Version May 2024
INTRODUCTION — Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies, characterized by the shared features of proximal skeletal muscle weakness and evidence of muscle inflammation [1-5]. DM, unlike PM, is associated with a variety of characteristic skin manifestations. A form of DM, termed clinically amyopathic DM (CADM, also known as dermatomyositis sine myositis), is a condition in which patients have characteristic skin findings of DM without objective muscle weakness. (See "Clinical manifestations of dermatomyositis and polymyositis in adults".)
There is growing understanding that the term PM is falling out of favor given the increased understanding that many cases that would previously have been classified as PM are now better classified as the antisynthetase syndrome (without a characteristic rash), overlap myositis (with another connective tissue disease), or immune-mediated necrotizing myopathy (IMNM). While it is now understood that muscle biopsy features are distinct in IMNM, previous studies often labeled these cases as PM, assuming the primary inflammation had been missed on the muscle biopsy sampling.
The diagnosis and differential diagnosis of DM and "PM" in adults will be reviewed here. The pathogenesis, clinical manifestations, and treatment of these diseases and of the related disorders that occur in children (known as juvenile DM and PM); malignancy in patients with DM and PM; and the clinical manifestations, diagnosis, and treatment of inclusion body myositis (IBM) are discussed separately.
●(See "Pathogenesis of inflammatory myopathies" and "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations".)
●(See "Clinical manifestations of dermatomyositis and polymyositis in adults".)
●(See "Initial treatment of dermatomyositis and polymyositis in adults" and "Treatment of recurrent and resistant dermatomyositis and polymyositis in adults".)
●(See "Cutaneous dermatomyositis in adults: Overview and initial management" and "Management of refractory cutaneous dermatomyositis in adults".)
●(See "Juvenile dermatomyositis and polymyositis: Treatment, complications, and prognosis".)
●(See "Malignancy in dermatomyositis and polymyositis".)
●(See "Clinical manifestations and diagnosis of inclusion body myositis" and "Management of inclusion body myositis".)
DIAGNOSTIC APPROACH
When to suspect the diagnosis — The diagnosis of dermatomyositis (DM) or polymyositis (PM) should be suspected in patients who present with proximal muscle weakness. The suspicion for DM in particular should be further increased if the patient has a cutaneous eruption suggestive of DM. Both PM and DM often have elevated creatine kinase (CK) at presentation, which may also raise suspicion of the disease in a patient presenting with weakness leading to difficulty in activities of daily living.
The diagnosis of clinically amyopathic DM (CADM) should be suspected in a patient presenting with a characteristic cutaneous eruption in the absence of muscle weakness, although some patients may have mild abnormalities in muscle-derived enzymes, electromyography (EMG), muscle magnetic resonance imaging (MRI), or muscle biopsy. Notably, despite lacking clinically meaningful muscle involvement, these patients may have associated systemic disease including interstitial lung disease or malignancy. (See "Cutaneous dermatomyositis in adults: Overview and initial management".)
Initial evaluation — In patients suspected of having DM or PM, the initial presentation generally determines the nature and scope of testing that may be required to confirm the diagnosis and to exclude other disorders that may cause similar weakness, cutaneous eruptions, or multisystem disease. In addition, the initial workup often involves input from different subspecialists and may include referral to a dermatologist, a rheumatologist, a pulmonologist, and/or a neurologist (neuromuscular specialist), depending on the predominant symptoms and clinical findings.
The evaluation begins with a careful history and physical examination, as well as selected laboratory testing. Additionally, the evaluation should help to differentiate patients with DM and PM from those with other conditions, including other systemic rheumatic diseases and metabolic, infectious, or malignant disorders, other causes of photosensitive or pruritic cutaneous eruptions, as well as neuropathies, neuromuscular diseases, and other myopathies, including drug-induced myopathy, metabolic myopathies, muscle dystrophies, and inclusion body myositis (IBM). (See 'Differential diagnosis' below.)
History — Patients should be questioned regarding the duration, mode of onset, location, and severity of weakness and/or cutaneous eruptions. The patient should be asked about their ability to carry out various activities that they commonly perform, such as climbing stairs, getting up from a chair, and carrying heavy groceries or other objects. The severity and distribution of myalgia, as well as the presence or absence of significant muscle or joint pain, swelling, or stiffness, should be assessed. True muscle weakness should be distinguished from complaints of fatigue or shortness of breath with exertion and from limitation due to joint disease. (See "Approach to the patient with muscle weakness" and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Clinical manifestations'.)
Regarding the skin, a history of photosensitivity and pruritus should be ascertained. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Skin findings in dermatomyositis'.)
Patients should be asked about a history of dysphagia, which may suggest esophageal involvement, and of cough or shortness of breath, which may occur due to pulmonary involvement. Symptoms of other systemic rheumatic diseases, particularly systemic lupus erythematosus (SLE) and systemic sclerosis (SSc), that may identify an overlap syndrome should be sought, and patients should be queried about cutaneous eruptions, photosensitivity, pruritus, fever, and Raynaud phenomenon. Patients should also be queried regarding whether they have been experiencing any symptoms suggesting malignancy and what screening and other testing has been performed for screening for cancer. The timing of the use of any drugs that may cause myopathy, particularly statins, should be determined. (See 'Differential diagnosis' below and "Malignancy in dermatomyositis and polymyositis" and "Drug-induced myopathies" and "Statin muscle-related adverse events".)
Physical examination — Examination of the skin, muscles, and joints is central. Characteristic findings on skin examination should be identified, if present, but they are sometimes subtle. A thorough examination of the skin should be performed with particular attention to the scalp, face, eyelids, hands, feet, extensor surfaces, upper chest and back, arms, lateral thighs, and joints. A nailfold examination should also be performed, evaluating for periungual erythema as well as nailfold capillary abnormalities. A general physical examination should be performed, with particular attention to the heart and lungs, which should be carefully auscultated for pulmonary crackles. A thorough joint examination should be carried out to detect signs of inflammatory arthritis. A detailed neurologic and neuromuscular examination is critical to determine the severity and distribution of weakness and of muscle tenderness, as well as the presence or absence of other abnormal neurologic findings that may be indicative of a clinical myositis mimic. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Skin findings in dermatomyositis' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Muscle weakness'.)
Laboratory testing and imaging — We obtain the following general laboratory tests:
●CK – CK is the most sensitive muscle enzyme and should be tested and followed in all patients with suspected DM or PM. Patients may also have an elevation of other muscle-derived enzymes including lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and alanine aminotransferase (ALT).
●Aldolase may occasionally be useful in cases in which the CK is normal, as a small percentage of patients may have an elevated aldolase level, which suggests there may be perimysial involvement. However, we do not routinely test for aldolase. Elevations in aldolase may be suggestive of fasciitis, connective tissue disease-associated interstitial lung disease [6], or a myositis associated with chronic graft-versus-host disease (GVHD) or checkpoint inhibitor immunotherapy.
●Complete blood count with differential.
●Comprehensive metabolic panel.
