Idiopathic Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of systemic autoimmune conditions characterised by immune-mediated muscle inflammation, leading to progressive proximal muscle weakness. The major subtypes are dermatomyositis (DM), polymyositis (PM), immune-mediated necrotising myopathy (IMNM), anti-synthetase syndrome (ASS), and overlap myositis. IIM are rare conditions with significant morbidity, including interstitial lung disease (ILD), dysphagia, malignancy association, and cardiac involvement. Early diagnosis and prompt immunosuppressive treatment are essential to prevent irreversible muscle damage.

IIM are autoimmune conditions driven by aberrant innate and adaptive immune activation targeting skeletal muscle. The pathophysiology differs between subtypes, underpinning their distinct clinical features and treatment responses.

Dermatomyositis

• Type I interferon-driven innate immune response — plasmacytoid dendritic cell activation, interferon signature in blood and muscle
• Complement-mediated microangiopathy — perimysial capillary deposition of complement membrane attack complex (MAC) → perifascicular atrophy
• Perivascular CD4+ T cell and B cell infiltrate
• Myositis-specific antibodies (MSAs) are central to subclassification — anti-MDA5, anti-TIF1γ, anti-Mi-2, anti-NXP2 each define distinct clinical phenotypes

Polymyositis / Overlap Myositis

• CD8+ cytotoxic T cell-mediated invasion of non-necrotic muscle fibres expressing MHC class I
• T cell-driven endomysial inflammation
• PM is now considered rare — most cases previously labelled PM represent IMNM or ASS with re-analysis

Immune-Mediated Necrotising Myopathy (IMNM)

• Anti-HMGCR antibodies (often statin-triggered) or anti-SRP antibodies
• Direct antibody-mediated muscle fibre necrosis with macrophage infiltration
• Minimal lymphocytic inflammation — differs from DM/PM
• Severe CK elevation (often >5000 IU/L), profound weakness; statin cessation alone insufficient — requires aggressive immunosuppression

Anti-Synthetase Syndrome (ASS)

• Autoantibodies against aminoacyl-tRNA synthetases — anti-Jo-1 most common (~70% of ASS); others: anti-PL-7, anti-PL-12, anti-EJ, anti-OJ
• Classic triad: ILD + inflammatory arthritis + myositis; mechanic's hands common
• ILD often the dominant and most dangerous feature — UIP, NSIP, or OP pattern

IIM present with a combination of muscle, skin, pulmonary, articular, and systemic features. The clinical phenotype varies significantly between subtypes and is closely linked to myositis-specific antibody (MSA) profile.

Core Muscle Features

Dermatomyositis — Skin Features

• Heliotrope rash — violaceous/purple discolouration of upper eyelids, often with periorbital oedema; pathognomonic
• Gottron's papules — erythematous papules/plaques over MCP, PIP, DIP joints, elbows, knees; pathognomonic
• Gottron's sign — macular erythema over extensor surfaces without papules
• V-sign (anterior chest/neck), shawl sign (posterior shoulders/neck)
• Periungual nailfold capillary changes — dilated loops, haemorrhages
• Amyopathic dermatomyositis (ADM) — classic DM skin features with no or minimal clinical myositis; but can have severe ILD (especially anti-MDA5)

Anti-Synthetase Syndrome Features

• Mechanic's hands — hyperkeratosis, fissuring, and scaling of the lateral fingers and palmar surfaces; non-pruritic
• Interstitial lung disease (ILD) — often the presenting feature; can be rapidly progressive (especially anti-MDA5)
• Inflammatory arthritis — typically non-erosive, small joint polyarthritis
• Raynaud's phenomenon common
• Fever at disease onset

Investigation of suspected IIM is multifaceted — confirming muscle inflammation, identifying the specific MSA subtype, excluding mimics, screening for malignancy, and assessing for systemic complications including ILD.

