Autoimmune Necrotizing Myopathy
Autoimmune necrotizing myopathy is an inflammatory myopathy characterized by prominent myofiber necrosis with minimal inflammatory cell infiltration, often associated with anti-SRP or anti-HMGCR antibodies, leading to progressive proximal muscle weakness.
Autoimmune Necrotizing Myopathy (AINM / IMNM)
Autoimmune necrotizing myopathy (ANM) — also called immune-mediated necrotizing myopathy (IMNM) — is a subtype of the idiopathic inflammatory myopathies (IIM) characterised histopathologically by prominent myofibre necrosis and regeneration with minimal or absent inflammatory cell infiltration in skeletal muscle [1][2]. Unlike classical polymyositis or dermatomyositis where T-cell or perivascular inflammatory infiltrates dominate, the hallmark of IMNM is antibody- and complement-mediated destruction of muscle fibres rather than a cell-mediated autoimmune attack.
Breaking down the name:
- Auto-immune = the immune system attacks "self" tissue
- Necrotizing (Greek nekros = death) = myofibre death/necrosis is the dominant pathological finding
- Myopathy (Greek myo = muscle, pathos = disease) = disease of muscle
This distinction is clinically important because IMNM tends to be more severe, more refractory to treatment, and has a different autoantibody profile compared to other inflammatory myopathies [1][2].
Key Distinction from Polymyositis
IMNM was historically lumped under "polymyositis," but it is now recognised as a separate entity. On muscle biopsy, polymyositis shows endomysial CD8+ T-cell infiltrates surrounding and invading non-necrotic fibres, whereas IMNM shows scattered necrotic and regenerating fibres with macrophage predominance and sparse lymphocytic infiltration. This has direct therapeutic implications — IMNM often requires more aggressive immunosuppression [1][2].
| Parameter | Detail |
|---|---|
| Incidence | ~20% of all IIM cases; rare overall (IIM incidence ~2–10/million/year) |
| Age of onset | Bimodal — can occur at any age but peaks in adults 40–60 years; anti-SRP+ tends to present younger (30–50 years), anti-HMGCR+ older (>50 years) |
| Sex | Female predominance overall (F:M ≈ 2:1); anti-HMGCR+ IMNM shows less female predominance |
| Geography | Anti-HMGCR+ IMNM is increasingly recognised worldwide due to widespread statin use. In Hong Kong, statin prescriptions are very common for the ageing population, making this a relevant clinical entity |
| Racial variation | Anti-SRP+ IMNM may be more common and more severe in African Americans |
Risk Factors
-
Statin use — the strongest and most clinically important association
- Statins (HMG-CoA reductase inhibitors) can trigger anti-HMGCR antibody production
- Mechanism: statins upregulate HMGCR expression in regenerating myofibres → HMGCR becomes an autoantigen → autoimmune attack perpetuates even after statin withdrawal
- Not all statin myopathy is IMNM — most statin-related muscle symptoms are self-limiting toxic myopathy that resolves on discontinuation. IMNM is the autoimmune form that persists
- ~60–70% of anti-HMGCR+ IMNM patients have prior statin exposure, but 30–40% develop it without ever taking statins (particularly younger patients and children)
-
Malignancy — IMNM can be cancer-associated, particularly in seronegative IMNM and in older patients [1][2]
-
Connective tissue disease overlap — can occur in the context of other autoimmune conditions
-
Genetic predisposition — HLA-DRB111:01 associated with anti-HMGCR+ IMNM; HLA-DRB108:03 associated with anti-SRP+ IMNM
Hong Kong context: Given the high prevalence of statin prescriptions for dyslipidaemia and cardiovascular risk reduction in our ageing population, and the local prevalence of NPC (nasopharyngeal carcinoma), clinicians in HK should have a low threshold for considering IMNM in a patient on statins who develops persistent proximal weakness, and should perform NPC screening (including EBV serology and nasopharyngoscopy) as part of the malignancy workup [2].
3. Anatomy and Function: The Skeletal Muscle
Understanding why IMNM presents as it does requires understanding normal skeletal muscle anatomy:
- Motor unit = a lower motor neuron + all the muscle fibres it innervates
- Muscle fibres are organised into fascicles (bundles wrapped in perimysium), and each fibre is surrounded by endomysium
- Each muscle fibre (cell) is a multinucleated syncytium with peripherally located nuclei
- The sarcolemma (cell membrane) contains ion channels, receptors, and structural proteins (dystrophin, etc.)
- Inside: myofibrils composed of repeating sarcomeres (actin + myosin = the contractile machinery)
- Proximal muscles (shoulder girdle, hip girdle) are larger, more metabolically active, and have higher energy demands
- They are therefore more vulnerable to:
- Inflammatory/immune-mediated damage
- Metabolic insults
- Ischaemic injury from capillary dropout
- This is why patients struggle with tasks requiring proximal strength: climbing stairs, rising from a chair, combing hair, reaching overhead
- Creatine kinase (CK) is the most abundant intracellular enzyme in skeletal muscle
- When myofibres are damaged (necrosis), CK leaks into the bloodstream → CK is typically markedly elevated in IMNM, often > 10× ULN, sometimes > 50× ULN [1][2]
- Other enzymes: LDH, AST, ALT, aldolase also leak out (AST/ALT elevation may be mistakenly attributed to liver disease)
4. Aetiology and Pathophysiology
IMNM is classified by serological subtype (based on autoantibodies), which is the most clinically useful framework:
| Subtype | Autoantibody | Approximate Proportion | Key Associations |
|---|---|---|---|
| Anti-SRP+ IMNM | Anti-signal recognition particle (SRP) | ~30–40% of IMNM | More severe, refractory disease; younger onset; cardiac involvement; aggressive disease refractory to high-dose steroids and immunosuppressants [3] |
| Anti-HMGCR+ IMNM | Anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) | ~30–40% of IMNM | Statin exposure in majority; older patients; can occur without statins |
| Seronegative IMNM | Neither anti-SRP nor anti-HMGCR | ~20–30% of IMNM | Higher malignancy association; may overlap with other CTD; more heterogeneous |
High Yield — Autoantibody-defined Subsets
Anti-SRP antibody defines a subset with severe myopathy with muscle fibre necrosis / endomysial fibrosis with minimal inflammatory infiltrates on histology, and aggressive disease refractory to high-dose steroids and immunosuppressants [3].
Anti-HMGCR antibody identifies statin-triggered autoimmune myopathy — the disease does not resolve with statin cessation (unlike toxic statin myopathy) because the autoimmune process has become self-perpetuating.
Note that autoAb defines homogeneous clinical subsets of disease [3].
4.2 Pathophysiology — From First Principles
Anti-HMGCR+ pathway:
- Patient takes a statin → statin inhibits HMG-CoA reductase (HMGCR) in hepatocytes (intended therapeutic effect)
- Statins also cause upregulation of HMGCR protein expression in muscle fibres (compensatory response to enzyme inhibition)
- In genetically susceptible individuals (HLA-DRB1*11:01), the overexpressed HMGCR is presented as a neoantigen by MHC class II → loss of immune tolerance → generation of anti-HMGCR autoantibodies
- These antibodies bind HMGCR on the sarcolemma of muscle fibres → activate complement → membrane attack complex (MAC) formation on myofibres → myofibre necrosis
- Necrotic fibres undergo regeneration → regenerating fibres express even more HMGCR → more antigen presentation → self-amplifying cycle of destruction and regeneration
- This is why stopping the statin does not stop the disease — the autoimmune loop is self-perpetuating
Anti-SRP+ pathway:
- Signal recognition particle (SRP) is an intracellular ribonucleoprotein involved in targeting nascent proteins to the endoplasmic reticulum
- Mechanism of loss of tolerance to SRP is less well understood — possibly triggered by viral infection or other immune activation events
- Anti-SRP antibodies may directly bind muscle fibres, activate complement, and cause severe necrosis
- Histologically: severe myopathy with muscle fibre necrosis / endomysial fibrosis with minimal inflammatory infiltrates [3]
The dominant effector mechanism in IMNM is humoral (antibody-mediated) rather than cell-mediated:
Key pathological features explained:
- Necrotic fibres: pale, eosinophilic, loss of cross-striation → direct complement-mediated killing
- Regenerating fibres: basophilic, centrally located nuclei → attempted repair
- Macrophage-predominant infiltration: macrophages phagocytose necrotic debris (this is clean-up, not the primary attack)
- Sparse T-cell infiltrates: this distinguishes IMNM from polymyositis where CD8+ T cells are the primary effectors
- MAC (C5b-9) deposition on non-necrotic fibres: visible on immunohistochemistry — this is a characteristic finding
- Capillary involvement: anti-SRP+ IMNM may show pipestem capillaries (capillary wall thickening) but not the perifascicular atrophy seen in dermatomyositis
If untreated or refractory:
- Repeated cycles of necrosis → regeneration → necrosis
- Eventually fatty replacement and fibrosis of muscle (seen on MRI as T1 hyperintensity)
- Irreversible weakness if treatment is delayed
- This is why early diagnosis and aggressive treatment are essential
Why IMNM Has Very High CK Levels
Because the pathology is direct myofibre necrosis (sarcolemma rupture by MAC), CK leaks massively. In contrast, in dermatomyositis where the primary target is intramuscular vasculature (causing ischaemic damage), CK may be only moderately elevated. In IBM, CK is often only mildly elevated because the process is slow and chronic. IMNM typically has the highest CK among all IIMs (often > 5,000–10,000 U/L, sometimes > 50,000 U/L).
5. Classification
Idiopathic inflammatory myopathies are classified as [1][2]:
| Subtype | Key Features |
|---|---|
| Dermatomyositis (DM) | Characteristic skin rash + proximal weakness; perifascicular atrophy on biopsy; anti-Mi2, anti-MDA5, anti-NXP2, anti-TIF1γ |
| Polymyositis (PM) | Proximal weakness without skin rash; endomysial CD8+ T-cell infiltration; diagnosis of exclusion (increasingly rare as a standalone diagnosis) |
| Inclusion body myositis (IBM) | Very rare in HK; proximal + distal muscle weakness; failed response to treatment; Dx inclusion bodies in muscle biopsy [1] |
| Autoimmune necrotizing myopathy (IMNM/AINM) | Prominent necrosis, minimal inflammation; anti-SRP or anti-HMGCR; often severe and refractory |
| Overlap myositis | When a/w collagen vascular disease [2]; anti-synthetase syndrome now often classified here |
| Cancer-associated myositis (CAM) | Can occur with any IIM subtype; DM > PM in malignancy risk |
| Clinically amyopathic dermatomyositis (CADM) | Cutaneous manifestations, but no muscle weakness for ≥6 months [1]; anti-MDA5 |
High Yield — GC Lecture Classification
Brief introduction on the subtypes of IIM: Dermatomyositis, Polymyositis, Anti-synthetase syndrome, Immune-mediated necrotizing myopathy [4]. This is from the GC rheumatology interactive tutorial and represents the examinable classification framework.
| Feature | Anti-SRP+ IMNM | Anti-HMGCR+ IMNM | Seronegative IMNM |
|---|---|---|---|
| Age of onset | Younger (30–50 y) | Older (> 50 y) | Variable |
| Statin association | No | Yes (~60–70%) | No |
| CK level | Very high (often > 10,000) | High (often > 5,000) | Variable |
| Severity | Severe, refractory [3] | Moderate to severe | Variable |
| Cardiac involvement | More common | Less common | Variable |
| ILD | Uncommon | Rare | Uncommon |
| Malignancy risk | Low | Low–moderate (esp. if statin-naïve) | Highest among IMNM subtypes |
| Skin manifestations | Seldom have skin manifestations [3] | None | None |
| Response to treatment | Aggressive disease refractory to high-dose steroids and immunosuppressants [3] | Moderate response | Variable |
| Histology | Necrosis + fibrosis, minimal inflammation | Necrosis + regeneration, MAC deposition | Necrosis, macrophage infiltration |
6. Clinical Features
| Symptom | Pathophysiological Basis | Notes |
|---|---|---|
| Progressive proximal muscle weakness | Necrosis of proximal muscle fibres (shoulder/hip girdle) → loss of contractile units | Symmetrical weakness in shoulder girdle + hip girdle [1]; difficulty rising from chairs, climbing stairs, lifting arms overhead, combing hair |
| Rapid onset (days to weeks in severe cases) | Acute complement-mediated necrosis → rapid destruction | Faster progression than DM/PM in many anti-SRP+ cases |
| Myalgia (muscle pain) | Necrotic fibres release inflammatory mediators (prostaglandins, bradykinin) → nociceptor activation | ± pain (if acute, usually mild) [2]; pain is NOT a dominant feature — weakness is |
| Dysphagia | Bulbar muscle involvement [1] — pharyngeal and oesophageal striated muscle weakness → impaired swallowing | Dysphagia, dysphonia, HOV (hoarseness of voice) [1]; can lead to aspiration |
| Dysphonia / Hoarseness of voice | Weakness of laryngeal muscles | Less common but characteristic of severe disease |
| Dyspnoea | (1) Respiratory muscle weakness (diaphragm, intercostals) → hypoventilation; (2) Rare ILD (more common in anti-synthetase); (3) Cardiac involvement in anti-SRP+ | Ask about exertional dyspnoea, orthopnoea |
| Fatigue | Generalised muscle dysfunction + systemic inflammation | Non-specific but very common |
| Dark urine | Myoglobinuria from massive muscle necrosis (rhabdomyolysis) → myoglobin excreted in urine | "Cola-coloured urine" — indicates severe disease; risk of AKI |
| Arthralgia | Systemic autoimmune inflammation; overlap with CTD features | Less prominent than in antisynthetase syndrome |
What Patients Cannot Do — Functional Impact
- Cannot rise from a squat or low chair without using arms
- Cannot climb stairs
- Cannot lift arms above head (combing hair, reaching shelves)
- Cannot lift head off pillow (neck flexor weakness — a red flag for severity)
- In severe cases: cannot swallow (choking episodes), cannot breathe adequately
Difficulty in swallowing food with episodes of choking — this is a key clinical scenario from the GC interactive tutorial case [4].