●Erythrocyte sedimentation rate (ESR) and/or C-reactive protein (CRP) – The CRP and/or ESR are often normal or only mildly elevated, even in patients with active muscle disease [5]. Markedly elevated CRP and/or ESR in a patient with an inflammatory myopathy should raise the suspicion of a coexisting infection, underlying malignancy, or could be seen in some cases of acute/subacute antisynthetase syndrome or anti-melanoma differentiation-associated gene 5 (MDA5)-positive disease.
●Thyroid-stimulating hormone (TSH).
We also perform the following serologic tests, which may support the diagnosis if positive:
●Myositis-specific autoantibodies (often available as a panel) including anti-Jo-1 – Myositis-specific autoantibodies are associated with particular clinical syndromes within the idiopathic inflammatory myopathy spectrum, which includes DM and PM. These tests are positive in 45 to 85 percent of patients [7]; thus, a negative myositis panel does not rule out a diagnosis of DM or PM. Many experts use myositis autoantibody panels to determine what particular myositis phenotype may best fit the patient rather than to make a diagnosis of myositis. (See "Overview of and approach to the idiopathic inflammatory myopathies", section on 'Myositis-specific autoantibodies'.)
In many clinical settings, serologic test panels for idiopathic inflammatory myopathy are available from reference laboratories, in an effort to eliminate the need to order the numerous autoantibody tests individually. However, the results are often not available for several weeks, and treatment decisions may need to be made before the specific serologic profile is known. In addition, many commercially available myositis panels have varying degrees of accuracy. Hence, using careful clinical judgment based on the patient's examination and other studies is crucial.
●Myositis-associated autoantibodies including anti-Ro/SSA, anti-La/SSB, anti-Sm, anti-ribonucleoprotein (RNP) antibodies, anti-PM-Scl, and anti-Ku antibodies – Myositis-associated autoantibodies are those found in patients with other systemic rheumatic diseases that can be associated with myositis. (See "Overview of and approach to the idiopathic inflammatory myopathies", section on 'Myositis-associated autoantibodies'.)
●Antinuclear antibodies (ANA) – While ANA detected by standard immunofluorescence methods may be present in up to 60 percent of patients with DM or PM, ANA testing is not diagnostic for DM or PM [8]. An anti-cytoplasmic ANA pattern is typical of both anti-MDA5 and all antisynthetase autoantibodies.
Chest radiographs and pulmonary function tests (PFTs) with diffusing capacity of carbon monoxide should be performed in all patients with suspected or confirmed DM or PM, regardless of pulmonary symptoms. For patients in whom abnormalities are found on chest radiograph or PFTs, a high-resolution chest computed tomography (CT) scan with interstitial lung disease protocol should be completed. A high-resolution chest CT should be performed in all patients with a high risk of interstitial lung disease, such as patients with antisynthetase, MDA5, and overlap antibodies (eg, myositis-associated antibodies). This is discussed further below. (See 'Assessment for interstitial lung disease' below and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Interstitial lung disease'.)
Subsequent evaluation based on clinical presentation — Our diagnostic approach varies according to the following patterns of clinical features:
Major patterns of disease
●Patients with characteristic skin findings and muscle weakness – Patients with characteristic clinical and laboratory findings that are pathognomonic of DM, which mainly include symmetric proximal muscle weakness with "specific" or pathognomonic cutaneous findings (eg, Gottron papules and/or a heliotrope eruption) in the setting of a marked elevation of muscle enzymes, in whom evidence suggesting an alternative diagnosis is lacking, may not require additional testing to confirm the diagnosis, including a skin or muscle biopsy (see "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Characteristic findings' and 'Diagnosis' below). Some experts, however, also obtain an EMG and/or MRI in this setting to further document that findings characteristic of an inflammatory myopathy are present (see 'Electromyography' below and 'Skeletal muscle imaging' below). Others obtain a skin biopsy to confirm histopathologic changes compatible with a diagnosis of DM.
●Patients with muscle weakness without characteristic skin findings – Patients presenting with symmetric proximal muscle weakness and elevated muscle enzymes without skin findings usually require a muscle biopsy to demonstrate either typical histopathologic findings of PM or an alternative diagnosis such as muscular dystrophy, metabolic myopathy, necrotizing myopathy, or IBM. EMG and/or muscle MRI may be performed to identify a site for muscle biopsy in some cases. Given the high sensitivity and specificity of anti-hydroxymethylglutaryl (HMG) coenzyme A reductase (HMGCR) autoantibodies, patients with these antibodies and a clinically consistent phenotype, especially with previous statin exposure, do not generally need to undergo muscle biopsy. (See 'Muscle biopsy' below.)
In patients with muscle weakness that is atypical (ie, asymmetric or more distal), in whom it may be difficult to distinguish between a myopathic and neuropathic disorder causing weakness, an EMG and/or MRI should be performed before a muscle biopsy. If myopathy is suggested by EMG or MRI, muscle biopsy should then be performed. (See 'Testing for myopathy' below.)
●Patients with cutaneous features without weakness – A distinct group of patients with "clinically amyopathic dermatomyositis" (CADM), historically called "dermatomyositis sine myositis," have classic cutaneous findings of DM without clinical evidence of muscle weakness [9-12]. There are two subsets within this group. One subset, referred to as "hypomyopathic dermatomyositis" (HDM), has subtle evidence of myositis upon investigation by laboratory, EMG, muscle biopsy, or imaging despite a lack of clinical muscle weakness; the other subset, classified as "amyopathic dermatomyositis" (ADM), includes patients without clinical weakness or laboratory or muscle study abnormalities.
Patients who present with cutaneous signs of DM but with normal strength and normal muscle enzymes often undergo a skin biopsy to rule out other cutaneous eruptions that mimic DM, although a diagnosis of cutaneous lupus erythematosus (CLE) cannot be excluded based on skin biopsy as described below. In such patients, EMG, MRI, or even a muscle biopsy may reveal occult muscle disease, but such studies are generally not needed in patients with classic cutaneous presentations if the clinician can be confident that there is no muscle weakness present. Findings on skin biopsy in patients with DM and the further evaluation of patients with CADM are described separately. Notably, patients with CADM may still have associated malignancy or systemic disease despite lacking muscle involvement; hence, further evaluation in these patients is still required. (See 'Skin biopsy' below and "Cutaneous dermatomyositis in adults: Overview and initial management".)
Testing for myopathy — As discussed above, the approach to additional testing for myopathy depends on the need for more diagnostic certainty. For example, in patients with characteristic skin findings of DM, proximal muscle weakness, and elevated CK, such testing may not be necessary in absence of a possible alternative diagnosis. However, for other patients in whom there is more uncertainty, additional testing is often required.
EMG and nerve conduction studies (NCS) are most useful early in the evaluation of a patient presenting with weakness in determining if the weakness is due to generalized myopathy versus motor neuron disease or some form of neuropathy. It is therefore most useful in patients with asymmetric or distal muscle weakness. However, if a CK is markedly abnormal (eg, greater than 2000 units/L), an EMG may not be required to confirm the presence of a muscle disease rather than a neuropathic disorder. An EMG can also be performed to help identify an appropriate muscle to target for patients in whom there is difficulty localizing an appropriate muscle group for biopsy. (See "Approach to the patient with muscle weakness" and "Muscle examination in the evaluation of weakness".)
