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Dermatomyositis and polymyositis (myositis)

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Dermatomyositis and Polymyositis: Overview

Dermatomyositis (DM) and Polymyositis (PM) are idiopathic inflammatory myopathies (IIMs) – systemic autoimmune connective tissue diseases primarily characterized by inflammation of skeletal muscles (myositis) [1, 2]. Dermatomyositis distinctively involves characteristic skin manifestations in addition to muscle inflammation [1, 2]. Polymyositis primarily affects the muscles, though systemic features can occur [1, 2]. These conditions lead to progressive muscle weakness, fatigue, and sometimes pain.

Key features include [1, 2]:

  • Muscle Weakness: Typically symmetrical and proximal (affecting shoulders, hips, neck flexors), leading to difficulty with activities like rising from chairs, climbing stairs, lifting objects, or combing hair. Weakness of pharyngeal and respiratory muscles can cause dysphagia (swallowing difficulty) and respiratory compromise. Muscle soreness (myalgia) can occur but weakness is usually the dominant feature.
  • Skin Manifestations (Dermatomyositis only):
    • Heliotrope rash: Violaceous (purple-ish) discoloration and edema of the eyelids.
    • Gottron's papules/sign: Erythematous or violaceous papules or plaques over bony prominences, especially knuckles (MCPs, PIPs, DIPs), elbows, knees.
    • Other rashes: Photosensitive rash on face/neck/upper chest ("V-sign", "shawl sign"), periungual erythema/telangiectasias, cuticular hypertrophy, mechanic's hands (cracked, rough skin on palms/fingers).
  • Systemic Involvement: Both DM and PM can involve internal organs, including:
    • Lungs: Interstitial lung disease (ILD) is a common and serious complication, especially in patients with certain autoantibodies (like anti-Jo-1).
    • Heart: Myocarditis, conduction abnormalities, pericarditis.
    • Joints: Arthralgia or non-erosive arthritis.
    • Gastrointestinal tract: Esophageal dysmotility.

The underlying pathogenesis involves immune system dysfunction, with T-cell mediated muscle fiber injury thought to be prominent in PM, while DM is associated with complement activation, microvascular injury, and B-cell involvement [1]. Autoantibodies targeting various cellular components, including tRNA synthetases (like Jo-1), are frequently found and associated with specific clinical subtypes [1, 2].

Inflammatory myopathies like dermatomyositis and polymyositis cause muscle weakness and sometimes pain, distinct from the widespread pain and tender points of fibromyalgia shown here.

Diagnosis of Dermatomyositis & Polymyositis (including Anti-Jo-1)

Diagnosis of Dermatomyositis (DM) and Polymyositis (PM) involves clinical evaluation, laboratory tests, electrodiagnostic studies, and often muscle/skin biopsy [1, 2]. Consultation with a rheumatologist or neurologist is typically required.