| Sign | Pathophysiological Basis | Notes |
|---|---|---|
| Proximal muscle weakness on examination | Direct necrosis of proximal muscle groups | MRC grading: often grade 3–4/5 proximally; distal strength relatively preserved (unlike IBM) |
| Neck flexor weakness | Anterior neck flexors are a "semi-proximal" group frequently affected in IIM | Test: ask patient to lift head against resistance while supine — a bedside test for severity |
| Intact sensation | The disease targets muscle, not peripheral nerves | Normal sensory examination helps distinguish from neuropathy |
| Normal or depressed deep tendon reflexes | Reflex arc involves muscle contraction — weakened muscle produces weaker contraction; no UMN involvement | NOT brisk (would suggest UMN pathology) |
| No fasciculations | Fasciculations indicate LMN denervation (anterior horn cell disease); IMNM is a myopathic process | Helps distinguish from motor neuron disease |
| No skin rash | IMNM typically lacks the cutaneous features of dermatomyositis | Anti-SRP is almost exclusively described in polymyositis, i.e. seldom have skin manifestations [3]. No heliotrope rash, no Gottron's papules |
| Muscle wasting (late finding) | ± wasting/contractures (ONLY if chronic) [2] — chronic cycles of necrosis → fatty/fibrotic replacement | Not present early; indicates delayed diagnosis or refractory disease |
| Muscle tenderness | Acute necrotic fibres release inflammatory mediators | Variable; may be mild or absent |
| Raynaud's phenomenon | If overlap with CTD features; vasospasm of digital arteries → white (ischaemia) → blue (cyanosis) → red (reactive hyperaemia) | Pain and numbness of fingers on exposure to cold [4] |
| System | Manifestation | Mechanism | IMNM Subtype Most Affected |
|---|---|---|---|
| Cardiac | Myocarditis, cardiomyopathy, arrhythmias, heart failure | Antibody/complement-mediated damage to cardiac myocytes (which also express the target antigens) | Anti-SRP+ (more common) |
| Pulmonary | ILD (≥10%): a/w anti-Jo1 Ab (anti-synthetase) [3]; respiratory muscle weakness → hypoventilation | ILD: autoimmune alveolitis; hypoventilation: diaphragm weakness | ILD is uncommon in pure IMNM; more common in overlap/antisynthetase |
| Oesophageal | Dysphagia, nasal regurgitation, aspiration, aspiration pneumonia [3] | Striated muscle of upper oesophagus affected | All subtypes |
| Renal | AKI from rhabdomyolysis → myoglobin cast nephropathy | Massive CK elevation → myoglobinuria → tubular obstruction and injury | Severe anti-SRP+ cases |
| Joints | Arthralgia, non-erosive arthritis | Systemic autoimmune inflammation | Overlap cases |
High Yield — Cancer Association in IIM
Adult form a/w malignancy: 5× risk in dermatomyositis, 2× risk in polymyositis [2]. IMNM (particularly seronegative) also has an increased malignancy risk.
1/3 malignancy (lung, breast, gastric, NPC) – usually diagnosed within 1 year of DM/PM [1].
In Hong Kong, NPC screening is essential. Always think about this in any new IIM diagnosis.
Malignancy can occur before, with, or after onset of inflammatory myopathy [2][3].
7. Distinguishing IMNM from Other Causes of Proximal Weakness
This is critical for clinical approach. When you see a patient with proximal muscle weakness, the differential is broad:
| Feature | IMNM | Dermatomyositis | Polymyositis | IBM |
|---|---|---|---|---|
| Skin rash | Absent | Heliotrope, Gottron's, shawl/V sign [1] | Absent | Absent |
| CK level | Very high (often > 10×) | Moderate | Moderate–high | Normal–mildly elevated |
| Weakness pattern | Symmetric proximal | Symmetric proximal | Symmetric proximal | Proximal + distal [1] |
| Onset speed | Can be rapid | Subacute | Subacute | Insidious (years) |
| Biopsy | Necrosis + regeneration, minimal inflammation, MAC | Perifascicular atrophy, perivascular B/CD4+ T cells | Endomysial CD8+ T cells invading non-necrotic fibres | Rimmed vacuoles, inclusion bodies, endomysial CD8+ |
| Treatment response | Often refractory (esp. anti-SRP+) [3] | Usually responds | Usually responds | Failed response to treatment [1] |
| Autoantibodies | Anti-SRP, anti-HMGCR | Anti-Mi2, anti-MDA5, anti-TIF1γ, anti-NXP2 | Non-specific | None (anti-cN1A in some) |
8. Clinical Approach to a Patient with Suspected IMNM
-
Characterise the weakness:
- Onset and duration (acute/subacute/chronic?)
- Distribution (proximal vs distal vs both?)
- Progression (getting worse? stable? fluctuating?)
- Functional impact (stairs, rising from chair, combing hair, swallowing?)
-
Ask about associated symptoms:
- Skin rash? (→ DM)
- Dysphagia, choking? (→ bulbar involvement)
- Dyspnoea? (→ respiratory muscle weakness, ILD, cardiac)
- Dark urine? (→ rhabdomyolysis)
- Joint pain? Raynaud's? Dry eyes/mouth? (→ CTD overlap)
- Fever? Weight loss? (→ malignancy, antisynthetase)
-
Drug history [5]:
- Statins — most important! Which statin, duration, timing of onset relative to statin initiation
- Other myotoxic drugs: fibrates, colchicine, hydroxychloroquine, checkpoint inhibitors (pembrolizumab, nivolumab)
- Methotrexate lung → hypersensitivity reaction from csDMARD [5] (if patient is on immunosuppressants for other reasons)
- NSAIDs → GI bleeding [5] (relevant for anaemia workup)
-
Past medical history:
- Prior malignancy? (the GC case mentions history of breast cancer 5 years ago [4])
- Other autoimmune diseases?
- Family history of autoimmune or neuromuscular disease?
-
Cancer red flags (especially in older patients):
- Weight loss, night sweats, constitutional symptoms
- New lumps, bleeding, change in bowel habit
- In Hong Kong: epistaxis, nasal obstruction, hearing loss (NPC)
- General inspection: muscle bulk (wasting?), skin (rash? — should be absent in pure IMNM)
- Power testing (MRC grading):
- Neck flexors (lift head off pillow)
- Shoulder abduction and flexion (deltoid)
- Hip flexion (iliopsoas)
- Compare proximal vs distal
- Reflexes: normal or reduced (myopathic pattern)
- Sensation: should be intact
- Swallowing assessment: if dysphagia reported
- Respiratory: respiratory rate, use of accessory muscles, chest expansion
- Cardiovascular: signs of heart failure (if cardiac involvement)
- Full systemic examination: looking for malignancy, other CTD features
Conditions that are associated with Raynaud's phenomenon: connective tissue disease patients often present with multi-system complaints [4].
High Yield Summary
Autoimmune Necrotizing Myopathy (IMNM) — Key Points:
- Definition: Subtype of IIM characterised by myofibre necrosis with minimal inflammation on biopsy, distinct from PM/DM
- Pathophysiology: Antibody + complement-mediated (humoral), NOT T-cell mediated → MAC deposition → sarcolemma rupture → massive CK elevation
- Three subtypes by autoantibody: Anti-SRP+ (severe, refractory), Anti-HMGCR+ (statin-associated), Seronegative (highest malignancy risk)
- Clinical features: Symmetric proximal weakness, very high CK (often > 10× ULN), NO skin rash, may have dysphagia and cardiac involvement
- Statin association: Anti-HMGCR+ IMNM — statin upregulates HMGCR → neoantigen → autoimmune cycle that persists after statin withdrawal
- Cancer screening essential: Especially NPC in Hong Kong; malignancy can occur before, with, or after onset [2]
- Anti-SRP+: Aggressive disease refractory to high-dose steroids and immunosuppressants [3]
- Distinguish from: DM (skin rash), PM (CD8+ T-cell infiltrate on biopsy), IBM (distal weakness, non-responsive, inclusion bodies), toxic statin myopathy (resolves with statin cessation)
Active Recall - Autoimmune Necrotizing Myopathy
[1] Senior notes: Maksim Medicine Notes.pdf (Rheumatology — Idiopathic inflammatory myopathies, p.318) [2] Senior notes: Ryan Ho Neurology.pdf (10.4.3 Inflammatory Myopathies, p.194) [3] Senior notes: Ryan Ho Rheumatology.pdf (Autoantibody-defined subsets table, p.91) [4] Lecture slides: Block A - Rheumatology Interactive Tutorial.pdf (Case 2 — IIM subtypes and clinical presentations) [5] Senior notes: Block A - Hematology Interactive Tutorial.pdf (Drug history in RA patient, p.2)
Differential Diagnosis of Autoimmune Necrotizing Myopathy
When a patient walks into clinic — or, more likely, struggles to walk in — with progressive proximal muscle weakness and a markedly elevated CK, your job is not simply to label it "myopathy" and move on. You need a systematic framework to localise the lesion, stratify the differential, and then narrow it down. IMNM sits within a broader landscape of conditions that can mimic it. Let's work through this from first principles.
Before diving into the differential for IMNM specifically, recall that "weakness" can originate from any level of the neuraxis. The first clinical step is always localisation [6].
I learned to approach lower limb weakness by first localizing the lesion anatomically, asking whether it involves the cerebrum, brainstem, spinal cord, anterior horn cells, nerve roots, peripheral nerves, neuromuscular junction, or muscle. Key discriminating features include distribution of weakness, presence of upper versus lower motor neuron signs, sensory involvement, sphincter disturbances, and cortical signs [6].
The key "red line" for IMNM is: proximal weakness + very high CK + NO skin rash + NO sensory loss + NO UMN signs + biopsy showing necrosis without significant inflammation.
2. Differential Diagnosis of IMNM — Systematic Categorisation
The differential can be organised using the classic "VITAMIN-CDE" sieve adapted for myopathy, as presented across multiple sources [7][8][9]:
High Yield — GC Lecture Slide: Differential Diagnosis of Myopathy
From the GC neurology lecture [7]:
| Category | Examples |
|---|---|
| Infective | Viral (HIV, CMV, EBV…), pyomyositis |
| Neoplastic | Paraneoplastic |
| Inflammatory | Rheumatoid arthritis, Sjögren's syndrome |
| Congenital | Muscular dystrophy |
| Autoimmune | Dermatomyositis, necrotising autoimmune myositis |
| Trauma / toxin | Crush injuries / seizures causing rhabdomyolysis, glucocorticoids, colchicine, statins |
| Endocrine | Hypothyroidism, Cushing's syndrome, hypokalemia |
This is a high-yield exam table — know this categorisation.
Let me now expand each category, explaining why each mimics IMNM and how to distinguish them.
2A. Other Idiopathic Inflammatory Myopathies (IIMs)
These are the closest mimics because they share the core feature of autoimmune proximal weakness with elevated CK.
| Feature | How It Differs from IMNM |
|---|---|
| Cutaneous manifestations — usually appear before myopathy [1] | IMNM has NO skin rash. DM has heliotrope rash, Gottron's papules, shawl sign and V sign, mechanic's hands, holster's sign, calcinosis cutis, dilated nailfold capillaries [1] |
| CK | Moderate elevation (often < 10× ULN) vs IMNM's very high CK |
| Biopsy | Perifascicular atrophy, perivascular B cells and CD4+ T cells, complement deposition on capillaries [9] vs IMNM's necrosis with minimal inflammation |
| Autoantibodies | Anti-Mi2 (classic DM, good prognosis), anti-MDA5 (CADM + rapidly progressive ILD), anti-TIF1γ/anti-NXP2 (cancer-associated) vs anti-SRP/anti-HMGCR in IMNM |
| Cancer risk | 5× risk in dermatomyositis [2][3] — higher than IMNM overall |
Why the confusion arises: Both cause symmetric proximal weakness. But if there is NO rash, you cannot call it DM. The old habit of labelling everything without rash as "polymyositis" has led to many IMNM cases being misclassified.
| Feature | How It Differs from IMNM |
|---|---|
| Biopsy | Cellular infiltrate is predominantly within the fascicle with inflammatory cells invading individual muscle fibers; ↑ Cytotoxic CD8+ T lymphocytes; NO signs of vasculopathy [9] — this is a T-cell mediated attack on individual fibres. IMNM is antibody/complement-mediated necrosis |
| CK | Moderate–high, but rarely as extreme as IMNM |
| Treatment response | Generally responds to steroids better than anti-SRP+ IMNM |
| Current status | PM as a standalone entity is increasingly questioned; many previously labelled PM cases are now reclassified as IMNM, IBM, or anti-synthetase syndrome |
Why the confusion arises: Before myositis-specific antibodies were available, anything without a rash was called PM. Now, muscle biopsy and autoantibodies allow proper reclassification.
| Feature | How It Differs from IMNM |
|---|---|
| Age | Typically >50 years, male predominance |
| Weakness pattern | Proximal + distal muscle weakness [1] — specifically finger flexors (grip weakness) and quadriceps → asymmetric! This is very different from IMNM's purely proximal symmetric weakness |
| CK | Normal to mildly elevated (rarely > 5× ULN) — much lower than IMNM |
| Biopsy | Rimmed vacuoles, inclusion bodies [1], endomysial CD8+ T-cell infiltrates PLUS degenerative features |
| Treatment | Failed response to treatment [1] — does NOT respond to immunosuppression, unlike IMNM which at least partially responds |
| Onset | Insidious (years) — IBM creeps up over months to years, unlike IMNM which can be subacute [8] |
IBM vs IMNM — A Common Exam Trap
IBM is the inflammatory myopathy commonly misdiagnosed as PM [8]. Students often confuse IBM with IMNM because both lack skin rash. The distinguishing features are: IBM has distal involvement (finger flexors, quadriceps), low CK, insidious onset, rimmed vacuoles on biopsy, and does NOT respond to immunosuppression. IMNM has purely proximal weakness, very high CK, and necrosis without vacuoles on biopsy.
| Feature | How It Differs from IMNM |
|---|---|
| Autoantibody | Anti-Jo1 (most common), anti-PL7, anti-PL12, anti-EJ, anti-OJ |
| Clinical features | Classic triad/pentad: myositis + ILD + mechanic's hands + Raynaud's + non-erosive arthritis + fever [3] |
| ILD | Very common (>70%) and may be the dominant feature — vs ILD being uncommon in pure IMNM |
| Biopsy | Perifascicular necrosis + perimysial inflammation — differs from IMNM's scattered necrosis |
| Skin | Mechanic's hands: hyperkeratotic, fissured skin on palmar and lateral aspects of fingers [3] |
Why the confusion arises: Anti-synthetase syndrome involves myositis, so CK is elevated and there is proximal weakness. But the prominent ILD, mechanic's hands, and arthritis set it apart.
- When a/w collagen vascular disease [2] — the myositis occurs in context of SLE, systemic sclerosis (SSc), MCTD, RA, or Sjögren's
- Distinguished by features of the underlying CTD (malar rash for SLE, skin thickening for SSc, etc.)