Musculoskeletal imaging with MRI is a noninvasive method to establish the presence of muscle inflammation when features atypical for myopathy are present. MRI may be used to demonstrate muscle involvement in patients with classic DM skin disease in the setting of inconclusive muscle enzyme or strength testing. MRI may also be helpful in selecting a biopsy site if the physical examination or EMG do not provide sufficient guidance. (See 'Skeletal muscle imaging' below.)
Muscle biopsy is necessary to establish the diagnosis of PM, distinguishing it from other forms of myopathy described below in the section on differential diagnosis (see 'Differential diagnosis' below). However, as mentioned above, muscle biopsy may not be necessary in patients with myopathy and typical cutaneous signs of DM.
Electromyography — Characteristic abnormalities on EMG can support a diagnosis of inflammatory myopathy. However, such changes are not diagnostic of DM or PM, with similar findings occurring in various infectious, toxic, or metabolic myopathies. EMG is most helpful in distinguishing myopathic causes of weakness from neuropathic disorders (eg, amyotrophic lateral sclerosis [ALS], peripheral polyneuropathy) and myasthenia gravis, if one is unsure of the cause of weakness following careful history and examination. In patients with equivocal muscle weakness on exam, EMG may be useful in identifying a suitable muscle for biopsy. (See "Overview of electromyography" and 'Muscle biopsy technique' below.)
The EMG may show evidence of the following abnormalities, although these changes are not always present in all patients with inflammatory myopathy:
●Increased insertional activity and spontaneous activity in the form of fibrillation potentials, positive sharp waves, complex repetitive discharges
●Abnormal myopathic low-amplitude, short-duration polyphasic motor unit potential
●Early recruitment of motor unit potentials, in which there is an increased number of motor units firing rapidly in order to produce a low level of contraction
The findings of abnormal spontaneous activity such as insertional activity, fibrillation potentials, and complex repetitive discharges can be seen in a wide range of myogenic processes, including myositis patients with active inflammation and/or necrosis.
Certain technical considerations are important in the evaluation of patients with suspected inflammatory myopathy by EMG. Because involvement is typically bilateral and symmetrical, we perform a unilateral EMG study to detect changes suggestive of an active inflammatory myopathy and then obtain a muscle biopsy on the contralateral side. In patients who are having blood drawn for muscle enzyme testing, it should be drawn prior to performing the EMG because samples obtained soon after (within 24 to 36 hours) may be elevated due to the trauma associated with needle insertion into the muscle. Additionally, the electromyographer should sample multiple sites, including the paraspinals, if initial sites are unremarkable, before concluding that there are no myopathic changes because muscle involvement may not be generalized. (See "Muscle enzymes in the evaluation of neuromuscular diseases" and "Overview of electromyography".)
Skeletal muscle imaging — Skeletal muscle imaging with MRI is a noninvasive sensitive modality for evaluating patients with myopathy [13]. MRI can demonstrate areas of edema (which is consistent with but not diagnostic for muscle inflammation), atrophy, fatty replacement, and calcification (image 1). Unlike muscle biopsy, MRI can assess large areas of muscle (eg, both thighs), thereby avoiding problems with sampling error. It is, however, nonspecific and may not distinguish the changes of inflammatory myopathy from those that occur in rhabdomyolysis, muscular dystrophy, or metabolic myopathy. Additionally, denervation may also present with muscle edema on MRI radiographically indistinguishable from that of myositis.
The typical MRI ordered is a bilateral thigh MRI using a myositis protocol. Despite being nonspecific for myositis, MRI can be particularly useful in helping to determine whether muscle disease is active in a patient with known DM or PM, especially in situations in which the patient's serum muscle enzymes are not reflective of the patient's muscle disease activity.
Within myositis subgroups, some patterns may be present based on the associated autoantibody. As an example, in a study including patients with idiopathic inflammatory myopathies, MRIs of patients with immune-mediated necrotizing myopathy (IMNM) and anti-signal recognition particle (SRP) autoantibodies had more atrophy and fatty replacement than those with anti-HMGCR autoantibodies [14]. Muscle abnormalities were especially common in the lateral rotator and gluteal groups among patients with IMNM. Fascial involvement was most widespread in patients with DM. Fatty replacement of muscle tissue may begin early during the course of disease, particularly in IMNM [14].
MRI can be helpful in cases with normal CK but elevated aldolase; fasciitis may be seen (which is also observed in GVHD myositis and checkpoint inhibitor-induced myositis). Some MRI findings may also help distinguish DM and PM from other inflammatory myopathies. In IBM, the MRI may show a predilection for vastus lateralis and medialis with sparing of the rectus femoris in the thighs and of the deep finger flexors in the forearm. Also, in patients with IMNM, abnormalities in muscle are more common in the lateral rotator and gluteal groups. Fascial involvement may be more widespread in DM when compared with IBM [14].
Musculoskeletal ultrasound with power Doppler is another noninvasive technique that can be used to assess for the presence of myopathy. With inflammatory myopathies, muscle becomes hypoechoic, reflecting loss of normal muscle definition due to edema and fatty tissue infiltration. Hypervascular changes on power Doppler are seen in active early disease. While these changes are not specific for inflammatory myopathies, ultrasound can be an alternative to MRI in helping to establish the presence of muscle disease or in helping to locate an appropriate site for muscle biopsy. While it is less expensive and less time-consuming than MRI, it is operator-dependent, requiring an experienced ultrasonographer [15].
Muscle biopsy — The histologic features of both DM and PM include muscle fiber necrosis, degeneration, regeneration, and an inflammatory cell infiltrate. Certain characteristic findings in these two different diseases help to distinguish the disorders from each other and reflect their distinct pathophysiologic pathways [16].
Muscle histopathology
●DM muscle pathology – DM is characterized histopathologically by the following [5]:
•There is evidence of injury to capillaries and perifascicular myofibers (picture 1). Perifascicular atrophy and fibrosis are characteristic but may not be found in some patients biopsied early in the course of their illness. Rarely, abnormal muscle fibers are grouped in one portion of the fascicle, suggestive of microinfarction mediated by blood vessel dysfunction [17].
•The predominant inflammatory infiltrate is in the perimysial region (around the muscle bundles) and includes CD4+ cells, many of which (30 to 90 percent) are plasmacytoid dendritic cells rather than T cells, and it also includes macrophages and B cells. Unlike PM and IBM, invasion of non-necrotic fibers is not prominent.
•Expression of myxovirus resistance protein A (MxA), a type 1 interferon-inducible protein, in the sarcoplasm of muscle fibers and capillaries on immunostaining expression is the most sensitive histopathologic abnormality seen in DM [18].
•The terminal complement C5b-9 membrane attack complex is detectable in vessel walls before the appearance of inflammatory cell infiltration in DM but not in PM (picture 2) [17]. It is not known if the vasculopathy is mediated purely by complement or if the deposition of complement proteins and other immune complexes associated with this diagnosis is secondary to other pathophysiologic events [19,20].
