Necrotising Autoimmune Myopathy (NAM)

Necrotising autoimmune myopathy (NAM) — also termed immune-mediated necrotising myopathy (IMNM) — is a relatively recently characterised inflammatory myopathy that has been formally distinguished from dermatomyositis, polymyositis, and inclusion body myositis in the 2017 European Neuromuscular Centre (ENMC) international classification. NAM is defined by a unique clinicopathological triad: severe proximal muscle weakness, markedly elevated serum creatine kinase (CK), and a muscle biopsy demonstrating prominent myofibre necrosis and regeneration with strikingly sparse lymphocytic inflammatory infiltrate.

The identification of disease-specific autoantibodies — anti-HMGCR (3-hydroxy-3-methylglutaryl-coenzyme A reductase) and anti-SRP (signal recognition particle) — has refined diagnostic accuracy and improved understanding of pathogenic mechanisms. These antibodies are now considered central to the diagnosis and subclassification of NAM, with seronegative cases representing approximately 10–20% of all presentations.

Epidemiology in Australia

NAM is a rare condition with an estimated annual incidence of 1–2 per million population globally. Australian-specific incidence data are limited, though extrapolation from international registries suggests approximately 25–50 new cases per year nationally. Key epidemiological observations in the Australian context include:

• Anti-HMGCR NAM is the most common subtype in Australia, consistent with the widespread use of statin therapy (approximately 2.7 million Australians are prescribed statins through the PBS).
• Anti-SRP NAM accounts for approximately 20–30% of NAM cases and appears to have no significant racial predilection in Australian cohorts.
• Statin-naïve anti-HMGCR NAM has been reported in children and young adults, with some evidence suggesting increased susceptibility in individuals of Southeast Asian descent — relevant given Australia's multicultural population.
• Anti-SRP NAM has been associated with cardiac involvement including myocarditis in up to 15–25% of cases, a complication with significant morbidity requiring specialist cardiology input.
• The condition affects adults across a wide age range (20–80 years), with anti-HMGCR cases peaking in the 5th–7th decades (correlating with statin exposure), while anti-SRP cases may present at younger ages.

Australia's universal healthcare system facilitates access to the specialist investigations required for NAM diagnosis (anti-HMGCR and anti-SRP serology available through major immunology laboratories, and PBS-subsidised access to IVIG and rituximab), though delays in diagnosis remain a significant concern — the median time from symptom onset to NAM diagnosis in published series is 3–6 months.

NAM is a humoural (antibody)-mediated myopathy in which autoantibodies directed against intracellular muscle antigens drive complement activation and myocyte necrosis. This contrasts with dermatomyositis (microangiopathy) and polymyositis (T-cell-mediated cytotoxicity). Two pathogenic antibody systems have been identified:

Anti-HMGCR Antibodies

• Anti-HMGCR antibodies are detected in approximately 60% of NAM cases in Australian tertiary referral series.
• The HMGCR enzyme is the pharmacological target of statin drugs (HMG-CoA reductase inhibitors).
• In statin-exposed patients, regenerating muscle fibres overexpress HMGCR, presenting an immunogenic target. Anti-HMGCR antibodies bind to surface-expressed HMGCR on regenerating fibres and activate the classical complement pathway, culminating in C5b-9 membrane attack complex (MAC) deposition and myofibre necrosis.
• Statin-naïve anti-HMGCR NAM occurs in approximately 30–40% of anti-HMGCR-positive patients, more commonly in children, young adults, and individuals of non-Caucasian ethnicity. The mechanism in this group is less well understood but may involve genetic polymorphisms in HMGCR expression regulation.
• HLA associations: HLA-DRB1*11:01 is strongly associated with anti-HMGCR NAM (odds ratio approximately 3.6), suggesting a genetic susceptibility component.