• Essential
  Creatinine kinase (CK)

  Elevated in most IIM — severity ranges from mildly elevated (DM) to >50,000 (IMNM). Normal CK does not exclude DM, especially amyopathic DM.
• Essential
  Myositis-specific antibody (MSA) panel

  Includes anti-Jo-1, anti-MDA5, anti-TIF1γ, anti-Mi-2, anti-NXP2, anti-SRP, anti-HMGCR, anti-PL-7, anti-PL-12. Defines clinical subtype, prognosis, and guides malignancy/ILD screening.
• Essential
  Myositis-associated antibodies (MAA)

  Anti-Ro52, anti-U1RNP, anti-PM-Scl — associated with overlap syndromes. Often included in extended MSA panel.
• Essential
  ANA (antinuclear antibody)

  Positive in majority of IIM — non-specific; important for overlap syndromes.
• Essential
  Aldolase, LDH, AST, ALT

  Elevated in muscle disease — differentiate from hepatic cause; AST/ALT may be falsely elevated from muscle, not liver.
• Essential
  FBC, UEC, LFTs, ESR, CRP

  Baseline inflammatory markers; assess organ function prior to immunosuppression. CRP may be low in DM despite active disease.
• Essential — if MSA positive or dyspnoea
  HRCT chest

  Screen for ILD in all IIM. Patterns: NSIP (most common), UIP, OP. Mandatory if anti-Jo-1, anti-MDA5, anti-PL-7, or any ASS antibody positive.
• Recommended
  MRI muscle (thigh/pelvis protocol)

  Detects active muscle inflammation (oedema on STIR sequences), guides biopsy site, monitors disease activity and differentiates active myositis from damage.
• Essential — DM at diagnosis
  Malignancy screening

  CT chest/abdomen/pelvis; age/sex appropriate cancer screening (colonoscopy, mammography, Pap smear, PSA). Anti-TIF1γ and anti-NXP2 antibodies highest malignancy risk.
• Often required
  Muscle biopsy

  Gold standard for diagnosis — required when MSA negative or diagnosis uncertain. Pathological patterns differ by subtype: MAC deposition (DM), CD8 endomysial infiltrate (PM), necrosis with macrophages (IMNM).
• Recommended
  EMG/nerve conduction studies

  Myopathic pattern on EMG (small, polyphasic, short-duration motor units); helps exclude neurogenic causes; rarely required when MSA positive and CK elevated.
• Recommended
  Echocardiogram

  Cardiac involvement in IIM — myocarditis, conduction abnormalities, pericarditis. Particularly relevant in systemic disease.

Prognosis in IIM is strongly influenced by MSA subtype, presence of ILD, malignancy association, and severity of muscle involvement at presentation. Early recognition of poor prognostic features guides treatment intensity.

Poor Prognostic Features

• Dysphagia or respiratory muscle weakness at presentation
• Rapidly progressive ILD (particularly anti-MDA5)
• Malignancy-associated IIM (anti-TIF1γ, anti-NXP2)
• Cardiac involvement (myocarditis)
• Delayed diagnosis and treatment
• IMNM subtype (anti-SRP, anti-HMGCR) with severe weakness
• Failure to achieve CK normalisation within 3 months of treatment

Pharmacological management of IIM follows a treat-to-target approach with high-dose corticosteroids as induction therapy and steroid-sparing immunosuppressive agents for maintenance. Early introduction of steroid-sparing agents reduces corticosteroid toxicity.

Corticosteroids — Induction Therapy

IV Methylprednisolone — Severe or Rapidly Progressive Disease

Steroid-Sparing Agents — Maintenance

IVIG — Refractory Disease and Dysphagia

Tacrolimus — Anti-MDA5 and Severe ILD

Rituximab — Refractory IIM

Directed therapy in IIM addresses the specific MSA-defined subtype and systemic complications. The two most important directed treatment considerations are: (1) management of ILD, and (2) malignancy screening and treatment in malignancy-associated DM.

