Anatomical review

In order to best understand the systematic interpretation of data from the electrophysiological examination of nerves, the reader is invited to review the following foundational principles. These are explored in much greater detail elsewhere in this text.

Nerve conduction tests

A general overview of NCSs is presented to provide an understanding of their application and indications. Many excellent texts are available for details of the techniques.

Motor and sensory NCSs can provide data that are helpful in establishing the presence and location of pathological conditions in the PNS. The tests may indicate the anatomical level, such as a plexopathy, versus a localized peripheral mononeuropathy. Individual and multiple nerves may be assessed, and the findings compared with responses of the same nerves contralaterally. The site of pathology may be localized, such as median nerve compression at the wrist versus a lesion of the lateral cord of the brachial plexus.

Nerve conduction velocity is faster in myelinated fibers because of saltatory conduction. Disorders involving peripheral demyelination can thus be differentiated from impairments primarily involving axonal degeneration. A mild localized compressive disorder (neurapraxia) may be distinguished from a more severe lesion in which the axons and surrounding connective tissue have been completely disrupted (neurotmesis). ^, In the event that the findings of nerve conduction studies (NCS) and EMG are normal, the clinician may be able to rule out most conditions involving the PNS and look for central nervous system (CNS) or other pathology. Knowledge of the rationale for NCSs and EMG should help the therapist decide when the tests may be indicated and understand the reasoning behind reports of tests that have already been performed on patients.

Motor nerve conduction

In motor NCSs the peripheral nerve is stimulated at various sites and the evoked electrical response is recorded from a distal muscle supplied by the nerve (a measure of orthodromic conduction). Surface electrodes are usually used for both stimulating and recording. An example of electrode configuration for a motor NCS is shown in Fig. 31.1 (ulnar nerve study). The response represents the electrical activity of muscle fibers under the recording electrodes and is called the compound muscle action potential (CMAP). It is also called the M wave or M response. Measurements are taken of the latency (the time in milliseconds required for the impulse to travel from each stimulus site to the recording site) and the amplitude of the response in millivolts (mV). The shape and duration of the response are assessed, and motor nerve conduction velocity is calculated for each segment of interest by dividing the distance between stimulus sites (in millimeters) by the difference in latency measured at each respective site.

Electrode Location for Ulnar Motor Nerve Conduction.

AR , Active recording.

Velocities, latencies, and the shape and amplitude of the responses ( Fig. 31.2 ) are studied and compared with established normal values and often with values taken from tests of the uninvolved extremity (when possible). In infants and children, nerve conduction is slower than in adults and reaches adult values by age 4 years. Nerve conduction velocities gradually slow after 60 years of age but generally remain within the outer limits of normal. ^,

Velocities, Latencies, and the Shape and Amplitude of the Responses.

Sensory nerve conduction

Sensory nerve conduction can be measured from many superficial sensory nerves, such as the superficial radial and sural nerves. It can also be measured from mixed motor and sensory nerves. The stimulus is applied over the nerve in question, and the recordings taken from electrodes placed over a distal sensory branch of the nerve. The recordings are called sensory nerve action potentials (SNAPs). An example of recording and stimulation sites is shown in Fig. 31.3 . Both orthodromic and antidromic conduction can be assessed. Response latencies and amplitudes are measured, and sensory nerve conduction velocities are calculated for each segment by dividing the distance between two adjacent stimulus and recording sites, or two stimulus sites, by the latency (conduction time) between these same sites. Sensory nerve responses are considerably smaller than motor responses. Their amplitudes are generally measured in microvolts (μV). Sensory conduction velocities may be calculated with just one stimulation as, contrary to motor studies, there is no neuromuscular junction to account for in sensory nerves. Sensory recordings are more sensitive than motor recordings in cases of mixed sensory-motor neuropathies.

Example of Recording and Stimulation Sites.

F-wave latency

When a motor nerve is stimulated in the periphery, both orthodromic (peripherally to the muscle) and antidromic (centrally toward the spinal cord) impulses are generated. A proportion of the antidromic impulses will, as it were, “bounce off” the axon hillock and return as a recurrent discharge along the same neurons to activate the muscle from which the recording is taken. This activity is termed the F wave ( Fig. 31.4 ), and it is observed as a small wave occurring after the M wave. ^{, ,} No synapse is involved. Thus the F wave is not a reflex response, but rather only a measure of conduction along the motor neuron. Specific conditions of electropotential must exist at the soma-dendritic cell membrane to reactivate the efferent axon; therefore the occurrence of the F-wave response is inconsistent and variable in latency and waveform.

Example of F-Wave Study.

The F-wave latency can be useful in evaluating conduction in conditions usually involving the proximal portions of the peripheral neurons (e.g., radiculopathy, Guillain-Barré syndrome, or thoracic outlet syndrome [refer to Chapter 18 ]). Its value, however, has been questioned by some authors because of its variability. Normal values of F-wave latency are 22 to 34 msec in the upper extremity (stimulating at the wrist) and 40 to 58 msec in the lower extremity (stimulating at the ankle), depending on the height of the subject, with a bilateral difference in latency of no greater than 1 msec.

