Electrodiagnostic Medicine II: Clinical Evaluation and Findings

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Chapter 10 Electrodiagnostic Medicine II

Clinical Evaluation and Findings

Electrodiagnostic medicine is a unique area of medicine that differs from other medical and surgical specialities in many important ways. Electrophysiologic testing is an extension of the history and physical examination, and is tailored to the clinical scenario. A variety of tests can be performed, and it is incumbent on the electrodiagnostician to select those relevant to the clinical circumstances. Unlike other tests, such as an electrocardiogram, the testing procedure may change as data are acquired and new findings interpreted. Like any other clinical area of medicine, high-quality consultations are rendered by physicians with experience and technical competence coupled with an understanding of the peripheral nervous system.

Consultants in electrodiagnostic medicine provide referring physicians with critical information regarding neuromuscular disorders. Such conclusions are often a crucial part of the complex decision-making process that influences subsequent diagnostic and surgical interventions.

This chapter provides an overview of electrodiagnostic tests, the extent of such testing for different clinical scenarios based on recent guidelines, and the types of abnormalities one can expect to find with different neuromuscular disorders.

Clinical Assessment: History and Physical Examination

History

A directed history enables the electrodiagnostician to develop a conceptual framework for subsequent testing. The history should include information as to whether symptoms are confined to a single limb or more than one limb is involved. The character and nature of symptoms, as well as their duration and time of onset, are important to elicit. Pain and numbness indicate that the sensory system is involved. Motor weakness without pain and numbness indicates primarily involvement of the motor system, motor axons, neuromuscular junction, or muscles. For patients with more generalized symptoms, meaning symptoms in more than one limb, the electrodiagnostician should inquire about other neurologic complaints, such as dyspnea, bladder urgency, dysphagia, dysphonia, and visual changes. Previous cervical or lumbar spine conditions such as spinal stenosis or surgical interventions should be noted. Activities that aggravate or relieve a patient’s symptoms can often help differentiate musculoskeletal problems from entrapments, plexopathies, and radiculopathies.

Diabetes and alcohol are common causes of polyneuropathy in the United States, and an appropriate history would include questioning about such issues. Any family history of neurologic disorders is important to elicit for persons with generalized or unexplained symptoms. A brief recording of the person’s vocation is often helpful to identify persons at risk for overuse syndromes.

Important past medical history to elicit includes any history of malignancy, chemotherapy, or radiation, because such conditions and treatments can have peripheral neurologic implications. Previous surgeries for entrapment neuropathies, such as ulnar transposition or carpal tunnel release, are important as well. For instance, a transposed ulnar nerve complicates nerve conduction calculations for the segment across the elbow, because measurement is less precise, and stimulus sites above and below the elbow can be more difficult to find. Whether a person is thrombocytopenic, has bleeding disorders, or is receiving anticoagulants should be noted.

Physical Examination

Strength, sensation, and reflex testing are the most important aspects of the physical examination. Motor weakness is often seen in neuromuscular disorders. Subtle motor weakness can be elucidated by comparing sides and examining smaller muscles such as the extensor hallucis longus or the hand intrinsic muscles. Reduced reflexes indicate peripheral nervous system problems, whereas increased reflexes suggest a central nervous system disorder, particularly when seen in combination with spasticity and increased tone. An important caveat is that in acute spinal cord injury, there can be an initial reduction in reflexes because of spinal shock. Cranial nerve examination can identify weakness and should be performed if stroke or another generalized condition is suspected. Neck flexor or extensor weakness can be somewhat harder to grade.

In many cases, these signs can, when combined with the history, point to the correct condition. Other tests can further clarify the clinical picture. These include Phalen’s test for carpal tunnel syndrome. Palpation over the elbow to reproduce symptoms in someone with suspected ulnar neuropathy can be helpful. Straight leg raise testing for persons with suspected lumbosacral radiculopathy is useful when clearly positive. For persons with less clear conditions, palpation over the site of discomfort can help define trigger points or tendinopathies. Shoulder impingement signs can identify bursitis and bicipital tendonitis to help the electrodiagnostician in assessment for suspected C5 or C6 radiculopathy. Finklestein’s test for de Quervain’s tenosynovitis is useful for persons presenting with wrist pain. Lateral epicondylitis can mimic C6 radiculopathy. Lateral epicondylitis is characterized by reproduction of pain with palpation over the extensor forearm muscles and with resisted wrist extension. These examinations can help put the clinical scenario in the proper context.

