Dermatomyositis is an idiopathic inflammatory myopathy primarily affecting adults between 45-60 years of age, more prevalent in females, and characterised by proximal muscle weakness, distinctive skin rash, and presence of inflammatory infiltrates on muscle biopsy, with classic perifascicular atrophy.
Description
Dermatomyositis (DM) is an idiopathic inflammatory myopathy, which specifically targets the skin and skeletal muscle, leading to distinct cutaneous manifestations and muscle weakness. It is characterised by a pathognomonic skin rash that often precedes, accompanies or follows the muscle weakness. DM is part of a wider group of conditions known as inflammatory myopathies which also include polymyositis and inclusion body myositis.
Pathogenesis
The exact pathogenesis of DM is not well understood, but it is thought to be an autoimmune process triggered by environmental factors in genetically susceptible individuals. The autoimmune reaction is driven by CD4+ T cells and results in complement activation, leading to muscle fibre damage and necrosis. There is also vasculopathic damage specifically targeting the endomysial capillaries leading to ischemia and perifascicular atrophy.
Subtypes
Dermatomyositis can be categorised into several subtypes based on clinical features and age of onset:
- Classic Dermatomyositis: Characterised by muscle weakness and skin rash.
- Juvenile Dermatomyositis: Onset before the age of 18 and may present with calcinosis cutis.
- Amyopathic Dermatomyositis (ADM): Prominent skin manifestations with minimal or absent muscle involvement.
- Hypomyopathic Dermatomyositis (HDM): Minimal muscle symptoms with detectable muscle enzyme elevations and/or electromyographic changes.
- Dermatomyositis associated with malignancy: Associated with an underlying malignancy, most often occurring within one year of DM diagnosis.
Epidemiology, Risk Factors & Associations
- Predominantly affects adults between 45-60 years of age (7-10 per million), and children between 5-15 years of age (3.2 per million)
- More common in females with a female-to-male ratio of 2:1.
- Higher incidence in certain ethnic groups (African American and Hispanic)
- Can occur as part of a paraneoplastic syndrome – associated with malignancies such as ovarian, lung, pancreatic, stomach, and colorectal cancers, and non-Hodgkin’s lymphoma (15-32% of adult cases)
Clinical Features
- Proximal muscle weakness affecting the neck, shoulders, and pelvic girdle.
- Cutaneous manifestations including heliotrope rash (violaceous erythema on the eyelids), Gottron’s papules (red or violet, flat-topped papules over the knuckles), and shawl sign (erythema over the shoulders and back).
- Dysphagia due to involvement of pharyngeal and oesophageal muscles.
- Constitutional symptoms such as fever, fatigue, and weight loss.
Complications
- Known association with malignancy (15-32% of adult cases), most often diagnosed within one year of the diagnosis of DM.
- Interstitial lung disease occurs in up to 65% of cases.
- Dysphagia and aspiration pneumonia due to oropharyngeal muscle involvement.
- Cardiac complications: Myocarditis, conduction defects, and cardiomyopathy can occur.
Pathological Features
Histopathology
- Macroscopic: Muscle biopsy in DM may not reveal significant changes on gross examination.
- Microscopic: The hallmarks of DM on histopathology include perivascular and perifascicular inflammation, muscle fibre necrosis, and perifascicular atrophy. A unique feature is the presence of perifascicular atrophy, attributed to ischemia secondary to vascular damage.
Serology
- Presence of specific autoantibodies such as anti-Jo-1 and anti-Mi-2 antibodies are suggestive of DM. Anti-TIF1γ (p155) antibody is associated with malignancy-associated DM.
Biochemistry
- Elevation of muscle enzymes including creatine kinase (CK), aldolase, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) is a common feature.
Radiological Features
General Features
- Typically demonstrates symmetrical, bilateral muscle oedema and inflammation, predominantly in the proximal muscles (especially quadriceps and gluteus).
- Progresses to upper extremity, neck, & pharyngeal muscles
- Dense globular calcification may be seen extending along the fascial plane
- Associated findings:
- Osteonecrosis and osteopaenia related to corticosteroid therapy
XR
- Soft tissue calcification (20-50%):
- Classically sheet-like but may also be globular or amorphous
- Primarily juvenile form; much less common in adults & with modern therapy
- Rarely tumoral calcinosis: mass of calcification, may have fluid-fluid levels
- Acro-osteyolysis (non-specific)
MRI
- T1: Muscle atrophy and fatty infiltration may be seen in chronic disease.