H-reflex response

The H-reflex response latency ( Fig. 31.5 ) is a measure of the time for action potentials elicited by stimulating a nerve in the periphery to be propagated centrally over the Ia afferent (sensory) neurons to the posterior horn of the spinal cord, to be transmitted across the synapse to α in the anterior horn of the spinal cord, and then to travel distally over these neurons to activate the muscle. The response therefore measures conduction in both the afferent and efferent neurons. ^, It is also referred to as a “late” response (the other being the F wave).

Example of H-Reflex Study.

The H reflex is constant in latency and waveform, and it occurs with a stimulus usually below the threshold level required to elicit the M-wave response (Ia afferent fibers are larger in diameter than α and thus more sensitive to electrical stimulation [ES]). This monosynaptic reflex response is most easily found by stimulating the tibial nerve at the popliteal area and recording from the soleus muscle. Braddom and Johnson reported a mean latency of 29.8 msec (±2.74 msec) for the tibial nerve in normal adults, and a bilateral difference of no more than 1.2 msec. The H-reflex latency is a valuable measure of conduction over the S1 nerve root in differentiating suspected proximal plexopathy and radiculopathy from a herniated disc or foraminal impingement. Sabbahi and Khalil have reported a technique for recording the H reflex from the flexor carpi radialis muscle when stimulating the median nerve. In normal human beings older than 1 year, the H reflex is usually seen only in the tibial, femoral, and median nerves. It can be elicited from several nerves in infants and in conditions of CNS dysfunction in adults.

Blink reflex

In some conditions (e.g., facial palsy) or to establish the presence of a widespread neuropathy, it may be necessary to assess the conduction in the cranial nerves. The stimulation of the supraorbital nerve (branch of the trigeminal nerve, cranial nerve V) elicits two separate responses of the orbicularis oculi muscles, one early and ipsilateral (R1) and one late and bilateral (R2), via the facial nerve (cranial nerve VII). The early R1 response involves a synapse in the pons, whereas the late bilateral R2 responses involve a more complex pathway involving the pons and lateral medulla. Analysis of the respective latencies of each response will help in determination of the following:

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  Trigeminal neuralgia (cranial nerve V) or Guillain-Barré: all responses delayed

  
  
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  Acoustic neuroma or Bell palsy (cranial nerve VII): ipsilateral responses delayed

  
  
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  Pontine lesion: absence of R1

  
  
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  Medullar lesion: absence of R2

Hence, the blink reflex is a nerve conduction test that assesses a portion of the CNS and has shown some utility in assisting in the diagnosis of multiple sclerosis (see Chapter 19 ) and Wallenberg syndrome.

Clinical evoked potentials

Electrical potentials elicited by stimulation of nerves or sense organs in the periphery can be recorded from various sites as the impulses are transmitted centrally along the neuronal pathway and from the representative area of the brain. ^, SSEP procedures are particularly useful in assessing the integrity of afferent pathways in the CNS. They are helpful in differentiating among lesions in areas such as the plexus, spinal cord, brain stem, thalamus, and cerebral cortex. Evoked potential tests have the advantage of providing data about the integrity of both peripheral and central neuronal pathways, including transmission across axodendritic synapses.

The SSEP is valuable in assessing damage and continuity of spinal cord tracts in early spinal cord injury (SCI). For example, if an electrical stimulus is applied at the popliteal area over the tibial nerve, responses can be recorded with surface electrodes placed over the spine at the L3 and C7 spinal segments and from the lumbar representation of the contralateral sensory cortical area. Conduction time and other parameters of the response waveforms can be measured from the recordings.

This simplified example of an SSEP illustrates how conduction over sensory peripheral nerves and afferent pathways to the cerebrum can be studied. The median nerve is usually tested to evaluate the integrity of peripheral and central pathways and their synaptic connections as the impulses travel from the upper extremity to the contralateral cortical area.

In visual evoked potential (VEP) procedures, visual stimuli such as variable light flashes of changing patterns are applied to one or both eyes under highly controlled conditions. The response is recorded from the scalp over the representative area of the cerebral cortex. ^{, ,} The term pattern reversal evoked potentials (PREPs), a more descriptive term for these procedures, is recommended by the American Electroencephalographic Society. These tests and other VEP procedures are useful in assessing pathology of retinal photoreceptors, the optic nerve, and postchiasmal pathways. Abnormal conduction findings have been reported when VEP studies are used in demyelinating disorders such as multiple sclerosis and optic neuritis. The examiner may conclude that the patient is cortically blind because no response is recorded on the visual cortex. Although many causes for a stimulus not reaching the visual cortex are possible, the end result is considered blindness. If the cause for cortical inactivity is swelling or a neurochemical imbalance within a nuclear relay structure, once corrected, the individual may experience normal vision. A change in the reaction of the patient to the visual environment may reflect increased awareness and a change in coma scale rating. Similarly, just because an individual turns toward a light or visual stimulus does not mean an evoked potential reaches the visual cortex. Instead, the eyes as receptors and the visual tract to the brain stem may be intact even though a problem in the synaptic connections between or within the thalamus and visual cortex may exist.

