracellular system of tunnels called the sarcoplasmic reticulum, which is a repository for stored calcium. The depolarization of the T tubules triggers voltage-gated calcium channels, leading to an initial calcium influx into the muscle fiber. This process causes the further release of calcium stores in the sarcoplasmic reticulum. The accumulation of calcium in the muscle fiber then drives the contractile apparatus. The flux of ions within muscle fiber creates an electric field that can be recorded from the EMG needle.
In neuropathic conditions the motor axon is injured, leaving muscle fibers of entire motor units without innervation. The muscle fibers become hyperexcitable, leading to spontaneous depolarization and contraction of a single muscle fiber, which is detected on EMG as fibrillations or positive sharp wave potentials. The denervated muscle fibers also produce signaling molecules that induce nearby motor neurons to sprout axon twigs to innervate the orphaned fiber ( Fig. 2 ), which leads to enlarged motor units because then a single axon controls a greater number of motor fibers, as shown in Fig. 1 B.
In myopathic disorders the pathology localizes to individual muscle fibers. Inflammatory or degenerative processes result in the loss of muscle fibers within the motor units, meaning that for any given motor unit there will be fewer muscle fibers activated with each propagated action potential. The motor units are smaller (see Fig. 1 C).
Firing of a single motor neuron leads to the depolarization of many muscle fibers. If all of the muscle fibers in a motor unit depolarize at approximately the same time, their electric discharges will summate into a single potential seen on the EMG screen known as the motor unit action potential (MUAP), shown in Fig. 1 A. The shape of the MUAP will change in characteristic ways in neuropathic and myopathic conditions, and these changes can be used to localize the pathology.
Principles of muscle physiology
A brief review of basic nerve and muscle physiology serves as a foundation on which to build understanding of common EMG findings. Muscles are organized into motor units. A motor unit consists of a single alpha motor neuron and all of the muscle fibers it innervates, as shown in Fig. 1 A. One motor neuron may innervate several muscle fibers (in muscles requiring high precision such as extraocular muscles) or hundreds of muscle fibers (in muscles requiring steady power such as the gastrocnemius). The muscle fibers of a single motor unit are not situated adjacent to each other.
When a motor neuron fires, an action potential propagates down its axon and all of the axon twigs. When the action potential reaches the nerve terminal, the depolarization triggers opening of voltage-gated calcium channels. The influx of calcium then triggers the release of acetylcholine into the extracellular space by fusion of the storage vesicles with the muscle membrane. Acetylcholine diffuses across the neuromuscular junction and activates nicotinic receptors, which are ligand-gated sodium channels. The influx of sodium into the muscle cell triggers an action potential in the muscle fiber similar to that in the nerve. However, muscle fibers contain a system of tunnels (called transverse or “T” tubules), which are invaginations of the cell membrane into the depths of the muscle cell, allowing the action potential to propagate rapidly throughout the muscle cell. The lumen of the T tubule is contiguous with the extracellular space. T tubules lie adjacent to an intracellular system of tunnels called the sarcoplasmic reticulum, which is a repository for stored calcium. The depolarization of the T tubules triggers voltage-gated calcium channels, leading to an initial calcium influx into the muscle fiber. This process causes the further release of calcium stores in the sarcoplasmic reticulum. The accumulation of calcium in the muscle fiber then drives the contractile apparatus. The flux of ions within muscle fiber creates an electric field that can be recorded from the EMG needle.
In neuropathic conditions the motor axon is injured, leaving muscle fibers of entire motor units without innervation. The muscle fibers become hyperexcitable, leading to spontaneous depolarization and contraction of a single muscle fiber, which is detected on EMG as fibrillations or positive sharp wave potentials. The denervated muscle fibers also produce signaling molecules that induce nearby motor neurons to sprout axon twigs to innervate the orphaned fiber ( Fig. 2 ), which leads to enlarged motor units because then a single axon controls a greater number of motor fibers, as shown in Fig. 1 B.
In myopathic disorders the pathology localizes to individual muscle fibers. Inflammatory or degenerative processes result in the loss of muscle fibers within the motor units, meaning that for any given motor unit there will be fewer muscle fibers activated with each propagated action potential. The motor units are smaller (see Fig. 1 C).