Anti-SRP Antibodies

• Anti-SRP antibodies target the 54 kDa subunit of the signal recognition particle, a ribonucleoprotein complex involved in co-translational protein translocation across the endoplasmic reticulum.
• Anti-SRP NAM is statin-independent — there is no established association between statin exposure and anti-SRP antibody development.
• Anti-SRP antibodies activate complement via both the classical and alternative pathways, leading to MAC deposition on the myocyte surface and necrosis.
• Anti-SRP NAM is generally considered more severe than anti-HMGCR disease, with higher presenting CK levels, more rapid progression, greater treatment resistance, and higher rates of extramuscular complications including cardiac involvement (myocarditis, conduction abnormalities).
• HLA associations: HLA-DRB1*08:03 has been identified in Japanese anti-SRP cohorts; the Australian HLA association is not well characterised.

Seronegative NAM

• Approximately 10–20% of patients with clinicopathological features consistent with NAM are negative for both anti-HMGCR and anti-SRP antibodies on standard testing.
• Seronegative NAM remains a histopathological diagnosis — the biopsy must demonstrate necrotic and regenerating fibres with minimal inflammation and MAC deposition to support the diagnosis.
• Emerging antibodies under investigation include anti-Mi2, anti-NXP2, and anti-SAE, though these are typically associated with dermatomyositis rather than pure NAM.
• Seronegative NAM should be managed with the same aggressive immunotherapy approach as seropositive disease.

Shared Pathogenic Mechanisms

Both anti-HMGCR and anti-SRP antibodies share a final common pathway of complement-mediated myocyte injury. Key mechanistic steps include:

1. Autoantibody binding to intracellular target antigen (HMGCR on regenerating fibres surface; SRP on myocyte surface)
2. Classical complement pathway activation (C1q binding to antibody Fc regions)
3. C3 convertase formation and C3b amplification loop
4. C5 convertase generation → C5a release (pro-inflammatory) → C5b-9 MAC assembly on myocyte membrane
5. MAC pore formation → osmotic lysis → myofibre necrosis
6. Satellite cell activation → regenerating fibres with HMGCR upregulation (perpetuating anti-HMGCR disease)

This complement-mediated mechanism provides the rationale for IVIG (complement scavenging and anti-idiotypic antibody effects) and rituximab (B-cell depletion reducing autoantibody production) as therapeutic strategies.

Clinical Presentation

NAM typically presents with subacute to chronic progressive proximal muscle weakness, developing over weeks to months. The clinical phenotype differs between the two major subtypes:

Characteristic Features

• Proximal weakness: Difficulty rising from chairs, climbing stairs, lifting arms above head, combing hair. Proximal upper and lower limbs are affected symmetrically.
• Severity: Weakness is often severe at presentation — many patients have significant functional impairment (MRC grade 3–4/5 in proximal muscles).
• Dysphagia: Occurs in 20–50% of cases; may be the presenting feature; carries aspiration risk — assess with speech pathology referral.
• Fatigue: Profound and often disproportionate to the degree of weakness.
• Myalgia: Present in approximately 30–50% but is not a predominant feature; patients more commonly report weakness than pain.
• Respiratory involvement: Respiratory muscle weakness (diaphragm, intercostals) may occur, particularly in anti-SRP NAM; monitor forced vital capacity (FVC).
• No skin involvement: The absence of rash (heliotrope, Gottron's papules, mechanic's hands) distinguishes NAM from dermatomyositis and is a critical diagnostic feature.

Diagnostic Criteria

Differential Diagnosis

Laboratory Investigations

Treatment Principles

1. Combination immunotherapy is required — monotherapy with corticosteroids alone has a high relapse rate.
2. Initiate treatment early — delay correlates with irreversible muscle damage and fibrosis.
3. Anti-SRP NAM generally requires more aggressive upfront therapy than anti-HMGCR disease.
4. CK normalisation is the primary treatment target; complete clinical remission (normal strength + normal CK) is the long-term goal.
5. Treatment duration is prolonged — minimum 2 years of sustained remission before cautious tapering; many patients require treatment for 5+ years.
6. Premature discontinuation carries a 40–60% relapse rate.

First-Line Induction Therapy

Steroid-Sparing Agents (Commence within 4–8 Weeks)

A steroid-sparing agent should be started early to facilitate corticosteroid tapering and reduce cumulative steroid toxicity. Choice depends on patient factors including reproductive plans, hepatic and renal function, and comorbidities.

Second-Line / Refractory Disease — Rituximab

Treatment Response Monitoring

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