Auditory evoked potential tests are used to evaluate neurological function of the cochlear division of the auditory nerve (eighth cranial nerve), central auditory pathways and synapses in the brain stem, and the receptor areas on the cerebral cortex. ^, Brain stem auditory evoked potentials are frequently referred to as BAEPs. A series of high-intensity clicks is applied to auditory receptors in the ears through headphones, and several components of the response waveforms are recorded by using surface electrodes over the representative cortical areas. The BAEP is an effective test procedure for localizing and evaluating acoustic neuromas and other space-occupying lesions in the brain stem. This test is also used for assessment of brain damage in patients who are comatose as a result of traumatic brain injury (TBI). Robinson and Rudge recommend caution in using BAEP tests for this purpose because other factors, such as defective receptor organs, can cause abnormalities in BAEPs.

The evoked potential tests described in this chapter all require application of appropriate external stimuli that are rapidly repeated many times. The response is electronically averaged to sort out the desired signal from interference signals. The conduction times (latencies), waveform shape and amplitude, and sometimes conduction velocities are measured and compared with normal values. Absence of a response, increased latencies, decreased amplitudes, and slowing of conduction velocities are all abnormal findings. Normal values and details of techniques for the evoked potential tests are described elsewhere. ^,

Therapists with special interest and training administer the SSEP tests for neurological applications more frequently than other types of evoked potential tests. Because of the highly specialized techniques necessary to administer VEP tests for ophthalmological applications and BAEP tests for hearing dysfunction, they are usually performed by persons who specialize in these procedures.

Needle electromyography

Needle EMG complements the motor and sensory NCSs; it is most sensitive in the detection of denervation.

Unlike NCSs, which use the ES of the motor nerves to elicit muscle contraction, the needle EMG is used to record and analyze muscle activity at rest and during voluntary activation. It is particularly useful in identifying pathology of the lower motor neurons and of the muscle itself. EMG can also be used to identify abnormalities of motor neuron recruitment that are associated with certain disorders of the CNS, especially when NCS findings are normal—as would be the case in radiculopathies. The primary recording studied is the insertional activity, along with activity at rest and the motor unit action potential (MUAP), which is produced by the depolarization of single motor units during voluntary or reflex activity. Spontaneous electrical activity of single muscle fibers at rest is termed fibrillation and is diagnostic of denervation. For recording of muscle activity, small-diameter needles are inserted within the muscles to be studied. Three electrodes are required: active (negative), reference (positive), and ground. The needles may be monopolar, requiring a second needle or surface electrode for reference, or bipolar, containing both the active and reference electrodes (usually concentric in cross section). The ground electrode is typically placed on the surface of the skin. Most commonly the needles used are disposable. The activity detected in the muscle is displayed on the video display terminal of a computer (and can be stored and printed later). It is simultaneously played through an audio amplifier. The electromyographer can often identify pathological conditions by the characteristic “sounds” of the electrical activity of the muscle. Many excellent resources are available for readers interested in details of the equipment and procedures for EMG. ^{, ,} Details of contraindications and special precautions are described by Currier and Nelson.

In an EMG examination, four conditions are evaluated at each location: (1) activity during needle insertion (normally a brief burst [250 msec at most] of high-frequency activity that abates when the needle stops moving), (2) activity during rest (electrical silence is normal unless an electrode is placed directly over a motor endplate), (3) activity during minimal and gradually increasing voluntary contraction (biphasic or triphasic MUAPs of small amplitude composed of slow-twitch type I fibers that increase in frequency and are joined by higher-amplitude potentials as larger, predominantly fast-twitch type II motor units are recruited), and (4) activity during maximal activation (an interference pattern caused by the blending of potentials in which individual MUAPs cannot be identified), characterized as full (complete), reduced, or absent. Several locations in an individual muscle may be studied. The specific muscles to be studied are determined by clinical findings, and results must always be interpreted in the context of the total complex of signs and symptoms. In determining the specific location of a lesion, muscles located both proximally and distally to the suspected lesion site must be assessed. Studies may be repeated at intervals to determine if changes consistent with recovery (such as reinnervation) or exacerbation are present. EMG is typically used to help determine the presence (and extent) of the following:

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  Denervation

  
  
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  Reinnervation

  
  
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  Myopathic or neuropathic signs

  
  
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  Distribution or specific location of peripheral nerve pathology

Needle EMG is sensitive in determining the state of the axons. When axons are interrupted, there is denervation of the muscle cell. The findings in denervation and partial denervation include increased insertional activity, fibrillation potentials or positive sharp waves at rest, and a reduced or absent interference pattern. CNS dysfunction can result in no resting potentials, but if motor control was impaired, a decreased or abnormal interference pattern might be apparent because of difficulty in recruitment ( Fig. 31.6 ). Needle EMG also informs the clinician about muscle cell disorders such as myopathies. The primary finding in the case of a myopathy is fibrillation potentials and small-amplitude polyphasic MUAPs.