Firing of a single motor neuron leads to the depolarization of many muscle fibers. If all of the muscle fibers in a motor unit depolarize at approximately the same time, their electric discharges will summate into a single potential seen on the EMG screen known as the motor unit action potential (MUAP), shown in Fig. 1 A. The shape of the MUAP will change in characteristic ways in neuropathic and myopathic conditions, and these changes can be used to localize the pathology.
Electromyography
Electromyography is the most important part of the electrodiagnostic evaluation of possible myopathy. A small needle is inserted into various muscles to measure voltage. The most commonly used type of needle electrode contains a small concentric, bipolar electrode that measures the electrical potential difference between the edge and the inside of the needle. The needle consists of a cannula with a strip of conducting material that is insulated except at the tip. The needle cannula serves as one electrode. The conducting material inside the cannula serves as the other electrode. The concentric needle sizes range from 26 to 30 gauge and 2.5 to 5.0 cm in length. Within each muscle the needle must be moved several times to sample different locations within the muscle. With each movement the patient may feel a pinch similar to a blood draw. The electrical signal is amplified and then displayed on an oscilloscope screen in a plot of voltage versus time. The recording is converted to sound produced through a loudspeaker. Experienced electromyographers often rely on the audio characteristics when interpreting the results.
The number of muscles sampled varies based on the differential diagnosis. For example, testing for amyotrophic lateral sclerosis (ALS) will require sampling more muscles than carpal tunnel syndrome. Evaluation of a patient with myopathy depends on clinical history, but should include selected proximal muscles to establish a myopathic pattern as well as some more distal muscles to rule out a more diffuse process. In this case EMG might begin with proximal muscles such as deltoid, biceps, quadriceps, and iliopsoas as well as any muscle identified as weak during the neurologic examination. The electromyographer samples each muscle in the relaxed state, in a state of voluntary contraction and maximal contraction. Thus, a patient must be able to follow simple instructions to obtain optimal results.
Voluntary Activity
The electrical activity of the motor units closest to the needle placement is evaluated. The morphology of the motor unit potentials gives information about whether a process is neuropathic or myopathic. Of course, the motor neurons must fire action potentials in order for motor units to appear on the screen. The simplest way to produce these potentials is to ask the patient to contract the muscle voluntarily. In chronic neuropathic conditions, the MUAPs are enlarged (see Fig. 1 B), reflecting the expansion of the motor units themselves. After motor axons degenerate, neighboring motor neurons take over the orphaned muscle fibers. This takeover leads to higher amplitude motor units because the number of muscle fibers discharging simultaneously increases. The time duration of the MUAP also increases. The enlarged motor unit now covers a broader area of muscle tissue, so naturally more time is required to activate newly innervated fibers that are more distant than the native fibers. These characteristics come over the loudspeaker as a loud, dull thud. In myopathic processes the necrosis or apoptosis of muscle fibers leads to fewer muscle fibers in each motor unit, and produces motor units that appear small on EMG (see Fig. 1 C). The duration of the MUAP is shorter than usual because some of the most distant muscle fibers have degenerated. These MUAPs sound like high-pitched crackling. In both types of conditions the action potential waveforms appear polyphasic. Polyphasia refers to the number of times the waveform crosses the baseline, which is an indication that muscle fibers are firing asynchronously. If all of the fibers fire together, the action potentials will summate and there will be few phases. Polyphasia can occur in both neuropathic and myopathic conditions.
Recruitment
The other important parameter to assess during voluntary contraction is recruitment. Initially, the single neuron fires repetitively at a slow rate as long as the force is maintained. If more force recruited, the single neuron will begin to fire more rapidly. This action is similar to rowing a boat—faster frequency of rowing produces more driving force. If still more force is recruited, additional motor neurons will begin to fire; this is known as recruitment. The force produced by a muscle is a function of the number of motor units firing and the frequency at which they fire. Force can be increased either by recruiting additional motor units or by increasing the frequency of the units already firing. Normally the ratio between number of motor units firing and firing rate is 1:5. Once a single unit begins firing at 10 Hz, another will begin to fire. At about 15 Hz there will be 3 units firing, and so on. In this way additional motor units are recruited to increase the force generated. In neuropathic conditions there are fewer neurons and thus fewer motor units available, so recruitment is decreased. Thus, increased force is produced by driving the motor units to fire faster; this is known as decreased recruitment. In myopathic conditions there are normal numbers of motor units, but each individual unit is weaker than it would normally be. Thus, additional motor units are recruited at lower levels of force than they normally would be; this is known as early recruitment. The electromyographer will attempt to assess the recruitment pattern as normal, decreased, or early.
