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Diagnosis and differential diagnosis of dermatomyositis and polymyositis in adults
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Diagnosis and differential diagnosis of dermatomyositis and polymyositis in adults
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Nov 2016. | This topic last updated: Sep 28, 2016.

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 amyopathic DM (ADM, also known as dermatomyositis sine myositis), is a condition in which patients have characteristic skin findings of DM without weakness or abnormal muscle enzymes. (See "Clinical manifestations of dermatomyositis and polymyositis in adults".)

The clinical and serologic features of DM and PM vary among affected individuals and populations, depending upon immunogenetic characteristics and presumably upon potential environmental triggers [6,7]. The immune mechanisms and anatomic focus of injury within the muscle tissue in PM and DM appear distinct. The other major type of idiopathic inflammatory myopathy is inclusion body myositis (IBM). (See "Clinical manifestations and diagnosis of inclusion body myositis".)

The diagnosis and differential diagnosis of DM and PM in adults, as well as the electrophysiologic and pathologic findings on diagnostic testing, 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 IBM are discussed separately.

(See "Pathogenesis of inflammatory myopathies" and "Juvenile dermatomyositis and polymyositis: 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 "Initial management of cutaneous dermatomyositis" and "Management of refractory cutaneous dermatomyositis".)

(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 "Treatment and prognosis of inclusion body myositis".)

INITIAL EVALUATION — In patients suspected of dermatomyositis (DM) or polymyositis (PM) (eg, patients who present with progressive proximal muscle weakness and/or with a cutaneous eruption suggestive of DM), the initial diagnostic evaluation helps to determine 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.

The evaluation begins with a careful history and physical examination, as well as selected laboratory testing. Certain aspects of the evaluation are particularly helpful because they may reveal characteristic historical features, physical findings, the extent of disease, and multisystem involvement. 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, as well as neuropathies, neuromuscular diseases, and other myopathies, including drug-induced myopathy. (See "Clinical manifestations of dermatomyositis and polymyositis in adults" and 'Differential diagnosis' below.)

History – Patients should be questioned regarding the duration, mode of onset, location, and severity of weakness and 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 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'.)

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. Other symptoms of 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, Raynaud phenomenon, and other symptoms that may occur in DM, PM, or another systemic rheumatic disease. Patients should also be queried regarding whether they have been experiencing any symptoms suggesting malignancy and what screening and other testing has been performed. The timing of the use of any drugs that may cause myopathy, particularly statins, should be determined. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Overlap syndromes' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Clinical manifestations' and 'Differential diagnosis' below and "Malignancy in dermatomyositis and polymyositis" and "Drug-induced myopathies".)

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, fingers, extensor aspects, and joints. A general physical examination should be performed, with particular attention to the heart and lungs, which should be carefully auscultated for evidence of interstitial disease. 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. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Skin findings' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Muscle weakness' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Antisynthetase syndrome' and 'Differential diagnosis' below.)

Laboratory testing – The laboratory evaluation should include testing for muscle enzymes, including creatine kinase and aldolase, in addition to general laboratory testing. General laboratory testing usually includes a complete blood count with differential, creatinine, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), liver function tests, and thyroid-stimulating hormone (TSH; if hypothyroidism is in the differential diagnosis). (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Muscle enzymes'.)

Testing should also be performed for antinuclear antibodies (ANA); anti-Ro/SSA, anti-La/SSB, anti-ribonucleoprotein (RNP), and anti-Sm; and myositis-associated and myositis-specific antibodies including anti-Jo-1 and other anti-synthetase antibodies, as well as antibodies directed against the Mi2, SRP, PM/Scl, and Ku antigens. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Myositis-specific autoantibodies' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Autoantibodies and overlap syndromes'.)

Imaging – Chest radiographs should be performed in all patients with findings suggesting DM or PM to help detect the presence of pulmonary involvement, such as interstitial lung disease. This is of particular importance in patients suspected of having an antisynthetase syndrome. Pulmonary function tests and chest computed tomography (CT) scanning should be performed in patients with pulmonary symptoms or with abnormal chest radiographs demonstrating interstitial lung disease. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Lung disease' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Antisynthetase syndrome' and "Interstitial lung disease in dermatomyositis and polymyositis: Clinical manifestations and diagnosis".)