Typical findings in lower and upper motor neuron disorders and myogenic lesions. Myotonia shares many features common to myopathy in general in addition to myotonic discharges triggered by insertion of the needle or with voluntary effort to contract the muscle. Polymyositis shows combined features of myopathy and neuropathy, including (1) prolonged insertional activity, (2) abundant spontaneous discharges, (3) low-amplitude, short-duration, polyphasic motor unit potentials, and (4) early recruitment leading to a low-amplitude, full-interference pattern. EMG , Electromyography.

(From Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice . 3rd ed. New York; Oxford University Press; 2001.)

Although patient cooperation during EMG testing is important, some aspects of muscle electrical activity can be studied in the patient who is very young (infant), who is unable to move, or who has only involuntary or reflex activity. Insertional and resting potentials can always be evaluated. MUAPs appear during the contraction of muscle fibers activated both voluntarily and involuntarily in both isotonic or isometric conditions. In normal conditions MUAPs are seen with voluntary movement; however, reflex activation of muscle also produces MUAPs with certain normal characteristics. In CNS disorders, hypertonic or spastic muscles will produce recognizable MUAPs when they are actively contracting. The electromyographer can elicit a contraction by tapping on the muscle or tendon. For example, consider a patient who is recovering from a traumatic head injury with residual spastic hemiplegia. She is unable to cooperate with the EMG exam. The patient has abnormal extensor responses in the lower extremity with the exception of the ankle and foot, which appear flaccid. An EMG of the leg and foot muscles detects abnormal resting potentials, including fibrillation and positive sharp waves in muscles innervated by the fibular nerve. Tapping on the muscles fails to elicit MUAPs. Muscles in the tibial nerve distribution have no resting potentials and respond with bursts of identifiable MUAPs when the tendon is tapped. These findings would guide the physician and therapist in looking for a possible peripheral nerve lesion in addition to the CNS dysfunction. The treatment program in this situation would differ from that for a patient without peripheral nerve pathology.

In summary, results of the needle EMG are best presented in the form of a table that indicates the following:

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  Insertional activity: may be increased in acute denervation and myopathic processes

  
  
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  Spontaneous activity at rest: present (positive sharp waves and fibrillation potentials) in denervation

  
  
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  Fasciculations: may be present in motor neuron disorders

  
  
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  Analysis of MUAPs:

  
  
  
  
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    Amplitude: Low amplitude is seen in myopathy or nascent potential; large amplitude is a sign of chronicity.

    
    
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    Phases: Normal MUAPs are biphasic or triphasic. Polyphasia is indicative of denervation-reinnervation.

    
    
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    Recruitment pattern: A less-than-full recruitment is indicative of fewer motor units discharging, as can be seen in axon loss.

    
    
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    Firing rate: If elevated, fewer motor units are contracting more often to provide the same tension in a denervated muscle.

  
  

Justification and analysis of the basic principles and results after EMG studies should assist therapists managing patients with neurological dysfunction in planning and modifying therapeutic management programs. As electrical tests are being conducted, the findings are continuously studied and used by the physician and examiner as a guide in continuing with or modifying the plan for future tests based on whether they fit the characteristics usually identified with specific pathological conditions. As previously stated, the results of the electrodiagnostic tests must be correlated with other clinical findings and data.

Summary of clinical electroneuromyographic and nerve conduction studies

Instruments with computer-assisted analysis are now commonplace for studying electromyographic signals in great detail. ^, Parameters of the waveform, including amplitude, duration, frequency spectrum, number of turns, or phase polarity reversals and area (the integral or total voltage of the waveform), can be automatically analyzed. The data are then compared electronically with predetermined patterns of electrical changes, which correlate with categories of neuromuscular disorders such as myelopathies and neuropathies.

The following is a summary of the more characteristic EMG and nerve conduction changes associated with selected groupings of neurological disorders. The intent is to assist in the understanding of reports of these studies and recognize changes that may be seen in sequential tests during the course of the disorders. The following is a simplified grouping of electrical changes; actual electrodiagnostic studies show considerably more detail and frequent variations of these findings. ^{, ,}

Electrical testing in CNS disorders typically shows normal motor and sensory nerve conduction. In the EMG, spontaneous activity is typically not seen, and individual motor units seen on muscle contraction usually have normal parameters. The recruitment pattern may show a slower-than-normal MUAP discharge frequency with an incomplete and irregular interference pattern. In the presence of tremor and other involuntary movements, bursts of MUAPs occur, consistent with the muscle contraction pattern. In cases involving the brain stem, the blink reflex may show abnormalities. The tests are important in differential diagnosis between a CNS and a PNS problem, but often they are not used when clinical examinations demonstrate the problem to be definitively in the CNS.

In myelopathies, which include upper and lower motor neuron disorders (e.g., amyotrophic lateral sclerosis [ALS], poliomyelitis, cervical spondylitis, and syringomyelia), motor and sensory nerve conduction is usually normal, although mild slowing may be present. Motor amplitudes may be decreased. The characteristic EMG changes, which usually appear in the more chronic stages of the disorders, are increased amplitude and duration of MUAPs because of the variable impulse conduction time in sprouting axon terminals. An increased number of polyphasic potentials with increased duration is usually found. Spontaneous activity is often seen, and on strong contraction fewer rapidly firing large MUAPs are recruited, resulting in a single-unit or partial interference pattern. Fasciculations and denervation potentials are typically found in ALS. The distribution of the EMG abnormalities determines the extent of the condition.