- T2/STIR: Symmetric, bilateral and patchy, high signal intensity in the affected muscles. Calcific areas may be low signal.
- T1 Gad+: Enhancement may be seen due to inflammation and increased vascularity of the involved muscles. Fasciitis may precede myositis.
Grading and Staging
No specific grading and staging system exists for Dermatomyositis. Disease severity is often judged by the extent of muscle weakness and presence of extramuscular manifestations.
Diagnosis
Diagnostic Classification (Bohan & Peter)
| Criterion No. | Individual Diagnostic Criteria | Description / Key Findings |
|---|---|---|
| 1 | Symmetric proximal muscle weakness | Affects large proximal muscles (shoulder, hip girdle, neck flexors); difficulty climbing stairs, rising from sitting, or lifting arms. |
| 2 | Muscle biopsy evidence of myositis | Necrosis and regeneration of muscle fibres with inflammatory infiltrates (endomysial in polymyositis; perivascular/perifascicular in dermatomyositis). |
| 3 | Increased serum skeletal muscle enzymes | Elevated CK, aldolase, AST, ALT, or LDH indicating muscle breakdown. |
| 4 | Characteristic electromyographic (EMG) patterns | Myopathic changes—short-duration, low-amplitude polyphasic motor units, fibrillation potentials, and positive sharp waves. |
| 5 | Typical rash of dermatomyositis | Heliotrope rash, Gottron papules, shawl sign, V-sign, or mechanic’s hands. Defines dermatomyositis. |
| Diagnosis | Required Criteria |
|---|---|
| Definitive Dermatomyositis | Criterion 5 (rash) plus any 3 of 1–4 |
| Probable Dermatomyositis | Criterion 5 (rash) plus any 2 of 1–4 |
| Possible Dermatomyositis | Criterion 5 (rash) plus any 1 of 1–4 |
| Polymyositis | Same combinations without rash (criterion 5) |
Biopsy
MRI is valuable for selecting a biopsy site when dermatomyositis is suspected:
- Target STIR/T2 hyperintense muscle – Oedema marks active inflammation, maximising histological yield.
- Avoid T1 fatty replacement – Fibrofatty “burnt-out” muscle is low yield and nonspecific on biopsy.
- Pick moderate weakness (≈ MRC 4) – Clinically involved yet not end-stage tissue is most informative.
- Biopsy within the oedematous zone – Increases chance of sampling inflammatory infiltrates and myofibre necrosis.
- Steer clear of recent EMG/injection sites – Procedure-related change can mimic myositis and confound pathology.
- Prefer accessible muscles (e.g., deltoid, quadriceps) – Safer approach with adequate sample length and fewer complications.
- Use post-contrast if uncertain – Enhancement supports active inflammation and refines target selection.
- Beware denervation/exertional patterns – These can cause STIR hyperintensity without autoimmune myositis.
- In dermatomyositis, sample perifascicular/perimysial-predominant regions – Aligns imaging with characteristic pathology.
- Time biopsy before or early in steroid therapy – Treatment rapidly suppresses inflammatory signal and cellular yield.
Differential Diagnosis
- Polymyositis: Similar to DM, but lacks the characteristic skin rash.
- Systemic Lupus Erythematosus: Can have overlapping features, but typically presents with arthritis and renal involvement.
- Drug-induced myositis: History of drug exposure is crucial, statins being a common offender.
- Inclusion body myositis: More common in older individuals, and characterised by both proximal and distal muscle weakness and poor response to steroids.
Prognosis
The prognosis of DM varies widely depending on the extent of muscle weakness, presence of extramuscular manifestations, and associated malignancy. The five-year survival rate is approximately 70%. The presence of malignancy or interstitial lung disease portends a poorer prognosis.
Management
- Referral to rheumatology is essential for the management of DM.
- Immunosuppression with corticosteroids is the mainstay of treatment. Other immunosuppressive agents such as methotrexate or azathioprine may be used in refractory cases or to minimise steroid side effects.
- Regular physiotherapy is vital to prevent contractures and maintain muscle strength.
- Screening for associated malignancy is critical in all adult patients diagnosed with DM.