●PM muscle pathology – PM is characterized histopathologically by the following [5]:
•The cellular infiltrate is predominantly endomysial (within muscle fascicles) with inflammatory cells invading individual muscle fibers (picture 3) [21]. In contrast to DM, abnormal necrotic and regenerating muscle fibers are scattered throughout the fascicle and are not limited to one portion or perifascicular area. Muscle fiber size is variable. There are no signs of vasculopathy or immune complex deposition.
•Myofiber injury appears to be mediated directly by CD8+ cytotoxic T lymphocytes that surround and invade myofibers. Macrophages and myeloid dendritic cells are usually present, and in some patients, plasma cells are also seen. The inflammatory infiltrate can involve non-necrotic muscle fibers, but this finding is not always present. While the infiltrate is often endomysial, both perivascular and perimysial infiltrates may be observed. Inflammation is typically present, but these findings are nonspecific and can occur in muscular dystrophies, metabolic myopathies following rhabdomyolysis, and IBM [5].
•There is enhanced expression of class I major histocompatibility complex (MHC) antigens by the muscle fibers [22].
•Importantly, the above features are also evident in muscle biopsies in IBM, which is much more common than PM. As many as 20 to 30 percent of any given muscle biopsy in IBM may lack the canonical rimmed vacuoles and filamentous inclusions on electron microscope. Helpful features are demonstration of p62-positive inclusions with immunostaining and cytochrome oxidase-negative fibers. (See "Clinical manifestations and diagnosis of inclusion body myositis", section on 'Muscle biopsy'.)
●Myositis-specific autoantibodies pathology – Patients with myositis-specific autoantibodies may differ histopathologically from those lacking these autoantibodies, and there is histopathologic variation according to the particular myositis-specific autoantibody that is present [23-26]. For example, perimysial connective tissue fragmentation and inflammation, with myopathic changes in nearby perifascicular regions but with little endomysial or perivascular change, may be seen in patients with antisynthetase syndrome (eg, anti-Jo-1 autoantibodies) [24], but this is now recognized as a distinct form of idiopathic inflammatory myopathy separate from DM and PM. Anti-Jo-1 autoantibody-positive patients may thus exhibit the perifascicular atrophy typical of DM with more necrosis but no MxA expression and no membrane attack complex deposition on capillaries. Patients with anti-SRP antibodies and anti-HMGCR antibodies have necrotic and regenerating fibers with abnormal expression of MHC-1 and membrane attack complex on the sarcolemma of non-necrotic fibers without significant inflammation. Patients with autoimmune necrotizing myopathy have previously sometimes been classified with PM, but this is now known to be a distinct form of idiopathic inflammatory myopathy, called IMNM. Most patients have anti-HMGCR antibodies or anti-SRP antibodies, while some have no detectable myositis-specific autoantibody. In these patients with IMNM, there are few to no inflammatory cells, and, when present, they are localized to necrotic muscle fibers. Sometimes, inflammatory cells are found around small blood vessels, and thickened basement membranes are seen. Unlike DM, these patients do not exhibit perifascicular atrophy. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Myositis-specific autoantibodies'.)
Muscle biopsy technique — The biopsy should be obtained from a muscle that is weak on physical examination (Medical Research Council [MRC] grade 4 out of 5) but is not completely atrophic. The usual muscle targets for biopsy are the quadriceps in the legs or the deltoid or biceps in the arms. Muscles with longstanding antigravity weakness and marked atrophy and muscles recently subjected to EMG testing should be avoided. Biopsy of the calf muscles is discouraged because of the frequency of artifactual findings from that region including unrelated neurogenic atrophy from coincidental length-dependent polyneuropathy or S1 root irritation.
If physical examination and EMG fail to identify an appropriate muscle for biopsy, MRI may be useful. MRI may reveal areas of increased T2/short tau inversion recovery (STIR) signal in muscles that can then be selected for biopsy. Targeted study of the most accessible muscles (eg, deltoid, biceps, quadriceps) by MRI is one approach, but whole-body imaging is another option (image 1) [27].
We generally prefer an open biopsy to a closed-needle biopsy because larger specimens can be obtained and because muscle fiber orientation is better preserved. Muscle biopsy via a small incision using local anesthesia and a sharp-jawed surgical instrument (conchotome) may also yield a diagnostically adequate specimen with a low complication rate. The efficacy of this technique was evaluated in a report in which 149 muscle biopsies were obtained from 122 patients [28]. Only 4 biopsies (2.7 percent) failed to provide an adequate sample for histologic analysis. Eighty-three percent of patients who met clinical criteria for definite or probable DM or PM had biopsies that revealed myositis.
Proper processing of the muscle biopsy is essential. Once obtained, the muscle tissue should be preserved appropriately for later testing. Some tissue should be fixed for routine light and electron microscopy, and some tissue should be frozen in isopentane cooled in liquid nitrogen for biochemical assays. Important biochemical assays include:
●Testing for metabolic myopathies due to defects in carbohydrate, lipid, or purine metabolism
●Immunohistologic assays for mutant proteins including dystrophin for Duchenne/Becker dystrophy, merosin for congenital muscular dystrophy, and sarcoglycan for limb-girdle muscular dystrophy
The experience of the laboratory in processing muscle biopsy specimens correlates highly with the usefulness of diagnostic information obtained from the procedure. The team performing and processing the biopsy should be provided with detailed clinical information. The clinician requesting the biopsy should communicate directly with the surgeon and pathologist prior to the procedure. An example of detailed instructions for handling of muscle biopsy specimens is available [29].
Skin biopsy — Although a skin biopsy is usually not required to make a diagnosis of DM, it can be helpful in distinguishing the rash of DM from other conditions that can mimic DM, such as eczema or psoriasis.
A 3 to 4 mm punch biopsy is usually sufficient to obtain an adequate tissue sample for histologic examination on light microscopy with hematoxylin and eosin staining. Immunofluorescent studies are generally not required, and few experts perform these studies. Skin biopsy techniques are described in detail separately. (See "Skin biopsy techniques", section on 'Punch biopsy'.)
On light microscopy, DM skin lesions usually demonstrate mild atrophy of the epidermis with vacuolar changes in the basal keratinocyte layer, as well as a perivascular lymphocytic infiltrate in the dermis [30]. This is referred to as interface dermatitis. In addition, patients with DM frequently have increased dermal mucin in the dermis.
Direct immunofluorescence may reveal deposition of complement proteins and immunoglobulin at the dermal-epidermal junction that is generally not distinguishable on light microscopy from CLE. However, in DM, deposition of immunoglobulin, but not of complement, is less common than in CLE. Deposits of the membrane attack complex are found along the dermal-epidermal junction and within the walls of dermal blood vessels [31]. (See 'Differential diagnosis' below.)
Diagnosis — The diagnosis of PM and classic DM depends upon the presence of characteristic clinical and laboratory findings, including symmetric proximal muscle weakness, characteristic cutaneous findings in the case of DM, and elevated muscle enzymes. The diagnosis often relies upon further information from one or more diagnostic studies (eg, MRI of skeletal muscle, EMG, or muscle biopsy), which are selected based upon the clinical presentation and initial findings. In some cases, myositis-specific or myositis-associated autoantibodies are helpful in confirming the diagnosis and may provide management and prognostic information. (See 'Laboratory testing and imaging' above.)
