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Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Diagnosis

Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Diagnosis
Literature review current through: May 2024.
This topic last updated: Oct 25, 2023.

INTRODUCTION — Juvenile dermatomyositis (JDM) is the most common form of the rare idiopathic inflammatory myopathies (IIM; also called autoimmune or immune-mediated myopathies) affecting children [1,2]. JDM is characterized by symmetric, proximal muscle weakness and distinct rashes of heliotrope dermatitis and Gottron papules. It is primarily a capillary vasculopathy. Juvenile polymyositis (JPM) probably is not a single disorder but rather is a name given to forms of IIM (a subgroup of IIM) without skin disease. (See "Clinical manifestations of dermatomyositis and polymyositis in adults".)

The diagnosis of JDM and other IIM of childhood is reviewed here. The pathogenesis, clinical manifestations, and treatment of these disorders are discussed elsewhere. (See "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations" and "Juvenile dermatomyositis and polymyositis: Treatment, complications, and prognosis".)

CLASSIFICATION AND DIAGNOSTIC CRITERIA

Criteria and scoring for idiopathic inflammatory myopathies — The European Alliance of Associations for Rheumatology (EULAR; formerly known as European League Against Rheumatism)/American College of Rheumatology (ACR) developed and validated revised classification and diagnostic criteria for adult and juvenile IIMs in 2017 (no points are given for age of onset <18 years of age) (table 1) [3,4]. These criteria replace the longstanding Bohan and Peter classification schema and diagnostic criteria for the various forms of myositis, including JDM, that were proposed in 1975 [5].

The EULAR/ACR criteria include four variables related to muscle weakness and three related to skin manifestations, as well as variables related to laboratory measurements and other clinical manifestations. Muscle biopsies are infrequently performed in children. Thus, the EULAR/ACR criteria have one scoring system if no muscle biopsy is available and a separate scoring system if the patient has had a muscle biopsy.

The EULAR/ACR score in the absence of a muscle biopsy is interpreted as follows (table 1):

Definite IIM – Score of ≥7.5; level of probability ≥90 percent

Probable IIM – Score of ≥5.5 to <7.5; level of probability ≥55 to <90 percent

Possible IIM – Score of 5.3 or 5.4; level of probability ≥50 to <55 percent

Categories of idiopathic inflammatory myopathies — The categories of IIM include [5]:

Juvenile dermatomyositis (JDM) – Pediatric patients who meet the criteria for IIM are classified as having JDM if they have objective, symmetric, proximal greater than distal, muscle weakness that is chronic and usually progressive and a heliotrope rash (reddish-purple rash on the upper eyelids with periorbital edema), Gottron papules (erythematous, papulosquamous eruption over the dorsal surfaces of the knuckles), and/or Gottron sign (flat, erythematous rash over the dorsal surfaces of the knuckles) (picture 1).

Amyopathic/hypomyopathic JDM – Patients with amyopathic/hypomyopathic JDM lack clinically evident weakness but have the characteristic JDM cutaneous findings.

Adermatitic JDM – Patients with this rare form of JDM lack skin findings but have the same muscle pathology as patients with JDM.

Other IIM without skin findings – This category includes patients labeled as juvenile polymyositis (JPM). However, as the diagnostic tools become more sophisticated (eg, biopsies that demonstrate inflammatory dystrophies or autoantibodies that are markers of particular types of myositis such as anti-signal recognition particle [SRP] or anti-3-hydroxy-3-methylglutaryl-CoA reductase [HMGCR] for immune-mediated necrotizing myopathy [IMNM]), fewer patients are diagnosed with JPM, calling into question whether JPM is a specific entity. JPM probably is not a single disorder but rather is a name given to IIM without skin disease. IMNM seems to account for the cases previously labeled JPM [6].

Other forms of juvenile myositis in the differential diagnosis are discussed below. (See 'Other forms of myositis' below.)

