Idiopathic Inflammatory Myopathies
- Symmetric proximal weakness with a raised creatine kinase; dermatomyositis adds the heliotrope rash and Gottron's papules, and polymyositis, without them, is a diagnosis of exclusion
- A new adult dermatomyositis is a reason to look for cancer, and anti-TIF-1-gamma marks the subset to investigate hardest (41% cancer-associated, diagnostic OR 9.37)
- Interstitial lung disease can be severe, especially with the antisynthetase syndrome and anti-MDA5
- Myositis-specific antibodies define the subtype and predict cancer or lung disease; EMG, MRI and biopsy complete the diagnosis
- Juvenile dermatomyositis brings vasculopathy and calcinosis, which can ulcerate, become infected or restrict joints
- IVIG beat placebo 79% vs 44% in ProDERM, but 6 of 9 IVIG-related serious adverse events were thromboembolic and CK did not track the response
- “Adult dermatomyositis is paraneoplastic: screen, and let the antibody decide how hard
- “A 'polymyositis' that fails steroids is often inclusion-body myositis
- “Anti-HMGCR necrotising myopathy is the statin-associated one
- “Orthopaedic role: weakness, contractures, excision of problematic calcinosis, perioperative immunosuppression
Overview and Classification
The idiopathic inflammatory myopathies are autoimmune, immune-mediated muscle diseases with multisystem involvement. Dermatomyositis, polymyositis and their overlapping subtypes share symmetric proximal weakness and a raised creatine kinase. The course is chronic in two-thirds of adults, and management is multidisciplinary.
Dermatomyositis. The myopathy plus characteristic skin signs. It is a complement-mediated, interferon-driven microangiopathy, and it includes an amyopathic form, with skin disease only, and a juvenile form.
Polymyositis. The myopathy without the skin signs, driven by a T-cell-mediated endomysial process. It is a diagnosis of exclusion: inclusion-body myositis and the dystrophies must be excluded before the label is used.
- Dermatomyositis
- Yes - heliotrope rash, Gottron's papules
- Polymyositis
- No characteristic skin signs
- Dermatomyositis
- Complement-mediated microangiopathy (perifascicular)
- Polymyositis
- T-cell-mediated endomysial
- Dermatomyositis
- Strong paraneoplastic association - screen
- Polymyositis
- Weaker association
- Dermatomyositis
- Yes (calcinosis, vasculopathy)
- Polymyositis
- Rare
- Dermatomyositis
- Amyopathic form (skin only)
- Polymyositis
- Diagnosis of exclusion (exclude IBM, dystrophy)
Clinical Presentation
The weakness. It is symmetric and proximal, so the patient struggles to rise from a chair, climb stairs or lift the arms overhead. Chronic weakness can lead to joint contractures.
The skin in dermatomyositis. The heliotrope rash is a violaceous periorbital rash, and Gottron's papules sit over the knuckles. The shawl or V sign and mechanic's hands are the other signs to look for.
Juvenile dermatomyositis. The juvenile form is distinguished by a prominent vasculopathy and by calcinosis, dystrophic calcium deposits in the skin and soft tissues. Calcinosis is its hallmark complication: the deposits can ulcerate, become infected, restrict adjacent joints and cause significant morbidity. They differ in mechanism from the metastatic deposits of tumoral calcinosis, which is worth reading for the contrast.
Systemic Associations: Cancer and the Lung
In adults, dermatomyositis is strongly paraneoplastic. A new diagnosis should prompt an age- and risk-appropriate search for an occult malignancy, because the myositis can be the presenting sign of an underlying cancer.
Stratify the search by antibody. "Screen for cancer" is true but blunt, because the risk is concentrated in an antibody-defined subset. A meta-analysis of 18 studies and 1,962 patients found that 41% of adults with anti-TIF-1-gamma had cancer-associated disease (95% CI 36 to 45), with a diagnostic odds ratio for cancer of 9.37 (95% CI 5.37 to 16.34) and no heterogeneity between studies (I-squared zero).
The other end of the scale. Anti-Mi-2 marks classic dermatomyositis with a good prognosis and a low cancer risk, and anti-MDA5 points to rapidly progressive lung disease rather than to cancer. The antibody result should therefore set the intensity of the work-up, telling you whom to investigate hard and whom to reassure.
What nobody knows. No study establishes how long to keep screening or at what interval, so no surveillance schedule is given here.
Interstitial lung disease. This is the second critical association, a major complication that can be rapidly progressive and life-threatening, particularly with the antisynthetase syndrome and anti-MDA5. Lung involvement must be actively assessed and treated.
