Case 20

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Case 20

HISTORY AND PHYSICAL EXAMINATION

A 32-year-old white woman developed progressive weakness in the legs over 2 months. She noticed gradually increasing difficulty climbing stairs and standing from a sitting position. She began using a walker for assistance 4 days before admission to the hospital. She reported muscle pain in both legs but denied any ocular, bulbar, or sphincteric symptoms. There was no history of skin rash or arthralgia. Three weeks earlier, she was found to have “abnormal liver function tests” and received a diagnosis of possible “toxic hepatitis.” She was taking no medications and denied any history of alcohol or drug abuse.

On examination, she was in no apparent distress. There was no skin rash. She had normal cranial nerves. She had significant difficulty getting up from a chair and needed assistance when walking. There was generalized mild muscle tenderness. Manual muscle examination revealed significant symmetrical proximal more than distal muscle weakness, worse in the legs. Deep tendon reflexes were normal. Sensation was normal. Using the Medical Research Council (MRC) grading system, her muscle strength was rated as follows:

	Right	Left
Deltoid	4/5	4/5
Triceps	4/5	4/5
Biceps	4/5	4/5
Hand grip	4+/5	4+/5
Iliopsoas	4−/5	4−/5
Quadriceps	4−/5	4−/5
Hamstrings	4−/5	4−/5
Ankle dorsiflexion	4+/5	4+/5
Plantar dorsiflexion	4+/5	4+/5

Abnormal laboratory values were: creatine kinase (CK) 14 857 units/liter (U/L) (normal, <140 U/L), aldolase 589.1 U/L (normal, 3.5–17.5 U/L), lactate dehydrogenase (LDH) 1475 U/L (normal, 60–210 U/L), serum alanine aminotransferase (ALT) 478 U/L (normal, 8–54 U/L), and serum aspartate aminotransferase (AST) 566 U/L (normal, 10–50 U/L). Normal laboratory studies included the following: Westergren erythrocyte sedimentation rate (ESR; 10 mm/h), serum gamma-glutamyl transferase (GGT) level, antinuclear antibody (ANA), rheumatoid factor, thyroxine (T₄), thyroid-stimulating hormone (TSH), complete blood count (CBC), bilirubin, alkaline phosphatase, electrolytes, and serum protein electrophoresis. Negative results included the following: human immunodeficiency virus (HIV) antibody, urine myoglobin, serum lyme titers, toxoplasma immunoglobulin M (IgM) and Venereal Disease Research Laboratory (VDRL).

An electrodiagnostic (EDX) examination was performed.

Please now review the Nerve Conduction Studies and Needle EMG tables.

QUESTIONS

1. In this disorder fibrillation potentials are most common in:

A. Paraspinal muscles.

B. Proximal muscles of the lower extremities.

C. Proximal muscles of the upper extremities.

D. Distal muscles.

E. Anterior neck muscles.

2. Motor unit action potentials (MUAPs) seen in the chronic form of this disorder are:

A. Short in duration and low in amplitude.

B. Long in duration and complex.

C. Normal.

D. All of the above.

3. Increasing weakness developed in a 45-year-old woman with polymyositis who was taking corticosteroids. Which of the following findings suggests steroid myopathy rather than relapsing polymyositis?

A. Rising CK.

B. Fibrillation potentials.

C. Short-duration, low-amplitude MUAPs.

D. Complex repetitive discharges.

E. Normal insertional activity.

Case 20: Nerve Conduction Studies

Case 20: Needle EMG

EDX FINDINGS AND INTERPRETATION OF DATA

Pertinent EDX findings in this case include the following:

1. Normal sensory and motor nerve conduction studies (amplitudes, distal latencies, and conduction velocities), including F-wave latencies.

2. Fibrillation potentials in all muscles tested in the lower and upper extremities, including the paraspinal muscles.

3. Prominent MUAP changes in many muscles tested, mostly resulting in short-duration, low-amplitude, and polyphasic MUAPs.