DIAGNOSIS

When to suspect — JDM is suspected if there is both chronic, progressive muscle weakness that is symmetric (eg, both upper extremities or both lower extremities) and affects proximal muscles more than distal and/or characteristic cutaneous findings, including heliotrope rash, Gottron papules, and nailfold capillary abnormalities (picture 1 and picture 2) [7]. The diagnosis is more challenging when there is lack of cutaneous manifestations (adermatitic JDM), immune-mediated necrotizing myopathy (IMNM) and other forms of inflammatory myopathy, or lack of clinically evident weakness (amyopathic/hypomyopathic JDM). (See "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Clinical manifestations'.)

Our diagnostic approach — The diagnosis of JDM and other forms of IIM is primarily a clinical one (table 1) (see 'Classification and diagnostic criteria' above), although specific testing helps confirm the diagnosis (see 'Specific tests' below). The authors use the following approach in making the diagnosis of JDM and other IIM in their practice (algorithm 1):

Initial evaluation in all patients – All patients with suspected JDM or other IIM are examined for the characteristic cutaneous findings of JDM, including the rashes and nailfold capillary changes, and muscle strength testing is performed to evaluate for symmetric proximal muscle weakness. The diagnosis of JDM is suspected if cutaneous findings and weakness are present, amyopathic/hypomyopathic JDM if rash is present but weakness is absent, and adermatitic JDM or other IIM if weakness is present but cutaneous manifestations are absent. (See "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Muscle weakness' and "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Cutaneous manifestations' and 'Muscle strength testing' below and 'Nailfold capillaroscopy' below.)

Patients whose muscle weakness is not typical of an IIM and who lack characteristic cutaneous findings should be evaluated for other myopathic disease or disorders due to denervation or neuropathy. (See 'Differential diagnosis' below.)

Subsequent testing – The ensuing testing depends upon the IIM suspected after the initial evaluation:

Patients with characteristic proximal muscle weakness but no cutaneous findings (suspected adermatitic JDM or other IIM):

-Muscle biopsy – In these patients, we go straight to muscle biopsy, in addition to performing muscle enzyme levels and magnetic resonance imaging (MRI), to differentiate among JDM without skin findings (called "adermatitic" JDM by some), other IIM without skin findings, and inflammatory dystrophies. (See 'Muscle biopsy' below and 'Measurement of muscle enzymes' below and 'Magnetic resonance imaging' below.)

-Antibody testing – We also perform myositis-specific antibody (MSA) and myositis-associated antibody (MAA) testing (best done by immunoprecipitation, but commercial line blot assays are more widely available). (See 'Antibody testing' below and "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Immunologic mechanisms'.)

Patients with characteristic cutaneous findings with or without proximal muscle weakness (suspected JDM, hypomyopathic JDM, amyopathic JDM, or other IIM):

-Muscle enzyme levels In these patients, we measure serum muscle enzyme levels. These studies often provide evidence of the presence of myositis but not always (sensitivity approximately 60 to 80 percent; low specificity). Thus, further testing should still be performed even if levels are not elevated. (See 'Measurement of muscle enzymes' below.)

-MRI – We next perform short tau inversion recovery (STIR) sequence MRI of the whole body, or of the thigh and shoulder girdle muscles, to document evidence of muscle inflammation and confirm a diagnosis of JDM in a patient with typical clinical findings. If the MRI findings are consistent with JDM, we usually do not perform any further testing. (See 'Magnetic resonance imaging' below.)

Treatment should not be delayed if these additional testing modalities are not available, the presentation is typical, and disease is moderate to severe.

Patients with unusual clinical features or in whom the diagnosis remains uncertain – In these patients, we perform a muscle biopsy. Atypical clinical features include absence of rash, normal nailfold capillaries, and responses indicating lactic acidosis with exercise suggesting metabolic disease. (See 'Muscle biopsy' below.)

Electromyography (EMG) is a classically used test, but we almost never use it anymore because of its painful, invasive nature and availability of other diagnostic testing. (See 'Electromyography' below.)

Specific tests — Elevation of muscle enzymes and STIR (fat-suppressed, T2-weighted) MRI are indicative of muscle edema and inflammation. A muscle biopsy may be used to confirm the diagnosis whenever the clinical evaluation and initial laboratory testing are inconclusive. As with any test, there are false positives and false negatives. Thus, any of these tests can be "normal" in a patient with juvenile IIM. (See 'Our diagnostic approach' above.)