Peripheral neuropathies show a variety of electrical changes depending on the type and location of the pathology. In a proximal pathology (e.g., radiculopathy), motor and sensory nerve conduction generally remain normal, except F waves and H-reflex responses in specific spinal segments. If motor nerve roots are compromised, spontaneous activity and increased polyphasic potentials appear, and reduced recruitment of MUAPs results in an incomplete interference pattern. In more chronic stages MUAP amplitude and duration can be increased. As the lesion improves, spontaneous activity decreases and the recruitment patterns become more normal. If only sensory roots are injured, no EMG changes occur. Again, the distribution of the EMG abnormalities (all in one myotome) is pathognomonic, especially in the presence of denervation potentials in the corresponding paraspinal muscles.

Lesions of peripheral nerves, which range from a focal mononeuropathy to plexopathy, frequently show abnormalities in motor and sensory nerve conduction, depending on which components of the nerve are involved. In the EMG, spontaneous activity, particularly fibrillation and positive sharp waves, is common. If the lesion is complete, no MUAPs are found. The presence of even a few MUAPs suggests a more optimistic prognosis. Often the location of the lesion can be identified by the distribution of the electrical changes. With regenerating axons, low-amplitude polyphasic MUAPs gradually appear. In the chronic stage, the amplitude and duration of MUAPs are often increased. Spontaneous activity decreases with reinnervation, but it may persist for several years.

Generalized, systemic peripheral polyradiculoneuropathies can be divided into primarily demyelinating, primarily axon loss, or mixed axonal-demyelinating polyneuropathies. Some involve mostly sensory nerves (e.g., hereditary sensory neuropathy types I to IV, Sjögren syndrome, Friedreich ataxia), and others mostly motor nerves (e.g., chronic inflammatory demyelinating polyneuropathy [CIDP], lead neuropathy), but most involve both sensory and motor nerves. In the primarily demyelinating type, such as Guillain-Barré syndrome, motor and sensory nerve conduction become markedly slow and F-wave latencies are delayed. EMG changes usually do not occur, except for a reduced recruitment pattern consistent with weak muscle contraction or conduction block (when the demyelination is such that the impulse does not propagate). With primarily axonal polyneuropathies, such as uremic neuropathy, isoniazid or cisplatin toxicity, and lead poisoning, motor and sensory nerve conduction is mildly slowed or may remain normal. The duration and amplitude of the response, however, decrease. During advanced stages, many polyneuropathies develop both demyelinating and axonal pathology (e.g., diabetic neuropathy, which is by far the most commonly encountered polyneuropathy). On EMG, spontaneous activity is commonly seen. These electrical changes generally become more severe with worsening of the pathology, but they also improve if the pathology is reversed. From a patient management standpoint, remyelination occurs at a much more expedient pace than reinnervation.

Again, the scope of the NCS electromyographic test is to determine the presence and extent of a neurological dysfunction. Attributing a cause to the dysfunction (e.g., diabetes versus alcoholism) requires other tests.

With myopathic disorders, motor and sensory nerve conductions are generally normal unless neural tissue is also affected. In advanced stages, however, severely atrophied muscles can produce decreased amplitude and distorted nerve conduction responses. The characteristic findings on EMG are short-duration, low-amplitude potentials. Some spontaneous potentials, particularly fibrillations and positive sharp waves, may be found but are much more frequent in the inflammatory myopathies such as polymyositis. Specific myotonic potentials appear in certain myopathic disorders (e.g., myotonia congenita). The recruitment pattern shows many low-amplitude MUAPs, appearing in a full pattern, with little voluntary effort. This type of recruitment pattern is referred to as early recruitment.

Neuromuscular junction disorders involve the synapse between axons and myocytes. In LEMS, the pathology is in the presynaptic membrane (decreased release of acetylcholine [ACh]), whereas, in myasthenia gravis, the pathology involves the postsynaptic membrane. The two are differentiated by the response to RNS or jitter with single-fiber EMG (see later). In LEMS, the amplitude of the responses increases with repetitive stimulation (more quanta of ACh released), whereas, in myasthenia gravis, the responses decrease (all ACh receptors saturated). Sensory NCS findings are typically normal. Table 31.1 provides a summary of typical findings.

Single-fiber electromyography

Electrical activity can be recorded from two or more muscle fibers innervated by the same motor unit by using a specially designed single-fiber needle electrode. SFEMG is, at this time, the most sensitive test for evaluation of neuromuscular transmission defects such as myasthenia gravis and myasthenic syndrome. It is also used to evaluate peripheral neuropathies, motor neuron diseases, and myopathies. It is typically provided only in tertiary centers and requires a high level of patient participation.