4. Early MUAP recruitment in proximal muscles.

These findings are consistent with a diffuse myopathy associated with fibrillation potentials, compatible with a necrotizing myopathy, such as an inflammatory myopathy (e.g., polymyositis).

DISCUSSION

Classification and Pathology

The inflammatory myopathies are a heterogeneous group of disorders that share a common pathologic feature: inflammatory cells in muscles. They comprise three major categories of muscle disease, polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM), conditions that are clinically, histologically, and pathogenetically distinct. Inflammatory myopathies are often classified into two major types (Table C20-1), a primary and secondary (associated with systemic or other identifiable disorders).

Table C20-1 General Classification of Inflammatory Myopathies

Primary

Inclusion body myositis

Secondary

With connective tissue diseases (overlap syndromes), e.g., scleroderma, systemic lupus erythematosus

With cancer (paraneoplastic), e.g., breast, ovary, lung, and gastrointestinal malignancies

Giant cell and granulomatous disorders, e.g., sarcoidosis

Infection, e.g., trichinosis, cysticercosis

Others

Eosinophilic syndromes, e.g., diffuse fascitis of Schulman

Local nodular myositis

Polymyositis and dermatomyositis are organ-specific autoimmune disorders in which the skeletal muscles and skin (in DM only) are the primary target(s). In IBM, there is evidence that the disorder is primarily degenerative while the immune mechanisms are secondary.

Pathologically, the inflammatory myopathies are characterized by mononuclear cell inflammation, segmental muscle fiber necrosis, and muscle fiber regeneration. The pathologic findings in PM and DM have both common and diverse features. Both also are different from the findings in inclusion body myositis. Table C20-2 shows the major differences among these three primary inflammatory myopathies.

Table C20-2 Pathologic and Treatment Response Differences Between Polymyositis, Dermatomyositis, and Inclusion Body Myositis

Clinical Features

Polymyositis and dermatomyositis affect patients of all ages, with a predilection to women. In the United States, these disorders are twice as common among blacks than whites (incidence rate of 0.77 versus 0.32 per 100000, respectively). Most patients present with muscle weakness that develops subacutely over weeks to months, and affects predominantly the proximal pelvic and shoulder girdle muscles, including the neck flexors. Dysphagia and myalgia/muscle tenderness are common, each occurring in one-third of patients. Extramuscular manifestations are not uncommon. They include involvement of the lungs (interstitial lung disease) and the heart (cardiomegaly, congestive heart failure, and conduction defects), and manifestations of diffuse necrotizing vasculitis (especially in juvenile DM).

In DM only, there is an associated skin rash that can be the presenting symptom. Two classic rashes are characteristic. The first is a “heliotrope rash,” an erythematous, violaceous (hence the name) rash over the malar and periorbital areas that may extend to involve other sun-exposed areas, such as the dorsum of the hands, knees, elbows, or forehead. The second rash is Gottron papules, an erythematous papular rash over the knuckles of the fingers. Subcutaneous calcinosis complicates up to half of children with juvenile DM and may be present in chronic DM. These lesions are most often seen in the buttocks, thighs, knuckles, and elbows (Figure C20-1). They may be painful, ulcerate through the skin, or get infected.

Figure C20-1 Calcinosis in the popliteal fossa of a patient with a long history of dermatomyositis.

In contrast to PM and DM, IBM affects patients over the age of 50, has a male predominance, and is more common in the white than black population. The onset is insidious and it progresses slowly, evolving over years. It is the most common inflammatory myopathy in patients over the age of 50. Clinically, IBM has a unique muscle involvement that easily distinguishes it from the other inflammatory myopathies. There is usually asymmetrical involvement of the finger flexors and wrist flexors in the upper extremities, and the knee extensors and ankle dorsiflexors in the lower extremities (Figure C20-2). Dysphagia is common and afflicts up to 60% of patients as the disease progresses.