We summarize the diagnostic features for the main disease categories below:
●Dermatomyositis – The diagnosis of classic DM can be made without a muscle or skin biopsy in patients with clinical and laboratory findings that are particularly characteristic of this disorder, which mainly include symmetric proximal muscle weakness in the setting of marked elevation of muscle enzymes with classic DM rashes, in whom evidence suggesting an alternative diagnosis is lacking. An example of such a patient is one with cutaneous findings that are relatively specific for DM, such as Gottron papules or a heliotrope eruption, in the absence of another explanation. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Characteristic findings'.)
If there is any diagnostic uncertainty, an EMG and/or MRI may help further document that findings characteristic of an inflammatory myopathy are present. Similarly, a muscle biopsy can also confirm the diagnosis. Skin biopsy can be performed to rule out other dermatologic conditions. Myositis-specific autoantibodies can also help in confirming the diagnosis but are not present in all cases and may not be widely available. (See 'Electromyography' above and 'Skeletal muscle imaging' above and 'Skin biopsy' above and 'Muscle biopsy' above.)
●Clinically amyopathic dermatomyositis – A distinct group of patients have CADM, historically called "dermatomyositis sine myositis," in which they present with classic cutaneous manifestations of DM without clinical evidence of muscle weakness [9-12]. CADM comprises 10 to 30 percent of DM cases [32,33]. There are two subsets within this group. One subset, referred to as "hypomyopathic dermatomyositis" (HDM), has subclinical evidence of myositis upon investigation by laboratory, EMG, muscle biopsy, or imaging despite a lack of clinical muscle weakness; the other subset, classified as "amyopathic dermatomyositis" (ADM), includes patients without clinical weakness or laboratory or muscle study abnormalities.
The diagnosis of CADM is a clinical one, although it may be suggested by a skin biopsy that rules out other etiologies of rash that can mimic cutaneous DM (other than CLE). In such patients, EMG, MRI, or muscle biopsy may reveal occult muscle disease, but such studies are generally not needed in patients with classic clinical presentations, if the clinician is confident that there is no muscle weakness present. (See "Cutaneous dermatomyositis in adults: Overview and initial management", section on 'Patient evaluation'.)
Rarely, these patients may develop myopathy over time; thus, they require ongoing follow-up. In addition, these patients may have or develop pulmonary disease or a malignancy [32-34]; accordingly, a thorough evaluation should be performed including PFTs and an assessment for malignancy. (See 'Assessment for interstitial lung disease' below and 'Screening for malignancy' below.)
Anti-MDA5-associated DM refers to a subset of patients positive for MDA5 antibodies who typically have distinctive mucocutaneous features, a lower incidence of muscle weakness, and a higher incidence of interstitial lung disease, including a rapidly progressive type with high mortality [35]. In this cohort, careful and frequent pulmonary screening is crucial. (See 'Assessment for interstitial lung disease' below and "Overview of and approach to the idiopathic inflammatory myopathies", section on 'Myositis-specific autoantibodies'.)
●Polymyositis – The diagnosis of PM can be made in patients presenting with symmetric proximal muscle weakness and elevated muscle enzymes but typically requires a muscle biopsy to demonstrate typical histopathologic findings consistent with PM. EMG or MRI may be required to help confirm the presence of myopathy and/or guide the location for the muscle biopsy. (See 'Muscle biopsy' above.)
Occasionally, the clinician is faced with the diagnostic challenge of a patient with nonspecific or subtle suspicious features, such as weakness or rash in the setting of mildly elevated CKs, without another clear diagnosis. Thus, it is often appropriate to involve a rheumatologist, dermatologist, and/or neurologist with experience in these diseases when making the diagnosis.
Classification criteria, which were developed for use in clinical and epidemiologic research, have sometimes been used historically for the purpose of diagnosis but have limitations when used for this purpose. (See "Overview of and approach to the idiopathic inflammatory myopathies", section on 'Classification criteria'.)
DIFFERENTIAL DIAGNOSIS — Dermatomyositis (DM) and polymyositis (PM) must be distinguished from other conditions that cause muscle weakness, with or without muscle enzyme elevation. Clinically amyopathic DM (CADM) must be distinguished from other causes of rash.
The differential diagnosis of PM and classic DM thus includes other inflammatory myopathies; muscular dystrophies; metabolic myopathies; congenital myopathies; motor neuron disease; myasthenia gravis; other systemic rheumatic diseases; and a variety of inherited, metabolic, drug-induced, endocrine, and infectious myopathies. Notably, none of these disorders is associated with the skin lesions that are characteristic of DM. Additionally, in DM and PM, myalgia can be absent, mild, or quite severe (table 1) [36].
Features that may suggest an alternative diagnosis include:
●An asymmetric or distal presentation of weakness
●Intermittent symptoms
●Muscle pain in the absence of muscle weakness
●Marked muscle atrophy
●Calf hypertrophy
●Scapular winging
●Family history of muscle disease
●A history of medication that could be associated with myopathy
●Neuropathic symptoms or findings
While it is beyond the scope of this review to provide a comprehensive list of all possible alternative diagnoses, we present several here:
●Antisynthetase syndrome – Up to 30 percent of patients with immune-mediated myopathy have a constellation of clinical findings termed the "antisynthetase syndrome" [37-39]. These findings include relatively acute disease onset, constitutional symptoms (eg, fever and weight loss), myositis, Raynaud phenomenon, "mechanic's hands" (picture 4), arthritis that is generally nonerosive, and interstitial lung disease [40]. Approximately 30 percent of patients with antisynthetase syndrome will also have cutaneous eruptions typical of DM. Not all patients with antisynthetase antibodies or even those classified as having antisynthetase syndrome have all manifestations of this syndrome. Affected patients have antibodies to one of the aminoacyl-transfer RNA (tRNA) synthetase enzymes, most commonly histidyl-tRNA synthetase (anti-Jo-1 autoantibodies) [41-43].
●Inclusion body myositis – Inclusion body myositis (IBM) is the inflammatory myopathy most commonly misdiagnosed as PM, and, in cases of "refractory PM," the correct diagnosis often proves to be IBM. In contrast to PM, IBM generally has a more insidious onset, asymmetric weakness, and more prominent forearm muscles (table 2). The combination of muscle atrophy and weakness of the wrist and deep finger flexors (eg, flexor pollicis longus) with relative sparing of the deltoids in the upper extremities strongly favors the diagnosis of IBM over PM. Less sensitive, but helpful when detected, is weakness of the quadriceps that is as severe and worse than hip flexors on Medical Research Council (MRC) grading scale and weakness of anterior tibial muscles in the legs [44,45]. Furthermore, in many patients with IBM, the muscle involvement is asymmetric, particularly in early disease. (See "Clinical manifestations and diagnosis of inclusion body myositis".)