FURTHER DIAGNOSTIC STUDIES — The diagnosis of dermatomyositis (DM) or polymyositis (PM) often also relies upon further information from one or more diagnostic studies, in addition to blood tests and a chest radiograph, which are selected based upon the clinical presentation and initial findings (see 'Initial evaluation' above and 'Diagnosis' below). These studies include:

Muscle biopsy (see 'Muscle biopsy' below)

Skin biopsy (see 'Skin biopsy' below)

Electromyography (see 'Electromyography' below)

Magnetic resonance (MR) imaging (see 'MRI' below)

Muscle biopsy — DM and PM can be distinguished from each other and other forms of myopathy by their histopathologic findings. Particular attention should be given to the techniques used for the biopsy of muscle tissue. (See 'Muscle histopathology' below and 'Muscle biopsy technique' below.)

Muscle histopathology — 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 [8]. (See 'DM muscle pathology' below and 'PM muscle pathology' below and "Pathogenesis of inflammatory myopathies".)

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. Abnormal muscle fibers are usually grouped in one portion of the fascicle, suggestive of microinfarction mediated by blood vessel dysfunction [9].

The predominant inflammatory infiltrate is in the perimysial region 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 inclusion body myositis (IBM), invasion of nonnecrotic fibers is not prominent.

Overexpression of type I interferon-inducible genes and proteins, especially in the perifascicular region, is present, and gene expression profiling has demonstrated increased expression of various genes that are induced by interferon-alpha and interferon-beta [10,11].

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) [9]. 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 [12,13].

PM muscle pathology — PM is characterized histopathologically by the following [5]:

The cellular infiltrate is predominantly within the fascicle, with inflammatory cells invading individual muscle fibers (picture 3) [14]. In contrast to DM, abnormal necrotic and regenerating muscle fibers are scattered throughout the fascicle and are not limited to one portion. 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. Perivascular, perimysial, or endomysial 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 antigens by the muscle fibers [15].

In patients with autoimmune necrotizing myopathy, which is sometimes classified with PM, scattered necrotic muscle fibers are present. There are few 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.

MSA and histopathology — Patients with myositis-specific antibodies (MSA) may differ histopathologically from those lacking these antibodies, and there is histopathological variation according to the particular MSA that are present [16-19]. 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 anti-Jo-1 antibodies [17]. Anti-Jo-1 antibody-positive patients may thus exhibit the perifascicular atrophy typical of DM but not the characteristic capillary pathology. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Antisynthetase antibodies'.)

By contrast, patients with anti-signal recognition particle (SRP) antibodies have the capillary pathology but not the perifascicular atrophy. Patients with anti-SRP antibodies are reported to have less intramuscular inflammation than do other patients with DM or PM. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Anti-SRP antibodies'.)

Muscle biopsy technique — The biopsy should be obtained from a muscle that is weak on physical examination but is not atrophied. The usual muscle targets for biopsy are the quadriceps or the deltoid. Biopsying the muscle contralateral to one shown to be abnormal by electromyography (EMG) increases the likelihood of a diagnostic biopsy. We avoid muscles with severe weakness, marked atrophy, or recent EMG testing. In addition, biopsy of the calf muscles is discouraged because of the frequency of artifactual findings in biopsies from that region. (See 'Electromyography' below.)

If physical examination and EMG fail to identify an appropriate muscle for biopsy, magnetic resonance imaging (MRI) may be useful. MRI may reveal areas of increased T2 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) [20]. (See 'MRI' below.)

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 [21]. Only four 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 (see "Approach to the metabolic myopathies")

Immunohistologic assays for mutant proteins including:

Dystrophin for Duchenne/Becker dystrophy

Merosin for congenital muscular dystrophy

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 [22].

Skin biopsy — In patients with DM, characteristic findings may also be seen on skin biopsy, although these findings are very similar on light microscopy to changes that can be seen in systemic lupus erythematosus. (See 'Skin histopathology' below and 'Skin biopsy technique' below.)

Skin histopathology — 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 [23]. This is referred to as interface dermatitis. In addition, patients with DM frequently have increased dermal mucin.

Lesions of DM are difficult to distinguish histologically from those of systemic lupus erythematosus (SLE), requiring careful clinical examination of the skin for confirmation of the diagnosis [23]. This is particularly important in patients with amyopathic DM who often present after being misdiagnosed as having LE; the distinction must be made based upon clinical rather than histopathologic features. Findings that are more common in DM than LE include a predominance of CD4+ and chemokine receptor CXCR3-positive cells, as well as increased endothelial cell expression of the interferon-inducible MxA protein [24].