Figure C20-2 Bilateral asymmetrical hand weakness and atrophy (A) with severe bilateral asymmetrical quadriceps weakness, necessitating an assisted knee brace (B), in a 67-year-old man with inclusion body myositis.

Up to a quarter of patients with DM/PM have an associated connective tissue disease. The “overlap syndrome” links PM and DM with other connective tissue disease such as scleroderma, systemic lupus erythematosus, Sjögren syndrome, rheumatoid arthritis, and mixed connective tissue disease.

Laboratory features include an elevated serum CK in 90% of patients (at least 5 to 10 times normal values). LDH, ALT, and AST may be elevated, which can lead to the erroneous diagnosis of liver disease. In these situations, the ALT/AST ratio is useful. In hepatocellular disease, the ratio is greater than 1 while in myopathies, the ratio should be reversed (i.e., ALT/AST < 1). Also, measuring serum GGT activity is helpful in excluding concomitant hepatic disease, since this enzyme highly specific for hepatocellular disease and has a low level or is absent in muscle. Serum aldolase may also be elevated, but the utility of this enzyme in the diagnosis and follow-up of myopathies is of limited value for two reasons. First, aldolase is less sensitive or specific than CK since it is present in lower amounts in skeletal muscle. Second, serum aldolase is elevated in primary muscle as well as liver disease. ESR is normal or mildly elevated. Autoantibodies, such as ANA, SSA, or SSB, are positive in overlap syndromes. Anti-Jo-1 antibody is the most prevalent in DM/PM, occurring in 20% of patients and in 50 to 75% of patients with associated interstitial lung disease.

The incidence of malignancy in patients with DM and PM older than 40 years of age is higher than that expected for the general population, suggesting that DM and PM may be paraneoplastic. The increased risk in patients older than 40 years of age is 6-fold for DM and 2-fold for PM compared with that of the general population. The neoplasms are variable, but most are reported as carcinoma of the breast, ovary, and lung and gastrointestinal tract.

The diagnoses of DM and PM are confirmed based on the combination of clinical, laboratory, electrophysiologic, and pathologic findings. In 1975, Bohan and Peter proposed a classification that has been used since, with some revisions, in confirming the diagnoses of PM and DM. Based on these criteria, the confidence limits in the diagnosis of PM or DM range from definite to probable to possible (Table C20-3). The differential diagnoses of PM include IBM, polymyalgia rheumatica, metabolic myopathies (such as acid maltase deficiency), and limb girdle muscular dystrophy. In dermatomyositis, the presence of typical skin rash combined with muscle weakness often is diagnostic.

Table C20-3 Criteria and Confidence Limits in the Diagnosis of Polymyositis and Dermatomyositis

*Criteria*
1. Clinical

Predominantly proximal (limb girdle and neck flexor muscles), usually symmetrical, muscle weakness progressing over weeks or months, with or without myalgia, dysphagia or respiratory muscle involvement

2. Laboratory

Elevation of serum levels of skeletal muscle enzymes, particularly CK (MM isoenzyme), and sometimes aldolase, ALT, and LDH

3. Electromyography

Multifocal needle EMG changes of myopathy (short, small, polyphasic motor unit potentials), fibrillations and, sometimes, complex repetitive discharges^*

4. Muscle biopsy

Necrosis affecting all types of muscle fibers, phagocytosis, muscle fiber regeneration, and lymphocytic infiltration in the absence of cytoplasmic inclusion bodies^†

5. Dermatological

Lilac (heliotrope) discoloration of the eyelids and/or Grotton signs (scaly, erythematous dermatitis over the dorsum of the hands, particularly over the metacarpophalangeal and proximal interphalangeal joints which may extend to knees, elbows, medial malleoli, neck, face and upper trunk)

Confidence Limits
Diagnosis	Polymyositis	Dermatomyositis
Definite	4 criteria without rash	3 or 4 criteria plus rash
Probable	3 criteria without rash	2 criteria plus rash
Possible	2 criteria without rash	1 criteria plus rash

* See electrodiagnosis discussion for details.