During a carefully controlled minimal voluntary contraction, a 25 μm–diameter needle is inserted into the muscle, and several potentials from muscle fibers within the recording area are stored. Equipment with a trigger and delay line is necessary to “time lock” the tracings of the potentials. The slightly different conduction time or interpulse interval (IPI) required for impulses to be transmitted from a single motor neuron to each of its terminal endplates, cross the neuromuscular junction, and activate the muscle fiber is called jitter. This time difference is collected from several tracings and is converted into a mean consecutive time difference (MCD), which normally ranges from 5 to 55 msec. Values shorter or longer than this range are considered abnormal. The impulses from some axons to their muscle fibers may fail to be transmitted. This is referred to as blocking. Another capability of SFEMG is the measure of fiber density, that is, the average number of muscle fibers within the needle recording area. Fiber density is increased in reinnervation and also with certain myopathies because of axonal collateralization or splitting.

Macroelectromyography

A variation of SFEMG uses a macroelectrode to record the majority of muscle fibers of a single motor unit as they are triggered by an initial potential, which is then time locked with all the other muscle fiber potentials recorded from a different part of the same or a nearby needle. ^{, , , ,} Two recording channels are used. Maximal amplitude of the potentials from several muscles has been reported by Stalberg. The findings are analyzed, along with findings of jitter, fiber density, and conventional EMG, to evaluate the status and prognosis of various neurological and neuromuscular disorders, such as motor neuron disease, peripheral nerve lesions, and myopathies.

In summary, the astute PT in the presence of a patient reporting weakness and/or numbness will refer that patient appropriately for electrophysiological testing based on the findings of a judicious clinical examination. In reading the report of an electrophysiological consultation, the PT will first correlate the results with the findings of the physical examination of the patient, determine whether the studies performed are complete (i.e., there are sufficient data to rule in the condition but also sufficient data to rule out other conditions), and correlate the findings with the conclusion. There is evidence that PTs performing NCSs and EMG tend to follow guidelines consistently.

Kinesiological electromyography

KEMG measures muscle activation during movement, whether it is purposeful, involuntary, dynamic, or relatively static. It is the method by which the therapist-examiner determines a muscle’s (or muscle group’s) onset, cessation, relative intensity, and activation sequencing during functional activities such as walking. Because normal movement depends on the CNS’s ability to execute motor programs through muscle action, KEMG provides the therapist with insight, in real time, into motor function, motor control, and motor learning.

Persons with neuromuscular disorders typically exhibit control errors, including the inability to initiate, execute, or terminate movement. Selective control may be absent or abnormal with errors in muscle timing, intensity, and sequencing. Spasticity or synergistic muscle action may impede smooth execution of tasks and prevent purposeful movement. With increased emphasis on evidence-based practice, KEMG can provide objective documentation of abnormal control and intervention outcomes and provide insight into optimizing strategies for improved functional performance. Many orthopedic surgeons rely on KEMG testing to supplement clinical evaluation in planning surgical interventions (muscle transfers and releases) in children with cerebral palsy (CP), in patients with TBI, and in those who have had a stroke.

KEMG interpretation depends on the examiner’s understanding of the instrumentation chosen for testing, including electrode selection, recording techniques, signal processing, and time and intensity normalization. Coupled with three-dimensional motion (infrared camera and motion capture) and force plate analysis, external moments are calculated that define internal force demands on muscles during functional activities such as walking. KEMG delineates the muscles that participate in meeting the internal force demand.

Neuromuscular electrical stimulation

NMES is often used as a tool in the management of neurological dysfunction. In NMES, muscle contraction is elicited by depolarization of the motor neurons. Electrodes may be placed over the muscle to be stimulated or over the motor nerve that controls the muscle. In NMES, muscle contraction is elicited by depolarization of the motor neurons. Electrodes may be placed over the muscle to be stimulated or over the motor nerve that controls the muscle. Firing order of neurons is a result of neuronal size, proximity of the electrical stimulus, and the intensity of stimulation. Muscle recruitment patterns triggered by ES differ from those observed in normal muscle activation. In a voluntary muscle contraction, motor units fire asynchronously, with a larger proportion of type I, fatigue-resistant muscle fibers of the smaller motor units being recruited first. The order of muscle fiber firing occurs as a result of motor neuron size and the anatomy of synaptic connections. Conversely, an electrically stimulated muscle contraction elicits initial responses from larger motor units, which contain a greater number of fatigable, type II muscle fibers. The type II fibers are innervated by larger-diameter neurons that have a lower threshold for ES than smaller neurons. A study of healthy subjects demonstrated recruitment of these higher-threshold motor units at relatively low NMES training levels. In voluntary exercise a much greater exercise intensity is required for activation of these larger motor units.

Synchronous recruitment of muscle fibers is obtained with ES. This does not occur with volitional activation. During a sustained volitional contraction, motor units periodically “drop out” and then “drop in” to reduce fatigue. With NMES, once recruited the motor units will continue to fire until the stimulus is ended. This, coupled with the early recruitment of fatigable motor units, accounts for fatigue being a major problem in the use of NMES. It also is one reason functional activities performed under control by stimulation are much less smooth and balanced than when they are performed volitionally. At the same time, relatively low levels of NMES can recruit motor units that volitionally would be recruited only with maximal effort. This provides support for observed increases in strength with low NMES training intensities. Numerous potential benefits have been identified for NMES, such as improvement in ROM, edema reduction, treatment of disuse atrophy, and improvement of muscle recruitment for muscle reeducation.