Cannon et al.23 examined the ability of physical examination and delineation of upper limb musculoskeletal disorders (myofascial pain, shoulder impingement, lateral epicondylitis, de Quervain’s tenosynovitis) to predict the outcomes of electrodiagnostic testing (normal study, cervical radiculopathy, or another electrodiagnostically confirmed diagnosis). They found that the total prevalence of musculoskeletal disorders was 42%. The prevalence in those with a normal study was 69%, compared with 29% in those with cervical radiculopathy (p <0.0001) and 45% in those with another diagnosis (p = 0.02). While the prevalence of certain musculoskeletal disorders made having a normal electrodiagnostic evaluation significantly more likely, the high prevalence among both patients with normal studies and those with radiculopathy and other disorders limited the usefulness of this information in precisely predicting study outcome. Consequently the presence of upper limb musculoskeletal disorders should not preclude electrodiagnostic testing when it is otherwise indicated.

These investigators found similar results for the lower limb.22 In a sample of 170 patients who were referred for suspected lumbosacral radiculopathy, they found the total prevalence of musculoskeletal disorders (myofascial pain, trochanteric bursitis/iliotibial band syndrome, or plantar fasciitis) in the sample was 32%. The prevalence in those with a normal study was 55%, compared with 21% in those with lumbosacral radiculopathy (p <0.0001). As in the case for suspected cervical radiculopathy, the high prevalence among both patients with normal studies and those with radiculopathy and other disorders limits the usefulness of this information in predicting study outcome. Because it is common for patients to have two or more problems, the presence of a musculoskeletal disorder should not preclude electrodiagnostic testing when radiculopathy is suspected.22

An algorithmic approach to using physical examination and symptom information to tailor the electrodiagnostic evaluation is shown in Figure 10-1. In this approach, the patient’s physical examination signs of sensory loss and weakness create a conceptual framework for approaching these sometimes daunting problems. For the purposes of this discussion, generalized findings are defined as being present in two or more limbs. While helpful, there are many exceptions to this taxonomy. The electrodiagnostician must refocus the diagnostic effort as data are acquired. Sensory loss is a key finding on examination for someone presenting with generalized symptoms, and provides a branch point in the algorithmic approach to tailoring the electrodiagnostic study (see Figure 10-1). Myopathies, neuromuscular junction disorders, motor neuron disease, and multifocal motor neuropathy are all characterized by preservation of the sensory system. In contrast, polyneuropathies, bilateral radiculopathies, myelopathies, and central nervous system disorders frequently result in reduced sensation. In persons with no sensory loss or weakness on examination, the electrodiagnostician should maintain a heightened suspicion for myofascial pain syndrome or fibromyalgia, polymyalgia rheumatica, or multiple other musculoskeletal disorders.11

Purpose of Electrodiagnostic Testing

Electrodiagnostic testing excludes conditions in the differential diagnosis and alters the referring diagnostic impression 42% of the time.71 Electrodiagnostic testing can to some extent suggest severity or extent of the disorder beyond the clinical symptoms. Involvement of other limbs can be delineated, or the involvement of multiple roots can be demonstrated, such as in the case of lumbosacral spinal stenosis. Finally, there is utility in solidifying a diagnosis. For example, an unequivocal radiculopathy on electromyography (EMG) provides greater diagnostic certainty and identifies avenues of management.

The value of any test depends on the a priori certainty of the diagnosis in question. For a condition or diagnosis for which there is great certainty before additional testing, the results of the subsequent tests are of limited value. For instance, in a patient with acute-onset sciatica while lifting, L5 muscle weakness, a positive straight leg raise, and magnetic resonance imaging (MRI) showing a large extruded L4–L5 nucleus pulposus, an EMG test will be of limited value in confirming the diagnosis of radiculopathy because the clinical picture is so convincing. In contrast, an elderly diabetic patient with sciatica, having limited physical examination findings and equivocal or age-related MRI changes, presents an unclear picture. In this case, electrodiagnostic testing is of high value, placing in perspective the imaging findings and excluding diabetic polyneuropathy as a confounding condition.

Types of Electrodiagnostic Tests

Motor and Sensory Nerve Conduction Studies

Electrodiagnostic testing consists of EMG and nerve conductions. Standard nerve conduction studies (NCSs) that are used in the evaluation of patients typically include motor nerve conductions, sensory nerve conductions, F-waves, and H-reflexes. It is important to properly perform these tests and compare the data with well-derived normative values. Enough testing should be performed to properly delineate the conditions being sought and eliminate other conditions in the differential diagnosis.

