Polymyositis and Dermatomyositis

Polymyositis (PM) and dermatomyositis (DM) are systemic autoimmune inflammatory myopathies characterized by proximal muscle weakness and, in DM, distinctive cutaneous manifestations. These conditions result from immune-mediated destruction of skeletal muscle and represent major causes of acquired myopathy in adults. The incidence is approximately 1–2 cases per million annually, with DM occurring twice as frequently as PM; peak incidence occurs in the fifth to sixth decades, though juvenile DM represents a distinct subset with better prognosis. Clinical recognition is essential as early aggressive immunosuppression can substantially modify disease course and prevent irreversible muscle atrophy and functional disability. These conditions are frequently tested on USMLE Step 2 CK, particularly regarding diagnostic criteria, association with malignancy, and management paradigms.

The pathogenesis of polymyositis and dermatomyositis involves complex interactions between genetic susceptibility, environmental triggers, and dysregulated adaptive and innate immunity, resulting in T-cell and antibody-mediated muscle destruction.

In polymyositis, CD8+ cytotoxic T lymphocytes infiltrate and directly attack MHC class I-expressing muscle fibers, representing the primary mechanism of muscle injury. These autoreactive T cells recognize muscle-associated antigens presented on the sarcolemmal surface, leading to perforin- and granzyme-mediated cytotoxic granule release and subsequent myocyte apoptosis. The T-cell infiltrate is predominantly located in the perifascicular and interfascicular regions, with invasion of intact muscle fibers by cytotoxic cells—a pathognomonic feature. This CD8+ infiltration is accompanied by CD4+ T helper cells and macrophages that amplify tissue destruction through cytokine production. The presence of clonally expanded T-cell populations in affected muscle suggests antigen-driven selection and sustained activation against muscle-associated autoantigens.

Dermatomyositis primarily involves type I interferon (IFN-α/β)-mediated immune activation with prominent involvement of innate immunity. Myoepithelial cells and capillary endothelium express elevated interferon-stimulated genes (ISGs), including MxA and OAS, detectable by immunohistochemistry and constituting the "interferon signature." This IFN-driven response involves activation of plasmacytoid dendritic cells (pDCs) through toll-like receptors (TLRs), particularly TLR7 and TLR9, leading to robust type I IFN production. Unlike PM, DM features prominent B cell and antibody pathology; circulating and muscle-infiltrating B cells produce autoantibodies (discussed below) that deposit in muscle capillaries. The hallmark DM pathology includes perifascicular muscle atrophy, capillary dropout with muscle ischemia, and MAC (complement component C5b-9) deposition on capillaries—indicating complement-mediated microvascular injury. This explains the distinctive cutaneous involvement in DM, as dermis shares similar capillary pathology.

Both PM and DM are characterized by circulating myositis-specific antibodies (MSAs) and myositis-associated antibodies (MAAs), which serve as disease markers and likely contribute to pathogenesis. Anti-synthetase antibodies (most commonly anti-Jo-1 directed against histidyl-tRNA synthetase) are present in ~30% of myositis cases and associated with "synthetase syndrome"—characterized by myositis, arthritis, interstitial lung disease, Raynaud's phenomenon, and mechanic's hands. Other MSAs include anti-Mi-2 (associated with classic DM and better prognosis), anti-TIF1-γ (associated with DM and occult malignancy), and anti-CADM-140/MDA5 (increasingly recognized, associated with DM, interstitial lung disease, and poor prognosis). These antibodies likely facilitate immune activation by forming immune complexes, activating complement, and promoting cellular inflammation. The presence of specific antibodies correlates with distinct clinical phenotypes and prognostic patterns.

Resting skeletal muscle minimally expresses MHC class I molecules on myocytes; however, in both PM and DM, muscle fibers aberrantly express HLA-A, -B, and -C, mediated by interferon-gamma (IFN-γ) produced by infiltrating T cells. This MHC upregulation permits enhanced antigen presentation to CD8+ T cells, perpetuating the cytotoxic response. Additionally, constitutive expression of MHC class II molecules on muscle fiber sarcolemma (normally absent) facilitates CD4+ T-cell recognition. Muscle fibers also express elevated levels of ICAM-1 (CD54) and VCAM-1 (CD106), adhesion molecules that facilitate lymphocyte infiltration and retention. These changes, combined with production of chemokines (especially CCL2/MCP-1 and CXCL10/IP-10) by muscle and infiltrating cells, create a pro-inflammatory microenvironment that sustains immune activation.