Diagnosis
The diagnosis integrates the clinical picture with elevated muscle enzymes, antibodies, electromyography, MRI and muscle biopsy. Myositis-specific and myositis-associated antibodies define subtypes and predict complications such as cancer or interstitial lung disease, and the antibody profile increasingly guides classification and risk stratification.
The antibody map. Each key myositis-specific antibody points to a subtype:
- Anti-Jo-1, and the other anti-tRNA-synthetases anti-PL-7 and anti-PL-12 - the antisynthetase syndrome: myositis, interstitial lung disease, mechanic's hands, Raynaud, non-erosive arthritis and fever
- Anti-Mi-2 - classic dermatomyositis, with the shawl/V sign and Gottron's papules
- Anti-MDA5 - clinically amyopathic dermatomyositis with rapidly progressive ILD, skin ulcers and palmar papules; mortality is high
- Anti-TIF-1-gamma, and anti-NXP2 - cancer-associated dermatomyositis, the highest adult malignancy risk
- Anti-SRP and anti-HMGCR - immune-mediated necrotising myopathy, with a very high CK and necrosis with sparse inflammation; anti-HMGCR is the statin-associated one
MRI. It shows muscle oedema and guides the choice of biopsy site.
The muscle biopsy. Three patterns separate myopathies that look alike at the bedside, and the third changes the answer when a "polymyositis" fails to respond:
- Dermatomyositis - the hallmark is perifascicular atrophy, atrophic fibres at the fascicle periphery from the microangiopathy. There is perivascular and perimysial inflammation, complement (MAC, C5b-9) on capillaries with capillary dropout, and MxA, an interferon marker, in the perifascicular fibres.
- Polymyositis - endomysial CD8+ T cells surround and invade non-necrotic fibres that aberrantly express MHC-I, the CD8/MHC-I complex. There is no perifascicular atrophy.
- Inclusion-body myositis - the same endomysial CD8/MHC-I picture, plus rimmed vacuoles, protein aggregates (p62/TDP-43) and COX-negative fibres. Clinically it is distal and asymmetric (finger flexors, quadriceps), affects older patients and responds poorly to steroids.
A "polymyositis" that fails steroids is often inclusion-body myositis.
A granulomatous mimic. A granulomatous myopathy with weakness and raised muscle enzymes can mimic these diseases closely, so musculoskeletal sarcoidosis belongs in the differential.
Management and Orthopaedic Relevance
Medical treatment. Management is medical and multidisciplinary, with rheumatology or neurology and, where relevant, oncology and respiratory medicine. Corticosteroids are combined with steroid-sparing immunosuppression: methotrexate, azathioprine or mycophenolate. Intravenous immunoglobulin is efficacious, particularly in refractory dermatomyositis, and newer agents targeting the interferon/JAK pathway or B cells are aimed at refractory disease.
The IVIG trial. ProDERM randomised 95 adults with active dermatomyositis to IVIG 2.0 g/kg or placebo every four weeks. At 16 weeks 79% (37 of 47) met the primary endpoint against 44% (21 of 48) on placebo, a 35-percentage-point difference (95% CI 17 to 53). That is the evidence behind the word "efficacious", and it is worth quoting in place of the adjective.
The thrombosis signal. Of nine IVIG-related serious adverse events, six were thromboembolic. That matters when the same patient is being consented for an operation and considered for thromboprophylaxis, and the risk IVIG adds to an already immobile patient is set against the general framework in venous thromboembolism.
CK is a poor guide to response. Creatine kinase did not differ meaningfully between the IVIG and placebo groups despite the clinical improvement. It is used to make the diagnosis, but a CK that has not fallen is not evidence that treatment has failed.
The orthopaedic role. The surgeon deals mostly with the consequences of the disease and its treatment, always alongside the treating rheumatology or neurology team:
- Rehabilitation for weakness and the disability it causes
- Prevention and management of joint contractures
- Surgical excision of problematic calcinosis - ulcerating, infected or joint-restricting deposits, especially in juvenile dermatomyositis
- Attention to perioperative steroids and immunosuppression
Mnemonics & Memory Aids
MYOSITIS
Hook:MYOSITIS: Malignancy, Young (juvenile/calcinosis), Occult ILD, Symmetric proximal weakness, Immunosuppression+IVIG, Tests, Inflammatory skin (DM), Skin-sparing (PM).
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“An adult presents with symmetric proximal weakness, a violaceous rash around the eyes and scaly papules over the knuckles. What is the diagnosis and what must you not miss?”