Direct immunofluorescence may reveal deposition of complement proteins and immunoglobulin at the dermal-epidermal junction that is generally not distinguishable on light microscopy from systemic lupus erythematosus. However, in DM, deposition of immunoglobulin, but not of complement, is less common than in lupus. Deposits of the membrane attack complex are found along the dermal-epidermal junction and within the walls of dermal blood vessels [25].

Skin biopsy technique — Skin biopsy techniques are described in detail separately. A 4 mm punch biopsy is usually sufficient to obtain an adequate tissue sample for histological examination on light microscopy with hematoxylin and eosin staining. Immunofluorescent studies are generally not required, but some experts also perform these studies. (See 'Skin histopathology' above and "Skin biopsy techniques", section on 'Punch biopsy'.)

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. Certain technical considerations are important in the evaluation of patients with suspected inflammatory myopathy by EMG. (See 'EMG findings' below and 'EMG technical considerations' below.)

EMG findings — EMG is most helpful in distinguishing myopathic causes of weakness from neuropathic disorders, such as amyotrophic lateral sclerosis, peripheral polyneuropathy, or myasthenia gravis. It also helps identify the muscle group that is likely to provide useful information on biopsy. The biopsy is done on the side contralateral to that on which the EMG is performed to avoid needle-related artifact, given the usually symmetrical muscle involvement. (See "Overview of electromyography".)

The EMG may show evidence of increased membrane irritability with the following abnormalities, although these changes are not always present in patients with inflammatory myopathy:

Increased insertional activity and spontaneous fibrillations

Abnormal myopathic low-amplitude, short-duration polyphasic motor unit potential

Complex repetitive discharges

In addition, an early finding in myopathy is that of early recruitment, 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 but are much more frequently seen in inflammatory myopathies.

A normal EMG is unusual in a patient with otherwise typical DM or PM, occurring in one study in 16 of 153 patients (11 percent) [26].

Findings are not always generalized, and highly localized disease, although atypical, sometimes occurs in DM and PM.

EMG technical considerations — Because involvement is typically bilateral and symmetrical, we perform a unilateral EMG study to detect changes suggestive of an active inflammatory myopathy. We perform the biopsy, when indicated, in the contralateral muscle group to avoid needle-related artifact. (See "Overview of electromyography".)

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, 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".)

MRI — MRI of skeletal muscles is a non-invasive sensitive modality for evaluating patients with myopathy [27]. MRI can demonstrate areas of muscle inflammation, edema with active myositis, fibrosis, and calcification. Unlike muscle biopsy, MRI can assess large areas of muscle, thereby avoiding problems with sampling error (image 1). It is, however, nonspecific and may not distinguish the changes of inflammatory myopathy from those that occur in rhabdomyolysis, muscular dystrophy, or metabolic myopathy.

DIAGNOSIS — The diagnosis of dermatomyositis (DM) and polymyositis (PM) generally depends upon the presence of characteristic clinical and laboratory findings, including symmetric proximal muscle weakness and elevated muscle enzymes. Patients with DM may exhibit highly characteristic cutaneous manifestations, and cutaneous manifestations of DM may occur without muscle weakness in a subset of patients. Myositis-specific or myositis-associated antibodies may also be present. (See "Clinical manifestations of dermatomyositis and polymyositis in adults".)

The tests usually needed to make the diagnosis differ between groups of patients who can generally be described, respectively, as having the following patterns of features:

Highly characteristic clinical presentations (see 'Highly characteristic clinical presentation' below)

Muscle weakness and nonspecific but typical findings (see 'Nonspecific but typical presentation' below)

Atypical findings (see 'Atypical findings at presentation' below)

Cutaneous features without weakness (see 'Cutaneous features without weakness' below)

The diagnosis of DM can generally be made without a tissue biopsy in patients with clinical and laboratory findings that are particularly characteristic of this disorder. However, the diagnosis of DM or PM in patients with nonspecific or atypical findings, or of DM in patients with cutaneous features without weakness, usually also relies upon further information from one or more additional diagnostic studies, which are selected based upon the clinical presentation and initial findings.

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 'Classification criteria' below.)