  • Clinical Assessment: History of progressive proximal muscle weakness, characteristic skin rashes (in DM), and potential systemic symptoms. Physical exam assesses muscle strength, skin findings, and signs of organ involvement.
  • Laboratory Tests:
    • Muscle Enzymes: Creatine kinase (CK) is typically significantly elevated, reflecting muscle damage. Aldolase, AST, ALT, and LDH may also be high.
    • Inflammatory Markers: ESR and CRP may be elevated but can be normal.
    • Autoantibodies:
      • Antinuclear Antibodies (ANA): Often positive but non-specific.
      • Myositis-Specific Antibodies (MSAs): Crucial for diagnosis and prognosis. Examples include:
        • Anti-Jo-1: The most common MSA (present in ~20-30% of IIM), targeting histidyl-tRNA synthetase. Strongly associated with the "anti-synthetase syndrome" (myositis, interstitial lung disease, arthritis, Raynaud's, mechanic's hands).
        • Other anti-synthetase antibodies (anti-PL-7, anti-PL-12, etc.).
        • Anti-Mi-2: Associated with classic DM skin findings and often a good prognosis.
        • Anti-SRP: Associated with severe, rapidly progressive, necrotizing myopathy, often resistant to treatment.
        • Anti-MDA5: Associated with clinically amyopathic dermatomyositis (CADM) and rapidly progressive ILD.
        • Anti-TIF1-gamma / Anti-NXP2: Associated with cancer-associated myositis, especially in adults.
      • Myositis-Associated Antibodies (MAAs): Found in overlap syndromes (e.g., anti-PM-Scl in PM/SSc overlap, anti-U1-RNP in MCTD).
  • Electromyography (EMG) / Nerve Conduction Studies (NCS): EMG typically shows myopathic changes (short-duration, low-amplitude potentials, early recruitment, increased insertional activity, fibrillations/positive sharp waves). NCS are usually normal, helping differentiate from neuropathy.
  • Muscle MRI: Can show muscle edema (inflammation) and fatty infiltration (chronic damage), helping guide biopsy site selection.
  • Biopsy:
    • Muscle Biopsy: Often considered the definitive test. Shows characteristic inflammatory infiltrates (endomysial in PM, primarily perivascular/perifascicular in DM), muscle fiber necrosis/regeneration, and perifascicular atrophy (in DM).
    • Skin Biopsy: Confirms characteristic DM rash findings (interface dermatitis).
  • Cancer Screening: Appropriate age- and risk-based screening is crucial, especially in adult-onset DM and PM, due to the increased association with malignancy (paraneoplastic syndrome) [1, 2].

The presence of specific autoantibodies, like anti-Jo-1, is highly suggestive of an inflammatory myopathy, particularly the anti-synthetase syndrome subtype [1, 2].

 

Differential Diagnosis of Muscle Weakness

Condition Key Features / Distinguishing Points Typical Investigations / Findings
Inflammatory Myopathies (PM/DM) Subacute/chronic progressive symmetric proximal muscle weakness. Dysphagia, respiratory muscle weakness possible. DM: Characteristic skin rash (heliotrope, Gottron's). Elevated CK/aldolase. Myopathic EMG. Positive MSAs (Anti-Jo-1, Anti-Mi-2, etc.). Muscle biopsy shows inflammation/necrosis. MRI shows muscle edema.
Inclusion Body Myositis (IBM) Insidious onset, slowly progressive weakness, often asymmetric. Affects finger flexors, wrist flexors, quadriceps disproportionately. Typically older males (>50). Poor response to immunosuppression. CK mildly elevated or normal. EMG shows mixed myopathic/neurogenic features. Muscle biopsy shows endomysial inflammation, rimmed vacuoles, inclusions (diagnostic).
Muscular Dystrophies (e.g., Limb-Girdle, FSHD) Inherited disorders. Slowly progressive weakness in specific patterns (e.g., proximal limb-girdle, facial/scapular in FSHD). Family history often present. CK often very high (esp. dystrophinopathies). EMG myopathic. Muscle biopsy shows dystrophic changes. Genetic testing confirms specific type.
Myasthenia Gravis (MG) Fluctuating, fatigable weakness, worse with activity, improves with rest. Often affects ocular (ptosis, diplopia), bulbar, facial, limb muscles. Clinical history of fatigability. Positive AChR or MuSK antibodies. Repetitive nerve stimulation / Single-fiber EMG show neuromuscular junction defect. Tensilon test (rarely used now). Normal CK/muscle biopsy.
Lambert-Eaton Myasthenic Syndrome (LEMS) Proximal weakness, improves briefly with exertion (post-exercise facilitation). Autonomic symptoms (dry mouth). Often associated with small cell lung cancer. Positive VGCC antibodies. Repetitive nerve stimulation shows incremental response at high rates. Cancer screening essential.
Motor Neuron Disease (e.g., ALS) Progressive weakness with mixed upper (spasticity, hyperreflexia) and lower (atrophy, fasciculations) motor neuron signs. Bulbar onset common. Sensory exam normal. Clinical exam crucial. EMG shows widespread active/chronic denervation. CK often mildly elevated. Muscle biopsy shows neurogenic atrophy.
Endocrine Myopathies (e.g., Thyroid, Steroid) Proximal weakness, fatigue. Associated symptoms of underlying endocrine disorder (hyper-/hypothyroidism) or history of chronic steroid use. Abnormal thyroid function tests. History of steroid exposure. CK usually normal or mildly elevated. EMG may show myopathic features.
Metabolic Myopathies (e.g., Glycogen/Lipid Storage) Exercise intolerance, cramps, myalgia, sometimes weakness or rhabdomyolysis triggered by exertion or fasting. Forearm exercise test may be abnormal. Specific enzyme assays or genetic testing. Muscle biopsy shows storage material.
Drug/Toxin Induced Myopathy Myalgia, weakness developing after exposure to certain drugs (statins, fibrates, colchicine, alcohol, etc.). History of exposure. CK may be elevated. Symptoms improve on drug withdrawal.