Parameters of stimulation

With any ES used for patients with neuromuscular dysfunction, three parameters must be considered within the waveform: pulse (or phase) duration, pulse frequency, and pulse amplitude. Depending on the intent of the ES, instruments are available that allow all of these to be independently adjusted.

Additional parameters: Ramp time and on-off time ratio

On time and off time.

NMES for facilitation of muscle contraction should be used to supplement exercise, and goals for stimulation should be consistent with the goals of the exercise program. To simulate isotonic or isometric muscle contractions, as in voluntary movement for exercise, the stimulator must have the capability of setting cycles of on and off times. Each period of muscle contraction is followed by a period of relaxation ( Fig. 31.7 ). ^, In most cases a shorter on time than off time is desirable to avoid fatigue. For example, a 10-second on time may be followed by a 50-second off time in a cycle, resulting in an on-off ratio of 1:5. Packman-Braun investigated ratios of stimulation to rest time with NMES for wrist extension in a group of hemiplegic patients. Results supported the on-off time of 1:5 as being the most beneficial in training programs of 20 to 30 minutes because of the deleterious effects of fatigue with lower ratios (1:1, 1:2, 1:3, 1:4). If the goal is to reduce edema by providing a muscle pumping action, a ratio of 1:1 or 1:2 may be preferred, as the intent is to decrease the edema by continuous muscle pumping action. Lower ratios may be used when the goal is neuromuscular reeducation or endurance training.

Example of the relation between ramp times and on/off times. Each division on the horizontal axis equals 2 s. Note that the ramp-up time is considered part of the on time, whereas the ramp-down time is considered part of the off time.

(Reprinted from DeVahl J. NMES in rehabilitation. In: Gersh MR, ed. Electrotherapy in Rehabilitation . Philadelphia: FA Davis; 1992.)

Ramping.

Ramping is another modulation that can be set by the therapist. Ramp-up is the time taken by each train of pulses to increase amplitude or intensity sequentially from zero to maximum. Ramp-off is the period set at the end of the train of maximal intensity pulses to decrease sequentially from maximum to zero amplitude (see Fig. 31.7 ). Ramp time can be adjusted so that the stimulation more nearly resembles a pattern of gradually contracting and relaxing muscles. For patients with hypertonicity or spasticity and a goal of facilitation and strengthening the antagonist muscle, a longer ramp-up time may avoid or minimize activation of the stretch reflex in the hyperactive agonist muscle.

Duty cycle.

The term duty cycle is sometimes confused with the on-off time ratio. Duty cycle is the percentage of time a series or train of pulses is on out of the total on and off time in a cycle. ^, For example, if the train of pulses is on 10 seconds and off 30 seconds, the total cycle time is 40 seconds. The duty cycle would be 25% (10 seconds of the total 40-second cycle). The actual on time and off time of the pulses in a cycle is a more informative description than either the duty cycle or the on-off ratio.

Electromyographic biofeedback

Biofeedback is a general term used to describe the use of visual or auditory representation of physiological processes to allow an individual to modify those processes. EMGBF makes available to the patient information regarding the electrical activity of muscle. EMGBF has several well-documented applications, including alteration of physiological responses such as heart rate, temperature, and muscle tension. These applications may prove beneficial for patients with neurological dysfunction. An example would be relaxation to modify pain perception. The focus of this review is the use of EMGBF for improvement of active movement, which may include reduction of hypertonicity in addition to muscle reeducation. EMGBF units range from basic single-channel portable models to clinical units with multiple channels and multiple options for provision of feedback.

EMGBF may be used to assist a patient in attaining greater levels of muscle activation in paretic muscle, decrease levels of muscle activation in spastic muscle, or attain a balance between agonist and antagonist muscle pairs. For most practicing clinicians, EMG levels are monitored through the use of surface electrodes. Monitoring of activation of deep muscles is often not feasible. Attention to size and specific electrode placement is critical to ensure feedback that will be useful. Smaller electrodes allow specific placement, although higher impedance will be encountered. Skin must be carefully prepared to take this into account. Because the EMG information recorded represents the sum of action potentials from motor units between the electrodes, large interelectrode distance will increase the area of muscle recorded. This may be desirable for large muscle groups or when minimal activity is present.

Smaller interelectrode distances are preferable if interference from “crosstalk” or “volume conduction” from muscles or motor units not part of the target group is a risk. Basmajian and Blumenstein provide an excellent review of electrode placements.

Integrating neuromuscular electrical stimulation and electromyographic biofeedback

The use of EMG-triggered NMES, in which NMES is initiated once the patient has achieved a predetermined level of EMG activity in the targeted muscles, is an application that has been shown to have merit, ^, although a more recent systematic review showed no statistically significant differences between EMG-NMES and usual care in improving upper-extremity function of the affected extremity in people who had had a stroke. Threshold levels of EMG activity could gradually be increased as the patient gains the ability to activate muscles independently, with eventual discontinuance of the NMES as strength and active control allow. The success of this application has been shown in patients with hemiplegia in terms of increasing EMG activity and subsequent improvement in ROM and function in the involved arm and leg. In another study, patients who had had a stroke more than 1 year previously significantly improved wrist and finger extension strength and function after treatment with EMG-triggered stimulation compared with controls.