Serum muscle enzyme levels are generally lower in IBM than in PM, usually less than 10 times the upper limit of normal, although substantial elevations may occur. The presence of typical inclusion bodies on muscle biopsy is diagnostic of this disorder, but a single biopsy lacks the characteristic histopathologic features in 20 to 30 percent of patients [45]. Magnetic resonance imaging (MRI) findings may help distinguish PM from IBM. Whereas MRI changes suggestive of inflammation are noted along fascial planes in PM, more prominent involvement of the vastus lateralis and medialis with sparing of the rectus femoris is observed in IBM [14,46]. Fatty infiltration and muscle atrophy are more prominent in IBM than in PM, perhaps owing to the longer preclinical phase and to the relative refractoriness to treatment exhibited by IBM.
Anti-Ro/SSA antibodies are also seen in some patients with IBM, both with and without clinical features of Sjögren's disease. The significance of these antibodies is unknown. Finally, anti-cytosolic 5' nucleotidase IA (anti-cN1A, also known as anti-NT5c1A) antibodies may be seen in a subset of IBM patients and may be associated with a more severe phenotype. Although these autoantibodies are also noted in DM and Sjögren's disease, they can be helpful, when present, to help distinguish PM from IBM if there are otherwise subtle clinical features.
●Overlap syndrome – DM and PM may overlap with features of other systemic rheumatic diseases, particularly systemic sclerosis (SSc), systemic lupus erythematosus (SLE), and mixed connective tissue disease, and less often with rheumatoid arthritis and Sjögren's disease. Muscle involvement may at times be a manifestation of one of the other systemic rheumatic diseases, in which case, weakness and muscle biopsy changes are usually subclinical or mild [47-49]. (See "Undifferentiated systemic rheumatic (connective tissue) diseases and overlap syndromes".)
●Drug-induced myopathy – A number of drugs can produce a myopathy that can mimic the inflammatory myopathies. These include glucocorticoids, statins, antimalarials, antipsychotics, colchicine, penicillamine, alcohol, cocaine, and certain antiretroviral therapies (ARTs). Drug-induced myopathy can generally be distinguished from DM or PM based upon a careful clinical history of drug exposure, but, in patients in whom the diagnosis is uncertain, the distinction can usually be made by electromyography (EMG) and muscle biopsy if needed. (See "Drug-induced myopathies".)
●Immune-mediated necrotizing myopathy – Immune-mediated necrotizing myopathy (IMNM) is a distinct clinical subset of myositis that clinically resembles PM but is histologically distinct from PM or DM [50,51]. It is described most often following exposure to statins, but, unlike typical statin-induced myopathy, it persists following withdrawal of drug therapy [50]. Patients with IMNM associated with statin use are often anti-hydroxymethylglutaryl (HMG) coenzyme A reductase (HMGCR) positive. Another subset of IMNM occurs commonly in younger patients with a more severe and refractory phenotype, typically associated with anti-signal recognition particle (SRP) antibody. In addition, another form of IMNM may also be seen in association with an underlying autoimmune rheumatic disease, such as SSc or mixed connective tissue disease. IMNM may be seen in association with malignancy or may be idiopathic.
IMNM, similar to PM and DM, is characterized most often by proximal upper and lower extremity muscle weakness; however, patients also typically have very elevated muscle enzymes and a higher frequency of myalgia. EMG findings are typical of an inflammatory myopathy similar to PM and DM. In contrast to PM and DM, however, muscle biopsies show necrotic muscle fibers without significant inflammatory cell infiltrate around non-necrotic fibers, as well as a lack of perifascicular atrophy, and the deposition of membrane attack complex and major histocompatibility antigen on the sarcolemma of non-necrotic muscle fibers. IMNM has been associated with autoantibodies against HMGCR protein [52] and anti-SRP antibodies.
●Hypothyroidism – The myopathy associated with hypothyroidism can mimic the presentation of inflammatory myopathy, causing a subacute onset of proximal muscle weakness and elevated muscle enzymes. Coexisting clinical signs and symptoms of hypothyroidism, including delayed relaxation phase of the deep tendon reflexes, lack of fibrillation potentials on EMG with relatively normal appearing motor units, and the absence of inflammatory changes on muscle biopsy, all help to distinguish hypothyroid myopathy from PM. Thyroid-stimulating hormone (TSH) measurement is the preferred screening test. (See "Clinical manifestations of hypothyroidism".)
●HIV infection – In addition to causing weakness through chronic disease and cachexia, HIV infection is also associated with an inflammatory myopathy that may be indistinguishable from PM but may also have features of IBM [53]. The myopathy can either be a presenting manifestation of HIV infection or can occur in the later stages of infection. With the widespread use of combination ART, though, it is an uncommon occurrence [54]. Patients with HIV myopathy present with myalgias and muscle tenderness with elevated muscle enzymes. EMG findings in HIV myopathy are generally the same as those in idiopathic PM, and muscle biopsy demonstrates an endomysial mononuclear cell infiltrate with lesser accumulation of inflammatory cells (predominantly CD8 T cells and macrophages) around vessels or in the interfascicular space.
HIV myopathy appears to have a better prognosis than idiopathic PM, with some patients improving spontaneously. For those who do not, high-dose glucocorticoids, as used in idiopathic PM, have been shown to be effective [55-57]. Therapeutic options for glucocorticoid nonresponders are uncertain. Immunosuppressive agents such as methotrexate and azathioprine, which are employed in this setting in idiopathic PM, should be approached with caution in HIV-infected patients [58].
In HIV-infected patients with myopathy, other considerations include rhabdomyolysis and opportunistic infections (eg, toxoplasmosis). Older-generation nucleoside reverse transcriptase inhibitors, predominantly zidovudine, have also been associated with a myopathy that improves with discontinuation of the drug. However, these antiretroviral agents are uncommonly used, particularly in resource-abundant settings.
●Myasthenia gravis – Myasthenia gravis is a disorder of the neuromuscular junction, caused by antibodies to the acetylcholine receptor. Although the classic physical examination finding in myasthenia gravis is muscle fatigability (the development of muscle weakness as exercise proceeds), the disease can occasionally cause diffuse weakness without prominent fatigability symptoms. Myasthenia gravis is distinguished from myositis by the frequent presence of facial muscle weakness, normal muscle enzymes, characteristic EMG changes, and anti-acetylcholine receptor antibodies. Myasthenia gravis can also occur in association with use of checkpoint inhibitor immunotherapy. (See "Clinical manifestations of myasthenia gravis" and "Toxicities associated with immune checkpoint inhibitors", section on 'Neurologic'.)
In contrast to myasthenia gravis, DM and PM rarely involve the oculobulbar muscles. A condition related to myasthenia gravis, known as the Lambert-Eaton syndrome, can mimic DM and PM more closely because the oculobulbar muscles are usually spared.
●Muscular dystrophy – The muscular dystrophies are an inherited group of progressive myopathic disorders resulting from defects in a number of genes required for normal muscle function. Patients with muscular dystrophy occasionally have a prominent endomysial inflammatory cell infiltrate, which may cause diagnostic confusion with PM, particularly in dystrophic disorders such as limb-girdle and facioscapulohumeral muscular dystrophies. However, the inflammatory cell infiltrate in muscular dystrophy is typically limited to areas adjacent to necrotic muscle fibers, in contrast to the tendency of PM to involve non-necrotic muscle fibers.