Highly characteristic clinical presentation — The diagnosis of DM can generally be made without a tissue biopsy in patients with clinical and laboratory findings that are particularly characteristic of this disorder, including symmetric proximal muscle weakness and marked elevation of muscle enzymes, in whom evidence suggesting an alternative diagnosis is lacking. Examples of such patients include the following:

Patients with a cutaneous finding that is relatively specific for DM, such as Gottron’s papules or a heliotrope eruption, in the absence of another explanation (see "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Characteristic dermatomyositis findings')

Patients with the extramuscular manifestations of the antisynthetase syndrome including rash, polyarthritis, Raynaud phenomenon, and interstitial lung disease, particularly if an antisynthetase antibody (most often anti-Jo-1) is present (see "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Antisynthetase syndrome')

Some experts also obtain an electromyogram (EMG) and/or magnetic resonance imaging (MRI) in this setting to further document that findings characteristic of an inflammatory myopathy are present. (See 'Electromyography' above and 'MRI' above.)

Nonspecific but typical presentation — Additional data are usually required to confirm the diagnosis of DM or PM in patients with the typical but nonspecific presentation of symmetric proximal muscle weakness and elevated muscle enzymes, in the absence either of highly specific cutaneous manifestations or myositis-specific antibodies (MSA). We generally obtain tissue confirmation of the diagnosis by muscle biopsy in such patients. In patients in whom there is difficulty localizing an appropriate muscle group for a muscle biopsy, EMG should be performed to identify an appropriate muscle to target. MRI may be helpful in selecting a biopsy site if the physical examination and EMG do not provide sufficient guidance. (See "Clinical manifestations of dermatomyositis and polymyositis in adults" and 'Muscle biopsy' above and 'Electromyography' above and 'MRI' above.)

In selected patients with typical clinical findings and skin changes, a skin biopsy will sometimes be adequate for confirming a diagnosis of DM if highly characteristic histopathologic findings are present. (See 'Skin biopsy' above.)

Atypical findings at presentation — In patients with atypical findings, in whom it may be difficult to distinguish between a myopathic and neuropathic disorder causing weakness, an EMG and/or MRI should be performed, depending upon the differential diagnosis suggested by the findings. (See 'Electromyography' above and 'MRI' above.)

In patients in whom the EMG and/or MRI suggest a myopathy and in those in whom another form of myopathy is under consideration, a muscle biopsy should be performed to confirm the diagnosis of DM or PM and to exclude alternative diagnoses, such as inclusion body myositis (IBM), metabolic or mitochondrial myopathy, muscular dystrophy, necrotizing myositis, or drug-induced myopathy. (See 'Muscle biopsy' above and 'Differential diagnosis' below.)

Atypical features that may suggest an alternative diagnosis include:

An asymmetric or distal presentation of weakness

Intermittent symptoms

Painful muscles

Marked muscle atrophy

Family history of muscle disease

A history of medication that could be associated with myopathy

Neuropathic symptoms or findings

Cutaneous features without weakness — Patients who present with cutaneous signs of DM but with normal strength and normal muscle enzymes require a skin biopsy to confirm the diagnosis. In such patients, EMG and MRI may reveal occult muscle disease, but such studies are generally not needed in patients with classic clinical 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 clinically amyopathic DM (ADM) are described separately. These patients may develop myopathy over time, so close follow-up is needed. In addition, these patients may have or develop pulmonary disease or a malignancy; thus, a thorough evaluation should be performed including pulmonary function testing and an assessment for malignancy. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Skin findings' and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Amyopathic dermatomyositis' and "Initial management of cutaneous dermatomyositis", section on 'Patient evaluation' and 'Skin biopsy' above and 'Electromyography' above and 'MRI' above.)

DIFFERENTIAL DIAGNOSIS — Dermatomyositis (DM) DM and polymyositis (PM) must be distinguished from other conditions that cause muscle weakness, with or without muscle enzyme elevation. The differential diagnosis of DM and PM thus includes other inflammatory myopathies, motor neuron disease, myasthenia gravis, and the muscular dystrophies, as well as a variety of inherited, metabolic, drug–induced, endocrine, and infectious myopathies. Notably, none of these disorders are associated with the skin lesions that are characteristic of DM. Additionally, in DM and PM, myalgia tends to be mild, unlike the more prominent muscle pain that occurs in polymyalgia rheumatica, fibromyalgia, and viral or bacterial myositis. (table 1) [28]. (See "Approach to the patient with muscle weakness" and "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Skin findings'.)