NCSs have been found to be safe. In a well-designed study involving patients with dual-chamber pacemakers and implanted cardiac defibrillators, NCSs including median motor testing with Erb’s point stimulation on the left side of the body resulted in no change in the function of these devices, nor did any stimulation show up on any of the electrocardiogram tracings.123 These investigators concluded that conventional NCSs have no effects on these implanted devices, and their findings should diminish the anxiety of electrodiagnosticians when working with such patients.123 These findings further support the safety of performing such testing in persons with implanted cardiac devices and dispel some of the concern expressed in previous recommendations.6

Sensory nerve conductions should always be incorporated when assessing a patient.7 In the upper limb, multiple sensory nerves are easily accessible and allow assessment for both entrapments and polyneuropathies. In the lower limbs, perhaps the most readily accessible is the sural nerve. Indeed, in the recent position paper on polyneuropathies, this nerve was felt to be an excellent nerve for screening a patient for distal symmetric polyneuropathy.58

Motor NCSs should be performed in almost all situations.7 Examiners should assess the morphology of the waveform, its latency, amplitude, and conduction velocity. Conduction should be performed across any suspected sites of entrapment or injury. Enough nerves should be studied to be able to determine whether a generalized condition is present.

The elderly frequently have symptoms that prompt electrodiagnostic testing, and for this reason understanding the implications of nerve conduction testing in this age-group is vital to understand and interpret normal from abnormal.60 Falco et al.60 examined the upper limb nerves using standardized testing in a group of healthy elderly persons. They found that upper limb conductions compared favorably with those reported for younger groups. Age had some influence on sensory NCS latencies and amplitudes These researchers concluded that age has smaller effects on NCSs than was previously thought.60 These investigators meticulously controlled temperature, and this might have contributed to the NCS findings, because the limbs of the elderly are often somewhat colder than younger adults. They stressed the need for temperature control. It is also possible that their stringent selection criteria for “healthy” selected out a special group of the elderly who were in markedly better health than their peers.

In a similar companion study, these investigators61 studied lower limb NCSs in healthy elderly persons. In a similarly well-designed study they found that nerve conduction velocities and distal latencies did not change with age from 60 to 89 years of age. Distal amplitudes did change significantly with a reduction in the elderly groups. What was particularly striking was that the sural sensory response was obtained in 98% of the elderly and the superficial peroneal sensory (medial dorsal cutaneous nerve) in 90%. This means that an unobtainable sural response should be considered abnormal and not simply attributed to age-related changes.61 The same reservations, however, as stated above for the upper limb studies pertain here as well.

Nerve conduction parameters are often obtained serially over time to monitor a disease process. The issue of stability and reproducibility of NCS testing is an important one. In a large multicenter trial, the reproducibility of NCSs if temperature and distances are well controlled is excellent, demonstrating excellent intraclass correlation coefficients ranging from about 0.7 to 0.9.12 The major sources of variability were the patients, with the sites of testing showing remarkable stability of NCS parameters. This means that NCSs done with meticulous technique and control for temperature can be used to judge changes in nerve function caused by disease processes. For large-scale clinical trials the authors point out the ability to reduce sample sizes because of the accuracy and stability of NCSs.12

H-Reflexes

The H-reflex is an electrophysiologically recorded Achilles muscle stretch reflex. It is typically generated by recording over the gastrocnemius and soleus muscles, and stimulating the tibial nerve in the popliteal fossa. Care must be taken to perform this test correctly. It is a submaximally elicited reflex response. The stimulus duration is 1 ms, and the stimulus should be slowly increased by 3- to 5-mA increments. The patient should be relaxed, and the stimulus frequency should be less than once per second. The H-reflex is consistent in latency and morphology, and occurs when the motor response over the gastrocsoleus is submaximal. As the stimulus current is gradually increased, the H-reflex will reaches its maximum amplitude, and then extinguish as the motor response becomes maximal. Many researchers have evaluated its sensitivity and specificity with respect to lumbosacral radiculopathies, and generally found a range of sensitivities from 32% to 88%.95,98,102,119,140 The specificity, however, has been reported at 91% for H-reflexes in lumbosacral S1 radiculopathy.102 The H-reflex can also help separate S1 radiculopathy from L5 radiculopathy, the latter being more likely to have a normal reflex.102

H-reflexes are also helpful for assessing for polyneuropathy or confirming sciatic neuropathy. It is important to remember that delay in the H-reflex can occur with lesions anywhere along its course, including the sciatic nerve, the lumbosacral plexus, or the S1 root. H-reflex studies of other nerves have been mentioned in the literature, but as of yet are not commonly used in clinical practice and remain experimental.