ILD Management in IIM

• HRCT chest at diagnosis — mandatory for all IIM with positive anti-Jo-1, anti-MDA5, anti-PL-7, or any ASS antibody
• Pulmonary function tests (PFTs) — FVC, DLCO at baseline and every 3–6 months
• Pulmonology co-management for all IIM-ILD
• Anti-MDA5 rapidly progressive ILD — requires urgent combined immunosuppression (triple therapy: high-dose prednisolone + tacrolimus + MMF/cyclophosphamide)
• Cyclophosphamide IV pulse for rapidly progressive ILD unresponsive to above
• Pirfenidone/nintedanib — antifibrotic agents under investigation for IIM-ILD; not standard first-line therapy
• Supplemental oxygen if hypoxic; monitor for respiratory failure

Malignancy Screening — Dermatomyositis

• All adult DM patients require thorough malignancy screening at diagnosis
• Anti-TIF1γ and anti-NXP2 antibody positivity carries highest malignancy risk (ovarian, breast, GI, lung)
• Baseline: CT chest/abdomen/pelvis + PET-CT in high-risk patients
• Age/sex-appropriate screening: colonoscopy (if ≥45 years or symptoms), mammography, cervical smear, PSA
• Repeat malignancy screening annually for ≥3 years after DM diagnosis
• Treating underlying malignancy can lead to IIM remission in paraneoplastic cases

Dysphagia Management

• Speech pathology assessment and modified diet texture — all patients with dysphagia
• Videofluoroscopic swallow study (VFSS) if clinical dysphagia
• Nasogastric or PEG feeding if aspiration risk is high
• IVIG for acute severe dysphagia — rapid onset of benefit
• Aggressive immunosuppression — dysphagia is a poor prognostic feature and should drive treatment escalation

Statin Cessation in Anti-HMGCR IMNM

• Cease statin immediately on diagnosis of IMNM — but this alone is insufficient treatment
• Anti-HMGCR IMNM requires aggressive immunosuppression regardless of statin cessation
• Statins should not be restarted — alternative lipid-lowering therapy (ezetimibe, PCSK9 inhibitors) can be considered once disease is controlled

Non-pharmacological management in IIM focuses on rehabilitation, exercise, and monitoring of functional recovery. Unlike some autoimmune conditions, exercise is safe and beneficial in IIM and does not exacerbate inflammation.

Exercise Rehabilitation

• Graded resistance exercise is safe in stable IIM — does not worsen inflammation and improves muscle strength and function
• Physiotherapy referral — graduated resistance training program, initially supervised
• During active flares — gentle range-of-motion exercises; avoid high-intensity exercise until CK normalising
• Pool therapy/hydrotherapy — beneficial for joint and muscle function with low mechanical stress
• Monitor CK with exercise commencement — ensure no exacerbation of muscle inflammation

Swallowing and Nutritional Support

• Speech pathology — formal swallowing assessment, texture modification recommendations
• Dietitian — nutritional support, weight management, calcium/vitamin D requirements
• Ensure adequate protein intake to support muscle recovery and prevent corticosteroid-related muscle wasting

Respiratory Physiotherapy — ILD

• Breathing exercises and airway clearance techniques
• Pulmonary rehabilitation program for established ILD
• Smoking cessation — essential in all ILD patients
• Oxygen therapy if resting or exertional hypoxia

Corticosteroid Complication Prevention

• Calcium 1200 mg/day + vitamin D 1000 IU/day — all patients on prolonged corticosteroids
• Bisphosphonate therapy (alendronate or risedronate) if DEXA T-score <−1.5 or prolonged steroids anticipated
• Blood pressure and glucose monitoring — corticosteroid-induced hypertension and diabetes
• Weight management — corticosteroid weight gain exacerbates myopathy
• Pneumocystis jirovecii pneumonia (PJP) prophylaxis — co-trimoxazole 480 mg/day when on ≥2 immunosuppressive agents or prednisolone >20 mg/day for >1 month

Monitoring in IIM tracks disease activity (CK, functional strength, PFTs), treatment response, and immunosuppression toxicity.

