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.

Inclusion Body Myositis (IBM)

2. Epidemiology

Risk Factor	Explanation
Age > 50	The strongest demographic risk factor; incidence increases with age
Male sex	Male predominance, unlike other inflammatory myopathies
Caucasian ethnicity	Higher prevalence in Western populations
HLA-DRB1*03:01 (HLA-DR3)	The strongest genetic association; present in ~75% of IBM patients vs ~25% of controls. This HLA allele is involved in antigen presentation to CD8+ T cells, linking to the autoimmune component
Possible viral triggers	Some historical associations with HIV, HTLV-1 (theoretical, not well established)
Family history	Rare familial forms exist (hereditary inclusion body myopathy, which is a separate entity — not truly inflammatory)

HK Context

IBM is explicitly noted as "very rare in HK" ^[2]. In HK clinical practice, when encountering an elderly patient with treatment-refractory myopathy, IBM should still be considered in the differential but is far less common than in Western populations. In HK, the idiopathic inflammatory myopathies you encounter are predominantly PM and DM, often with associations to malignancy (especially NPC in this locality).

3. Anatomy and Function

Understanding the specific muscles targeted by IBM is critical because it produces a highly characteristic pattern that distinguishes it from all other inflammatory myopathies.

Key muscles affected in IBM ^[1]:

Muscle Group	Specific Muscles	Function	Clinical Consequence
Quadriceps	Rectus femoris, vastus lateralis/medialis/intermedius	Knee extension, maintaining upright posture	Frequent falls, difficulty rising from chairs, difficulty climbing stairs, knee buckling
Volar forearm muscles	Finger flexors (flexor digitorum profundus, flexor digitorum superficialis)	Grip strength, pinching	Weak grip, difficulty opening jars, turning keys
Ankle dorsiflexors	Tibialis anterior, extensor digitorum longus	Dorsiflexion of foot (foot clearance during gait)	Foot drop, tripping
Axial muscles (later)	Neck flexors, trunk muscles, pharyngeal muscles	Head support, swallowing	Head drop, dysphagia

"Quadriceps, volar muscles, dorsiflexors → axial muscles" — this describes the typical order of progression ^[1]

4. Aetiology and Pathophysiology

IBM has a complex, incompletely understood pathogenesis involving two intertwined processes:

This is the feature that truly sets IBM apart from PM and DM:

Rimmed vacuoles
- Rimmed vacuoles visible on modified Gomori trichrome stain ^[1]
- These represent collections of autophagic debris — essentially, the cell's garbage disposal system (autophagy) is failing
- The "rim" is composed of membranous material and granular debris
- Why do they form? → Impaired autophagosome-lysosome fusion leads to accumulation of autophagic vacuoles
Congophilic inclusions (amyloid-like protein deposits)
- Congophilic inclusions (amyloidogenic proteins) ^[1]
- Congo red stain shows apple-green birefringence under polarised light — the same staining pattern seen in amyloidosis
- These deposits contain:
  - Amyloid-β (Aβ) — the same protein seen in Alzheimer's disease plaques
  - Phosphorylated tau — also seen in Alzheimer's
  - TDP-43 — also seen in ALS and frontotemporal dementia
  - p62/SQSTM1 — an autophagy receptor (its accumulation indicates autophagic failure)
- Why do these accumulate? → Impaired proteostasis (protein quality control) — the proteasome and autophagy pathways cannot keep up with the rate of misfolded protein production
Mitochondrial dysfunction
- Cytochrome c oxidase (COX)-negative fibres are common on muscle biopsy
- These represent muscle fibres with dysfunctional mitochondria (specifically Complex IV)
- Mitochondrial DNA deletions accumulate with age, and this process is accelerated in IBM
- Consequence: reduced ATP production → contributes to muscle fibre atrophy and weakness

Why Does IBM Not Respond to Immunotherapy?

The key insight is that IBM is not purely inflammatory. Even if you suppress the CD8+ T cell infiltration with immunosuppression, the degenerative process (amyloid deposition, rimmed vacuoles, mitochondrial dysfunction) continues unabated. This is why trials of prednisone, IVIG, methotrexate, and biologics have all failed to show meaningful benefit. "Immuno-therapy not beneficial" ^[1]. You are treating one arm of the disease while the other arm progresses unchecked.