† See Table C20-2 for differences between PM, DM, and inclusion body myositis.

Nearly all patients with DM and PM respond favorably to corticosteroids. The initial recommended dose of prednisone is 1.0 to 1.5 mg/kg/day. One must treat with adequate prednisone as long as there is evidence of active disease. This should be titrated based on objective serial clinical evaluations and CK determinations. The effectiveness of intravenous immunoglobulin is proven in DM, but it is also effective in PM. Other immunosuppressive agents such as azathioprine, methotrexate, and cyclophosphamide also are effective. The prognosis is favorable for children and with early diagnosis and treatment. A significant number of patients require a small dose of prednisone for maintenance.

In contrast to PM/DM, IBM is a treatment-resistant myositis. No immunosuppresive or immunomodulating agent has been shown to alter the natural course of disease.

Electrodiagnosis

Electrodiagnostic Findings of Myopathies in General

Sensory nerve conduction studies (NCS) in myopathy are normal, except in certain myopathies in which an associated peripheral polyneuropathy may occur (such as myotonic dystrophy or Kearns-Sayre syndrome). Similarly, motor NCSs usually are normal; however, motor studies may reveal low-amplitude compound muscle action potentials (CMAPs) when recording severely affected muscles. Examples include median and ulnar motor NCSs in adults with myotonic dystrophy, advanced Duchenne muscular dystrophy or critical illness myopathy. Proximal motor NCSs, such as musculocutaneous and femoral studies, may be low in amplitudes if performed on children with Duchenne muscular dystrophy. F waves and H reflexes are universally normal except when there is an associated polyneuropathy.

Repetitive nerve stimulations (RNS) generally are normal in myopathy, with certain exceptions. In non-dystrophic myotonic disorders, rapid RNS (usually at 20 Hz) may result in a decrementing response, particularly after prolonged stimulation. Also, during a paralytic attack of hyperkalemic periodic paralysis, rapid RNS may result in an increment of the low-amplitude baseline CMAP.

Electrodiagnostic findings in myopathies generally are limited to those attained with needle EMG. This has led few electromyographers to perform only needle EMG, without NCSs, on patients with suspected myopathy. However, this is not recommended because other neuromuscular disorders, such as neuromuscular junction disorders and early anterior horn cell disorders, may result in MUAP changes similar to those seen in myopathy.

The changes seen on needle EMG in myopathy include one or more of the following.

1. Abnormal insertional and spontaneous activities.

• Fibrillation potentials. These are commonly seen in neurogenic disorders (hence, the designation “denervation potentials”), but they also frequently accompany certain necrotizing myopathies, namely the inflammatory myopathies and the progressive muscular dystrophies. Morphologically, fibrillation potentials seen in myopathies do not differ from the ones observed in neurogenic disorders, except that fibrillation potentials associated with myopathies tend to fire at slower rates. The pathogenesis of these potentials in myopathy is discussed in a forthcoming section.

• Myotonic discharges. These potentials are induced by needle insertion and usually are “waxing and waning” in character because of variability in frequency and amplitude (Figure C20-3). They are associated with a distinctive sound that can be compared with the sound of a diving airplane (“dive-bomber”). These discharges, generated by single muscle fibers, can be accompanied by clinical myotonia (percussion myotonia, grip myotonia, or lid myotonia). Myotonic discharges are specific for certain myopathies, including myotonic dystrophy, non-dystrophic myotonias (such as myotonia congenita), and acid maltase deficiency (see Table 2-4, p. 33).