Spontaneous Activity
Electrical discharges that occur spontaneously in relaxed muscle can be useful for determining if there is pathologic condition and whether it localizes to muscle or peripheral nerve.
Normal spontaneous activity and insertional activity
During the evaluation the patient is asked to relax the muscle and the electromyographer evaluates spontaneous activity. Normal muscles produce spontaneous discharges for less than 2 seconds after each needle movement, due to the disruption of muscle fiber membranes by the needle; this is known as insertional activity. If it persists longer than 2 seconds, it is abnormally increased and is a nonspecific finding that may be caused by neuropathic or myopathic conditions. After insertional activity dissipates, normal muscle fibers are electrically silent except in certain regions located near the endplate of the neuromuscular junction. Here spontaneous electrical activity known as miniature endplate potentials (MEPP) and end-plate spikes (EPS) can be recorded in normal muscle. MEPP are low-amplitude, short-duration discharges that correspond to the spontaneous release of a single vesicle of acetylcholine at the endplate. MEPP occur continuously and overlap one another, giving an audio quality like listening to a sound of the ocean in a seashell. EPS are higher amplitude spikes with waveform morphology similar to fibrillation. The difference is that EPS are irregular, whereas fibrillations fire rhythmically. The physiologic generator of EPS is controversial. One hypothesis is that EPS measure discharges from intramuscular nerves. Alternatively, muscle spindles contain small muscle fibers that regulate the sensitivity of the proprioceptive apparatus. These intrafusal muscle fibers are innervated by a separate set of gamma motor neurons, not by the alpha motor neurons that innervate skeletal muscle. Some investigators have postulated that these intrafusal muscle fibers may be the origin of EPS. In summary, therefore, there are 3 types of spontaneous activity seen in normal muscles: insertional activity, MEPP, and EPS.
Abnormal spontaneous activity
Other spontaneous discharges are abnormal. Their morphology can be useful in determining the source of the pathology. Abnormal spontaneous activity will originate from one of two sources: individual muscle fibers or peripheral nerve axons. The size and morphology of the electrical potential suggests the source of the abnormality. A discharge originating in a single muscle fiber will be small relative to a discharge of a single motor neuron. An abnormal discharge in the motor neuron will propagate down its axon to activate all the muscle fibers in the motor unit, possibly as many as hundreds of muscle fibers. Therefore, discharges originating from a peripheral nerve axon will be higher amplitude and longer duration than those originating from a single muscle fiber. As might be imagined, discharges originating from the peripheral nerve will be indistinguishable from the morphology of motor unit potentials that were voluntarily initiated. The muscle acts the same whether the action potential originates normally in the brain or pathologically due to hyperexcitable peripheral nerves. The difference is that motor units firing under voluntary control tend to fire slower and more irregularly than those originating from pathologic peripheral nerves.
Abnormalities originating in muscle fibers
Abnormal spontaneous discharges originating from the muscle fibers include fibrillation, positive sharp waves, complex repetitive discharges (CRDs), and myotonia.
Fibrillation
A fibrillation is the spontaneous firing of a single muscle fiber, and results when a muscle fiber is disconnected from the motor neuron. Fibrillation of skeletal muscle is not visible through the skin surface, and is detectable only by electrodiagnostic testing. The EMG appearance of fibrillation is a low-amplitude, short-duration potential with an initial negative deflection ( Fig. 3 ). Fibrillation typically repeats in a rhythmic pattern, but can also occur irregularly. The exact mechanism producing fibrillations is not known, but several hypotheses have been proposed. In fibrillating muscle fibers there are local segments of abnormal muscle fiber membrane that are probably not more than millimeters in length. These abnormal regions are the origin of fibrillation potentials. Such abnormal segments most often occur near the neuromuscular junction, but can occur anywhere along the length of the muscle fiber. Random, spontaneous depolarization of these abnormal segments that is sufficient to trigger an action potential results in irregular fibrillation. By contrast, rhythmic fibrillation is produced by regular, oscillating fluctuations in membrane potential. These spontaneous depolarizations and oscillating potentials may be caused by altered sodium conductance in the abnormal segment of membrane. Although denervated muscle fibers exhibit altered expression of acetylcholine receptors compared with a normal muscle fiber, fibrillations are probably not caused solely by hypersensitivity to circulating acetylcholine because fibrillation persists in the presence of tubocurare.