Feature	Inflammatory Component	Degenerative Component
Pathological hallmark	CD8+ T cell invasion of non-necrotic fibres	Rimmed vacuoles, congophilic inclusions
Mediators	CTLs, macrophages, pro-inflammatory cytokines	Aβ, tau, TDP-43, p62
Mechanism	Autoimmune attack on MHC-I-expressing muscle	Proteostatic failure + mitochondrial dysfunction
Response to immunotherapy	Partial (may reduce inflammation histologically)	None
Analogy	Similar to polymyositis	Similar to Alzheimer's disease (in muscle)

5. Classification

IBM is classified as one of the major subtypes of idiopathic inflammatory myopathy (also called autoimmune inflammatory myopathies, AIM):

Subtype	Key Features
Dermatomyositis (DM)	Skin rash (heliotrope, Gottron's papules) + proximal weakness; humoral immunity (complement-mediated microangiopathy); a/w malignancy (5× risk) ^[4]
Polymyositis (PM)	Proximal weakness without skin rash; CD8+ T cell-mediated; diagnosis of exclusion
Inclusion body myositis (IBM)	Proximal + distal weakness; oldest onset; male predominance; rimmed vacuoles; treatment-refractory ^[1]
Immune-mediated necrotizing myopathy (IMNM)	Severe proximal weakness, very high CK; muscle necrosis with minimal inflammation; a/w anti-SRP or anti-HMGCR (statin-associated) antibodies ^[2]
Anti-synthetase syndrome	Myositis + ILD + mechanic's hands + Raynaud's + arthritis; anti-Jo-1 or other anti-synthetase antibodies ^[5]
Overlap myositis	Myositis occurring in context of another CTD (SLE, scleroderma, MCTD)
Clinically amyopathic DM (CADM)	Skin manifestations of DM but no clinical muscle weakness for ≥ 6 months; anti-MDA-5 → rapidly progressive ILD ^[2]

This distinction is critical:

Feature	Sporadic IBM (sIBM)	Hereditary IBM (hIBM)
Inheritance	Acquired (sporadic)	Autosomal recessive (most common: GNE myopathy)
Age of onset	> 50 years	20–30 years
Inflammation	Present (CD8+ T cells)	Absent or minimal
Rimmed vacuoles	Present	Present
Quadriceps	Affected early	Characteristically SPARED (until very late)
Response to immunotherapy	Poor	Not applicable

Hereditary IBM is an entirely different disease entity — it is a myopathy with rimmed vacuoles but without significant inflammation. The quadriceps-sparing pattern is almost pathognomonic for GNE myopathy (the most common form of hIBM).

6. Clinical Features

A. Symptoms (with Pathophysiological Basis)

B. Signs (with Pathophysiological Basis)

Feature	IBM	PM/DM
Proximal weakness	Yes (quadriceps > hip flexors)	Yes (symmetric, shoulder/hip girdle)
Distal weakness	Yes (finger flexors, dorsiflexors)	No (or late, mild)
Symmetry	Often asymmetric	Symmetric
Facial weakness	Uncommon	Uncommon
Ocular weakness	No	No

"Slowly progressive, life expectancy not significantly affected" ^[1]
IBM is a chronic, indolent disease — it does NOT typically shorten life expectancy significantly
However, it causes significant disability over time:
- Mean time to requiring a cane: ~5–10 years from onset
- Mean time to wheelchair dependence: ~10–15 years
Most common cause of death (when related to IBM): aspiration pneumonia from dysphagia
Unlike PM/DM, there is no association with malignancy in IBM