- Myositis-associated autoantibodies (suggestive of other CTDs): anti-Ro, anti-La, anti-Sm, anti-RNP [10]
2B. Drug-Induced Myopathy
This is the most important practical differential for anti-HMGCR+ IMNM, because the initial question is always: "Is this just statin toxicity, or is this true autoimmune disease?"
| Feature | Statin Toxic Myopathy | Anti-HMGCR+ IMNM |
|---|---|---|
| Mechanism | Direct pharmacological toxicity (mitochondrial dysfunction, coenzyme Q10 depletion) | Autoimmune — anti-HMGCR antibodies + complement |
| CK level | Usually < 10× ULN; may be normal with myalgia only | Usually > 10× ULN, often > 50× |
| Response to statin withdrawal | Resolves within days to weeks | Does NOT resolve — persists or worsens |
| Anti-HMGCR antibody | Negative | Positive |
| Biopsy | Non-specific type II fibre atrophy; no necrosis | Necrosis, regeneration, MAC deposition |
The key teaching point: If you stop the statin and the patient improves → toxic myopathy. If you stop the statin and weakness persists or CK stays high → think IMNM, check anti-HMGCR antibody.
| Drug | Mechanism | Key Clues |
|---|---|---|
| Glucocorticoids [7][8] | Type IIb fibre atrophy (catabolic effect on muscle protein); NO inflammation, NO CK elevation | Proximal weakness with normal CK; temporal relationship with steroid use; Cushing's features |
| Colchicine [7] | Disrupts microtubules → vacuolar myopathy; often combined with neuropathy | Concomitant neuropathy (neuromyopathy); typically in patients with CKD (reduced clearance) |
| Hydroxychloroquine | Lysosomal dysfunction → vacuolar myopathy | Very long-term use; curvilinear bodies on biopsy |
| Checkpoint inhibitors (pembrolizumab, nivolumab) | Immune-mediated — can trigger IMNM-like picture | Oncology patients; temporally related to immunotherapy initiation |
| Fibrates | Similar to statins; additive risk when combined | Check drug interactions |
| Zidovudine (AZT) | Mitochondrial myopathy | HIV patients; ragged red fibres on biopsy |
Steroid Myopathy — The Ironic Trap
Glucocorticoid-induced myopathy [7][8] is particularly tricky because steroids are used to TREAT inflammatory myopathies. A patient on high-dose prednisolone who develops worsening proximal weakness might have (a) relapsing IMNM, or (b) steroid myopathy. The key distinguishing feature: CK is NORMAL in steroid myopathy (because there is no necrosis, just atrophy) but CK is elevated in active IMNM. This distinction guides whether to increase or decrease the steroid dose — a critical clinical decision.
These are systemic conditions where proximal weakness is a secondary feature.
| Condition | Mechanism | Key Distinguishing Features |
|---|---|---|
| Hypothyroidism [7][8] | Reduced metabolic rate → impaired muscle energy metabolism; ↓Na⁺/K⁺-ATPase activity; accumulation of glycosaminoglycans in muscle | Myopathy associated with hypothyroidism can mimic inflammatory myopathy with a subacute onset of proximal muscle weakness and elevated CK [8]; but patient also has fatigue, cold intolerance, constipation, weight gain, delayed relaxation of reflexes; check TFT |
| Cushing's syndrome [7][9] | Glucocorticoid excess → protein catabolism, type IIb fibre atrophy | Proximal weakness with normal CK; cushingoid habitus; moon face, striae, central obesity |
| Hypokalemia [7][9] | K⁺ depletion → muscle cell hyperpolarisation → impaired depolarisation and contraction | Muscle weakness, paralysis (proximal muscle myopathy); rhabdomyolysis [11]; check serum K⁺; episodic weakness; ECG changes (U waves, flattened T waves) |
| Hypophosphataemia | Phosphate depletion → impaired ATP synthesis → energy failure in muscle | Chronic alcoholism, refeeding syndrome, diabetic ketoacidosis recovery |
| Hypercalcaemia | ↑Ca²⁺ impairs neuromuscular transmission and muscle contractility | Check serum Ca²⁺; associated lethargy, confusion, constipation |
Why these matter: All are treatable causes that must be excluded before committing to an IMNM diagnosis. A simple TFT and electrolyte panel rules out most of them. The GC lecture on myopathy explicitly lists hypothyroidism, Cushing's syndrome, and hypokalemia as endocrine differentials [7].
| Agent | Mechanism | Key Distinguishing Features |
|---|---|---|
| Viral: HIV, CMV, EBV, influenza, Coxsackie [7][9] | Direct viral invasion of myocytes → inflammation and necrosis | Acute febrile illness; self-limiting; diffuse myalgia > weakness; check viral serology |
| Pyomyositis (Staph. aureus) | Bacterial abscess within muscle | Localised pain, swelling, fever; often in immunocompromised; MRI shows focal abscess |
| Parasitic: Trichinella, Toxoplasma | Larval/parasite invasion of muscle | Travel history; eosinophilia; periorbital oedema (trichinosis) |
Why the confusion arises: Viral myositis can cause elevated CK and diffuse weakness, but it is typically acute, self-limiting, and associated with a systemic viral syndrome. IMNM is subacute to chronic and progressive.
Myasthenia Gravis (MG)
| Feature | How It Differs from IMNM |
|---|---|
| Mechanism | NMJ post-synaptic disorder caused by antibodies to ACh receptors [8] — blocks neuromuscular transmission |
| Weakness character | Fatigable — gets worse with repetitive use, improves with rest; this is the hallmark |
| Distribution | Facial muscle weakness [8] (ptosis, diplopia, dysphagia) — ocular involvement is very common |
| CK | Normal muscle enzymes [8] |
| EMG | Decremental response on repetitive nerve stimulation [8]; IMNM shows myopathic potentials |
| Autoantibodies | AChR antibodies [8] or anti-MuSK |
Why the confusion arises: Both cause proximal and bulbar weakness (dysphagia). But MG has fatigability, ocular involvement, and normal CK. IMNM has constant weakness, typically spares eyes, and has very high CK.
| Condition | Key Distinguishing Features from IMNM |
|---|---|
| Muscular dystrophy (Duchenne, Becker, limb-girdle) [7][8][9] | Inherited group of progressive myopathic disorders resulting from defects in genes required for normal muscle function [8]; positive family history; childhood/adolescent onset (DMD/BMD); elevated CK but genetic testing is diagnostic; biopsy shows dystrophic changes, not necrosis-regeneration with MAC |
| Myotonic dystrophy [5][8] | Proximal myotonic myopathy (PROMM) presents with myotonia, slowed relaxation following a normal muscle contraction and positive family history [8]; distal weakness + wasting in classic DM1 (temporalis, sternocleidomastoid, forearms); myotonia (delayed relaxation); multisystem (cataracts, cardiac conduction defects, DM, testicular atrophy) [5]; genetic testing (CTG repeat expansion) |
| Metabolic myopathies (glycogen storage, lipid storage, mitochondrial) | Episodic weakness or exercise intolerance; second-wind phenomenon (McArdle disease); ragged red fibres on biopsy (mitochondrial); specific enzyme assays |
Why the confusion arises: Limb-girdle muscular dystrophy (LGMD) can present in adulthood with proximal weakness and elevated CK — closely mimicking IMNM. Genetic testing and biopsy are essential for differentiation.
Muscular Dystrophy Mimicking IMNM
An adult presenting with proximal weakness and high CK may have late-onset LGMD. Key clues pointing away from IMNM: family history of similar weakness, very slowly progressive course (years), absence of myositis-specific autoantibodies, and dystrophic features on biopsy (not pure necrosis-regeneration). Always consider genetic myopathies in the differential, especially if immunosuppressive treatment fails.
| Feature | How It Differs from IMNM |
|---|---|
| Weakness pattern | Both UMN AND LMN signs (brisk reflexes + fasciculations + wasting); IMNM has neither UMN signs nor fasciculations |
| Fasciculations | Present in MND; absent in IMNM |
| CK | Can be mildly elevated in MND (from denervation), but not massively elevated |
| EMG | Denervation pattern (fibrillation + fasciculation + large motor unit potentials); IMNM shows myopathic pattern (small, polyphasic, short-duration potentials) |
| Sensory | Normal in both |
Paraneoplastic [7] myopathy can manifest as proximal weakness in the context of an occult malignancy, sometimes independent of direct IIM. However, IMNM itself (especially seronegative) can BE the paraneoplastic manifestation. This creates conceptual overlap.
- Adult form a/w malignancy: 5× risk in dermatomyositis, 2× risk in polymyositis [2][3]
- Types: adenocarcinoma of cervix, lung, ovaries, pancreas, bladder, stomach, NPC (this locality) [2][3]
- Temporal relationship: can be diagnosed before, with or after diagnosis of inflammatory myopathy [2][3]
- Potential association between cancer and dermatomyositis and the rationale of cancer screening [4]
- In seronegative IMNM patients, the malignancy association is highest — always perform a thorough malignancy screen
| Cause | Mechanism | Distinguishing from IMNM |
|---|---|---|
| Crush injuries / seizures [7] | Mechanical destruction of muscle | History of trauma, prolonged immobilisation, seizure |
| Severe exercise (exertional rhabdomyolysis) | Energy depletion → sarcolemma rupture | History of extreme physical exertion |
| Drugs / toxins (statins, alcohol, cocaine, amphetamines) | Direct myotoxicity | Drug/toxin exposure history |
| Hypokalemia [11] | Severe K⁺ depletion → cell membrane instability → muscle breakdown | Electrolyte panel |
| Malignant hyperthermia | Uncontrolled Ca²⁺ release from sarcoplasmic reticulum (RYR1 mutation) → sustained contraction → necrosis | Triggered by anaesthetic agents (halothane, isoflurane) [2]; perioperative setting |
Key point: Rhabdomyolysis gives massively elevated CK and myoglobinuria — identical to severe IMNM. The distinction is that rhabdomyolysis is acute and has an identifiable trigger, whereas IMNM is subacute/chronic and progressive without an obvious precipitant (or persists after trigger removal).
| Category | Condition | CK | Weakness Pattern | Key Distinguishing Feature |
|---|---|---|---|---|
| IIM | DM | Moderate ↑ | Proximal, symmetric | Skin rash (heliotrope, Gottron's) |
| PM | Moderate–high ↑ | Proximal, symmetric | CD8+ T-cell infiltrate on biopsy | |
| IBM | Normal–mild ↑ | Proximal + distal | Insidious onset, rimmed vacuoles, treatment-refractory | |
| Anti-synthetase | Moderate–high ↑ | Proximal | ILD, mechanic's hands, arthritis, fever | |
| Overlap | Variable | Proximal | Features of underlying CTD | |
| Drug-induced | Statin toxic | Mild–moderate ↑ | Proximal | Resolves with statin cessation |
| Steroid myopathy | Normal | Proximal | On steroids; no necrosis; CK normal | |
| Colchicine | Mild ↑ | Proximal ± distal | Neuromyopathy; CKD | |
| Endocrine | Hypothyroidism | Mild–moderate ↑ | Proximal | TFT abnormal; delayed relaxation reflexes |
| Cushing's | Normal | Proximal | Cushingoid features | |
| Hypokalemia | Variable (may ↑↑) | Proximal, episodic | Low K⁺; ECG changes | |
| NMJ | Myasthenia gravis | Normal | Fatigable; ocular/bulbar | AChR Ab; decremental EMG |
| Genetic | Muscular dystrophy | High ↑ | Proximal (LGMD) | Family history; young onset; genetic test |
| Myotonic dystrophy | Mild ↑ | Distal > proximal | Myotonia; family history; CTG repeat | |
| Infectious | Viral myositis | Mild–moderate ↑ | Diffuse | Acute, febrile, self-limiting |
| Pyomyositis | Mild ↑ | Focal | Focal abscess; fever | |
| Other | MND/ALS | Mild ↑ | UMN + LMN | Fasciculations; UMN signs |
| Rhabdomyolysis | Very high ↑↑↑ | Variable | Acute precipitant; resolves | |
| Paraneoplastic | Variable | Proximal | Occult malignancy on screening |
When facing a patient with subacute proximal weakness + very high CK + no rash:
- Exclude drug-induced → Stop statins/other offenders; if improves → toxic myopathy; if persists → consider IMNM
- Exclude endocrine/metabolic → TFT, electrolytes (K⁺, Ca²⁺, PO₄), cortisol
- Exclude infection → Viral serology, blood cultures if febrile
- Check myositis-specific antibodies → anti-SRP, anti-HMGCR, anti-Jo1, anti-Mi2, anti-MDA5
- EMG → Myopathic pattern (rules out neuropathy, NMJ)
- MRI muscles → Oedema pattern guides biopsy site
- Muscle biopsy → The definitive test: necrosis + regeneration + minimal inflammation + MAC deposition = IMNM
- Malignancy screen → CT TAP, tumour markers, NPC screen (EBV serology, nasopharyngoscopy in HK), age-appropriate cancer screening
High Yield Summary — Differential Diagnosis of IMNM
- Closest mimics within IIM: DM (has skin rash), PM (CD8+ T-cell infiltrate), IBM (distal weakness, insidious, refractory), anti-synthetase (ILD, mechanic's hands)
- Drug-induced: Statin toxic myopathy (resolves with cessation) vs anti-HMGCR+ IMNM (persists); steroid myopathy (normal CK — the ironic trap)
- Endocrine: Hypothyroidism (check TFT), Cushing's, hypokalemia (check K⁺)
- NMJ: Myasthenia gravis — fatigable weakness, ocular involvement, normal CK
- Genetic: LGMD and myotonic dystrophy — family history, genetic testing
- Infectious: Viral myositis — acute, febrile, self-limiting
- Malignancy: Must screen in ALL IIM patients, especially seronegative IMNM; NPC in Hong Kong
- Key distinguishing features of IMNM: Very high CK (> 10× ULN), NO rash, subacute progression, necrosis without inflammation on biopsy, anti-SRP or anti-HMGCR positive, does NOT resolve with statin cessation
Active Recall - Differential Diagnosis of IMNM
References
[1] Senior notes: Maksim Medicine Notes.pdf (Rheumatology — Idiopathic inflammatory myopathies, p.318) [2] Senior notes: Ryan Ho Neurology.pdf (10.4.3 Inflammatory Myopathies, p.194) [3] Senior notes: Ryan Ho Rheumatology.pdf (Autoantibody-defined subsets table and malignancy association, p.91–92) [4] Lecture slides: Block A - Rheumatology Interactive Tutorial.pdf (Case 2 — IIM subtypes, learning objectives) [5] Senior notes: Adrian Lui Pediatrics Notes.pdf (Myotonic Dystrophy and Inflammatory Myopathies, p.145) [6] Senior notes: learning_points_output.txt (Neurology — Two Cases of Lower Limb Weakness, Learning Point 1) [7] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (Differential Diagnosis of Myopathy table, p.38) [8] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Differential diagnosis of IIM, p.1757) [9] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (Differential diagnosis of myopathies table, p.706) [10] Senior notes: Ryan Ho Rheumatology.pdf (Diagnosis section — autoantibodies, p.92) [11] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Hypokalemia complications, p.27)
Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Autoimmune Necrotizing Myopathy
1. Diagnostic Criteria
Diagnosing IMNM is not a single-test exercise. It requires integrating clinical findings, serology, electrophysiology, imaging, and — critically — muscle biopsy. Let's walk through the relevant criteria frameworks.