• Complex repetitive discharges (CRDs). CRD is a composite waveform that contains several distinct spikes and often fires at a constant and fast rate of 30 to 50 Hz. These discharges are recognized as polyphasic and complex potentials; hence the former name “bizarre repetitive potentials” (Figure C20-4). CRD remains uniform from one discharge to another, a feature that helps distinguishing it from myokymic discharge. CRDs are spontaneous discharges of muscle fibers during which a single muscle fiber spontaneously depolarizes, which is followed by ephaptic spread to adjacent denervated fibers. Often, a circus movement is created, which leads to a recurrent discharge (Figure C20-5). The chain reaction eventually blocks resulting in abrupt cessation. These recurrent discharges give a distinctive machine-like sound over the loudspeaker during needle EMG. Complex repetitive discharges are most often seen in myopathies and neuropathic disorders such as radiculopathies. They accompany most commonly chronic conditions but may be observed in subacute disorders.

2. Changes in MUAP morphology.

Figure C20-3 Myotonic discharge. Note the change in both amplitude and frequency of the discharge. This discharge is recorded at a compressed screen (sweep speed = 200 ms).

(From Sethi RK, Thompson LL. The electromyographer’s handbook, 2nd ed. Boston, MA: Little, Brown, 1989, with permission.)

Figure C20-4 Complex repetitive discharge. Note that the complex (circled) is stable and remains exactly the same between discharges with a constant firing rate. (A) The discharge is shown as a triggered rastered form. (B) The five rasters are superimposed. Note that the complex superimposes perfectly reflecting its uniform configuration.

Figure C20-5 Pathophysiology of a complex repetitive discharge (CRD): an ephaptic transmission from muscle fiber to muscle fiber that creates a circus movement without an intervening synapse.

(Courtesy of Dr. David Preston.)

Although MUAP changes were suspected to exist in myopathy, Kugelberg (1949) is credited for a major contribution to our current understanding of the characteristic MUAP changes that occur in myopathy. These MUAP changes reflect the disintegration of motor unit structure in myopathy caused by muscle fiber loss, the variation in muscle fiber diameter, and an increased amount of connective tissue, along with the presence of regenerated and reinnervated muscle fibers. In general, MUAPs in myopathy are short in duration, low in amplitude and polyphasics (Figure C20-6). These units are very small compared with both normal MUAPs and sprouted (“neurogenic”) MUAPs (Figure C20-7). Typical MUAP changes seen in myopathy include:

• Short-duration MUAPs. In myopathy, there is a shift toward a shorter mean MUAP duration (Figure C20-8). This is probably caused by the loss of muscle fibers within the motor unit. Short-duration MUAPs are the most consistent MUAP change associated with myopathy.

• Polyphasic MUAPs. In myopathy, MUAPs with more than four phases are increased in number, resulting in an increased percentage of polyphasic potentials, beyond the 10 to 20% seen in healthy individuals. This is probably caused by variability in muscle fiber conduction velocities and desynchronization of muscle action potentials within the territory of the motor unit.

• MUAPs with satellite (linked) potentials. Sometimes, an MUAP may be divided into two or more time-locked sections that are separated by baseline. The MUAP portion with the greatest duration or amplitude is considered the main body of the MUAP, while the remaining portions are called “linked” or “satellite” potentials (Figures C20-8 and C20-9). These MUAPs are easily seen with trigger/delay techniques. Including the satellite potentials may yield very long duration values that may be misleading. Therefore, these satellite potentials should be excluded from duration analysis.

• Low-amplitude MUAPs. This is the least reliable and most debated MUAP change in myopathy. Low-amplitude MUAPs are caused by the loss of muscle fiber and collagen tissue replacement, which results in increasing the distance between many muscle fibers and the recording electrode.

Figure C20-6 Typical short-duration, low-amplitude, and polyphasic MUAP, recorded in a raster form, from the deltoid of a 55-year-old patient with a 3-month history of proximal weakness, elevated CK, and elevated anti-Jo1 antibody. Subsequent work-up revealed polymyositis with interstitial lung disease.