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 and more prominent distal muscle weakness (table 2). Unlike PM, the combination of muscle atrophy and weakness of the wrist and finger flexors in the upper extremities with weakness of the quadriceps and anterior tibial muscles in the legs is characteristic [29,30]. 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 [30]. Magnetic resonance imaging (MRI) findings may help to distinguish PM from IBM. Whereas MRI changes suggestive of inflammation are noted along fascial planes in PM, such changes are observed throughout the muscle in IBM [31]. 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.

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 antiretrovirals. 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" and "Statin myopathy".)

Necrotizing myopathy – Autoimmune necrotizing myopathy is a rare disorder that clinically resembles PM but that is histologically distinct from PM or DM [32,33]. It is described most often following exposure to statins, but, unlike typical statin-induced myopathy, it persists following withdrawal of drug therapy [32]. It may also be seen in association with an underlying autoimmune rheumatic disease, such as scleroderma or mixed connective tissue disease, may be seen as a paraneoplastic syndrome, or may be idiopathic. (See "Paraneoplastic syndromes affecting peripheral nerve and muscle", section on 'Acute necrotizing myopathy' and "Statin myopathy", section on 'Neuromuscular disorders'.)

The autoimmune necrotizing myopathies, similar to PM and DM, are characterized most often by proximal upper and lower extremity muscle weakness. Also, EMG findings are typical of an inflammatory myopathy, and muscle enzymes are elevated. 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. Some of these patients exhibit antibodies to signal recognition peptide or to another antibody, anti-200/100 [34]. The latter antibody, seen in patients with statin-induced necrotizing myopathy, recognizes the hydroxymethylglutaryl (HMG)-coenzyme A reductase protein [35]. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Myositis-specific autoantibodies'.)

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 electromyogram with relatively normal appearing motor units, and the absence of inflammatory changes on muscle biopsy, all help to distinguish hypothyroid myopathy from polymyositis. Thyroid-stimulating hormone (TSH) measurement is the preferred screening test. (See "Hypothyroid myopathy".)

HIV infection – In addition to causing weakness through chronic disease and cachexia, human immunodeficiency virus (HIV) infection is also associated with an inflammatory myopathy that is indistinguishable from PM. The myopathy either can be a presenting manifestation of HIV infection or can occur in the later stages of infection. With the widespread use of combination antiretroviral therapy (ART), though, it is an uncommon occurrence [36]. 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 polymyositis, 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 steroids, as used in idiopathic PM, have been shown to be effective [37-39]. Therapeutic options for steroid non-responders 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 [40].

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-rich 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. (See "Clinical manifestations of myasthenia gravis".)

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. (See "Clinical features and diagnosis of Lambert-Eaton myasthenic syndrome".)

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. (See "Limb-girdle muscular dystrophy" and "Facioscapulohumeral muscular dystrophy".)

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 [41]. The evaluation of patients with suspected metabolic myopathy is discussed in detail separately. (See "Approach to the metabolic myopathies" and "Metabolic myopathies caused by disorders of lipid and purine metabolism".)

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 [42,43]. (See "Viral myositis" and "Pyomyositis" and "Causes of rhabdomyolysis", section on 'Inflammatory myopathies'.)

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 about 1000 units/L). (See "Clinical features of amyotrophic lateral sclerosis and other forms of motor neuron disease" and "Diagnosis 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 (MSA) is lower with the familial disorder [44].

Chronic GVHD – Muscle disease that is clinically and histopathologically similar to PM has been well-documented in patients with chronic graft-versus-host disease (GVHD) [45]. The estimated frequency is 0.6 percent. The onset of myositis is typically more than one year after transplantation [46]. (See "Clinical manifestations, diagnosis, and grading of chronic graft-versus-host disease".)

Focal myositis – Rare patients present with focal myositis that usually but not always progresses to the typical generalized form over time [47].

Amyloid myopathy – Amyloid myopathy can occur in immunoglobulin-related or familial amyloidosis [48]. Other manifestations of the particular form of amyloidosis are typically present, unlike in DM or PM. The diagnosis may be confirmed by biopsy, if needed. (See "Musculoskeletal manifestations of amyloidosis".)

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. (See "Sarcoid: Muscle, bone, and vascular disease manifestations", section on 'Myopathy'.)

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".)

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 amyotrophy and idiopathic lumbosacral radiculoplexus neuropathy" and "Diabetic muscle infarction".)