These were originally designed for PM and DM, not IMNM (which wasn't recognised as a separate entity until the 2000s). However, they remain the foundation upon which modern criteria are built:
Bohan and Peter criteria (1975): PM require all of 1–4, DM require any 3 in 1–4 + 5 [1]
1. Symmetrical weakness of limb-girdle muscles and anterior neck flexors 2. Muscle biopsy: typical of myositis 3. Muscle enzyme elevation (esp. CK) 4. EMG: typical of myositis (spontaneous fibrillation; polyphasic low-amplitude motor unit potential) 5. Cutaneous manifestations of DM: e.g. heliotrope rash, Gottron's papules [1]
Limitation for IMNM: These criteria were designed before IMNM was recognised. IMNM patients satisfy criteria 1, 3, and 4, but their biopsy pattern (necrosis without significant inflammation) doesn't fit the "typical myositis" expected in criterion 2 under the original framework. This led to years of IMNM being misclassified as PM.
2017 EULAR/ACR classification criteria for IIM: include age of onset, antibodies (only anti-Jo1 now), different scoring with / without muscle biopsy [1]
This is a probability-based scoring system using a decision tree and weighted variables:
| Variable | Score (without biopsy) | Score (with biopsy) |
|---|---|---|
| Age of onset ≥18 years vs ≥40 years | 1.3 / 2.1 | 1.5 / 1.5 |
| Muscle weakness: proximal UL | 0.7 | 0.7 |
| Muscle weakness: proximal LL | 0.5 | 0.8 |
| Dysphagia / oesophageal dysmotility | 0.7 | 0.6 |
| Anti-Jo1 positive | 3.9 | 3.9 |
| ↑CK or ↑LDH or ↑AST/ALT | 1.3 | 1.4 |
| Muscle biopsy: endomysial infiltration of mononuclear cells surrounding but not invading myofibres | N/A | 1.7 |
| Muscle biopsy: perifascicular atrophy | N/A | 1.9 |
| Muscle biopsy: rimmed vacuoles | N/A | 3.1 |
| Skin features (heliotrope, Gottron's, Gottron's sign) | 3.1–3.7 | 3.1–3.7 |
A probability score ≥ 55% without biopsy or ≥ 90% with biopsy classifies a patient as having IIM.
Relevance to IMNM: The 2017 criteria classify someone as having IIM in general, but do not specifically sub-classify IMNM. Sub-classification into IMNM requires the additional step of autoantibody profiling + characteristic biopsy findings.
These are the most specific diagnostic criteria for IMNM and remain the standard used in clinical practice:
| Criterion | Requirement |
|---|---|
| Clinical | Subacute or acute onset of proximal, symmetric muscle weakness |
| Muscle enzymes | Markedly elevated CK (typically > 10× ULN, often > 50× ULN) |
| EMG | Myopathic pattern (no neuropathic features) |
| Muscle biopsy (essential) | Prominent myofibre necrosis with myophagocytosis (macrophages engulfing dead fibres) + regenerating fibres + sparse or absent lymphocytic inflammatory infiltrate + no perifascicular atrophy |
| Additional biopsy features | MAC (C5b-9) deposition on sarcolemma of non-necrotic fibres; MHC class I upregulation on non-necrotic fibres; capillary C5b-9 deposition (anti-SRP+); pipestem capillaries (anti-SRP+) |
| Autoantibodies | Anti-SRP and/or anti-HMGCR (present in ~60–80% of cases; ~20–30% are seronegative) |
| Exclusion | Must exclude other IIM subtypes (DM by absence of rash/perifascicular atrophy; PM by absence of endomysial CD8+ T-cell invasion; IBM by absence of rimmed vacuoles); must exclude non-autoimmune causes (drug toxicity that resolves, endocrine, genetic) |
High Yield — Biopsy Is the Gold Standard for IMNM
Unlike DM where characteristic skin findings may suffice clinically, IMNM requires muscle biopsy for definitive diagnosis. The biopsy pattern of necrosis + regeneration + minimal inflammation + MAC deposition is what definitively separates IMNM from PM (CD8+ T cells invading fibres), DM (perifascicular atrophy), and IBM (rimmed vacuoles). Without biopsy, you can suspect IMNM (very high CK + positive anti-SRP/HMGCR + no rash), but you cannot confirm it.
Here is a comprehensive step-by-step algorithm integrating the clinical approach, investigations, and criteria:
High Yield — GC Interactive Tutorial Investigation Sequence
The GC rheumatology interactive tutorial case demonstrates the investigation approach in sequence [4]:
- CK 1570 U/L (NR 22–198) — elevated muscle enzymes
- ANA positive with a titre of 1/160 — marker of autoimmunity
- Myositis specific antibody test reported as positive for anti-Jo1 antibody — identifies subtype
- EMG shows low amplitude motor-units potentials with occasional fibrillation potentials, compatible with inflammatory myopathy — confirms myopathic process
- PFT shows reduced FVC 70% predicted and DLCO 64% predicted — assesses respiratory muscle and parenchymal involvement
- HRCT reveals ground glass opacities predominantly over the lower lobes. No honeycombing — characterises ILD
This same systematic sequence applies to investigating IMNM, though the antibody profile and biopsy findings will differ.
3. Investigation Modalities — Detailed Breakdown
3A. Blood Investigations
| Enzyme | Rationale | Expected Findings in IMNM | Interpretation Notes |
|---|---|---|---|
| Creatine kinase (CK) | Released from damaged muscle membrane [12][13]; most sensitive and specific marker of muscle damage | Usually > 10× ULN, may even be > 50–100× ULN in severe cases [10]; often 5,000–50,000+ U/L | CK values: rarely normal (~5%) [10]. If CK is normal, seriously reconsider the diagnosis |
| LDH | Cytoplasmic enzyme released from damaged cells (less specific — also from RBCs, liver, heart) | Elevated | Non-specific; contributes to the enzyme profile |
| AST / ALT | Released from damaged muscle as well as liver | Elevated (may be mistaken for liver disease!) | Always check CK alongside AST/ALT — if CK is very high and AST/ALT mildly elevated, the "transaminitis" is likely from muscle, not liver |
| Aldolase | Enzyme in glycolytic pathway; released from muscle | Elevated | Less commonly measured but can be elevated when CK is borderline |
CK values in context [12][13]:
- 200–1000 IU/L: most myopathies, SMA/MND, rigorous exercise, IM injection, after EMG/muscle biopsy, tonic-clonic seizures, statin/neuroleptic-induced
- 1000–10000 IU/L: inflammatory myopathies, acute rhabdomyolysis, DMD/BMD
- In IMNM, CK commonly exceeds 10,000 IU/L — this is a distinguishing feature
AST/ALT Trap
A common clinical error: an elevated AST/ALT in a patient with IMNM is reflexively attributed to liver disease, triggering unnecessary hepatology investigations. Always check CK first. If CK is markedly elevated, the "liver enzymes" are almost certainly of muscular origin. AST and ALT are present in skeletal muscle and leak out during necrosis, just like CK.
| Marker | Expected in IMNM | Notes |
|---|---|---|
| ESR | Often mildly to moderately elevated | Non-specific; can be elevated in any inflammatory/autoimmune process |
| CRP | Often mildly to moderately elevated | ↑↑ESR/CRP [10] — general markers of inflammation |
| Investigation | Purpose | What You're Looking For |
|---|---|---|
| CBC [10] | Baseline; exclude haematological malignancy | Usually normal; anaemia of chronic disease possible |
| RFT [10] | Renal function — rhabdomyolysis can cause AKI | ↑Creatinine, ↑urea if myoglobin-induced AKI |
| LFT [10] | Distinguish muscle-origin transaminases from true liver disease | GGT and bilirubin should be NORMAL if the transaminitis is from muscle |
| TFT [10][12][13] | R/o thyroid myopathy [10] — hypothyroidism can mimic inflammatory myopathy | ↑TSH, ↓fT4 → hypothyroid myopathy |
| Electrolytes [12][13] | Exclude hypokalemia, hypocalcaemia, hypophosphataemia | Any of these can cause myopathy and even rhabdomyolysis |
| Urinalysis | Myoglobinuria | Dipstick positive for "blood" but no RBCs on microscopy → myoglobin! |
This is where the diagnostic precision for IMNM lies. The autoantibody profile sub-classifies the IIM and predicts prognosis and treatment response.
a) Myositis-Specific Antibodies (MSAs) — Only found in myositis
| Antibody | Associated IIM Subtype | Clinical Significance |
|---|---|---|
| Anti-SRP | IMNM | Severe myopathy with muscle fibre necrosis / endomysial fibrosis with minimal inflammatory infiltrates; aggressive disease refractory to high-dose steroids and immunosuppressants [3] |
| Anti-HMGCR | IMNM | Statin-associated autoimmune myopathy; persists after statin cessation; ~30–40% are statin-naïve |
| Anti-Jo1 | Anti-synthetase syndrome | Mechanic's hands, ILD, Raynaud's, non-erosive arthritis, fever [3]; most common anti-synthetase antibody; included in 2017 EULAR/ACR criteria [1] |
| Anti-Mi2 | DM (classic) | Good prognosis, responds well to therapy [3] |
| Anti-MDA5 | CADM | Cutaneous ulceration, erythematous painful palmar macules/papules, alopecia, oral ulcers [3]; associated with rapidly progressive ILD |
| Anti-TIF1γ | DM (cancer-associated) | Strongest cancer association in adult DM |
| Anti-NXP2 | DM | Confers a higher risk of malignancy [14] |
| Anti-SAE | DM | Initially amyopathic, then develops myositis |
b) Myositis-Associated Antibodies (MAAs) — Found in other CTDs too
| Antibody | Associated CTD | Notes |
|---|---|---|
| Anti-Ro/SSA, Anti-La/SSB | Sjögren's, SLE | Found in other autoimmune rheumatic diseases associated with myositis [14] |
| Anti-Sm | SLE | Generally negative in JDM and JPM [14] |
| Anti-U1 RNP | MCTD | Overlap myositis |
| Anti-PM/Scl | SSc-myositis overlap | Identify a small, distinct subgroup of myopathies with a protracted disease course often complicated by pulmonary interstitial fibrosis and/or cardiac involvement [14] |
c) ANA
An anti-nuclear antibodies test is reported as positive with a titre of 1/160 [4].
- ANA is positive in ~60–80% of IIM patients overall
- In IMNM specifically, ANA may be negative (especially anti-HMGCR+ cases) — ANA negativity does NOT exclude IMNM
- A positive ANA is non-specific and must be interpreted in context
3B. Electrophysiology
Purpose: To r/o neuropathic disorders [10] — the primary role of EMG is to confirm the process is myopathic, not neuropathic.
Myositis: defined as 2 out of 3 of: ↑muscle enzymes, EMG typical of myositis, Muscle biopsy [10].
| EMG Finding | Significance | Pathophysiology |
|---|---|---|
| Spontaneous fibrillation potentials at rest [1][10] | Indicates muscle membrane instability → denervated or necrotic fibres firing spontaneously | In IMNM, necrotic fibres lose sarcolemma integrity → uncontrolled depolarisation |
| Polyphasic or short-duration potentials on voluntary contraction [1][10] | = Myopathic motor unit potentials → fewer functioning fibres per motor unit; the surviving fibres produce smaller, fragmented signals | In IMNM, many fibres in each motor unit are necrotic → the motor unit potential becomes smaller and polyphasic because fewer fibres contribute synchronously |
| Salvos of repetitive potentials on mechanical stimulation of nerve [10] (complex repetitive discharges) | Groups of fibres fire in synchronized bursts | Reflects irritability of regenerating fibres |
| Low amplitude motor-units potentials with occasional fibrillation potentials, compatible with inflammatory myopathy [4] | Consolidates the myopathic pattern | From the GC tutorial case |
| Normal nerve conduction velocities | Rules out neuropathy | Motor and sensory nerve conduction should be normal in a pure myopathic process |
EMG Pattern: Myopathic vs Neuropathic
| Feature | Myopathic (IMNM, DM, PM) | Neuropathic (GBS, CIDP, MND) |
|---|---|---|
| Motor unit potentials | Small, short, polyphasic | Large, long, polyphasic |
| Spontaneous activity | Fibrillations, positive sharp waves | Fibrillations, fasciculations |
| Recruitment | Early recruitment (many small units fire early) | Reduced recruitment (few large units fire) |
| NCS | Normal | Abnormal (↓velocity in demyelinating; ↓amplitude in axonal) |
Why early recruitment in myopathy? Because individual motor units are weakened (fewer functioning fibres), the CNS compensates by activating MORE motor units earlier to generate force. In neuropathy, entire motor units are lost, so fewer units are available → reduced recruitment.
- Typically normal unless severe muscle necrosis and atrophy are present [14]
- Purpose: rule out concurrent neuropathy (important in colchicine-induced neuromyopathy, diabetic neuropathy, etc.)
- If NCS is abnormal → reconsider the diagnosis or look for overlap with neuropathic conditions
3C. Imaging
MRI: sensitive but non-specific [10].
| Sequence | Finding in Active IMNM | Interpretation |
|---|---|---|
| T2-weighted / STIR | Patchy ↑T2W indicating inflammation, oedema [10]; diffuse bilateral symmetrical signal hyperintensity in proximal muscles (thighs, upper arms) | Active muscle oedema/necrosis → increased water content → bright on T2/STIR |
| T1-weighted | Fatty replacement (hyperintense streaks within muscle) | Chronic damage → irreversible fatty infiltration replacing necrotic muscle. If present, indicates delayed diagnosis |
| Post-gadolinium T1 | Enhancement of actively inflamed/necrotic areas | Helps distinguish active from chronic disease |
Why MRI?