●Myotonic dystrophy – Proximal myotonic myopathy (PROMM), also known as myotonic dystrophy type 2, must also be considered in the differential diagnosis. It presents in adults, but, unlike in PM, the majority of patients exhibit myotonia, a slowed relaxation following a normal muscle contraction, and a family history of the disorder. There is often early involvement of the finger flexors in addition to proximal muscle groups, and other findings may be present, such as cataracts, mild cognitive impairment, and glucose intolerance. Genetic testing can confirm the diagnosis. (See "Myotonic dystrophy: Etiology, clinical features, and diagnosis".)
●Inherited metabolic myopathies – Inherited metabolic myopathies include disorders of carbohydrate and lipid metabolism, such as carnitine deficiency and myoadenylate deaminase deficiency. These diseases are characterized by intermittent episodes of acute muscle pain and tenderness, usually induced by exertion. The episodes are often accompanied by myoglobinuria with red or brown urine. Occasional patients develop chronic weakness after years of repeated acute episodes [59]. The evaluation of patients with suspected metabolic myopathy is discussed in detail separately. (See "Approach to the metabolic myopathies", section on 'Overview of clinical manifestations'.)
●Other muscle disease – A variety of other myopathies may be mimics of PM. The following considerations and diagnoses should be noted:
•Acute myopathy – Acute viral or bacterial infections (pyomyositis), immobilization, and trauma can be characterized by acute, fulminant presentations that are often complicated by rhabdomyolysis. However, rhabdomyolysis has been described only infrequently in case reports in the inflammatory myopathies [60,61].
•Amyotrophic lateral sclerosis – Amyotrophic lateral sclerosis (ALS, also called motor neuron disease) can present with muscle weakness, but ALS has a number of clinical features that differ from DM and PM, including presentation with distal rather than proximal weakness, most often with asymmetric onset; presence of long-tract signs, such as hyperreflexia and muscle spasticity; absence of myopathic changes on EMG; and normal or only mildly elevated muscle enzymes (creatine kinase [CK] up to approximately 1000 units/L). (See "Clinical features of amyotrophic lateral sclerosis and other forms of motor neuron disease".)
•Familial idiopathic myositis – Muscle weakness occurring in multiple family members is not always due to inherited metabolic defects or dystrophies but may rarely result from the development of idiopathic myositis in several members of the same family. The clinical features of familial idiopathic inflammatory myopathy are similar to those of sporadic disease, although the frequency of myositis-specific autoantibodies is lower with the familial disorder [62].
•Chronic graft-versus-host disease – Muscle disease that is clinically and histopathologically similar to PM has been well documented in patients with chronic graft-versus-host disease (GVHD) [63]. The estimated frequency is 0.6 percent. The onset of myositis is typically more than one year after transplantation [64]. (See "Clinical manifestations and diagnosis of chronic graft-versus-host disease", section on 'Musculoskeletal'.)
•Focal myositis – Rare patients present with focal myositis that usually but not always progresses to the typical generalized form over time [65]. As distinct from DM or PM, these patients usually have marked pain in the affected limb.
•Amyloid myopathy – Amyloid myopathy can occur in immunoglobulin-related or familial amyloidosis [66]. Other manifestations of the particular form of amyloidosis are typically present. The diagnosis can be confirmed by the demonstration of typical amyloid fibers on Congo red staining of a muscle biopsy. (See "Myopathies of systemic disease", section on 'Amyloid myopathy'.)
•Sarcoid myopathy – Myopathy may occur in one of several patterns, including an inflammatory myopathy, in a majority of patients with sarcoidosis, but it is usually asymptomatic. It is distinguished from DM or PM by the presence of other clinical and histologic manifestations of sarcoid. Approximately 50 percent of patients with granulomas on biopsy are ultimately found to have IBM [67-70]. (See "Sarcoid myopathy".)
•Myositis associated with checkpoint inhibitor immunotherapy – Myositis can also be observed in association with checkpoint inhibitor immunotherapy and is discussed separately. (See "Toxicities associated with immune checkpoint inhibitors", section on 'Rheumatologic and musculoskeletal' and "Rheumatologic complications of checkpoint inhibitor immunotherapy", section on 'Inflammatory myopathies'.)
•Trichinellosis – Parasitic infections, particularly trichinellosis, may cause severe muscle pain and enzyme elevations, but it is distinguished by the history of ingesting inadequately cooked meat and by the presence of eosinophilia. Muscle biopsy can confirm the diagnosis, if needed. (See "Trichinellosis", section on 'Clinical manifestations'.)
•Diabetic amyotrophy and diabetic muscle infarction – Diabetic amyotrophy is characterized typically by acute, asymmetric, focal onset of pain followed by weakness involving the proximal leg, with associated autonomic failure and weight loss. Some patients present with distal involvement. The clinical findings in the setting of type 2 diabetes and the electrodiagnostic findings help to distinguish it from DM or PM. The diagnosis of diabetic muscle infarction is suggested clinically by the acute or subacute onset of muscle pain, swelling, and associated tenderness, often in the muscles of the thigh and calf, and may be confirmed by biopsy of the affected area. (See "Diabetic muscle infarction" and "Diabetic amyotrophy and idiopathic lumbosacral radiculoplexus neuropathy".)
●Other skin disease – Lesions of DM are difficult to impossible to distinguish histologically from those of cutaneous lupus erythematosus (CLE), requiring careful clinical examination of the skin for confirmation of the diagnosis [30]. This is particularly important in patients with CADM, who often present after being misdiagnosed as having CLE; the distinction must be made based upon clinical rather than histopathologic features. Findings that are more common in DM than CLE include pruritus and a predominance of CD4+ and chemokine receptor CXCR3-positive cells, as well as increased endothelial cell expression of the interferon-inducible myxovirus resistance protein A (MxA) [71].
Patients with CLE may also develop facial erythema and other photosensitive eruptions, resembling some of the cutaneous features seen in DM. However, unlike in patients with CLE, the midfacial erythema seen in DM does not spare the nasolabial folds, helping with this distinction (picture 5A-B). Furthermore, DM may present with additional cutaneous clues that are not seen in CLE, for example, poikiloderma of the lateral thigh ("holster sign"). Another distinguishing feature between DM and CLE is the frequency of an adverse reaction to antimalarial therapy, with the prevalence in DM being much greater than in CLE [72,73]. Although these disorders may be distinguished by the severity of muscle involvement in classic DM or PM compared with CLE, by the organ systems involved, and by the serologic studies present, a careful cutaneous examination is the ultimate arbiter; hence, referral to a dermatologist with expertise in DM can be valuable.
Scalp involvement in DM may resemble psoriasis or seborrheic dermatitis. Unlike in DM, a thicker, silvery or micaceous scaling may be present in psoriasis, and a more yellow, greasy scaling may be evident in seborrheic dermatitis. Unlike in DM, both scalp psoriasis and seborrheic dermatitis lack any evidence of poikiloderma. Finally, scalp DM may be distinguished from both of these disorders on skin biopsy. The scalp disease in DM exhibits characteristic vacuolar interface changes, but psoriasis and seborrheic dermatitis do not.