Needle or Pin Electromyography

One of the most important means of evaluating for neuromuscular diseases is EMG using a monopolar pin or coaxial needle (in this chapter, “needle” is used to mean either pin or needle). We will deal primarily with needle EMG and not surface EMG. Surface EMG has no clinical utility over standard needle EMG.104 EMG testing, a vital part of electrodiagnostic evaluations, provides highly useful information and, although somewhat uncomfortable, poses minimal risks to the patient. The American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) supports the Occupational Health and Safety Administration rule that mandates the use of gloves and universal precautions. Disposable needles are now readily available and are sufficiently inexpensive that they should be used. It is recommended that surface electrodes be cleaned with a 1:10 dilution of household bleach or 70% isopropyl alcohol solution between patients. Disposable surface electrodes are inexpensive, widely available, and easy to use.6 Needlestick injuries are reported by the majority of electrodiagnosticians to have occurred at least once.103 The most common preventable reason for injury was a perceived lack of time.103 Using a device to hold the plastic sleeve that the needle is packaged in, then replacing the needle using a one-handed technique, is recommended to help prevent needlesticks.

In most cases, it makes no difference whether the skin is prepared before needle insertion.6 Some examiners use alcohol as an antiseptic, and certainly if the skin is not clean then alcohol represents a useful skin preparation. There are no clear contraindications with respect to lymphedema as long as caution is used to sterilize the skin and not traverse an infected space or an area of clearly taut skin that could weep serous fluid after the insertion. Care should also be used to avoid cellulitis.8 The needle should not penetrate infected skin or open ulcerations or wounds. Needle EMG is not listed by the American Heart Association as a procedure requiring prophylactic antibiotic treatment to prevent endocarditis.4,6 Hemorrhagic complications are quite rare and are not usually symptomatic when they occur.67

In one case series, minor paraspinal muscle hematomas were noted on MRI performed shortly after needle EMG examination in 4 of 17 patients who were not receiving anticoagulants. These hematomas were small and of no clinical significance.24 In this report, the authors state that in their review of the literature, there has never been a case report of paraspinal hematoma compressing the spinal roots or the epidural space.24 Although clinicians should always weigh carefully the risk-to-benefit ratio of testing, a person with appropriate levels of anticoagulation for venous thromboembolism can be safely studied with little risk of neurologic complications. Appropriate pressure should be applied to the area after testing to prevent intramuscular hematoma. The paraspinal muscles are such an important region to study, not only for persons with suspected radiculopathy but in neuromuscular evaluations as well, that electrodiagnosticians should examine them whenever reasonably possible.

Needle EMG is a somewhat painful procedure and can be distressing to some patients. Patient education can help reduce anxiety. Recently a prospective, double-blind, randomized, placebo-controlled crossover study showed that 400 mg of ibuprofen taken 2 hours before EMG in a group of healthy volunteers reduced significantly the perceived pain immediately after the EMG. It did not, however, reduce the memory of pain a day after study.57 Another means of decreasing pain is to use a cold topical spray agent to the skin over the muscle being tested and to pinch the skin slightly before inserting the needle.

As noted previously, the electrodes for standard EMG generally fall into two categories. Monopolar needles are those in which the needle is coated with Teflon except for the tip, and the differences in potential from the tip of the electrode to a nearby surface electrode are recorded. Concentric needles are those with a fine wire running through the center of an insulated shaft that is electrically referenced to an outer metal shaft. Concentric needles are most useful for quantitative motor unit analysis, and are now commercially available in disposable form. Background interference is minimized with concentric needles, and the ground electrode can remain in one place for a given limb.

It is important that the examiner use a one-handed technique when resheathing needles. The plastic sheath should be held by a clip or taped to some part of the electrodiagnostic instrumentation such that one hand can replace the needle, thereby preventing needlestick injuries.

The needle examination consists of four separate types of evaluations, which assess for the following:

Insertional activity is examined by moving the needle through the muscle briefly and observing the amount and duration of electrical noise produced. This sound is mechanically evoked muscle depolarizations caused by the advancement of the needle. Insertional activity and spontaneous activity are usually examined using three or four insertions in each of four muscle quadrants. Smaller muscles, such as the abductor pollicis brevis, can be adequately assessed with fewer needle movements. After brief small movement of the needle, insertional activity usually persists for no more than a few hundred milliseconds.85 Insertional activity can be decreased in the case of atrophied muscle or placement of the needle into fatty tissue. Increased insertional activity is activity that lasts for more than about 300 ms after the needle stops advancing. It is sometimes difficult to sort this out from spontaneous activity, and such determination is a qualitative assessment. Diffuse abnormal insertional activity with prolonged trains of positive sharp waves (PSWs) in essentially every muscle, yet without any symptoms or disability, has been described as “EMG disease.”138 This is a rare condition that is infrequently encountered in clinical practice.