Figure C20-7 Relative average durations and amplitudes of some motor unit action potentials (MUAPs) seen in myopathic and neurogenic disorders.

(From Daube J. AAEM minimonograph 11: needle electromyography in clinical electromyography. Muscle Nerve 1991;14:685–700, with permission.)

Figure C20-8 Several MUAPs recorded from the biceps muscle, recorded in a raster form, of the same patient as Figure C20-5. Note the short-duration, low-amplitude, and polyphasic MUAPs (solid arrow and dashed-line arrow) and the MUAP with satellite (linked) potentials (bulleted arrow).

Figure C20-9 MUAP recording from the quadriceps of a 61-year-old woman with a 3-year history of partially responsive chronic polymyositis. Note the two complex MUAPs that are polyphasic and with satellite potentials (arrow and bulleted arrow) leading to an initial misdiagnosis of motor neuron disease. These complex MUAPs with satellite potentials may yield long-duration values that may be misleading. Therefore, satellite potentials should be excluded from duration analysis and only the MUAP portion with the greatest duration or amplitude (main body of the MUAP) should be measured.

The alterations in MUAP configuration that occur in myopathy are not absolute. Instead, the MUAP changes form a continuum that ranges from normal to grossly abnormal MUAPs, with many MUAPs falling at points on the continuum between these two. Hence, identifying a myopathic process by needle EMG is one of the most difficult tasks for electromyographers. Evaluation of a patient with suspected myopathy requires meticulous analysis of the morphology of many MUAPs, within different areas of the muscle, in many muscles and in multiple extremities including the paraspinal muscles. Automated analysis of MUAPs, using computer-assisted EDX equipment that incorporates triggered and delayed sweeps, has allowed better determination of MUAP changes, especially those associated with myopathy (Figure C20-10).

Figure C20-10 Quantitative MUAP analysis (Multi-MUP system, Medtronic) from the deltoid muscle of a 65-year-old man with polymyositis. Thirteen MUAP were collected for analysis. Note the shift to the left of both amplitude and duration depicting short-duration and low-amplitude MUAPs compared to normal (bell-shaped curves).

3. Changes in MUAP recruitment.

The EDX assessment of recruitment is the most subjective parameter studied in the EMG laboratory, and attainment of accurate results is highly dependent on experience. Interpretation of recruitment is a particularly difficult task in myopathy. Interference pattern computer-assisted analysis, which typically judges the “turns and amplitudes,” has gained some popularity but has not been used extensively in routine EDX studies.

The recruitment of MUAPs in various myopathies may take one of these forms:

• Normal recruitment. This is common in mild to moderate myopathies.

• Early recruitment. There is increased recruitment of MUAPs in relation to effort, which results from the inability of an individual MUAP to generate any significant force (because of the loss of muscle fibers). This phenomenon sometimes is referred to as an “increase in the interference pattern.” An early recruitment pattern is the most common recruitment abnormality seen in myopathy.

• Reduced recruitment with a rapid firing rate. This recruitment, which imitates neurogenic recruitment, is the least common in myopathy. It occurs in advanced myopathy when loss of muscle fibers is severe, resulting in the functional loss of many motor units.

In summary, the EDX evaluation in patients with suspected myopathy is challenging to electromyographers because the findings may be subtle and patchy. Meticulous care in analyzing MUAP configuration and recruitment and a rather extensive sampling of muscles in both upper and lower extremities are needed in most cases.

Despite its complexity, there are many advantages to the use of EDX studies in the diagnosis of myopathy:

1. The EDX study excludes other neuromuscular disorders (neuromuscular junction, peripheral nerve, anterior horn cell), which sometimes mimic a myopathy.

2. The EDX study permits widespread muscle sampling and may detect abnormalities that are regional (such as in facioscapulohumeral muscular dystrophy or IBM) or patchy (such as in polymyositis). This is in contrast to muscle biopsy, which is usually limited to analysis of one or two muscle specimens.