Drug-induced cutaneous DM should be suspected particularly in patients who have been treated with hydroxyurea, the most commonly reported cause, but it may also occur with anti-tumor necrosis factor (TNF) therapy, statins, and multiple other agents [74,75].
POST-DIAGNOSTIC EVALUATION — Additional studies are important in the evaluation of a given patient once the diagnosis of dermatomyositis (DM) or polymyositis (PM) has been made. All patients with DM or PM should be assessed for interstitial lung disease and malignancy. Further investigation for esophageal or cardiac involvement depends upon the symptoms and findings that are present.
Assessment for interstitial lung disease — Chest radiographs and pulmonary function tests (PFTs) with diffusion capacity for carbon monoxide should be performed in all patients with DM or PM. In patients who have pulmonary symptoms such as cough or dyspnea, in patients with an abnormal lung examination suggestive of interstitial lung disease, and in patients with antisynthetase or melanoma differentiation-associated gene 5 (MDA5) antibodies who are at increased risk for interstitial lung disease, high-resolution computed tomography (HRCT) of the chest should also be obtained. (See "Interstitial lung disease in dermatomyositis and polymyositis: Clinical manifestations and diagnosis".)
Screening for malignancy — All adult patients with DM or PM should undergo a thorough medical history; physical examination with breast, rectal, and pelvic examinations; laboratory testing; and age-appropriate cancer screening tests (eg, mammography and colonoscopy). Patients at increased risk of malignancy for reasons unrelated to DM or PM may require additional studies, including further imaging. Furthermore, many experts recommend a comprehensive malignancy screening rather than a focused or age-appropriate cancer screening in patients with DM, especially in patients with myositis-specific autoantibodies associated with higher risk of malignancy. Specific recommendations are discussed in detail separately. (See "Malignancy in dermatomyositis and polymyositis".)
Evaluation for esophageal dysfunction — In patients suspected to have esophageal dysmotility, such as those with dysphagia, and therefore at risk for aspiration, esophageal motility studies or swallowing studies should be performed. (See "Overview of gastrointestinal motility testing", section on 'Technique'.)
Evaluation for cardiac involvement — In patients with clinical suspicion of cardiac muscle involvement, such as signs or symptoms of heart failure or of conduction abnormalities, we obtain an echocardiogram and electrocardiogram. Cardiac magnetic resonance imaging (MRI) can be performed to evaluate for myocarditis in selected patients. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Cardiac involvement'.)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Dermatomyositis and polymyositis".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Dermatomyositis (The Basics)" and "Patient education: Polymyositis (The Basics)")
●Beyond the Basics topics (see "Patient education: Polymyositis, dermatomyositis, and other forms of idiopathic inflammatory myopathy (Beyond the Basics)")
SUMMARY AND RECOMMENDATIONS
●Clinical presentation – The diagnosis of classic dermatomyositis (DM) or polymyositis (PM) should be suspected in patients with proximal muscle weakness. Clinically amyopathic DM (CADM) should be suspected in patients with a characteristic cutaneous eruption without muscle weakness. (See 'When to suspect the diagnosis' above.)
●Initial evaluation – The initial evaluation begins with a careful history, a physical examination, and a laboratory examination, with particular attention to the following (see 'Initial evaluation' above):
•Characterization of muscle weakness, including assessment of severity and the degree of symmetry and of relative proximal versus distal involvement
•Skin examination
•Assessment for other organ involvement (eg, pulmonary symptoms, esophageal dysfunction)
•Muscle enzyme measurement, including creatine kinase (CK), general laboratory testing, and serologic testing (including myositis-specific autoantibodies, myositis-associated autoantibodies, antinuclear antibodies [ANA])
•Chest radiograph and pulmonary function tests (PFTs) with diffusing capacity of carbon monoxide followed by high-resolution chest computed tomography (CT) scan with interstitial lung disease protocol depending on initial pulmonary studies
●Testing for myopathy – The approach to additional testing for myopathy depends on the need for more diagnostic certainty. For example, in patients with characteristic skin findings of DM and proximal muscle weakness, such testing may not be necessary. However, for other patients in whom there is more uncertainty, additional testing is often required (see 'Testing for myopathy' above):
•Electromyography – Electromyography (EMG) and nerve conduction studies (NCS) are most useful early in the evaluation of a patient presenting with weakness in determining if the weakness is due to generalized myopathy versus motor neuron disease or some form of neuropathy. EMG shows evidence of muscle membrane irritability, usually including increased insertional activity, spontaneous fibrillations, positive sharp waves, and complex repetitive discharges. (See 'Electromyography' above.)
•Imaging – Skeletal muscle imaging with magnetic resonance imaging (MRI), typically of the bilateral thighs, can demonstrate areas of edema (which is consistent with but not diagnostic for muscle inflammation), atrophy, fatty replacement, and calcification (image 1). (See 'Skeletal muscle imaging' above.)
•Biopsy – Histologic features of muscle biopsies in DM and PM include muscle fiber necrosis, degeneration, regeneration, and inflammatory cell infiltration. Skin changes characteristic of DM have the histopathologic features of interface dermatitis (ie, inflammation at the dermal-epidermal junction) with variable levels of dermal mucin deposition. (See 'Muscle biopsy' above and 'Skin biopsy' above.)
●Diagnosis – The diagnostic features of the main disease categories are as follows:
•Dermatomyositis – The diagnosis of classic DM can be made without a muscle or skin biopsy in patients with clinical and laboratory findings that are particularly characteristic of this disorder, which include symmetric proximal muscle weakness in the setting of marked elevation of muscle enzymes and characteristic cutaneous eruptions, in whom evidence suggesting an alternative diagnosis is lacking.
•Clinically amyopathic dermatomyositis – The diagnosis of CADM is based upon the presence of classic cutaneous manifestations of DM without clinical evidence of muscle weakness. Skin biopsy may be performed to rule out other etiologies of rash that can mimic cutaneous DM. EMG, MRI, or muscle biopsy may reveal occult muscle disease, but such studies are generally not needed in patients with classic clinical presentation.
•Polymyositis – The diagnosis of PM can be made in patients presenting with symmetric proximal muscle weakness and elevated muscle enzymes but typically requires a muscle biopsy to demonstrate typical histopathologic findings consistent with PM. EMG or MRI may be required to help confirm the presence of myopathy and/or guide the location for the muscle biopsy.
In some cases, myositis-specific or myositis-associated autoantibodies are helpful in confirming the diagnosis and may provide management and prognostic information. (See 'Laboratory testing and imaging' above.)
●Differential diagnosis – The differential diagnosis of DM and PM includes inflammatory myopathies; muscular dystrophies; metabolic myopathies; congenital myopathies; motor neuron disease; myasthenia gravis; other systemic rheumatic diseases; and a variety of inherited, metabolic, drug-induced, endocrine, and infectious myopathies. The differential diagnosis of CADM includes other eruptions that may mimic cutaneous DM. (See 'Differential diagnosis' above.)
●Post-diagnostic evaluation – All patients should be assessed for interstitial lung disease and malignancy. Further investigation for esophageal or cardiac involvement depends upon the symptoms and findings that are present. (See 'Post-diagnostic evaluation' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Marc L Miller, MD, who contributed to an earlier version of this topic review.
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