Spontaneous activity consists of electrical discharges occurring after the needle movement has stopped. Daube and Rubin36 highlight the usefulness and importance of auditory pattern recognition in the delineation of spontaneous activity of different types. A variety of potentials can be seen when the needle is not moving. If the needle is in an end-plate region, then end-plate noise can be heard. This noise is generated from miniature end-plate potentials, which sound like noise generated by a seashell held close to the ear. End-plate spikes are fully propagated action potentials from the end-plate region. These are typically biphasic potentials with an initially negative (upward) deflection. They can be mistaken for fibrillation potentials, but they fire irregularly, a distinction easily made visually on the display screen as well as by listening carefully. They tend to have a sputtering sound and are associated with excessive pain. The needle should be moved to a new location after it enters an end-plate region, to reduce pain and enter muscle more conducive to assessment for fibrillation potentials.

Fibrillation potentials represent spontaneous discharges of single muscle fibers, and are the most important EMG finding in clinical EMG. They are usually of short duration, and they can be biphasic (PSWs) or triphasic (fibrillation potentials).35,53,85 They represent individual muscle fiber depolarizations. One hypothesis is that PSWs arise from suprathreshold single muscle fiber discharges induced by and originating in close proximity to a perielectrode crushed membrane that then propagate away from the electrode.56 A smaller population of PSWs conforms to that of a blocked fibrillation potential.56 Such muscle membrane irritability can be caused by the mechanical stimulation of the needle, but persists after needle movement has stopped. Fibrillations result from motor axonal loss that is not balanced by reinnervation. They are seen in any condition causing denervation, including nerve disease, inflammatory myopathies, and direct muscle trauma.112 It is an interesting phenomenon that in persons with complete spinal cord injury, muscles innervated by roots below the level of the lesion demonstrate fibrillation potentials.86,131 Such prevalence of these findings in the legs of spinal cord–injured individuals with complete myelopathies can make electrodiagnostic testing for such patients indeterminate. Similar fibrillations and positive waves can be seen in stroke patients as well, and such findings should be interpreted with caution in the hemiplegic limb.66,77

Fibrillation potentials as well as PSWs are graded using a 0 to 4 grading scheme.68 This grading scale is described as follows:

The finding of a fibrillation or PSW in only one area of the muscle that is not easily reproducible is probably of uncertain significance and can represent an end-plate spike. The density of fibrillation potentials does not necessarily correlate with the degree of nerve damage and loss of axons. The compound muscle action potential (CMAP) gives a better estimate of the proportion of axons remaining. The innervation ratio is the average size of the motor unit expressed as a ratio between the total number of extrafusal muscle fibers and the number of innervating motor axons. For small extraocular muscles, this ratio is 1 to 3. For large muscles such as the gastrocnemius, this ratio increases to 1934 muscle fibers per motor axon. This means that the loss of relatively few motor axons results in many fibrillating muscle fibers.85

In a sample of persons with lumbosacral radiculopathy, the interrater reliability of identifying spontaneous activity on needle EMG was disquietingly low.81 For individual muscles, agreement on the presence of spontaneous activity was 64% for the paraspinal muscles, yet up to 92% in the vastus medialis (Spearman ρ = 0.655, p <0.01).81 Training level played a strong role in reliability, with residents scoring much lower on concordance than attending physicians. This study points out the need to closely supervise residents and fellows when doing needle EMG. The paraspinal muscles in particular posed identification difficulties and underscore the need to examine firing rate to ensure that discharges are firing regularly before calling them fibrillations or PSW.55

Motor unit morphology involves placing the needle near a group of muscle fibers such that the rise time of the motor unit action potential (MUAP) is sharp (less than 300 μs). At this location, a proper quantification of the MUAP can be made. Polyphasic potentials are those with greater than four phases.85 Serrated potentials have the same clinical relevance, but the turns do not cross the baseline. Polyphasic potentials are associated with reinnervation of denervated motor units when the duration of these MUAPs is increased. This is seen in chronic motor axonal loss from an entrapment, a radiculopathy, or other axonal polyneuropathy. If the polyphasic potentials are short in duration, they can be associated with myopathies or neuromuscular junction disorders. If profound axonal loss has occurred and the remaining axons are reinnervating many muscle fibers, polyphasic satellite potentials can be seen that are of short duration and appear myopathic. Large motor units, recruited with low force and greater than 6 mV in amplitude, are seen in chronic or old axonal loss conditions where there has been substantial reinnervation and reorganization of the motor unit. Remember that motor unit morphology assessment is generally a qualitative statement and less clear than the presence or absence of fibrillations, particularly for less skilled electrodiagnosticians. Such motor unit changes, however, when clear and profound can be very helpful, particularly in the proper clinical context. Quantitative EMG is necessary to precisely characterize motor unit morphology, but is rarely necessary in routine clinical EMG testing.