3. The EDX study may identify specific features of myopathy and helps in attaining a final diagnosis (see forthcoming section). These include myotonia (as seen with myotonic dystrophy) and fibrillation potentials (as seen in the inflammatory myopathies).

4. When abnormal, The EDX study is often able to identify muscle(s) that are significantly affected by the pathological process, thereby guiding the clinician to a suitable site for biopsy (usually from the contralateral side).

5. The EDX study may be used to follow the progress of certain myopathies, such as an inflammatory myopathy during treatment or in cases of relapse.

The EDX examination, however, has two relevant limitations in the diagnosis of myopathy:

1. In general, the diagnostic sensitivity of EDX in myopathy is not high. This is because needle EMG is dependent on the muscle action potential and is not related to the contractile function of muscle. In general, needle EMG more readily detects myopathies in which muscle necrosis has occurred and tends to miss myopathies in which the integrity of both muscle fibers and their action potentials is spared. Thus, needle EMG findings in myopathy are diverse. At one end of the spectrum, if myonecrosis is prominent, such as in muscular dystrophy or inflammatory myopathy, the MUAPs are small and the EMG is abnormal and sensitive. At the other extreme, the needle EMG may be normal (or the changes too subtle to detect) in certain myopathies, such as the metabolic and endocrine myopathies, because the integrity of most muscle fibers and their action potentials is maintained. Thus, a normal needle EMG never excludes a myopathy.

2. The EDX findings in myopathy are not always specific enough to provide a specific diagnosis. In other words, the needle EMG changes in many myopathies are similar, which may prevent the electromyographer from making a specific diagnosis of the myopathic process. Hence, the EDX examination is certainly not the final step in the work-up in many of these patients. Except in certain situations, as in a patient with a classic rash of dermatomyositis or in assessment of a sibling of a patient with documented myotonic dystrophy, a muscle biopsy or genetic testing is necessary to identify the exact cause of myopathy.

Certain needle EMG features may accompany the MUAP changes and are helpful in the differential diagnosis of myopathies. Two features are instrumental in the accurate diagnosis of myopathy: fibrillation potentials and myotonic discharges. Based on these electrical potentials and the MUAP changes associated with myopathy, the EDX findings in myopathy may be easily divided into six general categories (Table C20-4):

• Myopathies commonly presenting with normal EMG.

• Myopathies commonly presenting with MUAP changes and fibrillation potentials.

• Myopathies commonly presenting with MUAP changes only.

• Myopathies commonly presenting with fibrillation potentials only.

• Myopathies commonly presenting with MUAP changes and myotonic discharges.

• Myopathies commonly presenting with myotonic discharges only.

Table C20-4 Patterns of Needle EMG Findings in Myopathies

Electrodiagnostic Findings in Polymyositis/Dermatomyositis

Although the clinical manifestations of PM and DM are diffuse, the EDX findings, similar to their pathologic counterparts, are frequently patchy. Thus, a meticulous needle EMG often is required to identify these abnormalities. This should include sampling of the proximal and distal muscles, the upper and lower extremity muscles, and the paraspinal muscles.

Needle EMG findings during the active phases of PM/DM consist of the following.

1. Increased insertional activity with fibrillation potentials. These potentials, when associated with PM and DM, have certain features:

• They are patchy in distribution and are not present in every muscle sampled. Thus, extensive needle EMG often is required, particularly in early or mild cases, to allow identification of these potentials.

• They are detected in nearly all patients with PM or DM, as long as an extensive search is performed. In more than two-thirds of patients with PM/DM, fibrillation potentials are present in all or at least half of the sampled muscles. In the minority of patients, these potentials are limited to only a few muscles.

• They have a predilection to the paraspinal muscles in the cervical, thoracic, and lumbar regions, presumably because of the propensity of the disease to affect proximal and truncal muscles. In fact, fibrillation potentials are found in the paraspinal muscles in 90 to 100% of patients during the active phase of disease.