Test performed in all patients with suspected JDM/IIM — Tests used to make the diagnosis of JDM/IIM in all patients include muscle strength testing and nailfold capillaroscopy. (See 'Our diagnostic approach' above.)

Muscle strength testing — Testing of weakness is essential in assessing any child with suspected JDM or other IIM because symmetric proximal muscle weakness is a diagnostic requirement [5,8]. Any decrement in proximal muscle strength is considered a positive finding. Distal muscles are often weak in JDM and other IIM, but distal muscle weakness in the absence of proximal muscle weakness should prompt suspicion of other diagnoses.

A standardized confrontational (manual) muscle strength testing score of eight muscle groups, the MMT8, has been validated for use in assessing strength in JDM [9] and is also used in assessing patients suspected of having another form of IIM. Tested muscles comprise the deltoid, biceps, wrist extensors, quadriceps, ankle dorsiflexors, neck flexors, gluteus medius, and gluteus maximus [10]. For the MMT8, muscle strength is scored on a 0 to 10 point scale (Kendall scale) (table 2) [11,12].

The Childhood Myositis Assessment Scale (CMAS) is a clinical assessment tool that provides an objective measure of muscle strength (table 3). The CMAS tests function and endurance by having a child complete a number of different physical maneuvers that assess primarily axial and proximal muscle groups [13]. A score is assigned from 0 to 51, with a higher score indicating greater strength. In a validation study of 10 children with JDM or other IIM and 12 clinicians, the CMAS was found to correlate with the clinician global assessment of disease activity, manual muscle testing, and serum creatine kinase (CK) level, with excellent intra- and inter-rater reliability [13].

Nailfold capillaroscopy — Abnormalities in nailfold capillaroscopy suggest an underlying connective tissue disease such as JDM, mixed connective tissue disease, or scleroderma (systemic sclerosis). Nailfold capillaroscopy is easily performed at the bedside or in clinic. A drop of oil or water-soluble gel is placed on the patient's fingernail bed, and a light microscope, dermatoscope, otoscope, or ophthalmoscope (set on the highest level of magnification) is used to assess nailfold capillary dilatation, tortuosity, dropout (loss of capillary loops), and the total number of capillaries per millimeter (picture 2 and picture 3) [14].

In one study, nailfold capillary microscopy demonstrated a lower density of capillaries (4.9 versus 6.9 capillaries per millimeter) and increased capillary width in 18 children with connective tissue disease, including 8 with JDM, compared with 17 healthy control children [15].

In another study of 84 children with a variety of rheumatologic disorders and 34 healthy controls, nailfold capillaroscopy demonstrated significant dropout and dilatation of nailfold capillaries in patients with childhood dermatomyositis or scleroderma compared with children with other rheumatologic diseases and controls [16]. Of the 25 patients with JDM for whom muscle biopsies were available, there was good correlation between the severity and number of vascular abnormalities on biopsy (eg, nonnecrotizing vasculitis or significant loss of capillary bed) and an increased likelihood of having nailfold capillaroscopic abnormalities.

In patients with JDM, abnormal nailfold capillaroscopy correlated with increased skin involvement, but not muscle weakness, in several studies [14,17]. However, in one study, loss of nailfold capillary density correlated with both muscle weakness and skin disease activity over time [7]. The likelihood of nailfold changes increases with the duration of untreated disease [14].

Tests performed in selected patients — Muscle enzyme levels, MRI, antibody testing, and muscle biopsy are performed in select patients. (See 'Our diagnostic approach' above.)

Measurement of muscle enzymes — The muscle enzymes routinely tested in the evaluation of myopathy include:

Creatine kinase (CK)

Lactate dehydrogenase (LDH)

Aldolase

Alanine aminotransferase (ALT)

Aspartate aminotransferase (AST)

Elevated serum muscle enzyme levels are markers for muscle damage and, when present, support the diagnosis of JDM or other IIM. However, these enzymes may arise from sources other than muscle, even in patients with muscle weakness. In addition, normal levels do not exclude these diagnoses.