When to Refer

• Rheumatology: All newly suspected IIM — subspecialty-level diagnosis, MSA interpretation, and treatment required
• Pulmonology: All IIM with ILD or respiratory symptoms — co-management, PFT monitoring, antifibrotic consideration
• Speech pathology: All patients with dysphagia — swallowing assessment and dietary modification
• Oncology: If malignancy identified — particularly in DM where treatment of malignancy may treat myositis
• Physiotherapy: All patients — graduated rehabilitation program
• Dermatology: If skin lesions require biopsy confirmation or DM skin management is complex

IIM present unique management challenges across several populations.

Juvenile Dermatomyositis (JDM)

• Most common IIM in children — predominantly DM phenotype; PM is rare in children
• Key difference from adult DM: malignancy association is rare in JDM
• Calcinosis (dystrophic calcium deposits in skin and muscle) — more common in JDM; painful and disfiguring; hard to treat
• Vasculopathy more prominent — skin ulceration, digital infarcts
• Treatment: high-dose corticosteroids + methotrexate (paediatric standard of care); IVIG for refractory cases
• Paediatric rheumatology referral — JDM management is subspecialty-level

IIM in Pregnancy

• IIM can flare during pregnancy; new-onset IIM in pregnancy requires urgent specialist management
• Prednisolone is safe in pregnancy at lowest effective dose
• Azathioprine — considered relatively safe in pregnancy; used when necessary
• Methotrexate and MMF — absolutely contraindicated in pregnancy; must be ceased ≥3 months before conception
• IVIG — safe in pregnancy; can be used for flare management
• Anti-Ro/SSA positivity — risk of neonatal lupus/heart block; cardiac monitoring of fetus

Elderly Patients with IIM

• Higher malignancy risk — particularly in new-onset DM after age 60
• Corticosteroid toxicity more pronounced — infection risk, osteoporosis, hyperglycaemia, delirium
• Frailty — impairs rehabilitation and functional recovery
• Polypharmacy — increased drug interaction risk with immunosuppressants
• Start corticosteroids at lower initial dose if very frail; escalate as tolerated

IIM with Overlap CTD

• IIM frequently overlaps with RA, SLE, systemic sclerosis (SSc), and Sjögren's syndrome
• Anti-PM-Scl antibodies — SSc-myositis overlap; ILD common
• Anti-U1RNP — mixed connective tissue disease (MCTD) overlap
• Treatment must address both conditions; hydroxychloroquine useful in SLE-overlap myositis
• Rheumatology management essential — overlap syndromes are complex

IIM management requires careful stewardship to balance the significant risks of under-treatment (permanent muscle damage, respiratory failure, malignancy) against the risks of over-treatment (immunosuppression toxicity).

GP Role

• Recognise IIM clinical syndrome — proximal weakness + elevated CK + skin features (DM)
• Initial workup: CK, MSA panel, ANA, FBC, UEC, LFTs before specialist review
• Expedite rheumatology referral — IIM is a serious condition requiring specialist management
• Manage corticosteroid complications — osteoporosis prophylaxis, glucose monitoring, infection surveillance
• Coordinate malignancy screening in DM — CT chest/abdomen/pelvis + age-appropriate cancer screening
• Maintain long-term medication monitoring once stable — FBC, LFTs 3-monthly

Follow-up in IIM is long-term and requires coordination between rheumatology, pulmonology, and general practice. Most patients require maintenance immunosuppression for years.

Remission and Medication Cessation

• IIM remission — CK normal, no active weakness, stable PFTs, no skin activity; may be achieved in ~50% at 3 years
• Medication cessation — attempt only after sustained remission (≥2 years); slow taper of steroid-sparing agent; high relapse risk if stopped prematurely
• Patients with ILD rarely achieve complete disease cessation — maintenance MMF/azathioprine typically continued

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