- Guides biopsy site [10] — you want to biopsy muscle that is oedematous (active disease) but NOT completely replaced by fat (end-stage, unhelpful)
- Assesses extent and distribution of disease
- Monitors treatment response — resolving oedema indicates improving disease
- D/dx: muscular dystrophy, metabolic, rhabdomyolysis [10] — helps refine the differential
Distribution pattern in IMNM: Tends to affect posterior thigh compartment (hamstrings > quadriceps) and medial compartment more than lateral. This differs from IBM (which preferentially affects quadriceps and forearm flexors).
| Modality | Purpose | Findings to Look For |
|---|---|---|
| CXR | Baseline screen for ILD | Faint reticulation over bilateral lower zones [4] |
| HRCT thorax | Gold standard for ILD characterisation | Ground glass opacities predominantly over the lower lobes. No honeycombing [4] → suggests NSIP pattern (non-specific interstitial pneumonia), which is the most common ILD pattern in IIM |
Investigations of interstitial lung disease [4] — explicitly a GC learning objective.
ILD is less common in pure IMNM compared to anti-synthetase syndrome or DM with anti-MDA5, but must be screened for because it affects management and prognosis.
PFT shows a reduced forced vital capacity of 70% predicted and diffusion capacity of the lungs for carbon monoxide of 64% predicted [4].
| Parameter | What It Measures | Expected Finding | Interpretation |
|---|---|---|---|
| FVC | Lung volumes (restrictive pattern?) | ↓ in ILD or respiratory muscle weakness | If FVC is ↓ but DLCO is normal → likely respiratory muscle weakness. If BOTH are ↓ → ILD |
| DLCO | Gas exchange efficiency across alveolar membrane | ↓ in ILD (parenchymal damage reduces diffusion surface area) | ↓DLCO out of proportion to ↓FVC → suggests parenchymal disease (ILD), not just weak muscles |
| MIP/MEP (maximal inspiratory/expiratory pressures) | Respiratory muscle strength directly | ↓ in IMNM if diaphragm/intercostal weakness | Helps distinguish "can't breathe because muscles are weak" from "can't breathe because lungs are diseased" |
3E. Muscle Biopsy — The Definitive Investigation
Muscle Bx: at muscle that is most affected [12][13]. Done on weak but not atrophied muscle → guided by P/E, EMG ± MRI [10].
- Open biopsy (traditional) or needle biopsy (less invasive, but smaller sample)
- Site selection: Choose a muscle that is clinically weak (reduced MRC grade) but NOT severely atrophied or replaced by fat
- Commonly biopsied muscles: deltoid or quadriceps (vastus lateralis)
- Avoid the muscle used for EMG on that side (needle trauma causes artefactual inflammation)
- MRI guidance optimises yield — biopsy the area showing T2/STIR hyperintensity
| Finding | Description | Pathophysiological Explanation |
|---|---|---|
| Myofibre necrosis | Pale, eosinophilic fibres with loss of cross-striation; may show hyaline degeneration | MAC-mediated sarcolemma rupture → cell death; cytoplasm becomes homogeneous as proteins denature |
| Myophagocytosis | Macrophages surrounding and engulfing dead fibres | Macrophages are recruited to clear necrotic debris — this is clean-up, NOT the primary attack |
| Regenerating fibres | Basophilic, small calibre, centrally placed nuclei | Satellite cells (muscle stem cells) activate → proliferate → fuse into new myotubes; basophilia from abundant ribosomal RNA |
| Minimal lymphocytic infiltration | Sparse T cells (if any); NO endomysial CD8+ T-cell invasion of non-necrotic fibres | This is what distinguishes IMNM from PM — the attack is humoral (antibody/complement), not cell-mediated |
| MAC (C5b-9) deposition on sarcolemma | Immunohistochemistry shows C5b-9 deposits on the surface of non-necrotic fibres | Complement activation has occurred → MAC assembles on sarcolemma → next step would be pore formation and necrosis. Catching it on non-necrotic fibres shows the "attack in progress" |
| MHC class I upregulation | Diffuse sarcolemmal MHC-I expression on non-necrotic fibres | Normal muscle does NOT express MHC-I. Upregulation indicates immune activation — the fibres are presenting antigens and are under immune attack |
| Pipestem capillaries (anti-SRP+) | Thickened capillary walls with hyalinisation | Complement-mediated endothelial damage in small vessels |
| No perifascicular atrophy | Atrophy is NOT preferentially at fascicle edges | Perifascicular atrophy is the hallmark of DM (ischaemia from perimysial vasculopathy); its ABSENCE helps exclude DM |
| No rimmed vacuoles | No autophagic vacuoles with eosinophilic rim | Rimmed vacuoles are the hallmark of IBM; their ABSENCE helps exclude IBM |
High Yield — Biopsy Pattern Comparison Across IIMs
| Feature | IMNM | DM | PM | IBM |
|---|---|---|---|---|
| Dominant pathology | Necrosis + regeneration | Perifascicular atrophy | CD8+ T-cell invasion | Rimmed vacuoles + inclusions |
| Inflammatory infiltrate | Macrophage-dominant, sparse | Perivascular/perimysial B + CD4+ T cells | Endomysial CD8+ T cells | Endomysial CD8+ T cells + degenerative changes |
| MAC deposition | On sarcolemma of non-necrotic fibres (characteristic) | On capillaries (capillary damage) | Absent or minimal | Absent |
| MHC-I expression | Diffuse | Perifascicular | Diffuse | Diffuse |
| Perifascicular atrophy | Absent | Present — hallmark | Absent | Absent |
| Investigation | Purpose | Expected Findings in IMNM |
|---|---|---|
| ECG | Screen for arrhythmias, conduction defects | Sinus tachycardia, ST/T changes (if myocarditis), conduction blocks |
| Echocardiography | Screen for ventricular dysfunction, myocarditis | ↓Ejection fraction, regional wall motion abnormalities (especially in anti-SRP+ IMNM) |
| Cardiac MRI | If echo abnormal — characterise myocardial inflammation/fibrosis | Late gadolinium enhancement (fibrosis/scar); T2 oedema (active myocarditis) |
| Troponin | Marker of myocardial injury | May be elevated from skeletal muscle (cross-reactivity) OR true cardiac involvement — interpret with echo |
ECG/echo: detect cardiac muscle involvement [12][13]. Cardiac assessment is essential because anti-SRP+ IMNM in particular can cause clinically significant myocarditis and cardiomyopathy.
Potential association between cancer and dermatomyositis and the rationale of cancer screening [4]. Thorough malignancy screen in elderly [10].
| Investigation | Target Malignancy | HK-Specific Relevance |
|---|---|---|
| CT TAP (chest/abdomen/pelvis) | Solid organ malignancies (lung, ovarian, gastric, pancreatic) | Standard baseline screen |
| EBV serology (VCA-IgA, EA-IgA) | NPC | NPC (this locality) [2] — high prevalence in Southern Chinese; must screen |
| Nasopharyngoscopy | NPC | Direct visualisation of nasopharynx |
| Mammography | Breast cancer | All female patients |
| Pap smear / HPV | Cervical cancer | All female patients of appropriate age |
| Colonoscopy | Colorectal cancer | Age-appropriate (>50 years) |
| PSA | Prostate cancer | Male patients >50 years |
| PET-CT | Occult malignancy | Consider if initial screen negative but high clinical suspicion (seronegative, older patient, constitutional symptoms) |
1/3 malignancy (lung, breast, gastric, NPC) – usually diagnosed within 1 year of DM/PM [1]. Repeat screening should be considered at 3–6 month intervals for the first 3 years, especially in seronegative IMNM and in older patients.
She has mild to moderate dysphagia upon speech therapist assessment [4].
- Bedside swallowing assessment by speech therapist
- Video fluoroscopic swallowing study (VFSS) if aspiration is suspected
- Identifies bulbar involvement and guides whether PO intake is safe vs need for NG/PEG feeding
| Step | Investigation | Key Findings for IMNM |
|---|---|---|
| 1 | CK + muscle enzymes | Very high (> 10× ULN) |
| 2 | Baseline bloods (CBC, RFT, LFT, TFT, electrolytes) | Exclude metabolic/endocrine mimics |
| 3 | Myositis-specific antibodies | Anti-SRP or anti-HMGCR positive |
| 4 | EMG | Myopathic pattern; fibrillations |
| 5 | MRI muscles | T2/STIR hyperintensity (oedema); guides biopsy |
| 6 | Muscle biopsy | Necrosis + regeneration + minimal inflammation + MAC deposition |
| 7 | HRCT + PFT | Screen for ILD |
| 8 | ECG + Echo | Screen for cardiac involvement |
| 9 | Malignancy screen | CT TAP, EBV/NPC screen, age-appropriate |
| 10 | Swallowing assessment | If dysphagia present |
High Yield Summary — Diagnosis of IMNM
- No single diagnostic test — diagnosis requires integration of clinical, serological, electrophysiological, and histopathological data
- Bohan and Peter criteria (1975): PM requires all 4 clinical criteria; DM requires 3 + skin [1] — historical but still examined
- 2017 EULAR/ACR criteria: Probability-based scoring for IIM overall; does NOT sub-classify IMNM
- ENMC criteria for IMNM: Requires biopsy showing necrosis + regeneration + minimal inflammation ± MAC deposition; anti-SRP or anti-HMGCR; exclusion of other causes
- CK: Usually > 10× ULN — rarely normal (~5%) [10]; highest among all IIM subtypes
- Autoantibodies: Anti-SRP (severe, refractory) and anti-HMGCR (statin-associated) are IMNM-specific
- EMG: Myopathic pattern — short, polyphasic MUPs + fibrillation potentials [1][10]
- MRI: T2/STIR hyperintensity guides biopsy; T1 fatty replacement indicates chronicity
- Muscle biopsy: Gold standard — necrosis, regeneration, macrophage predominance, sparse lymphocytes, MAC on sarcolemma, no perifascicular atrophy, no rimmed vacuoles
- Always screen for malignancy — especially NPC in HK — and for ILD and cardiac involvement
Active Recall - Diagnosis of IMNM
References
[1] Senior notes: Maksim Medicine Notes.pdf (Rheumatology — Idiopathic inflammatory myopathies, Diagnostic criteria, p.318) [2] Senior notes: Ryan Ho Neurology.pdf (10.4.3 Inflammatory Myopathies, p.194) [3] Senior notes: Ryan Ho Rheumatology.pdf (Autoantibody-defined subsets table, p.91) [4] Lecture slides: GC_Interactive tutorial (Rheum case 2) student copy.pdf (Learning objectives and Case P2 investigations); Senior notes: Block A - Rheumatology Interactive Tutorial.pdf (Case 2) [10] Senior notes: Ryan Ho Rheumatology.pdf (Diagnosis section — myositis criteria, autoantibodies, management, p.92) [12] Senior notes: Ryan Ho Neurology.pdf (10.4 Diseases of Muscles — Investigations, p.191) [13] Senior notes: Adrian Lui Pediatrics Notes.pdf (Inflammatory Myopathies — Investigations, p.143–145) [14] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (Serum autoantibodies and EMG, p.709)
Management of Autoimmune Necrotizing Myopathy
Before diving into specific drugs, let's establish why we treat IMNM the way we do. The management logic flows directly from the pathophysiology:
| Pathophysiological Target | Therapeutic Strategy | Rationale |
|---|---|---|
| Autoantibody production (anti-SRP, anti-HMGCR) | Deplete B cells (rituximab); reduce antibody titres (plasmapheresis, IVIG) | Antibodies are the primary effectors → removing them interrupts the attack |
| Complement activation → MAC deposition | High-dose steroids suppress complement cascade; IVIG scavenges complement | MAC formation is the terminal step causing sarcolemma rupture |
| Ongoing autoimmune stimulation | Steroid-sparing immunosuppressants (AZA, MTX, MMF, CYC) | Long-term immune suppression prevents recurrent cycles of necrosis |
| Persistent antigen exposure (HMGCR on regenerating fibres) | Stop the offending statin (necessary but NOT sufficient) | Removing the initial trigger, but the autoimmune loop is already self-sustaining |
| Muscle damage → functional impairment | Physiotherapy, rehabilitation | Preserve function, prevent contractures, promote recovery |
| Bulbar involvement → aspiration risk | Speech therapy, modified diet, PEG tube | Protect the airway |
| Malignancy driving the immune process | Treat the underlying malignancy | In paraneoplastic IMNM, treating the cancer may lead to myositis improvement |
Key Principle — IMNM Is Antibody-Mediated
Unlike PM/DM where steroids alone may suffice, IMNM is driven by autoantibodies and complement — a humoral mechanism. This is why IMNM often requires earlier and more aggressive use of B-cell–targeted therapies (rituximab, IVIG) compared to other IIMs. Anti-SRP: aggressive disease refractory to high-dose steroids and immunosuppressants [3] — knowing this changes your management escalation timeline.