A variation of this application is NMES triggered by positional feedback, such that NMES is initiated once the patient actively moves through a portion of the available ROM at a joint. The therapist may set the threshold angle in accordance with the patient’s goals and abilities. This method has been shown to be effective in improving wrist motion after stroke, although it was not as effective in altering control of the knee in a similar patient group.

Although discussions of EMGBF, NMES, and FES are often presented separately, the use of these modalities can be intertwined to achieve desired muscle control. NMES or FES may be initiated in the absence of active control (although lower levels of muscle activation may be discovered and facilitated with EMGBF). Once return of active control begins, EMGBF may be used to refine the control. An increase in muscle EMG should not be assumed to translate automatically to an improvement in functional use of that muscle in daily activities. Consequently, NMES and EMGBF need to be integrated into daily functional activities so that appropriate muscle activity is elicited and used in its appropriate functional context.

Upper-extremity management

Lower-extremity management

Evidence-based practice

Much of the literature evaluating the use of EMGBF and NMES involves patients with CVA. Although these studies have shown many significant results, interpretation of these findings in relation to what is recommended for clinical intervention is not as clear-cut. Improvements in generation of EMG activity and active movement are well documented, but the functional implications of these gains are not as well established. Clearly, in the current practice environment much of the focus is on function, so these techniques to improve muscle activation patterns must be put to functional use in the context of therapy. The therapist must consider the relevant factors that may predict success and critically evaluate outcomes during trial use of these modalities. Cost-benefit analyses must accompany any intervention using technology with the goal of regaining movement as quickly as possible and eliminating the use of equipment when practical.

Stroke

A number of studies have been published discussing the muscle recruitment problems observed after CVA. ^{, ,} Knowledge of these problems is a prerequisite for determination of the appropriate application of NMES. Delayed recruitment of the agonist and antagonist is a relatively consistent finding. Other findings include delayed termination of muscle activity once initiated, presence of co-contraction of agonist and antagonist muscles, ^, lack of co-contraction, and maintenance of agonist muscle contractions.

Sander and colleagues found that NMES application to the tibialis anterior muscle for 30 minutes three times a week for 4 weeks was effective in improving strength of the affected lower extremity.

Howlett and colleagues investigated the effect of FES in improving activity after stroke, and whether FES is more effective than training alone in a systematic review with meta-analysis. Following predetermined search and selection criteria, researchers included randomized and controlled trials up to June 22, 2014. Eighteen trials (19 comparisons) were eligible for inclusion in the review. Results of the study showed that FES had a moderate effect on activity compared with no or placebo intervention. Furthermore, researchers also noted that FES had a large effect on upper-limb activity and a small effect on walking speed compared with control groups, and appeared to have a moderate improvement in activity compared with both no intervention and training alone. In the study, researchers suggested that FES should be used in stroke rehabilitation to improve the ability to perform activities.

Most recently, Biasiucci and colleagues demonstrated that brain-computer interfaces (BCI) coupled with FES can elicit significant lasting arm motor recovery in patients with chronic stroke. BCI translate brain signals into planned movements in patients with limb paralysis. BCI alone has an unclear efficacy and mechanisms as to motor recovery after stroke. In this study, patients who received brain-actuated functional electrical stimulation (BCI–FES) exhibit a significant arm functional recovery post intervention. The recovery lasted for 6 to 12 months after the termination of therapy. The results of the study demonstrated a promising BCI–FES combination therapy to facilitate functional neuroplasticity leading to a significant arm functional recovery patients with chronic stroke.

Lee, Lee, and colleagues compared virtual reality (VR) combined with FES with cyclic FES for improving upper extremity function and health-related quality of life in patients with chronic stroke in a pilot, randomized, single-blind, controlled trial. In the study, participants were patients 3 months post-stroke. A VR-based wearable rehabilitation device was used in combination with FES. This device was used during the virtual activity-based training for the intervention group while the control group received cyclic FES only. Both groups received 20 sessions over a 4-week period. Results of the study have shown that FES with VR-based rehabilitation may be more effective than cyclic FES in improving distal upper extremity gross motor performance post-stroke.

In a study done by Kuznetsov and colleagues, a tilt-table with stepping features combined with FES was applied to patients with hemiparetic ischemic stroke 3 to 6 days post-stroke. Rehabilitation may be more effective if started early; however, early training is frequently limited because of orthostatic reactions. The researchers’ goal is to prevent these adverse reactions through robotic tilt-table training plus FES (ROBO-FES). The researchers compared the safety and feasibility of ROBO-FES and robotic tilt-table training (ROBO) against the control, tilt-table training alone. There were no serious adverse events occurred during the study. They concluded that robotic tilt-table exercise with or without FES is safe and more effective in improving leg strength and cerebral blood flow than tilt table alone in this patient population. Hocoma has produced a device with a tilt table with a stepping movement functionality with (ErigoPro®) or without FES (Erigo®) (see Fig. 31.8 ).

Erigo Pro.

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