Recognise
- Symmetric proximal muscle weakness + raised creatine kinase
- Dermatomyositis: heliotrope rash, Gottron's papules (shawl/V sign, mechanic's hands)
- Polymyositis: no skin signs (diagnosis of exclusion - exclude IBM/dystrophy)
Critical associations
- Adult dermatomyositis = paraneoplastic → screen, stratified by antibody (anti-TIF-1-gamma: 41% cancer, OR 9.37)
- Interstitial lung disease (anti-synthetase syndrome, anti-MDA5) - can be severe
- Juvenile DM: vasculopathy + calcinosis
Diagnosis
- Myositis-specific antibodies (subtype/risk stratification)
- EMG, MRI (oedema, biopsy guide), muscle biopsy
- Elevated muscle enzymes (CK)
Management
- Corticosteroids + steroid-sparing immunosuppression (methotrexate/azathioprine/MMF)
- IVIG 2 g/kg: 79% vs 44% response at 16 weeks (ProDERM, n=95) - but 6 of 9 serious adverse events thromboembolic, and CK did not track response
- Interferon/JAK and B-cell therapies in refractory disease
- Orthopaedic: weakness/contractures; excise problematic calcinosis
Evidence & Key Studies
Advances in the classification and management of idiopathic inflammatory myopathies
- The idiopathic inflammatory myopathies are immune-mediated disorders with multisystem involvement and a chronic course in two-thirds of adults; autoantibodies aid identification of disease subtypes and their associated severe complications such as cancer or interstitial lung disease.
- Patients should be managed in a multidisciplinary setting; intravenous immunoglobulin is efficacious in refractory dermatomyositis, improving skin and muscle disease activity.
- Advances include interferon-pathway biomarkers and therapies (anti-interferon monoclonals, JAK inhibitors) and B-cell (CD19 CAR-T) approaches for refractory disease.
Paraneoplastic dermatomyositis secondary to an underlying plasma-cell dyscrasia
- Dermatomyositis is often associated with malignancies; this case was paraneoplastic, secondary to an underlying (smouldering multiple myeloma) plasma-cell dyscrasia, reinforcing the malignancy association.
- Recognition of the dermatomyositis presentation (proximal weakness, skin changes, raised creatine kinase) led to investigation for secondary causes.
- Routine screening for secondary causes/malignancy is recommended as part of the dermatomyositis work-up, to allow early diagnosis before organ damage.
Trial of Intravenous Immune Globulin in Dermatomyositis (ProDERM)
- 95 adults with active dermatomyositis randomised 1 to 1 to IVIG 2.0 g/kg or placebo every 4 weeks for 16 weeks, with an open-label extension to 40 weeks. The primary endpoint was a Total Improvement Score of at least 20 with no confirmed deterioration.
- 79 percent of the IVIG group (37 of 47) reached the endpoint versus 44 percent on placebo (21 of 48) - a difference of 35 percentage points (95% CI 17 to 53, p less than 0.001). Secondary endpoints ran in the same direction, with one exception: change in creatine kinase did not differ meaningfully between groups.
- 282 treatment-related adverse events occurred over 40 weeks in the IVIG group - headache in 42 percent, pyrexia in 19 percent, nausea in 16 percent - and 6 of the 9 IVIG-related serious adverse events were thromboembolic. The trial was industry-funded and ran only 16 weeks to the primary endpoint.
Use of anti-transcriptional intermediary factor-1 gamma autoantibody in identifying adult dermatomyositis patients with cancer: a systematic review and meta-analysis
- 18 studies and 1,962 patients with dermatomyositis pooled from EMBASE, MEDLINE and the Cochrane Library to May 2018.
- Pooled prevalence of cancer-associated dermatomyositis among anti-TIF-1-gamma-positive adults was 0.41 (95% CI 0.36 to 0.45), with an overall diagnostic odds ratio for cancer of 9.37 (95% CI 5.37 to 16.34) and no heterogeneity (I-squared zero).
- The antibody therefore identifies a subset of adults warranting the most intensive malignancy work-up; the meta-analysis does not establish how long screening should continue or at what interval.
The multisystem nature, antibody-based subtyping (identifying cancer/ILD risk), chronic course, multidisciplinary management and the efficacy of IVIG in refractory dermatomyositis come from the cited Raaphorst review; the paraneoplastic malignancy association and the recommendation to screen for secondary causes from the cited Meel case. The randomised evidence for IVIG, including the thromboembolic signal and the failure of creatine kinase to track the clinical response, comes from ProDERM; the anti-TIF-1-gamma cancer figures from the Best meta-analysis. The dermatomyositis skin signs (heliotrope, Gottron's), the polymyositis diagnosis of exclusion, the three biopsy patterns, juvenile DM calcinosis, and the orthopaedic relevance (weakness, contracture, calcinosis excision) are standard, well-established teaching. No trial establishes an interval or a duration for malignancy surveillance after a dermatomyositis diagnosis, and none compares surgical excision of calcinosis against leaving it alone, so neither is quoted as though settled.