Other skin disease – Patients with systemic lupus erythematosus (SLE) may also develop facial erythema and other photosensitive eruptions, resembling some of the facial changes seen in DM. However, unlike in patients with SLE, the midfacial erythematous eruption that may be seen in DM often does not spare the nasolabial folds (picture 4A-B). These disorders are generally distinguished by the severity of muscle involvement in DM or PM compared with SLE, by the organ systems involved, and by the serologic studies. (See "Overview of cutaneous lupus erythematosus" and "Diagnosis and differential diagnosis of systemic lupus erythematosus in adults".)

Scalp involvement in psoriasis or seborrheic dermatitis may resemble scalp DM. 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. (See "Epidemiology, clinical manifestations, and diagnosis of psoriasis" and "Seborrheic dermatitis in adolescents and adults".)

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 statins and other agents [49,50].

POST-DIAGNOSTIC EVALUATION — Additional studies may be important in the evaluation of a given patient once the diagnosis of dermatomyositis (DM) or polymyositis (PM) has been made. The choice of studies for further investigation depends upon the symptoms and findings that are present. (See "Initial treatment of dermatomyositis and polymyositis in adults".)

Malignancy – All 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. Specific recommendations are discussed in detail separately. (See "Malignancy in dermatomyositis and polymyositis".)

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. (See "Clinical manifestations of dermatomyositis and polymyositis in adults", section on 'Cardiac disease'.)

Pulmonary disease – In patients suspected of pulmonary involvement based upon symptoms (eg, dyspnea or cough) or upon abnormalities on physical examination or on plain radiography of the chest, additional studies may include pulmonary function testing and computed tomography (CT) of the chest. (See "Interstitial lung disease in dermatomyositis and polymyositis: Clinical manifestations and diagnosis".)

Esophageal dysfunction – In patients suspected of esophageal dysmotility, such as those with dysphagia or with a question of aspiration pneumonia, esophageal motility studies should be performed. (See "Overview of gastrointestinal motility testing".)

CLASSIFICATION CRITERIA

Muscle disease — Several sets of classification criteria have been developed for dermatomyositis (DM) and polymyositis (PM) for the primary purpose of defining these disorders for clinical and epidemiologic research. The widely used classification criteria developed by Bohan and Peter in 1975 shaped thinking about these diseases and facilitated research for several decades, but they have several limitations [3,4,51-53]. These criteria defined DM and PM based upon the following features [3,4]:

Symmetric proximal muscle weakness

Typical cutaneous eruption of DM (the only feature distinguishing DM from PM)

Elevated serum muscle enzymes

Myopathic changes on electromyography (EMG)

Characteristic muscle biopsy abnormalities and the absence of histopathologic signs of other myopathies

Patients with the cutaneous eruption and at least three of the other four criteria met the requirements for definite DM according to these criteria, while requirements for definite PM were met by those with all four criteria other than the cutaneous features [3]. Patients with findings indicating the presence of other disorders that may present similarly were excluded. Those disorders included central or peripheral neurologic disease; muscular dystrophy; granulomatous myositis, such as sarcoidosis; infectious muscle disease (eg, trichinosis or toxoplasmosis); recent exposure to myopathic drugs or toxins; rhabdomyolysis, with history of a trigger; metabolic muscle disease; myopathic endocrinopathy; and myasthenia gravis. Patients who did not meet these criteria but who lacked any of the exclusions could potentially have been diagnosed with possible or probable DM or PM, depending upon the number of criteria met.

The proportion of inflammatory myopathy cases classified as pure adult PM has declined with advances in understanding of these disorders and with changes in definitions of PM and of overlap syndromes [51-53]. These advances and changes include:

Recognition of histologic and immunopathologic differences between DM and PM

Increased recognition of inclusion body myositis (IBM)

The discovery of multiple myositis-specific and myositis-associated autoantibodies

The discovery of autoantibody markers of overlap and other systemic rheumatic disease syndromes

Increased recognition of cases redefined or identified as overlap myositis

Amyopathic cutaneous disease — Several sets of classification criteria have also been proposed for amyopathic DM [54]. Dermatology experts have suggested the following [54,55]:

Biopsy-confirmed hallmark cutaneous manifestations of classic DM occurring for six months or longer.

No clinical evidence of proximal muscle weakness and no serum muscle enzyme abnormalities for six months or longer.

Results within normal limits, if more extensive muscle testing is carried out.

Absence of exclusion criteria. Exclusion criteria include treatment with systemic immunosuppressive therapy for two consecutive months or longer within the first six months after skin disease onset, or use of drugs known to be capable of producing DM-like skin changes (eg, hydroxyurea, statin cholesterol-lowering agents) at the onset of cutaneous DM changes.