Although complement activation occurs in both conditions, membrane attack complex (MAC/C5b-9) deposition on capillaries is the defining feature of dermatomyositis pathology. Complement activation may be triggered by immune complex deposition or direct complement activation through pattern recognition receptors. The resultant capillary endothelial damage leads to ischemia and perifascicular muscle atrophy. In polymyositis, MAC deposition is minimal; instead, T-cell infiltration and myocyte destruction predominate. The distinction in complement involvement explains the different pathological signatures and contributes to differential clinical features.

Environmental triggers, including viral infections (particularly picornaviruses, retroviruses), ultraviolet exposure (in DM), and potentially medications (statins, immune checkpoint inhibitors), may initiate autoimmunity in genetically predisposed individuals. Molecular mimicry mechanisms are proposed, wherein epitopes on infectious agents cross-react with self-antigens on muscle. Additionally, muscle damage from initial infection or injury may expose sequestered autoantigens and break immune tolerance. Genetic polymorphisms in HLA (particularly HLA-DRB1 and HLA-DQA1 alleles) and other immune regulatory genes predispose to myositis. The relative contribution of genetic versus environmental factors varies among individuals.

The majority of PM and DM cases are primary (idiopathic) autoimmune disorders without identified secondary cause. Genetic predisposition, mediated by HLA polymorphisms (HLA-DR3, HLA-DRB1*03, HLA-DQA1*05:01) and non-HLA genes (PTPN22, IRF5, STAT4), establishes baseline susceptibility. Environmental triggers remain incompletely characterized but likely include viral infections (enterovirus, paramyxovirus, hepatitis C virus, HIV), UV exposure (particularly for DM), and possibly medication-induced triggers.

Dermatomyositis carries significant malignancy risk (10–50% depending on age and geography), substantially higher than polymyositis (3–15%). The risk is highest in patients over age 50 and those with certain antibody profiles (anti-TIF1-γ, anti-MDA5, negative for anti-synthetase antibodies). Common associated malignancies include ovarian cancer (most frequent), lung cancer, gastric cancer, breast cancer, and lymphoma. The temporal relationship between myositis onset and malignancy detection is variable; malignancy may precede, coincide with, or follow myositis diagnosis by months to years. Clinically amyopathic DM (CADM or "DM sine myositis") with prominent skin findings but minimal muscle involvement carries particularly high malignancy risk. The mechanism linking myositis to malignancy likely involves tumor expression of myositis autoantigens (epitope spreading) or shared molecular pathways triggering cross-reactive immunity.

Statin-induced muscle injury is well-established, though frank myositis is uncommon; more frequent is statin-associated muscle symptoms (SAMS) and necrotizing autoimmune myopathy with anti-HMGCR antibodies. Immune checkpoint inhibitors (ipilimumab, nivolumab, pembrolizumab) can trigger myositis as immune-related adverse events, often with severe manifestations including myocarditis. Interferon-α (used for hepatitis C and malignancy) can trigger myositis, particularly in genetically predisposed individuals. L-tryptophan contaminated with eosinophilia-myalgia syndrome agent caused a historic outbreak of myositis with eosinophilia.

Myositis features can overlap with systemic lupus erythematosus (SLE), systemic sclerosis (scleroderma), Sjögren's syndrome, and mixed connective tissue disease (MCTD). Approximately 15–20% of myositis patients have features of other connective tissue diseases. Anti-synthetase syndrome represents an important overlap phenotype with myositis, arthritis, ILD, Raynaud's, and mechanic's hands, occurring in ~30% of myositis patients.

Juvenile dermatomyositis (JDM) occurs in children aged 2–14 years (peak 5–7 years) and represents a distinct entity with different antibody profiles, better muscle prognosis but higher risk of calcinosis and lipodystrophy. Anti-synthetase antibodies are less common in JDM; anti-NXP2 and anti-MDA5 are relatively more frequent.

HIV infection can trigger inflammatory myositis, particularly during immune reconstitution inflammatory syndrome (IRIS) with antiretroviral therapy. Hepatitis C virus is associated with myositis, particularly in certain populations.

Symmetric proximal muscle weakness affecting the hip and shoulder girdle muscles is the pathognomonic clinical feature of both PM and DM. The weakness typically develops subacutely over weeks to months, though acute presentations over days can occur (especially in severe disease). Patients report difficulty rising from chairs, climbing stairs, lifting objects overhead, or combing hair. The weakness is non-fatigable, distinguishing it from myasthenia gravis—strength does not deteriorate with repetitive exertion. Distal muscles are typically spared early, though advanced disease may involve forearm flexors and foot dorsiflexors. Neck flexor weakness occurs in ~50% of patients and is relatively specific for myositis. The distribution follows a characteristic pattern reflecting myopathic pathology: hip extensors and abductors, shoulder abductors, and knee extensors are particularly affected. Creatine kinase elevation correlates with weakness severity, though exceptions exist.