Fibrillation is typically evidence of a neuropathic disorder, although there are some exceptions. Certain myopathic processes, particularly inflammatory myopathies or certain muscular dystrophies, can develop fibrillation. Such fibrillation may be caused by inflammatory damage to the distal motor axon that makes the terminal branches of the nerve hyperexcitable. Alternatively, segmental necrosis may occur between the insertion or origin of the fiber and its neuromuscular junction, effectively severing the distal part of the fiber from its endplate. Certain muscular dystrophies including Duchenne muscular dystrophy and limb girdle muscular dystrophies also cause segmental necrosis and fibrillation. Furthermore, fibrillation can be seen in multifocal motor neuronopathy, which is a demyelinating condition in which the axon is typically preserved.
Positive sharp waves
Positive sharp waves are a second type of spontaneous activity. Like fibrillation, positive sharp waves are also low-amplitude, short-duration, very regular discharges arising from a hyperexcitable muscle fiber. The clinical significance of positive sharp waves is usually the same as that of fibrillation—it suggests denervation. Indeed, changing the needle position slightly often will convert a positive sharp wave into a fibrillation.
Complex repetitive discharges
A third type of spontaneous activity arising from muscle is a CRD. CRDs are long volleys of potentials of uniform amplitude and frequency that start and stop abruptly. On the loudspeaker these discharges sound like a mechanical hum, and may last minutes or up to half an hour in isolated cases. CRDs originate with ephaptic transmission of action potentials from one diseased muscle fiber to the adjacent muscle fiber. The initiating muscle fiber discharges an action potential and then abnormally transmits the depolarization to its neighbor, which in turn transmits the depolarization to its neighbor. Eventually the process will come back to the original muscle fiber and begin again. This cycle creates the repetitive nature of the discharge. The process must originate and propagate in muscle fibers because CRDs are not abolished by curare or nerve block. CRDs are most often associated with chronic neuropathic disorders, although they can be seen in some myopathic diseases. Loosely speaking and for practical purposes, then, CRDs may be thought of as a chronic form of fibrillation.
Myotonia
The final type of spontaneous discharge originating in muscle fibers is myotonia. Myotonia is a repetitive firing of muscle fibers at high frequency. The morphology of myotonic discharges is a repetitive train of muscle fiber discharges. Occasionally the discharges may have a morphology similar to MUAP. Myotonic discharges are thought to result from unstable membranes in muscle fibers. The frequency and amplitude of a myotonic discharge may fluctuate, giving it a waxing and waning sound over the loudspeaker that has been likened to a revving engine, motorcycle, or dive bomber. Myotonia must be differentiated from CRDs or neuromyotonia. CRDs have a steady sound, whereas myotonia waxes and wanes. Neuromyotonia does not have a waxing phase, but starts strong and dies out producing a “pinging” sound. The differential diagnosis of myotonia includes hereditary muscle diseases including myotonic dystrophy, paramyotonia congenita, myotonia congenita, hyperkalemic periodic paralysis, acid maltase deficiency, and other glycogen storage myopathies. It is also seen in some acquired muscle diseases such as hypothyroidism, colchicine toxicity, statin myopathy, and rarely in inflammatory myopathies. Rarely, myotonia will also occur in any neuropathic condition that causes fibrillation.
Abnormalities originating in peripheral nerve
There are 5 types of spontaneous activity arising from the peripheral nerve axon: fasciculation, grouped discharges, myokymia, cramp discharges, and neuromyotonia. Each of these discharges has the morphology of a MUAP on the EMG screen, because the abnormality originates in the motor axon. The difference between these types of discharges is the frequency at which they fire. The mechanism is thought to be destabilized, hypopolarized, or hyperexcitable motor neuron membranes.
Fasciculation
Fasciculation results from a single, spontaneous discharge that arises anywhere along the motor axon from the spinal cord to the terminal nerve twigs. Clinically, fasciculations are spontaneous twitching of a small segment of muscle that is seen under the skin’s surface. The morphology on the EMG screen is similar to that of a MUAP ( Fig. 4 ), and it has an audio quality like a dull thud.
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