Similar criteria for amyopathic DM (ADM), but which also included electrophysiologic studies, were proposed by an expert group of neurologists and rheumatologists [53]. These included:

Rash typical of DM (eg, heliotrope, periorbital edema, Gottron’s papules, Gottron’s sign, V-sign, shawl sign, holster sign)

Skin biopsy that demonstrates a reduced capillary density, deposition of complement membrane attack complex (MAC) on small blood vessels along the dermal-epidermal junction, and variable keratinocyte decoration for MAC

No objective weakness, normal creatine kinase (CK), and normal EMG

Muscle biopsy, if done, that does not reveal features compatible with definite or probable DM

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

The diagnosis of dermatomyositis (DM) or polymyositis (PM) should be suspected in patients who present with proximal muscle weakness. The evaluation of the patient suspected of having DM or PM 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

Muscle enzyme measurement, including creatine kinase (CK) and aldolase, and general laboratory testing

Systemic disease manifestations

Exclusion of other diseases, including drug-induced myopathy and malignancy

In patients in whom the initial evaluation is consistent with possible DM or PM, further studies should include testing for antinuclear and myositis-specific antibodies (MSA) and a chest radiograph. Additional diagnostic studies, including muscle or skin biopsy, electromyography (EMG), and magnetic resonance imaging (MRI) of skeletal muscle, depend upon the clinical presentation. (See 'Initial evaluation' above and 'Further diagnostic studies' above and 'Diagnosis' above.)

Histologic features of DM and PM include muscle fiber necrosis, degeneration, regeneration, and inflammatory cell infiltration. In DM, the cellular infiltrate is predominantly perifascicular and often perivascular. B lymphocytes and an increased number of plasmacytoid dendritic cells are also typical of DM. In PM, the cellular infiltrate in muscle is predominantly within the fascicle, in which there are increased numbers of cytotoxic CD8+ T cells. Skin changes that are characteristic of DM have the histopathological 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.)

EMG shows evidence of muscle membrane irritability, usually including increased insertional activity, spontaneous fibrillations, positive sharp waves, and complex repetitive discharges. EMG findings are helpful in confirming the presence of a myopathic process and in indicating which muscle groups are most involved. This information can be valuable in selecting a site for muscle biopsy. However, EMG findings are not specific for either DM or PM, and the EMG is normal in approximately 10 percent of patients. (See 'Electromyography' above.)

We diagnose DM without a biopsy in patients with symmetrical proximal muscle weakness, elevated muscle enzymes, a rash that is relatively specific for DM (such as Gottron’s papules or a heliotrope rash), and the absence of another explanation for the findings, as well as in patients with findings of a typical rash, myositis-specific antibodies, and other features characteristic of the antisynthetase syndrome. (See 'Highly characteristic clinical presentation' above.)

In patients suspected of DM or PM based upon proximal muscle weakness, elevated muscle enzymes, and a nonspecific rash, we generally make the diagnosis based upon muscle biopsy findings, which can distinguish between DM and PM and between DM or PM and other disorders. An EMG and muscle biopsy should be performed in patients who have atypical findings or in whom the diagnosis is uncertain in order to exclude other diagnoses such as inclusion body myositis (IBM), a metabolic myopathy, or a dystrophy. In selected patients with characteristic cutaneous and histopathologic findings on skin biopsy, we may diagnose DM without a muscle biopsy. (See 'Nonspecific but typical presentation' above and 'Atypical findings at presentation' above and 'Cutaneous features without weakness' above.)

DM and PM must be distinguished from other conditions that cause muscle weakness, with or without muscle enzyme elevation. The differential diagnosis of DM and PM includes other inflammatory myopathies, motor neuron disease, myasthenia gravis, and the muscular dystrophies. In addition, a variety of inherited, metabolic, drug–induced, endocrine, and infectious myopathies must also be considered. (See 'Differential diagnosis' above.)

In patients diagnosed with DM or PM, further studies to exclude malignancy depend upon an individualized assessment of the patient’s levels of risk for particular disorders. Patients with clinical evidence of other organ involvement, including cardiac, pulmonary, or esophageal disease in particular, should also undergo further evaluation to characterize abnormalities that may be present. (See "Malignancy in dermatomyositis and polymyositis" and 'Post-diagnostic evaluation' above.)

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