Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
The inflammatory myopathies are acquired diseases in which immune injury weakens muscle over days to months. They are the most treatable myopathies in adults, which is why recognising them matters: a patient with subacute proximal symmetric weakness deserves a workup for these conditions before the deficit becomes fixed. The family label hides real differences, because each major subtype injures muscle through a different mechanism and only some answer to immunosuppression.
The three major subtypes
Dermatomyositis pairs proximal weakness with a distinctive rash and behaves as a complement-mediated microangiopathy of muscle capillaries. Polymyositis produces similar weakness without any rash through cytotoxic T-cell attack on fibres, and it is defined by excluding everything else. Inclusion body myositis affects older adults with a different distribution, distal finger-flexor weakness in the arms with quadriceps weakness in the legs, often asymmetric, and combines inflammation with protein aggregation that resists immunotherapy.
| Feature | Dermatomyositis | Polymyositis | Inclusion body myositis |
|---|---|---|---|
| Rash | Present, often the presenting complaint | Absent | Absent |
| Weakness pattern | Proximal, symmetric | Proximal, symmetric | Finger flexors plus quadriceps, sometimes asymmetric |
| Typical onset | Any age, with a childhood peak | Middle adulthood, rare in children | Over 50 years |
| CK | Markedly raised in active disease | Markedly raised | Normal or mildly raised |
| Biopsy hallmark | Perifascicular atrophy | Endomysial cytotoxic infiltrates | Rimmed vacuoles |
| Treatment response | Good | Good | None to immunotherapy |
Beyond the trio sit several less common entities: overlap syndromes with connective tissue disease, drug-induced and HIV-associated myositis, immune-mediated necrotising myopathy with very high CK, and infectious or focal forms. These are diagnosed by their context rather than by the classic patterns above. Current classification has moved toward serotype-defined subsets, formalised in the 2017 EULAR/ACR criteria, because the antibody profile predicts organ involvement and course better than clinical labels alone.
Autoantibodies and what they predict
Myositis-specific antibodies appear in roughly a third of patients with dermatomyositis or polymyositis and shape both diagnosis and prognosis. Anti-Mi-2 is closely tied to classic dermatomyositis skin disease and a favourable treatment response. Anti-Jo-1, the commonest of the group, defines the anti-synthetase syndrome: myositis together with interstitial lung disease, arthritis, Raynaud phenomenon, and cracked mechanic’s hands, where the lung disease can dominate and threaten life. Anti-SRP and anti-HMGCR mark necrotising disease with severe weakness and very high CK, the latter persisting after statin withdrawal. Antibodies to extractable nuclear antigens point toward an overlap with systemic sclerosis, lupus, Sjogren disease, or mixed connective tissue disease.
Confirming the diagnosis
Serum CK, aldolase, and myoglobin confirm active fibre injury and give a baseline for monitoring; a falling CK tracks with effective immunosuppression. Electromyography shows small, brief motor unit potentials with early recruitment, and fibrillations with positive sharp waves when inflammation is active. Muscle biopsy separates the subtypes: perifascicular atrophy with capillary loss and complement deposition in dermatomyositis, cytotoxic invasion of otherwise healthy fibres in polymyositis, rimmed vacuoles with filamentous inclusions in inclusion body myositis. A sparse or normal biopsy does not overrule a convincing clinical picture in dermatomyositis, where infiltrates may be absent.
Treatment principles
Dermatomyositis and polymyositis share an escalation: weight-based oral prednisone induction, slow taper with steroid-sparing agents such as azathioprine, methotrexate, or ciclosporin if relapse occurs, maintenance on the lowest effective burden, and intravenous immunoglobulin for refractory disease or steroid intolerance. Inclusion body myositis stands apart. Trials of steroids, antimetabolites, and immunoglobulin have not altered its course, so care is supportive: physiotherapy, swallowing assessment, and fall prevention. Detailed protocols, dosing, and malignancy co-management are set out in the dermatomyositis treatment note, and the polymyositis and inclusion body myositis notes carry each disease in full.
Evidence anchors
- Dalakas MC. Inflammatory muscle diseases. N Engl J Med. 2015;372(18):1734-1747. doi:10.1056/NEJMra1402225
- Lundberg IE, Tjarnlund 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. Arthritis Rheumatol. 2017;69(12):2271-2282. doi:10.1002/art.40322
- Aggarwal R, Charles-Schoeman C, Schessl J, et al. Trial of intravenous immune globulin in dermatomyositis. N Engl J Med. 2022;387(14):1264-1278. doi:10.1056/NEJMoa2117912