3. Treatment Modalities — Detailed Breakdown
3A. Step 0: Immediate Measures
- Mandatory in all suspected anti-HMGCR+ IMNM
- Rationale: while stopping the statin alone will NOT cure established IMNM (the autoimmune cycle is self-perpetuating), continued statin use provides ongoing antigen stimulus → perpetuates the disease
- The patient should never be re-challenged with any statin (class effect — all HMG-CoA reductase inhibitors carry the risk)
- For cardiovascular risk management, switch to alternative lipid-lowering agents: ezetimibe (cholesterol absorption inhibitor — different mechanism, no HMGCR upregulation), PCSK9 inhibitors (evolocumab, alirocumab), or fibrates if needed
| Severity Marker | Mild–Moderate | Severe / Life-Threatening |
|---|---|---|
| MRC power grade | 4/5 proximally | ≤ 3/5 proximally; cannot walk; bedbound |
| CK | 5–10× ULN | > 50× ULN |
| Swallowing | Intact or mild dysphagia | Severe dysphagia, choking, aspiration risk |
| Respiratory | FVC > 80% predicted | FVC < 50% predicted; resp distress |
| Cardiac | Normal echo/ECG | ↓EF, arrhythmia, myocarditis |
| Rhabdomyolysis | No AKI | Myoglobinuria, rising creatinine, AKI |
- Severe disease: Initiate IV methylprednisolone pulse + consider IVIG simultaneously + early rituximab
- Mild-moderate disease: Oral prednisolone + steroid-sparing agent may suffice initially
Search for and treat any malignancy [10]
- If a malignancy is identified, treating the cancer is a critical component of management — in paraneoplastic IMNM, immunosuppression alone without addressing the tumour may be ineffective
- Temporal relationship: can be diagnosed before, with or after diagnosis of inflammatory myopathy [2]
Before starting immunosuppression, screen for:
| Screen | Rationale |
|---|---|
| HBV serology (HBsAg, anti-HBs, anti-HBc) | Reactivation risk with rituximab and other immunosuppressants; start antiviral prophylaxis (entecavir) if HBsAg+ or anti-HBc+ |
| HCV serology | Treatment of underlying HCV if present |
| TB screening (CXR + IGRA/Mantoux) | Risk of TB reactivation with steroids/immunosuppression; start isoniazid prophylaxis if latent TB |
| Bone density (DEXA) | Baseline before long-term steroid use |
| TPMT genotype (if planning azathioprine) | Thiopurine methyltransferase deficiency → risk of severe myelosuppression with azathioprine |
| Baseline CBC, RFT, LFT | Monitor for drug toxicity |
High dose steroids [10] — this is the universal first-line for all IIMs.
| Regimen | Dose | Duration | Indication |
|---|---|---|---|
| Oral prednisolone | 1 mg/kg/day (max 60–80 mg/day) | Start high, then taper after 4–8 weeks of improvement | Standard induction for mild–moderate IMNM |
| IV methylprednisolone pulse | 500–1000 mg/day for 3–5 days | Followed by oral prednisolone taper | Severe disease: bedbound, respiratory compromise, severe dysphagia, rhabdomyolysis with AKI |
How steroids work in IMNM (from first principles):
- Glucocorticoids bind the intracellular glucocorticoid receptor → translocate to nucleus → suppress transcription of pro-inflammatory cytokines (IL-1, IL-6, TNF-α)
- Reduce B-cell activity → decrease autoantibody production
- Suppress complement activation
- Stabilise lysosomal membranes → reduce tissue damage
- In IMNM, steroids provide the initial rapid immunosuppression to break the cycle of necrosis, but they are rarely sufficient alone (especially in anti-SRP+ disease)
Steroid taper: After clinical improvement (improving power, falling CK), taper gradually:
- Reduce by 10 mg/week from 60 mg → 40 mg
- Then by 5 mg/week from 40 mg → 20 mg
- Then by 2.5 mg/month from 20 mg → 5–10 mg maintenance (or off)
- Total taper duration: typically 6–12 months
- CK and clinical power guide the taper — if CK rises or power drops during taper, slow down or reverse
Steroid Myopathy vs Disease Relapse — The Management Dilemma
A patient on steroids whose weakness worsens may have either disease relapse (increase immunosuppression) or steroid myopathy (reduce steroids). The critical discriminator: check the CK. In disease relapse, CK rises. In steroid myopathy, CK is normal (because the mechanism is atrophy, not necrosis). This single blood test determines whether you turn the steroid dose up or down — a life-or-death distinction.
Side effects of chronic steroids and prophylaxis:
| Side Effect | Mechanism | Prophylaxis |
|---|---|---|
| Osteoporosis | ↓Osteoblast activity, ↑osteoclast activity | Calcium + Vitamin D supplementation; bisphosphonate if duration > 3 months |
| Glucose intolerance / DM | ↑Hepatic gluconeogenesis, ↓peripheral insulin sensitivity | Monitor blood glucose; start hypoglycaemics if needed |
| Gastric ulcer | ↓Prostaglandin synthesis → ↓mucosal protection | PPI co-prescription (especially if concomitant NSAIDs) |
| Immunosuppression → infection | Suppression of cell-mediated and humoral immunity | Vaccinations (before rituximab); PJP prophylaxis (co-trimoxazole) if high-dose steroids |
| Adrenal suppression | Exogenous steroid suppresses HPA axis | Do NOT stop steroids abruptly; taper slowly |
| Cushing's features | Redistribution of fat, protein catabolism | Minimize dose and duration; steroid-sparing agents |
| Cataracts, glaucoma | Direct lens/aqueous humour effects | Ophthalmology review |
| AVN of femoral head | ↓Blood supply to bone (mechanism unclear, possibly fat emboli or vasculitis) | Clinical monitoring; MRI if hip pain |
Started concurrently with or within 2–4 weeks of steroids. The rationale: allow steroid dose reduction while maintaining immunosuppression. These agents take weeks to months to reach full efficacy (they are "slow-acting"), so starting them early is essential.
High dose steroids + immunosuppressants, e.g. azathioprine, MTX, cyclophosphamide [10]
| Agent | Mechanism | Dose | Onset | Key Monitoring | Contraindications/Cautions |
|---|---|---|---|---|---|
| Azathioprine (AZA) | Purine analogue → inhibits DNA synthesis → suppresses lymphocyte proliferation (both T and B cells) | 2–3 mg/kg/day PO | 3–6 months to full effect | FBC (myelosuppression), LFT (hepatotoxicity); check TPMT genotype first (deficiency → fatal pancytopenia) | TPMT deficiency; concomitant allopurinol (inhibits xanthine oxidase → ↑active metabolite → toxicity) |
| Methotrexate (MTX) | Folate antagonist → inhibits dihydrofolate reductase → blocks DNA/RNA synthesis in rapidly dividing immune cells | 7.5–25 mg/week PO/SC + folic acid supplementation | 4–8 weeks | FBC, LFT, RFT; CXR at baseline (MTX lung) | Renal impairment (renally excreted → accumulates); pregnancy (teratogenic); active infection; liver disease; MTX lung → hypersensitivity reaction |
| Mycophenolate mofetil (MMF) | Inhibits inosine monophosphate dehydrogenase → blocks de novo purine synthesis → selective lymphocyte suppression (lymphocytes rely on de novo pathway, unlike other cells which can use salvage pathway) | 1–1.5 g BD PO | 2–3 months | FBC (cytopenias), LFT, GI symptoms (diarrhoea, nausea) | Pregnancy (teratogenic); concomitant azathioprine (overlapping mechanism → excessive myelosuppression) |
| Cyclophosphamide (CYC) | Alkylating agent → cross-links DNA → cytotoxic to rapidly dividing cells (especially B cells) | IV pulse: 0.5–1 g/m² monthly for 3–6 months; or PO 1–2 mg/kg/day | 2–4 weeks (faster than AZA/MTX) | FBC (leucopenia nadir at 10–14 days), urinalysis (haemorrhagic cystitis), LFT | Haemorrhagic cystitis (co-administer MESNA for IV pulse to neutralise acrolein); infertility (gonadotoxic); malignancy risk (↑bladder cancer with cumulative dose); avoid in young women if possible |
| Tacrolimus | Calcineurin inhibitor → blocks IL-2 transcription → suppresses T-cell activation | 0.05–0.1 mg/kg/day PO; target trough 5–10 ng/mL | 2–4 weeks | Drug trough levels, RFT (nephrotoxic), glucose (diabetogenic), BP (hypertension), Mg²⁺ (hypomagnesaemia) | Renal impairment; uncontrolled hypertension; concomitant nephrotoxic drugs |
Why Mycophenolate Is Increasingly Favoured Over Azathioprine
MMF selectively suppresses lymphocytes because they are uniquely dependent on the de novo purine synthesis pathway (which MMF blocks). Other cell types can use the salvage pathway and are relatively spared. This gives MMF a better side-effect profile than AZA (less bone marrow toxicity overall). However, AZA remains widely used because it is cheaper and has more long-term safety data.
Which agent to choose?
- AZA or MMF: First-line for most IMNM patients; good safety profiles for long-term use
- MTX: Good option, especially if there is concurrent arthritis (anti-inflammatory AND immunosuppressive); avoid if significant renal impairment or ILD (MTX can cause pneumonitis — confusing the picture)
- CYC: Reserved for severe, rapidly progressive, or refractory disease due to toxicity profile; sometimes used as induction for very aggressive anti-SRP+ IMNM
- Tacrolimus: Used in some Asian centres with good results in IMNM; may be particularly useful in anti-SRP+ disease refractory to first-line agents
3D. Step 3: Escalation for Refractory Disease
If the patient does not respond to steroids + one conventional immunosuppressant within 4–8 weeks (CK not falling, power not improving, or worsening):
IVIG for refractory disease [10]
| Parameter | Detail |
|---|---|
| Dose | 2 g/kg total dose, divided over 2–5 days |
| Frequency | Can be repeated monthly for 3–6 months, then tapered |
| Mechanism | Multi-pronged: (a) Anti-idiotypic antibodies neutralise pathogenic autoantibodies; (b) Fc receptor blockade on macrophages → reduced phagocytosis of antibody-coated fibres; (c) Complement scavenging → binds and inactivates C3b/C4b → blocks MAC formation; (d) Modulates cytokine networks; (e) Accelerates catabolism of pathogenic IgG via FcRn saturation |
| Why it works in IMNM | Because IMNM is antibody and complement mediated, IVIG directly counteracts BOTH effector mechanisms. This makes it particularly effective in IMNM compared to PM (which is T-cell mediated) |
| Key advantages | Rapid onset of action (days); well-tolerated; can be used as bridge therapy while waiting for immunosuppressants to take effect |
| Side effects | Headache, fever, chills (infusion reactions); aseptic meningitis; thromboembolic events (IVIG increases blood viscosity); renal failure (osmotic nephrosis, especially with sucrose-containing preparations) |
| Contraindications | IgA deficiency (risk of anaphylaxis from anti-IgA antibodies in the preparation); severe renal impairment |
| Monitoring | RFT before and after; hydration; slow infusion rate initially |
Anti-CD20 (still experimental) [10] — note: this was listed as experimental in the senior notes from ~2018–2020. By 2025–2026, rituximab is now considered standard of care for refractory IMNM based on accumulating evidence.
| Parameter | Detail |
|---|---|
| Dose | 1 g IV × 2 doses, 2 weeks apart (RA-protocol); or 375 mg/m² × 4 weekly doses (lymphoma-protocol) |
| Repeat cycles | Typically every 6 months based on clinical response and B-cell counts |
| Mechanism | Rituximab is a chimeric monoclonal antibody against CD20, a surface marker expressed on pre-B cells and mature B cells (but NOT on plasma cells or pro-B cells). Binding CD20 → antibody-dependent cellular cytotoxicity (ADCC) + complement-dependent cytotoxicity → B-cell depletion → reduced autoantibody production over subsequent weeks-months |
| Why it works in IMNM | IMNM is driven by autoantibodies produced by B cells. Depleting the B-cell precursors that differentiate into antibody-secreting plasma cells eventually reduces autoantibody titres. The effect is not immediate (existing long-lived plasma cells are NOT depleted by rituximab — they lack CD20), which is why combination with IVIG (for immediate antibody neutralisation) is logical |
| Key advantages | Highly effective in antibody-mediated diseases; durable responses (months); now well-established safety profile |
| Side effects | Infusion reactions (pre-medicate with paracetamol + antihistamine + methylprednisolone); ↑infection risk (especially bacterial, viral reactivation — HBV!); progressive multifocal leukoencephalopathy (PML — very rare, JC virus); hypogammaglobulinaemia with repeated cycles |
| Contraindications | Active severe infection; HBV without antiviral cover (risk of fulminant reactivation); severe hypogammaglobulinaemia |
| Pre-treatment | HBV serology mandatory; start entecavir prophylaxis if HBsAg+ or anti-HBc+; vaccinations (especially pneumococcal and influenza) should be given ≥2 weeks BEFORE rituximab (B-cell depletion renders vaccines ineffective) |
Why Rituximab Does Not Work Immediately
Rituximab depletes B cells, but long-lived plasma cells in the bone marrow do NOT express CD20 and are NOT depleted. These plasma cells continue producing autoantibodies for weeks to months even after B-cell depletion. This is why rituximab takes 2–4 months for clinical effect, and why bridging therapy with IVIG (which provides immediate antibody neutralisation and complement scavenging) is used in the interim.
3E. Step 4: Strategies for Persistent Refractory Disease
If the patient remains refractory after steroids + steroid-sparing agent + IVIG + rituximab:
| Parameter | Detail |
|---|---|
| Mechanism | Physical removal of circulating autoantibodies and complement components from the plasma |
| Protocol | Typically 5–7 exchanges over 10–14 days |
| Indication | Acute, severe IMNM with life-threatening features (severe rhabdomyolysis + AKI, respiratory failure, severe dysphagia with aspiration); bridge therapy |
| Limitation | Effect is temporary — antibodies are regenerated within days to weeks unless concurrent immunosuppression is given. Must be combined with definitive therapy |
| Side effects | Hypotension (from volume shifts), hypocalcaemia (citrate anticoagulant chelates calcium), infection (removal of protective immunoglobulins), coagulopathy (removal of clotting factors) |
- If AZA failed → try MMF or tacrolimus
- If MTX failed → try CYC
- Some case series report benefit with combination therapy (e.g., rituximab + MMF + low-dose steroids + monthly IVIG)
If the patient is truly refractory to all the above:
- Is the diagnosis correct? — Could this be IBM (which does NOT respond to immunosuppression), a muscular dystrophy, or a metabolic myopathy?
- Is there an occult malignancy driving the disease that has been missed?
- Is steroid myopathy contributing to the weakness (CK normal but weakness worsening on steroids)?