Although the majority of children with JDM or other IIM will have serum elevation of one or more of these enzymes, some patients may have normal values despite clinically evident weakness. This was illustrated in a retrospective study of 166 patients with JDM that reported normal serum levels of CK, LDH, and aldolase in 26, 13, and 17 percent of patients, respectively [18]. The muscle enzyme levels were more likely to be normal with prolonged disease duration prior to treatment, particularly beyond four months.

Magnetic resonance imaging — MRI, specifically T2-weighted and fat-suppressed (eg, STIR) images, shows muscle inflammation as edema (bright spots). Thus, MRI, either full body or of the thigh and shoulder muscles, is increasingly used in the diagnosis of childhood inflammatory myopathy to avoid the morbidity of muscle biopsy and EMG (picture 2). Whole-body MRI is also emerging as an important method of determining the extent of muscle disease and following patients with JDM and other IIM [19-21].

Of 122 children with various stages of JDM evaluated in one study from the United Kingdom and Ireland, 78 of 102 patients (76 percent) who underwent MRI had abnormalities consistent with myositis [22]. In this cohort, EMG was performed in only 8 percent of patients and muscle biopsy in 36 percent.

Another study of 19 patients with JDM (15 with active and 4 with inactive disease) and 5 healthy controls evaluated the role of MRI in the diagnosis of JDM [23]. All patients with clinically active disease had abnormal findings on MRI compared with none of the patients with inactive disease or controls. The abnormalities included increased signal intensity (on T2 or STIR images) of the affected muscle, perimuscular edema, enhanced chemical shift artifact, and increased signal intensity in subcutaneous fat. Higher muscle signal intensity on MRI correlated with decreased muscle strength. After therapy, signal intensity in the muscle returned to normal.

MRI has also been used to detect flares in muscle disease when other studies were less helpful [24].

Magnetic resonance spectroscopy (MRS) is a research tool that is not yet routinely available. It can detect irregularities in muscle function and metabolism in inflammatory myopathy. In a study of 13 children with JDM, MRS identified metabolic abnormalities in the quadriceps muscle of 10 patients with severe clinical weakness [25]. Two patients who had resolution of clinical weakness on prednisone treatment had normal findings, as did one patient with amyopathic JDM.

Muscle biopsy — For patients with unusual clinical features or in whom the diagnosis remains uncertain, we perform a muscle biopsy to further assess for muscle inflammation.

The chosen site for muscle biopsy should have evidence of muscle inflammation demonstrated by physical exam or preferably MRI. Open biopsy of an area that is actively inflamed, but not atrophied, is preferred by many pathologists, but needle biopsy may suffice in the hands of an experienced team.

The characteristic histologic findings of JDM include the following (picture 2) [5]:

Perifascicular atrophy.

Degeneration of type I and II muscle fibers.

Regeneration of muscle fibers with associated basophilia.

Centralization of nuclei in muscle fibers.

Perivascular mononuclear infiltrate.

Endothelial swelling and necrosis.

Reduced capillary number.

Tubuloreticular inclusions on electron microscopy.

Overexpression of major histocompatibility complex (MHC) class I molecules (an early event that may occur in the absence of lymphocytic infiltration and muscle damage). MHC class I expression is a highly sensitive marker of inflammatory myopathy but, unfortunately, is of low specificity [26]; it likely represents a reactive change to damage, perhaps endoplasmic reticular stress.

Possible poor prognostic features include capillary dropout (loss of capillary loops) and intimal changes in small arteries and arterioles that can lead to occlusion and muscle infarction [27]. A muscle biopsy score to help determine the course of JDM is available [28].

In JPM, as classically described, the muscle biopsy is different and often shows an inflammatory exudate consisting of mononuclear T cells, which accumulate in the endomysium and surround and invade muscle fibers [1]. IMNM, the most common IIM without skin disease or nailfold capillary changes seen in our clinic, is a disorder that was probably labelled as JPM in the past; it is characterized by fiber necrosis, activation of macrophages, and a sparse inflammatory infiltrate [6].