Myalgias occur in approximately 30–50% of myositis patients, often accompanying weakness or preceding it by weeks. The pain is typically described as deep, aching, and localized to affected proximal muscles. Muscle tenderness on palpation may be present but is less prominent than in polymyalgia rheumatica (PMR), a critical diagnostic distinction. Some patients experience severe, debilitating myalgia as the predominant symptom.

The heliotrope rash—a distinctive violaceous (purple-red) edema affecting the upper eyelids and periorbital region—is highly specific for dermatomyositis. The rash often has a fine, wrinkled appearance and may extend to cheeks and bridge of nose (butterfly distribution). Edema and erythema of the eyelids occur without conjunctivitis, distinguishing DM from lupus.

Gottron's papules represent scaly, violaceous papules overlying the knuckles, elbows, and knees. These are nearly pathognomonic for DM and are distinguished from lupus by their location over (rather than in) the knuckle spaces and by hyperkeratosis. Gottron's sign refers to erythema in similar distributions without papule formation.

Mechanic's hands consist of hyperkeratosis, fissuring, and irregularity of fingertips and lateral aspects of fingers, resembling those of manual laborers despite absence of such work. This is strongly associated with anti-synthetase antibodies and synthetase syndrome, though may occur in other myositis subsets.

Photosensitive rash predominantly affecting sun-exposed areas (V-neck distribution on chest, shoulders, face) occurs in ~40% of DM patients and may be the initial presentation. The rash may worsen with UV exposure and improve with sun avoidance.

Poikiloderma—a combination of hyperpigmentation, hypopigmentation, and atrophy creating a mottled appearance—affects sun-exposed areas, particularly the neck and upper chest ("shawl sign"), and represents chronic DM skin changes.

Other DM cutaneous features include periungual erythema and telangiectasia, nail fold capillary abnormalities resembling those in systemic sclerosis, and lipodystrophy (particularly in juvenile DM, manifesting as loss of subcutaneous fat in face and upper trunk).

Interstitial lung disease (ILD) occurs in 10–30% of myositis patients overall but is particularly common (40–60%) in anti-synthetase syndrome and anti-MDA5 positive patients. ILD can be asymptomatic (detected on imaging) or present with dyspnea, nonproductive cough, and restrictive pattern on pulmonary function testing. Acute, rapidly progressive ILD represents a medical emergency. Anti-MDA5 antibodies are especially associated with rapidly progressive ILD and poor prognosis, even when myositis is mild.

Arthritis and arthralgias occur in ~50% of patients, particularly those with anti-synthetase antibodies. The arthritis is typically non-erosive and predominantly affects small joints of hands, wrists, and knees. Arthritis may precede myositis onset.

Raynaud's phenomenon occurs in ~30% of myositis patients, especially with anti-synthetase antibodies, and may precede other manifestations by years. The mechanism involves vascular dysfunction and possible autoantibodies against vascular endothelium.

Cardiac involvement includes myocarditis (manifesting as arrhythmias, congestive heart failure, cardiogenic shock), conduction abnormalities, and heart block. Myocarditis is an important cause of morbidity and mortality, though often clinically silent. Incidence is likely underestimated due to insufficient screening. Anti-synthetase syndrome carries particular cardiac risk.

Dysphagia results from pharyngeal and esophageal muscle involvement, occurring in ~10–30% of patients. Weakness of cricopharyngeus and upper esophageal sphincter muscles predisposes to aspiration, a serious complication. Involvement of lower esophagus may cause reflux symptoms. Careful swallowing evaluation is essential in all myositis patients before oral intake.

Fever occurs in approximately 20–30% of patients, often accompanying acute disease exacerbations. Low-grade fever may persist chronically.

Constitutional symptoms including fatigue, malaise, and weight loss accompany active disease and improve with immunosuppression.

Calcinosis (calcium deposition in subcutaneous tissues and muscles) is a significant complication, particularly in juvenile DM (40–70%) and less common in adult DM (10–20%). Calcinosis may cause pain, functional impairment, infection, and ulceration. It typically develops months to years after disease onset and correlates with disease duration and severity.

Clinically amyopathic dermatomyositis (CADM or "DM sine myositis") presents with classic DM skin findings (heliotrope rash, Gottron's papules) but with minimal or no detectable muscle weakness on standard testing (though subclinical myositis may be evident on imaging or biopsy). CADM carries particularly high malignancy risk (25–50%) and higher risk of rapidly progressive ILD. Recognition is important as these patients may be initially misdiagnosed with dermatologic conditions.

Hypomyopathic myositis represents the opposite spectrum—prominent myositis with minimal cutaneous manifestations, approaching the PM phenotype.

**Immune checkpoint inhibitor-associated myositis