- Repeat muscle biopsy may be warranted to reassess the pathology
Supportive (for late bulbar S/S): PT, speech therapy, PEG tube [10]
| Supportive Measure | Indication | Rationale |
|---|---|---|
| Physiotherapy (PT) [10] | All patients | Maintains muscle strength, prevents contractures, promotes recovery of regenerating fibres; aerobic exercise improves cardiovascular fitness and fatigue; resistance exercise safe in stable/improving disease |
| Speech therapy [10] | Dysphagia, dysphonia | Swallowing exercises, safe swallowing techniques, modified diet consistency to reduce aspiration risk |
| PEG tube (percutaneous endoscopic gastrostomy) [10] | Severe, persistent dysphagia with aspiration risk; inability to maintain adequate nutrition PO | Provides enteral nutrition while bypassing the oropharyngeal stage of swallowing; prevents aspiration pneumonia |
| Respiratory support | Respiratory muscle weakness (↓FVC) | Non-invasive ventilation (NIV/BiPAP) for nocturnal hypoventilation; monitor FVC serially; ICU if FVC < 1L or acute respiratory failure |
| Occupational therapy | Functional impairment in ADLs | Assistive devices, home modifications, energy conservation techniques |
| Psychological support | Chronic illness, functional disability | Depression and anxiety are common; address early |
| Vasodilators for Raynaud's [10] | If overlap with Raynaud's phenomenon | Calcium channel blockers (nifedipine); keep hands warm; avoid cold exposure |
3G. Disease-Specific Considerations
- Stop statin permanently — the single most important first step
- Many anti-HMGCR+ patients respond reasonably well to steroids + one immunosuppressant (AZA or MMF)
- Add IVIG or rituximab if refractory
- Statin-naïve anti-HMGCR+ patients may have more aggressive disease requiring earlier escalation
- Lipid management: Switch to ezetimibe ± PCSK9 inhibitors; monitor cardiovascular risk closely
Anti-SRP: severe myopathy with muscle fibre necrosis / endomysial fibrosis with minimal inflammatory infiltrates on histology. Aggressive disease refractory to high-dose steroids and immunosuppressants [3]
- Anticipate refractoriness — do NOT wait 8 weeks on steroids alone before escalating
- Start steroids + immunosuppressant AND consider early IVIG + rituximab from the outset
- Cardiac screening is especially important (higher risk of myocarditis)
- Some experts advocate CYC induction for severe anti-SRP+ IMNM
- Highest malignancy association — thorough and repeated cancer screening essential
- Treatment generally follows the same algorithm (steroids + immunosuppressant ± IVIG/rituximab)
- If a malignancy is found, treating the cancer may improve the myositis
| Parameter | Frequency | Target |
|---|---|---|
| CK | Every 2–4 weeks during induction; every 1–3 months during maintenance | Normalisation or near-normalisation; CK should fall BEFORE power improves |
| Muscle power (MRC grading) | Every clinic visit | Progressive improvement; full recovery takes months |
| Autoantibody titres (anti-SRP/HMGCR) | Every 3–6 months | Falling titres correlate with response; rising titres may herald relapse |
| CBC, RFT, LFT | Every 2–4 weeks initially (for drug toxicity); monthly once stable | Detect myelosuppression (AZA, MTX, CYC), hepatotoxicity (MTX, AZA), nephrotoxicity (CYC, tacrolimus) |
| FVC | Serial if respiratory involvement | Improvement or stability; decline indicates worsening respiratory muscle weakness or ILD |
| Echo/ECG | Baseline + repeat if cardiac symptoms | Monitor for cardiomyopathy (anti-SRP+) |
| Malignancy re-screening | Annually for at least 3 years | New malignancy can emerge |
| Bone density (DEXA) | Baseline + annually if on long-term steroids | Osteoporosis prevention |
| Step | Treatment | Indication | Onset of Action |
|---|---|---|---|
| 0 | Stop statins; severity assessment; malignancy screen; pre-treatment screen | All patients | Immediate |
| 1 | High-dose prednisolone (± IV methyl-pred pulse) | All patients — induction | Days–weeks |
| 2 | Steroid-sparing agent (AZA / MTX / MMF) | All patients — started concurrently or early | Weeks–months |
| 3a | IVIG | Refractory; severe; as bridge therapy | Days |
| 3b | Rituximab | Refractory; antibody-mediated; now standard of care | Weeks–months |
| 4 | Plasmapheresis; switch immunosuppressant; CYC; combination therapy | Persistent refractory disease | Days (PLEX); weeks (agents) |
| Ongoing | PT, speech therapy, PEG, respiratory support, steroid prophylaxis | All patients as needed | Continuous |
High Yield — GC Lecture / Senior Notes Management Framework
Management of myositis [10]:
- High dose steroids + immunosuppressants, e.g. azathioprine, MTX, cyclophosphamide
- IVIG for refractory disease
- Anti-CD20 (still experimental) — note: now standard of care in 2025/2026 for refractory IMNM
- Supportive (for late bulbar S/S): PT, speech therapy, PEG tube
- Lung fibrosis: high dose steroids + immunosuppressants
- Search for and treat any malignancy
- Prognosis: variable depending on type of myositis, severity, delay in dx and autoAb
- Response to steroid: in general, overlap myositis > DM > PM [10] — IMNM (especially anti-SRP+) is at the refractory end of this spectrum
High Yield Summary — Management of IMNM
- Stop statins permanently in anti-HMGCR+ IMNM (necessary but NOT sufficient — autoimmune cycle persists)
- Induction: High-dose steroids (prednisolone 1 mg/kg/day or IV methylprednisolone pulse if severe)
- Steroid-sparing agent: Start early — AZA, MTX, or MMF first-line; CYC or tacrolimus second-line
- Escalation for refractory disease: IVIG (2 g/kg, rapid onset, bridges to rituximab); Rituximab (anti-CD20, depletes B cells, reduces autoantibody production — now standard of care, not "experimental")
- Anti-SRP+ IMNM: Anticipate refractoriness — escalate early; consider IVIG + rituximab from outset
- Steroid myopathy vs relapse: Check CK — normal CK = steroid myopathy (reduce steroids); rising CK = relapse (increase immunosuppression)
- Supportive care: PT, speech therapy, PEG tube, respiratory monitoring, steroid prophylaxis (Ca/VitD, bisphosphonates, PPI, PJP prophylaxis)
- Malignancy: Screen and treat; may improve paraneoplastic myositis
- Monitoring: CK (falls before power improves), power, drug toxicity, autoantibody titres, cancer re-screening
Active Recall - Management of IMNM
References
[2] Senior notes: Ryan Ho Neurology.pdf (10.4.3 Inflammatory Myopathies, p.194) [3] Senior notes: Ryan Ho Rheumatology.pdf (Autoantibody-defined subsets table — anti-SRP, p.91) [10] Senior notes: Ryan Ho Rheumatology.pdf (Management section — myositis treatment, prognosis, p.92)
Complications of Autoimmune Necrotizing Myopathy
The complications of IMNM arise from two broad sources: (A) the disease itself — direct consequences of ongoing autoimmune myofibre destruction and systemic immune activation — and (B) the treatment — side effects of the aggressive immunosuppression required to control the disease. Let's work through each systematically.
A. Disease-Related Complications
Two distinct mechanisms can cause respiratory compromise in IMNM, and distinguishing them is clinically critical because the treatments differ:
| Mechanism | Pathophysiology | Clinical Features | Key Investigation |
|---|---|---|---|
| Respiratory muscle weakness | Autoimmune necrosis of the diaphragm and intercostal muscles → reduced ability to generate negative intrathoracic pressure → hypoventilation → CO₂ retention and hypoxia | Orthopnoea (diaphragm most affected when supine), paradoxical abdominal breathing, morning headaches (CO₂ retention overnight), declining FVC | Serial FVC measurements (upright AND supine — >20% drop supine indicates diaphragm weakness); MIP/MEP; ABG showing type II respiratory failure |
| Interstitial lung disease (ILD) | Autoimmune alveolitis → fibrosis of alveolar-capillary membrane → impaired gas exchange | Progressive exertional dyspnoea, dry cough, bibasal fine inspiratory crackles | PFT: reduced FVC + reduced DLCO [4]; HRCT: ground glass opacities (active) ± honeycombing (fibrotic) [4] |
She has mild to moderate dysphagia upon speech therapist assessment… PFT shows a reduced forced vital capacity of 70% predicted and diffusion capacity of the lungs for carbon monoxide of 64% predicted… HRCT reveals ground glass opacities predominantly over the lower lobes. There was no honeycombing [4].
Why the distinction matters: Respiratory muscle weakness improves with immunosuppression + NIV support and can be fully reversible. ILD with established fibrosis (honeycombing) is irreversible, and the goal shifts to preventing progression. Ground glass opacities (active inflammation) may still respond to aggressive immunosuppression, whereas honeycombing does not.
ILD is less common in pure IMNM than in anti-synthetase syndrome or anti-MDA5+ DM, but it can occur — especially in overlap cases or anti-SRP+ IMNM. Always screen with PFT and HRCT.
Investigations of interstitial lung disease [4] — explicitly listed as a GC learning objective.
Respiratory Emergency — When to Escalate
If FVC falls below 1 litre or below 50% predicted, or if the patient has respiratory distress, they need ICU admission for monitoring and potential non-invasive ventilation (NIV/BiPAP) or even mechanical ventilation. Serial bedside FVC monitoring (like in GBS) is essential. Do NOT wait for an ABG to deteriorate before acting.
Difficulty in swallowing food with 2 episodes of choking in the past week [4]. Dysphagia, nasal regurgitation, aspiration, aspiration pneumonia [3].
| Step | Pathophysiology | Clinical Consequence |
|---|---|---|
| Pharyngeal striated muscle weakness | Autoimmune necrosis of the pharyngeal and upper oesophageal striated muscles → impaired propulsion of the food bolus | Difficulty swallowing solids → progressing to liquids; choking on food; nasal regurgitation (food comes out through the nose because the nasopharyngeal seal is weak) |
| Loss of airway protection | Weak suprahyoid muscles → poor laryngeal elevation → poor epiglottic closure during swallowing | Aspiration — food/liquid enters the trachea and lungs |
| Aspiration pneumonia | Aspirated material + oral bacteria → infection of the lower respiratory tract | Fever, productive cough, new consolidation on CXR; typically right lower lobe (due to anatomy of the right main bronchus being more vertical) |
Management: Speech therapist assessment (as in the GC case), modified diet consistency, PEG tube if severe and persistent, treat aspiration pneumonia with antibiotics (amoxicillin-clavulanate or piperacillin-tazobactam to cover oral anaerobes and Gram negatives).
This is one of the most dangerous acute complications of IMNM, particularly in severe anti-SRP+ disease:
| Step | Pathophysiology |
|---|---|
| 1. Massive myofibre necrosis | Antibody/complement-mediated sarcolemma rupture → CK often > 10,000–50,000 U/L |
| 2. Myoglobin release | Myoglobin (oxygen-binding protein in muscle) floods the bloodstream; normally bound to haptoglobin but overwhelms binding capacity |
| 3. Myoglobin filtered by kidneys | Free myoglobin is filtered into renal tubules |
| 4. Tubular obstruction and toxicity | Myoglobin precipitates in acidic urine → forms casts that obstruct tubules; myoglobin's haem moiety generates reactive oxygen species → direct tubular epithelial cell injury |
| 5. AKI | Oliguria → rising creatinine → uraemia; may require dialysis |
Clinical clues: Dark "cola-coloured" urine; urine dipstick positive for "blood" but no RBCs on microscopy (the dipstick detects the haem group in myoglobin, not actual haemoglobin from RBCs).
Prevention and management:
- Aggressive IV fluid resuscitation (target urine output > 200–300 mL/hr) to flush myoglobin through tubules
- Urine alkalinisation (IV sodium bicarbonate) — myoglobin precipitates less in alkaline urine
- Monitor for hyperkalaemia (K⁺ leaks from necrotic muscle) — may require urgent management (calcium gluconate, insulin-dextrose, salbutamol nebulisers, resonium, dialysis)
- Monitor for hypocalcaemia (calcium deposits in damaged muscle) and later hypercalcaemia (released during recovery phase)
- Dialysis if refractory AKI, life-threatening hyperkalaemia, or fluid overload
The heart is a muscle. In IMNM, particularly anti-SRP+ disease, the autoantibodies can target cardiac myocytes that express the same antigens (SRP is a ubiquitous ribonucleoprotein).
| Complication | Mechanism | Clinical Consequence |
|---|---|---|
| Myocarditis | Antibody/complement-mediated damage to cardiac myocytes | Chest pain, dyspnoea, heart failure, arrhythmias; ↑troponin; new wall motion abnormalities on echo |
| Cardiomyopathy | Chronic subclinical myocardial damage → fibrosis → progressive ventricular dysfunction | Dilated cardiomyopathy; ↓ejection fraction; heart failure |
| Arrhythmias / conduction defects | Inflammation or fibrosis involving the conduction system | Sinus tachycardia, atrial fibrillation, heart block, ventricular tachycardia; risk of sudden cardiac death |
Systemic involvement, eg. pulmonary fibrosis, arthralgia, Raynaud's phenomenon… [2] — cardiac involvement is part of the systemic autoimmune picture.
Monitoring: ECG + echocardiography at baseline and during follow-up. Cardiac MRI if clinical suspicion of myocarditis (late gadolinium enhancement = fibrosis; T2 oedema = active inflammation).
If IMNM is diagnosed late or remains poorly controlled, repeated cycles of necrosis → regeneration → necrosis eventually exhaust the muscle's regenerative capacity:
| Stage | Pathology | Imaging | Clinical Consequence |
|---|---|---|---|
| Early (active) | Necrosis + regeneration; satellite cells still active | MRI: T2/STIR hyperintensity (oedema) | Weakness is potentially reversible with treatment |
| Late (chronic) | Regenerative capacity exhausted → fatty replacement + fibrosis | MRI: T1 hyperintensity (fat signal replaces muscle) | Wasting/contractures (ONLY if chronic) [2] — weakness becomes irreversible even with effective immunosuppression |
This is why early, aggressive treatment is paramount. Once muscle is replaced by fat and fibrous tissue, no amount of immunosuppression will restore function.
Adult form a/w malignancy: 5× risk in dermatomyositis, 2× risk in polymyositis [2]. 1/3 malignancy (lung, breast, gastric, NPC) – usually diagnosed within 1 year of DM/PM [1].
Potential association between cancer and dermatomyositis and the rationale of cancer screening [4].
While the malignancy risk in IMNM overall is lower than in DM, seronegative IMNM has the highest cancer association among IMNM subtypes. This is both a complication and a co-morbidity:
- The malignancy may cause the IMNM (paraneoplastic — the tumour expresses muscle antigens, triggering cross-reactive autoimmunity)
- Temporal relationship: can be diagnosed before, with or after diagnosis of inflammatory myopathy [2]
- In HK: NPC (this locality) [2] must always be screened
- Cancer re-screening should be repeated annually for at least 3 years from diagnosis, as the malignancy may emerge after the myopathy
Patients with active IMNM are at increased risk of DVT and PE due to:
- Immobility (bedbound from severe weakness)
- Systemic inflammation (pro-thrombotic state — elevated inflammatory cytokines activate coagulation cascade)
- Active malignancy (if paraneoplastic — Trousseau syndrome)
Prevention: Prophylactic LMWH (enoxaparin) for hospitalised patients with reduced mobility; graduated compression stockings; early mobilisation with physiotherapy.