Recruitment is another important parameter to assess. For this evaluation, it is most important to examine low to moderate levels of muscle contractions. Full-force contractions fill the screen with overlapping motor unit potentials, rendering assessment of individual motor units impossible. At low force levels, there should be one or two firing units at about 10 Hz. As greater force is generated, more units will be recruited, and the firing rates for those already recruited increase. In normal situations, the ratio of highest firing rate to the number of units seen on a 100-ms display screen is less than five. This means that if the firing rate is 20 Hz, about four distinct motor units should be seen.85 When this firing ratio is increased to more than 10, it indicates a dropout of motor units and is termed reduced recruitment.

Reduced recruitment with a high firing ratio represents one end of a spectrum of recruitment findings, strongly suggestive of motor axonal loss or functional dropout resulting from conduction block. At the other end of a spectrum of recruitment findings is a situation in which many units are recruited to generate a low level of force. This is termed early or myopathic recruitment, and is seen in myopathies and neuromuscular junction disorders. Such recruitment is often seen with small, short-duration MUAPs that are characteristic for myopathies. At these two extremes of recruitment, findings are relatively specific for the underlying types of neuromuscular pathology. Beyond this, many recruitment findings are less clear. In the case of pain, poor cooperation, or upper motor neuron disorders, there can be slow firing rates with few units activated.

Other spontaneous discharges can be seen beyond fibrillations and positive waves that are more continuous and unique. One such discharge is the complex repetitive discharge (CRD). This machine-like discharge is of constant firing frequency and demonstrates consistent morphology with abrupt starts and stops. In Table 10-1, the firing characteristics are shown, along with some of the conditions in which they can be found. They are seen in conditions with chronic denervation, as well as in inflammatory myopathies. They are thought to be generated by a pacemaker muscle fiber with ephaptic conduction to other muscle fibers.

Table 10-1 Complex Repetitive Discharge Characteristics

Characteristic Details
Appearance May take any form, but this form is constant from one potential complex to the next
Rhythm Regular
Frequency 10-100 Hz
Amplitude 50-1000 μV
Stability Abrupt onset and cessation
Observed in Myopathies: Polymyositis, limb girdle dystrophy, myxedema, Schwartz-Jampel syndrome. Neuropathies: poliomyelitis, spinal muscular atrophy, amyotrophic lateral sclerosis, hereditary neuropathies, chronic neuropathies, carpal tunnel syndrome. “Normal”: iliopsoas, biceps brachii

Modified from Dumitru D: Needle electromyography. In Dumitru D, editor: Electrodiagnostic medicine, Philadelphia, 1995, Hanley & Belfus.

Myokymic potentials are grouped potentials that fire at regular rates. They have the characteristic sound of marching soldiers and are often seen in radiation plexopathies (Table 10-2). Myotonic discharges are fast firing with waxing and waning sounds somewhat like a dive bomber. The characteristics and conditions in which they are found are shown in Table 10-3.

Table 10-2 Characteristics of Myokymic Discharges

Characteristic Details
Appearance Normal motor unit action potentials
Rhythm Regular
Frequency 0.1-10 Hz
Burst frequency 20-250 Hz
Stability Persistent firing or occasional abrupt cessation
Observed in Facial: Multiple sclerosis, brainstem neoplasm, polyradiculopathy, Bell palsy, normal Extremity: radiation plexopathy, chronic nerve compression (carpal tunnel syndrome, radiculopathy), rattlesnake venom

Modified from Dumitru D: Needle electromyography. In Dumitru D, editor: Electrodiagnostic medicine, Philadelphia, 1995, Hanley & Belfus.

Table 10-3 Characteristics of Myotonic Discharges

Characteristic Details
Appearance Brief spikes, positive waveform
Rhythm Wax and wane
Frequency 20-100 Hz
Amplitude Variable (20-300 μV)
Stability Firing rate alterations
Observed in Myopathies: myotonic dystrophy, myotonia congenita, paramyotonia, polymyositis, acid maltase deficiency, hyperkalemic periodic paralysis Other: chronic radiculopathy, chronic peripheral neuropathy

Modified from Dumitru D: Needle electromyography. In Dumitru D, ed.itor: Electrodiagnostic medicine, Philadelphia, 1995, Hanley & Belfus.

Single-fiber EMG is a specialized technique with particular usefulness in the evaluation of persons with suspected neuromuscular disorders, myasthenia gravis, and Lambert-Eaton myasthenic syndrome (LEMS). A specialized electrode is used to examine two muscle fibers at a time and derive “jitter” values. It is a sensitive test for these disorders but can also be abnormal in persons with amyotrophic lateral sclerosis (ALS) and polyneuropathy.