Antibody testing — MSAs may be helpful in making the diagnosis of JDM, although the sensitivity of any one MSA is low. MSA testing, though, is useful in patients with unusual clinical features or an unusual clinical course, such as patients without the characteristic rashes (suspected IMNM) or those with scleroderma overlap features. The utility of testing may increase in the future with the discovery of additional MSAs [29]. MAAs often are indicative of overlap myositis (overlap with scleroderma, lupus, or arthritis, etc) when present. (See "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Immunologic mechanisms' and "Overview of and approach to the idiopathic inflammatory myopathies", section on 'Myositis-specific autoantibodies'.)

Electromyography — An EMG provides evidence of muscle inflammation, which supports a diagnosis of JDM or other IIM and helps to distinguish these disorders from muscle weakness caused by denervation (eg, due to motor neuron disease or myasthenia gravis) [30]. However, it is an invasive and uncomfortable test. Thus, it is reserved for cases where the diagnosis is uncertain or other testing (eg, MRI) is not available.

Other tests — Other blood tests are often performed in patients with suspected or established childhood inflammatory myopathy to assess for complications or disease activity. However, these studies are often not directly useful in establishing the diagnosis and are discussed in detail separately. (See "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Laboratory findings'.)

DIFFERENTIAL DIAGNOSIS — The diagnosis of JDM is relatively straightforward in the presence of the characteristic rash and symmetric proximal muscle weakness. However, JDM and other IIM without cutaneous manifestations are often difficult to distinguish from other causes of muscle weakness (eg, other myopathic diseases or disorders due to denervation or neuropathy).

Clinical features, such as the age at onset, the chronicity of the disorder, family history, and the predominant site of muscle involvement, are useful in differentiating JDM and other IIM from these disorders. In these cases, additional evaluation, including serum muscle enzyme levels, magnetic resonance imaging (MRI), electromyography (EMG), and muscle biopsy, is helpful in determining the etiology of muscle weakness.

Noninflammatory myopathy — Causes of noninflammatory myopathies include the following:

Muscular dystrophies are due to genetic causes and include Duchenne and Becker muscular dystrophy. There is often a family history of other affected members. Patients with Duchenne and Becker muscular dystrophy are usually male because the locus for the affected gene, dystrophin, is the X chromosome. Duchenne muscular dystrophy typically presents at an earlier age than those with JDM and other IIM (2 to 3 years of age versus 5 to 10 years of age). Genetic testing is available to confirm the diagnosis of many of these genetic myopathies, including Duchenne and Becker muscular dystrophy. (See "Duchenne and Becker muscular dystrophy: Clinical features and diagnosis" and "Myotonic dystrophy: Etiology, clinical features, and diagnosis".)

Metabolic myopathies are due to underlying deficiencies of energy production in muscle. These disorders include carnitine deficiency syndromes, defects of fatty acid transport and beta-oxidation enzymes, myoadenylate deaminase deficiency, and mitochondrial defects. Clinical features that differentiate these disorders from JDM and other IIM include marked fatigue with exercise, lactic acidosis, and myoglobinuria. Specific biochemical and immunohistochemical testing of muscle tissue is necessary for the diagnosis of these disorders. (See "Approach to the metabolic myopathies" and "Metabolic myopathies caused by disorders of lipid and purine metabolism" and "Mitochondrial myopathies: Clinical features and diagnosis".)

Other forms of myositis — Myositis may be caused by viral infection or another rheumatologic disease [31].

Viral infections (eg, influenza, coxsackie, and poliomyelitis) may cause acute and transient myositis. The history of a recent infection and the short, self-resolving clinical course distinguishes infectious myositis from JDM and other IIM. Viral myositis is often characterized by calf pain as opposed to the proximal muscle weakness seen in JDM and other IIM.

Other rheumatologic diseases, such as systemic scleroderma, systemic lupus erythematosus, mixed connective tissue disease, polyarteritis nodosum, and systemic juvenile idiopathic arthritis, may have myositis as a clinical manifestation of the disease. Differentiation among these disorders, JDM, and other IIM is usually not difficult, because of the presence of characteristic clinical features of each disease, although overlap syndromes may occur. (See "Juvenile systemic sclerosis (scleroderma): Classification, clinical manifestations, and diagnosis" and "Childhood-onset systemic lupus erythematosus (SLE): Clinical manifestations and diagnosis" and "Systemic juvenile idiopathic arthritis: Clinical manifestations and diagnosis" and "Mixed connective tissue disease".)