- Proximal weakness → unstable gait → high risk of falls
- Compounded by steroid-induced osteoporosis (treatment complication — see below)
- Falls with hip/vertebral fractures are a significant source of morbidity, especially in elderly patients
B. Treatment-Related Complications
Chronic high-dose steroid use is almost inevitable in IMNM management. The side effects are extensive and must be actively prevented:
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Osteoporosis → fractures | ↓Osteoblast activity + ↑osteoclast activity → bone loss | Calcium + Vitamin D; bisphosphonates (alendronate) if > 3 months use; DEXA monitoring |
| Steroid-induced diabetes | ↑Hepatic gluconeogenesis + ↓peripheral insulin sensitivity | Blood glucose monitoring; metformin or insulin if needed |
| Steroid myopathy | Type IIb fibre atrophy from protein catabolism | Monitor CK (normal in steroid myopathy) — key differentiator from disease relapse; minimise steroid dose |
| Immunosuppression → infections | ↓Cell-mediated and humoral immunity | PJP prophylaxis (co-trimoxazole) if high dose; monitor for TB, fungal infections |
| Cushing's syndrome | Exogenous hypercortisolism | Moon face, central obesity, striae, buffalo hump, thin skin, easy bruising — minimise dose/duration |
| Peptic ulcer / GI bleeding | ↓Prostaglandin-mediated mucosal protection | PPI prophylaxis |
| Adrenal suppression | Chronic exogenous steroids suppress HPA axis → adrenal atrophy | Never stop steroids abruptly; sick-day rules (stress dosing during illness/surgery) |
| AVN of femoral head | Disrupted blood supply to femoral head (mechanism debated: fat emboli vs microvascular thrombosis) | MRI if hip pain; orthopaedic referral |
| Cataracts / glaucoma | Posterior subcapsular cataracts; ↑intraocular pressure | Ophthalmology review annually |
| Psychiatric effects | Mood instability, insomnia, psychosis | Anticipate; consider psychiatry referral if severe |
| Agent | Key Complications | Mechanism | Monitoring |
|---|---|---|---|
| Azathioprine | Myelosuppression (leucopenia, pancytopenia); hepatotoxicity; ↑infection risk; nausea | Inhibits purine synthesis → affects all rapidly dividing cells; TPMT deficiency → cannot metabolise → toxic accumulation | FBC every 2 weeks initially → monthly; LFT; TPMT genotype before starting |
| Methotrexate | Myelosuppression; hepatotoxicity (steatohepatitis → cirrhosis with cumulative dose); pneumonitis (MTX lung); mucositis; teratogenicity | Folate antagonism → blocks DNA synthesis; idiosyncratic pulmonary hypersensitivity | FBC + LFT every 2–4 weeks; co-prescribe folic acid 5 mg weekly (reduces side effects without reducing efficacy); CXR if new cough/dyspnoea |
| Mycophenolate (MMF) | GI effects (diarrhoea, nausea — most common); cytopenias; ↑infection risk; teratogenicity | Inhibits inosine monophosphate dehydrogenase → blocks de novo purine synthesis in lymphocytes; GI effects from direct mucosal toxicity | FBC every 2 weeks initially; LFT |
| Cyclophosphamide | Haemorrhagic cystitis; myelosuppression; infertility; ↑risk of bladder cancer (cumulative); opportunistic infections | Acrolein metabolite is toxic to bladder urothelium; alkylates DNA of gonadal cells | FBC (nadir at 10–14 days); urinalysis; co-administer MESNA (binds acrolein in urine); encourage high fluid intake; sperm/egg banking in young patients |
| Tacrolimus | Nephrotoxicity (chronic tubulointerstitial nephritis); neurotoxicity (tremor, headache); hyperglycaemia; hypertension; hypomagnesaemia | Calcineurin inhibition → renal vasoconstriction → ↓GFR; direct pancreatic β-cell toxicity | Trough levels; RFT; glucose; Mg²⁺; BP |
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Infusion reactions | Cytokine release from lysed B cells (especially first dose) | Pre-medicate with paracetamol + antihistamine + IV methylprednisolone; slow initial infusion rate |
| HBV reactivation | B-cell depletion removes immune surveillance of latent HBV → viral rebound → fulminant hepatitis | Mandatory HBV screening pre-treatment; entecavir prophylaxis if HBsAg+ or anti-HBc+; continue for ≥12 months after last rituximab dose |
| Hypogammaglobulinaemia | Repeated B-cell depletion → reduced immunoglobulin production → acquired humoral immunodeficiency | Monitor IgG levels; if recurrent infections + low IgG → consider IVIG replacement |
| Progressive multifocal leukoencephalopathy (PML) | JC virus reactivation in immunosuppressed state → demyelinating CNS infection | Very rare but devastating; monitor for new neurological symptoms; MRI + CSF JC virus PCR if suspected |
| Increased infection risk | Humoral immune suppression → impaired opsonisation and bacterial clearance | Vaccinate before treatment (pneumococcal, influenza); low threshold for investigating febrile episodes |
| Complication | Mechanism |
|---|---|
| Headache / aseptic meningitis | Immune complex deposition in meninges; osmotic shift |
| Thromboembolic events (DVT, PE, stroke) | ↑Blood viscosity from high protein load; procoagulant factors in preparation |
| Renal failure | Osmotic nephrosis (especially with sucrose-containing formulations); renal tubular injury |
| Anaphylaxis | IgE-mediated reaction, especially in IgA-deficient patients (anti-IgA antibodies react with IgA in the IVIG preparation) |
| Haemolysis | Anti-A/anti-B isoagglutinins in IVIG preparations → passive haemolysis in non-O blood group recipients |
Prognosis: variable depending on type of myositis, severity, delay in dx and autoAb [10]. 5–10% mortality [15].
| Factor | Better Prognosis | Worse Prognosis |
|---|---|---|
| Autoantibody | Anti-HMGCR (especially statin-associated) | Anti-SRP (aggressive, refractory) [3] |
| Timing of treatment | Early diagnosis and treatment | Delayed diagnosis → irreversible fatty replacement |
| CK at presentation | Moderate elevation | Very high (> 50× ULN) with rhabdomyolysis |
| Organ involvement | Isolated myopathy | Cardiac involvement, ILD, severe dysphagia |
| Malignancy | No associated malignancy | Paraneoplastic — prognosis tied to cancer outcome |
| Age | Younger | Older (more comorbidities, higher malignancy risk) |
Response to steroid: in general, overlap myositis > DM > PM [10] — IMNM (especially anti-SRP+) is at the most refractory end of this spectrum. However, with modern combination therapy (steroids + immunosuppressant + IVIG + rituximab), many patients achieve good long-term outcomes.
High Yield Summary — Complications of IMNM
Disease-related:
- Respiratory failure: From respiratory muscle weakness (reversible) OR ILD (may be irreversible if fibrotic) — always differentiate the two
- Dysphagia → aspiration pneumonia: Pharyngeal/oesophageal striated muscle weakness; needs speech therapy assessment; PEG if severe
- Rhabdomyolysis → AKI: Massive CK → myoglobinuria → tubular obstruction/toxicity; treat with IV fluids, alkalinisation, monitor K⁺/Ca²⁺; may need dialysis
- Cardiac: Myocarditis, cardiomyopathy, arrhythmias — especially anti-SRP+; monitor with ECG/echo
- Irreversible muscle damage: Fatty replacement/fibrosis if chronic and untreated — irreversible even with immunosuppression
- Malignancy: Screen all patients; especially NPC in HK; seronegative IMNM has highest risk
- VTE: Immobility + inflammation + possible occult malignancy
- Falls/fractures: Weakness + steroid osteoporosis
Treatment-related:
- Steroids: Osteoporosis, DM, steroid myopathy (normal CK!), infections, Cushing's, adrenal suppression, AVN, cataracts
- Immunosuppressants: Myelosuppression, hepatotoxicity, infections, teratogenicity, haemorrhagic cystitis (CYC), nephrotoxicity (tacrolimus)
- Rituximab: HBV reactivation (screen before!), hypogammaglobulinaemia, PML (rare), infusion reactions
- IVIG: Thromboembolic events, renal failure, anaphylaxis (IgA deficiency), haemolysis
Active Recall - Complications of IMNM
References
[1] Senior notes: Maksim Medicine Notes.pdf (Rheumatology — Idiopathic inflammatory myopathies, p.318) [2] Senior notes: Ryan Ho Neurology.pdf (10.4.3 Inflammatory Myopathies, p.194) [3] Senior notes: Ryan Ho Rheumatology.pdf (Autoantibody-defined subsets table — anti-SRP, p.91) [4] Lecture slides: GC_Interactive tutorial (Rheum case 2) student copy.pdf (Further investigation findings, complications, management plan); Senior notes: Block A - Rheumatology Interactive Tutorial.pdf (Case 2 — ILD investigations, learning objectives) [10] Senior notes: Ryan Ho Rheumatology.pdf (Management section — prognosis, p.92) [15] Senior notes: Adrian Lui Pediatrics Notes.pdf (Inflammatory Myopathies — PM vs DM comparison, prognosis, p.146)
High Yield Summary
Autoimmune Necrotizing Myopathy (IMNM) — Key Points:
- Definition: Subtype of IIM characterised by myofibre necrosis with minimal inflammation on biopsy, distinct from PM/DM
- Pathophysiology: Antibody + complement-mediated (humoral), NOT T-cell mediated → MAC deposition → sarcolemma rupture → massive CK elevation
- Three subtypes by autoantibody: Anti-SRP+ (severe, refractory), Anti-HMGCR+ (statin-associated), Seronegative (highest malignancy risk)
- Clinical features: Symmetric proximal weakness, very high CK (often > 10× ULN), NO skin rash, may have dysphagia and cardiac involvement
- Statin association: Anti-HMGCR+ IMNM — statin upregulates HMGCR → neoantigen → autoimmune cycle that persists after statin withdrawal
- Cancer screening essential: Especially NPC in Hong Kong; malignancy can occur before, with, or after onset [2]
- Anti-SRP+: Aggressive disease refractory to high-dose steroids and immunosuppressants [3]
- Distinguish from: DM (skin rash), PM (CD8+ T-cell infiltrate on biopsy), IBM (distal weakness, non-responsive, inclusion bodies), toxic statin myopathy (resolves with statin cessation)
High Yield Summary — Differential Diagnosis of IMNM
- Closest mimics within IIM: DM (has skin rash), PM (CD8+ T-cell infiltrate), IBM (distal weakness, insidious, refractory), anti-synthetase (ILD, mechanic's hands)
- Drug-induced: Statin toxic myopathy (resolves with cessation) vs anti-HMGCR+ IMNM (persists); steroid myopathy (normal CK — the ironic trap)
- Endocrine: Hypothyroidism (check TFT), Cushing's, hypokalemia (check K⁺)
- NMJ: Myasthenia gravis — fatigable weakness, ocular involvement, normal CK
- Genetic: LGMD and myotonic dystrophy — family history, genetic testing
- Infectious: Viral myositis — acute, febrile, self-limiting
- Malignancy: Must screen in ALL IIM patients, especially seronegative IMNM; NPC in Hong Kong
- Key distinguishing features of IMNM: Very high CK (> 10× ULN), NO rash, subacute progression, necrosis without inflammation on biopsy, anti-SRP or anti-HMGCR positive, does NOT resolve with statin cessation
High Yield Summary — Diagnosis of IMNM
- No single diagnostic test — diagnosis requires integration of clinical, serological, electrophysiological, and histopathological data
- Bohan and Peter criteria (1975): PM requires all 4 clinical criteria; DM requires 3 + skin [1] — historical but still examined
- 2017 EULAR/ACR criteria: Probability-based scoring for IIM overall; does NOT sub-classify IMNM
- ENMC criteria for IMNM: Requires biopsy showing necrosis + regeneration + minimal inflammation ± MAC deposition; anti-SRP or anti-HMGCR; exclusion of other causes
- CK: Usually > 10× ULN — rarely normal (~5%) [10]; highest among all IIM subtypes
- Autoantibodies: Anti-SRP (severe, refractory) and anti-HMGCR (statin-associated) are IMNM-specific
- EMG: Myopathic pattern — short, polyphasic MUPs + fibrillation potentials [1][10]
- MRI: T2/STIR hyperintensity guides biopsy; T1 fatty replacement indicates chronicity
- Muscle biopsy: Gold standard — necrosis, regeneration, macrophage predominance, sparse lymphocytes, MAC on sarcolemma, no perifascicular atrophy, no rimmed vacuoles
- Always screen for malignancy — especially NPC in HK — and for ILD and cardiac involvement
High Yield Summary — Management of IMNM
- Stop statins permanently in anti-HMGCR+ IMNM (necessary but NOT sufficient — autoimmune cycle persists)
- Induction: High-dose steroids (prednisolone 1 mg/kg/day or IV methylprednisolone pulse if severe)
- Steroid-sparing agent: Start early — AZA, MTX, or MMF first-line; CYC or tacrolimus second-line
- Escalation for refractory disease: IVIG (2 g/kg, rapid onset, bridges to rituximab); Rituximab (anti-CD20, depletes B cells, reduces autoantibody production — now standard of care, not "experimental")
- Anti-SRP+ IMNM: Anticipate refractoriness — escalate early; consider IVIG + rituximab from outset
- Steroid myopathy vs relapse: Check CK — normal CK = steroid myopathy (reduce steroids); rising CK = relapse (increase immunosuppression)
- Supportive care: PT, speech therapy, PEG tube, respiratory monitoring, steroid prophylaxis (Ca/VitD, bisphosphonates, PPI, PJP prophylaxis)
- Malignancy: Screen and treat; may improve paraneoplastic myositis
- Monitoring: CK (falls before power improves), power, drug toxicity, autoantibody titres, cancer re-screening
High Yield Summary — Complications of IMNM
Disease-related:
- Respiratory failure: From respiratory muscle weakness (reversible) OR ILD (may be irreversible if fibrotic) — always differentiate the two
- Dysphagia → aspiration pneumonia: Pharyngeal/oesophageal striated muscle weakness; needs speech therapy assessment; PEG if severe
- Rhabdomyolysis → AKI: Massive CK → myoglobinuria → tubular obstruction/toxicity; treat with IV fluids, alkalinisation, monitor K⁺/Ca²⁺; may need dialysis
- Cardiac: Myocarditis, cardiomyopathy, arrhythmias — especially anti-SRP+; monitor with ECG/echo
- Irreversible muscle damage: Fatty replacement/fibrosis if chronic and untreated — irreversible even with immunosuppression
- Malignancy: Screen all patients; especially NPC in HK; seronegative IMNM has highest risk
- VTE: Immobility + inflammation + possible occult malignancy
- Falls/fractures: Weakness + steroid osteoporosis
Treatment-related:
- Steroids: Osteoporosis, DM, steroid myopathy (normal CK!), infections, Cushing's, adrenal suppression, AVN, cataracts
- Immunosuppressants: Myelosuppression, hepatotoxicity, infections, teratogenicity, haemorrhagic cystitis (CYC), nephrotoxicity (tacrolimus)
- Rituximab: HBV reactivation (screen before!), hypogammaglobulinaemia, PML (rare), infusion reactions
- IVIG: Thromboembolic events, renal failure, anaphylaxis (IgA deficiency), haemolysis
Inclusion Body Myositis
Inclusion body myositis is a chronic inflammatory myopathy characterized by slowly progressive asymmetric weakness, particularly of the finger flexors and quadriceps, with rimmed vacuoles and protein aggregates on muscle biopsy.
Overlap Myositis
Overlap myositis is an inflammatory myopathy that occurs in conjunction with features of another systemic autoimmune connective tissue disease, such as systemic lupus erythematosus, systemic sclerosis, or rheumatoid arthritis.