Extent of Electrodiagnostic Testing

Guidelines from the AANEM, published in 1999,5 provide valuable guidance regarding the extent of appropriate electrodiagnostic testing. The guidelines and recommendations are summarized in Table 10-4.5 Electrodiagnostic testing is uncomfortable for patients, and for this reason it is important to examine only the minimum number of nerves and muscles needed to make the diagnosis. Guidelines in electrodiagnostic medicine provide parameters for testing and give clinicians and insurers critical information regarding how many tests should be conducted in most cases (e.g., what is enough and what is excessive). The AANEM guidelines, synthesized from the scientific literature as well as expert clinical opinions, provide an important framework for establishing consistency in judging what constitutes sufficient testing. From time to time, cases occur for which these guidelines are inadequate, and additional testing is needed; however, for most cases these guidelines are entirely applicable. These guidelines were developed to improve electrodiagnostic patient care, as well as to combat unscrupulous providers of electrodiagnostic services in the United States who perform excessive studies far beyond what is necessary to make a diagnosis, simply for the purpose of increasing the electrodiagnostic fee.

In a policy statement, the AANEM addressed the necessary testing for the identification of distal symmetric polyneuropathy (DSP).58 In this critical review and policy synthesis, the importance of screening for DSP was highlighted. To do this with accuracy, these authors recommended that a sural sensory and peroneal motor study be conducted in the symptomatic limb. These tests are sensitive for DSP and are easily performed. It was recommended that if these two studies are normal, there is typically no need for additional testing for DSP. It is worth noting that acute inflammatory demyelinating polyneuropathy (AIDP) and chronic inflammatory demyelinating polyneuropathy (CIDP) might only demonstrate conduction abnormalities with the proximal plexus and roots involved such that the distal conductions are normal (especially early in the course).

Limitations of Electrodiagnosis

Unfortunately, very few electrodiagnostic findings are clearly specific for any single diagnostic entity. Repetitive nerve stimulation at 2 or 3 Hz can reveal decrements in neuromuscular junction disorders, but also in motor neuron disease, myopathies, peripheral neuropathies, and myotonic disorders.80 Fibrillations and PSWs are seen in polyneuropathies, motor neuron disease, inflammatory myopathies, radiculopathies, and entrapment neuropathies. Marked facilitation of the CMAP to more than 400% of the baseline amplitude after a brief contraction in persons with LEMS is one finding that is unique to this uncommon disease.74,80,120,135

The time course over which a disease process progresses and the time at which electrodiagnostic testing is conducted both play major roles in determining whether the electrodiagnostic testing can provide a reasonably certain diagnosis. It is frequently necessary to repeat the study if the diagnosis remains in question or if the clinical situation changes.

One important issue related to electrodiagnostic medicine is the possibility of false-positive results. In Table 10-5, the probabilities of finding false-positive results on the basis of chance alone are shown according to the number of independent measures.49 It is important to realize that if five measurements are performed, there is a 12% chance of having one false-positive result. If nine measurements are made, there is a 20% chance of a spuriously false-positive result. If there are two abnormal results when six or more tests are conducted, however, the likelihood that they are false-positive tests is quite low, less than about 1%. This underscores the need for electrodiagnosticians to critically examine their findings and not overdiagnose a disorder based on one subtle abnormality. If one electrodiagnostic parameter is markedly abnormal, far beyond the upper limit of normal, this can be compelling, particularly when that abnormality is consistent with the clinical impression. The study should be repeated to make certain of its validity. Two abnormalities indicating the same diagnosis, however, are far more likely to represent true findings and a clear underlying disorder.

Other common conditions, such as diabetes and its corresponding polyneuropathy, can make electrodiagnostic testing more difficult to interpret. With a diffuse polyneuropathy, the consultant is often unable to identify focal lesions against the backdrop of this disorder. In this scenario, the electrodiagnostician must assess the magnitude of the background polyneuropathy and make a judgment as to whether findings for the nerve in question are out of proportion to the background polyneuropathy. Caution is urged in this scenario to avoid overcalling entrapment neuropathies when a generalized polyneuropathy is present.

These issues are another reason why it is critically important that the electrodiagnostician have sufficient training to be able to interpret the findings appropriately in the clinical context of the patient.