 

Dermatomyositis and Polymyositis: Treatment

Treatment aims to improve muscle strength, resolve inflammation, manage skin manifestations (in DM), treat systemic complications (like ILD), and screen for/treat associated malignancy [1, 2].

It's crucial to distinguish idiopathic (primary) inflammatory myopathies from paraneoplastic (secondary) myositis, as the latter requires treatment of the underlying cancer for myositis improvement [1, 2]. Paraneoplastic myositis accounts for a significant portion (e.g., 14-30% or higher, especially in older adults) of new-onset DM/PM cases [1]. Cancers commonly associated include lung, ovarian, colorectal, prostate, breast cancers, and hematologic malignancies (hemoblastosis) [1]. Dermatomyositis presenting in individuals over 60 almost always warrants a thorough malignancy workup [1].

Treatment program for idiopathic DM/PM typically includes [1, 2]:

  1. Corticosteroids: High-dose oral prednisone (or equivalent) is the mainstay of initial therapy to rapidly control inflammation. Dose is gradually tapered based on clinical response and CK levels.
  2. Immunosuppressants (Steroid-Sparing Agents): Often added early, either for more severe disease or to allow corticosteroid tapering and reduce long-term steroid side effects. Common choices include methotrexate or azathioprine. Mycophenolate mofetil (MMF) or intravenous immunoglobulin (IVIg) may also be used. Cyclophosphamide might be considered for severe, refractory disease, especially with significant ILD. Rituximab is used in some refractory cases.
  3. Specific Therapies (DM Skin): Topical corticosteroids, antimalarials (hydroxychloroquine - caution with some skin subtypes), sun protection.
  4. Management of Complications: Treatment for ILD (may require cyclophosphamide or other agents), management of dysphagia (swallowing therapy, feeding tube if severe), physical therapy.
  5. General Measures: Addressing calcinosis (painful calcium deposits under skin, difficult to treat), ensuring adequate calcium/vitamin D intake (especially with steroid use), monitoring bone health.
  6. Therapeutic Exercise: Crucial component of rehabilitation.
    • In the acute phase with severe weakness/inflammation, gentle passive range-of-motion exercises are performed daily to prevent joint contractures (ankylosis from muscle shortening) and maintain mobility. Immobilization should be minimized unless necessary for pain control or specific joint issues.
    • As inflammation subsides and strength improves (recovery phase), a gradual program of active and resistive exercises is initiated under physiotherapy guidance to rebuild muscle strength, endurance, and function. Aerobic exercise is also incorporated.
Physiotherapy modalities like infrared therapy may help manage muscle pain associated with myositis or fibromyalgia.
Physiotherapy modalities like infrared therapy may help alleviate muscle pain or spasm, complementing exercise therapy in myositis management [1].

Treatment requires long-term monitoring and adjustments based on clinical status, muscle enzyme levels, and potential side effects of medications [1, 2].

References

  1. Dalakas MC, Hohlfeld R. Polymyositis and dermatomyositis. Lancet. 2003 Sep 20;362(9388):971-82. (Classic review, though older).
  2. Mammen AL. Dermatomyositis and Polymyositis: Clinical Presentation, Autoantibodies, and Pathogenesis. Ann N Y Acad Sci. 2010 Jun;1193:134-53. (Or cite newer reviews/textbook chapters like Kelley & Firestein's Rheumatology or Harrison's Internal Medicine).
  3. Lundberg IE, Tjärnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Ann Rheum Dis. 2017 Dec;76(12):1955-1964.