• When serial studies are performed, there is a rough correlation between the number of fibrillation potentials and the severity or level of activity of the disease (i.e., the degree of ongoing myonecrosis).

• Fibrillation potentials decrease after successful treatment. Along with the decline in serum CK, the disappearance of fibrillation potentials is among the earliest signs of a favorable response to treatment.

• Fibrillation potentials recur as PM or DM relapses. Thus, in patients treated with corticosteroids who develop increasing weakness, the presence of fibrillation potentials is generally consistent with recurring disease and not with “steroid myopathy,” which is not accompanied by these potentials. In these situations, comparative studies are important.

The exact cause of fibrillation potentials in the necrotizing and inflammatory myopathies is not well understood. Because fibrillation potentials are spontaneous action potentials that are generated by denervated muscle fibers, two possible explanations for their occurrence have been proposed:

• Segmental myonecrosis leading to effective denervation of the distant segments of muscle fibers as they become separated physically from the neuromuscular junction. The denervated segment of the muscle fiber generates the fibrillation potentials. This phenomenon may also initiate collateral sprouting to these segments, which may lead to fiber type-grouping (as seen histologically), increased fiber density (as measured by single-fiber EMG), and long-duration and complex MUAPs (as seen on conventional needle EMG).

• Damage to the terminal intramuscular motor axons, presumably by the inflammatory or necrotizing process, which results in denervation of some muscle fibers. Here again, collateral sprouting accounts for reinnervation (fiber type-grouping, increased fiber density, and long-duration, complex MUAPs).

2. Small MUAPs. In subacute PM or DM, the MUAPs frequently are short in duration, low in amplitude, and polyphasic in configuration, and they are frequently intermixed with normal MUAPs. However, in chronic cases of PM/DM (as well as in other chronic myopathies, such as IBM), it is common to find long-duration, complex MUAPs with many components and satellites (Figure C20-11). These potentials result from collateral sprouting or from significant variation of muscle fiber conduction velocities within the motor unit caused by segmental degeneration and regeneration.

Figure C20-11 MUAPs recording from the quadriceps of a 74-year-old man with 3-year history of inclusion body myositis. Although the MUAP in (A) is very low in an amplitude, it is polyphasic and complex, resulting in an increase in absolute duration to 25 ms. The increased duration of this unit may suggest a neurogenic disorder. (B) This MUAP has also a slightly prolonged duration (14.3 ms). It has a well-defined satellite, which precedes the main body of the MUAP. The satellite potential should not be counted in the measurement of the duration.

The EDX findings in polymyositis and dermatomyositis follow a cyclic pattern. Fibrillation potentials appear first at relapse and disappear early during remission, but abnormal MUAPs become evident later in relapse and last longer before resolution (Figure C20-12). This changing pattern must be recognized after treatment, when serial studies are performed on patients with PM/DM. For example, when fibrillations are not detected in a patient with PM/DM who is experiencing worsening weakness while taking corticosteroids, the diagnosis of iatrogenic “steroid myopathy” becomes more likely because the latter is not associated with fibrillation potentials.

Figure C20-12 Needle electromyography changes seen during the various phases of polymyositis and dermatomyositis.

(Adapted, with revisions, from Wilbourn AJ. Electrodiagnostic examination with myopathies. J Clin Neurophysiol 1993;10: 132–148, with permission.)

FOLLOW-UP

A muscle biopsy was obtained from the quadriceps muscle. The findings were diagnostic of inflammatory myopathy, particularly PM (Figure C20-13). No rimmed vacuoles were seen. The patient was started on prednisone 80 mg/day. She showed a dramatic improvement in strength, accompanied by a decline in CK. Prednisone was tapered slowly with no evidence of recurrence. One year later, she displayed normal strength and CK while taking prednisone 10 mg every other day.