Disorders of denervation or neuropathy — Lesions of the brain, spinal cord, anterior horn cell, lower motor neuron, and the neuromuscular junction may present with muscle weakness. Additional evaluation, including normal muscle enzyme levels, lack of inflammatory changes on MRI, or absence of histologic findings characteristic of JDM or other IIM on muscle biopsy, can differentiate these disorders from JDM and other IIM.

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".)

SUMMARY AND RECOMMENDATIONS

Classification and diagnostic criteria for idiopathic inflammatory myopathies (IIM) – The European Alliance of Associations for Rheumatology (EULAR; formerly known as European League Against Rheumatism)/American College of Rheumatology (ACR) criteria for IIM, which contain juvenile dermatomyositis (JDM) and other juvenile IIM, include four variables related to muscle weakness and three related to skin manifestations, as well as variables related to laboratory measurements and other clinical manifestations (table 1).

Diagnosis of JDM and other IIM – The diagnosis of JDM and other IIM is primarily a clinical one, although specific testing helps confirm the diagnosis (algorithm 1). For most patients, this includes obtaining muscle enzyme levels and magnetic resonance imaging (MRI), either full body or of the thigh and shoulder muscles. (See 'Our diagnostic approach' above.)

Clinical features – JDM is suspected if there is both chronic, progressive muscle weakness that is symmetric (eg, both upper extremities or both lower extremities) and affects proximal muscles more than distal (table 3) and/or characteristic cutaneous findings, including heliotrope rash, Gottron papules, and nailfold capillary abnormalities (picture 1 and picture 2 and picture 3). The diagnosis is more challenging when there is lack of cutaneous manifestations (adermatitic JDM); immune-mediated necrotizing myopathy (IMNM) and other forms of inflammatory myopathy; or lack of clinically evident weakness (amyopathic/hypomyopathic JDM). (See 'When to suspect' above and "Juvenile dermatomyositis and other idiopathic inflammatory myopathies: Epidemiology, pathogenesis, and clinical manifestations", section on 'Clinical manifestations' and 'Muscle strength testing' above and 'Nailfold capillaroscopy' above.)

Muscle enzyme levels Elevated serum muscle enzyme levels are markers for muscle damage and, when present, support the diagnosis of JDM or other IIM. However, these enzymes may arise from sources other than muscle, even in patients with muscle weakness. In addition, normal levels do not exclude these diagnoses. (See 'Measurement of muscle enzymes' above.)

Magnetic resonance imaging (MRI) – MRI may demonstrate inflammation of the characteristically involved muscle groups (picture 2) and is an important diagnostic tool in confirming the diagnosis in most patients. (See 'Magnetic resonance imaging' above.)

Muscle biopsy and electromyography (EMG) – These tests are invasive diagnostic procedures that rarely used. They are best reserved for cases in which the diagnosis is inconclusive despite thorough physical examination and investigation including serum muscle enzyme testing and MRI. (See 'Electromyography' above and 'Muscle biopsy' above and 'Our diagnostic approach' above.)

Antibody testing – Myositis-specific antibody (MSA) and myositis-associated autoantibody (MAA) testing is useful in patients with unusual clinical features or clinical course, such as patients without the characteristic rashes (suspected juvenile polymyositis [JPM]) or those with scleroderma overlap features. (See 'Antibody testing' above.)

Differential diagnosis The diagnosis of JDM is relatively straightforward in the presence of the characteristic rash and symmetric proximal muscle weakness. However, JDM and other IIM without cutaneous manifestations are often difficult to distinguish from other causes of muscle weakness (eg, other myopathic diseases or disorders due to denervation or neuropathy). These include muscular dystrophies, metabolic myopathy, infectious myositis, myositis associated with other rheumatic diseases, and disorders of denervation or neuropathy. Clinical features and additional studies differentiate JDM and other IIM from these other disorders that present with muscle weakness. (See 'Differential diagnosis' above.)

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References

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