Nerve Conduction and Needle Electromyelographic Studies: Proper Technique for Temperature, Age, and Height

The well-known effects of temperature on nerve conduction velocity, distal latency, amplitude, and neuromuscular transmission should be prevented by maintaining the upper limbs at 35° C and the lower limbs at 32° C.37 Normative reference values provide the electrodiagnostician with information that allows a determination as to whether a particular nerve conduction parameter is significantly different from that in a person without symptoms or known neurologic disorders. Besides temperature, age and height have been demonstrated to influence electrodiagnostic findings, and these factors must be taken into consideration. Buschbacher1620 has published extensive normative information derived from large samples, which account for height and age.

Anatomic variations in muscle and nerve pathways can confound electrodiagnostic interpretation as well. Two well-described variants are important for the electrodiagnostician. The Martin-Gruber anastomosis is formed by a branch from the median nerve, usually the anterior interosseus nerve, joining the ulnar nerve in the forearm. In this situation, the median CMAP with wrist stimulation is smaller than that with proximal stimulation. The ulnar nerve demonstrates a larger amplitude with wrist stimulation than with below-elbow or above-elbow stimulation. If a suspected conduction block is found in the forearm when testing the ulnar nerve, then median nerve conduction should be performed to exclude a Martin-Gruber anastomosis as the cause of CMAP reduction with proximal stimulation.

An accessory deep peroneal nerve is an anomalous part of the deep peroneal nerve that remains with the superficial peroneal nerve and innervates the extensor digitorum brevis after passing behind the lateral malleolus. It should be suspected if stimulation at the fibular head yields a larger CMAP than ankle stimulation when recorded over the extensor digitorum brevis. Stimulation of the accessory deep peroneal nerve behind the lateral malleolus will confirm the presence of such an anatomic variant.

Standards of Practice for Electrodiagnostic Medicine

The AANEM has addressed the need to set standards for the evaluation and interpretation of studies in response to the burgeoning and disquieting practice of decoupling nerve conductions and needle electromyography (EMG). It is the organization’s contention that electrodiagnostic studies should be performed by physicians properly trained in electrodiagnostic medicine, that interpretation of NCS data alone, absent face to face patient interaction and control over the process provides substandard care, and that performance of NCS without needle EMG has the potential of compromising patient care.9 Considerable concern exists that there has been a dramatic increase in NCSs without EMG testing.9 EMG testing is critical for the assessment of radiculopathies and plexopathies as well as to complement NCSs and give additional information about entrapments and nerve injuries. Interpreting NCS findings without the benefit of a focused history and examination is inappropriate as well.9

Communication of the consultation results to the referring physician and other health care providers is essential. The AANEM recommends that the electrodiagnostic report contain (1) a description of the problem and the reason for referral; (2) a focused history and examination; (3) tabular NCS and EMG information with normative reference values; and (4) temperature and limbs studied.8 It is recommended that the study results are defined as normal or abnormal, and a probable physiologic diagnosis is presented. Any limitations to the study should be included.8 An optional section after this summary can be a synthesis of the clinical and electrophysiologic information into clinical recommendations or a stronger set of conclusions. Such a section should be separate from the electrophysiologic testing. The EMG and NCS data and the conclusions they support should stand alone.

The standards for ethical and professional conduct were summarized in an AANEM publication.99 This was the synthesis of principles derived from the American Academy of Neurology and the American Medical Association. The electrodiagnostic consultant must be competent professionally and must place the safety, privacy, and patient’s best interests first. The consultant should perform evaluations that represent the prevailing standards of electrodiagnostic practice. Practices such as receiving a fee for making a referral or giving a fee in exchange for a patient referral (e.g., fee-splitting) are not appropriate.99 Expert witnesses should not provide services under a contingency fee arrangement; rather they should receive compensation for actual services provided. Other issues that are clearly important are compliance with state and federal regulations as well as Health Insurance Portability and Accountability Act privacy laws.99

Pediatric Electrodiagnosis

The pediatric examination poses challenges for even the most experienced electrodiagnostic physician. Diagnosis of pediatric neuromuscular disorders is a specialized and rapidly evolving area of medicine in which genetic testing plays an ever greater role in diagnosis. Interested readers are encouraged to examine specialized textbooks and recent articles, as much of this material is beyond the scope of this chapter.

In terms of equipment required for pediatric electrodiagnosis, a pediatric bipolar stimulating probe is available and is useful for small children.134 Monopolar needles are generally used because their diameter is smaller and insertional resistance is less, which is due to their Teflon coating. It is critically important to maintain a skin surface temperature of 36° to 37° C to avoid spurious results.134 There are strong advocates for and against the use of sedation and analgesia during electrodiagnostic examination.134 Performance of all conscious sedation and anesthesia should be done by specialists in pediatric anesthesia. When general anesthetic is required, children with possible neuromuscular diseases should not be given halogenated inhalation agents such as halothane because of the risks for malignant hyperthermia.128

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