High Yield Comparison: IBM vs PM vs DM

Feature	IBM	PM	DM
Age	> 50	Any (peak 40-50)	Any (peak 40-50)
Sex	M > F	F > M	F > M
Onset	Insidious (years)	Subacute (weeks-months)	Acute/subacute
Distribution	Proximal + distal, often asymmetric	Proximal, symmetric	Proximal, symmetric
Key muscles	Quadriceps, finger flexors, dorsiflexors	Shoulder/hip girdle	Shoulder/hip girdle
Skin	None	None	Heliotrope, Gottron's, shawl/V sign
CK	Normal to 10× ULN	Very high (10-50× ULN)	Moderate-high
Biopsy	Rimmed vacuoles, CD8+ T cells, congophilic inclusions	CD8+ T cell endomysial infiltration	Perifascicular atrophy, complement on capillaries
Antibody	Anti-cN1A	Various MSA	Anti-Mi-2, anti-MDA-5, anti-TIF1-γ, anti-NXP-2
Treatment response	Immuno-therapy not beneficial	Responds to steroids ± steroid-sparing	Responds to steroids ± steroid-sparing
Malignancy	No association	2× risk	5× risk (anti-TIF1-γ, anti-NXP-2 highest risk)

7. Associations and Comorbidities

Differential Diagnosis of Inclusion Body Myositis

A. Mimics Within the Idiopathic Inflammatory Myopathies (IIMs)

This is the single most important differential because IBM is very frequently misdiagnosed as PM, often for years ^[3].

Feature	IBM	PM
Age	> 50	Any age (peak 40–50)
Sex	M > F	F > M
Onset	Insidious over years	Subacute over weeks to months
Distribution	Proximal + distal; often asymmetric	Symmetrical proximal (shoulder girdle + hip girdle) ^[4]
Key muscles	Quadriceps, volar forearm (finger flexors), dorsiflexors ^[5]	Deltoids, hip flexors, neck flexors
Distal weakness	Early and prominent	Late and mild, if present
CK	Normal or up to 10-fold above ULN ^[5]	Usually > 10× ULN, often 10–50×
EMG	Mixed myopathic + neurogenic features	Pure myopathic (spontaneous fibrillation; polyphasic low-amplitude motor unit potential) ^[4]
Biopsy	Rimmed vacuoles, congophilic inclusions, CD8+ T cells invading non-necrotic fibres ^[5]	Endomysial CD8+ T cell infiltrates, no rimmed vacuoles ^[7]
Autoantibody	Anti-cN1A ^[5]	Various MSAs (anti-SRP, anti-Jo1)
Response to steroids	Immunotherapy not beneficial ^[5]	Responds to steroids ± steroid-sparing agents
Malignancy risk	None	2× risk ^[6]

Why is this distinction critical?

If you misdiagnose IBM as PM and treat with high-dose corticosteroids, the patient will not improve. Worse, prolonged steroid use causes steroid myopathy (which itself causes proximal weakness), compounding the problem. The clue should be: an elderly man with "PM" who does not respond to steroids should be re-evaluated for IBM.

Clinical Pearl — The 'Red Flag' for IBM Misdiagnosed as PM

A patient diagnosed with polymyositis who fails to respond to steroids should immediately raise suspicion for IBM. Other red flags: male sex, age > 50, distal (finger flexor) weakness, asymmetric involvement, and only modestly elevated CK. Always consider repeat muscle biopsy looking for rimmed vacuoles if initially absent.

B. Non-Inflammatory Myopathies

Key drugs that cause myopathy ^[1]^[3]^[10]:

Drug	Mechanism	Clinical Features	How to Distinguish from IBM
Glucocorticoids ^[3]	Type II fibre atrophy from catabolic protein breakdown	Proximal weakness (especially pelvic girdle), no pain, normal CK	Temporal relationship with steroid use; improves on dose reduction; CK is normal
Statins (HMG-CoA reductase inhibitors) ^[3]	Direct myotoxicity (mechanism incompletely understood; possibly mitochondrial dysfunction, impaired isoprenoid synthesis)	Myalgia ± weakness, CK elevated; ranges from simple myalgia to frank rhabdomyolysis	Usually resolves within weeks of stopping the statin; CK normalises. If weakness persists after statin withdrawal, consider anti-HMGCR IMNM
Colchicine ^[10]	Disrupts microtubule function → impaired autophagy and protein trafficking	Proximal myopathy + neuropathy (neuromyopathy), especially in renal impairment	Drug history; often combined myopathy + neuropathy; resolves on stopping drug
Alcohol ^[10]	Direct myotoxicity + nutritional deficiency	Acute (rhabdomyolysis) or chronic (proximal weakness)	Social history; acute form has very high CK + myoglobinuria

A particularly important scenario in HK: an elderly patient on both steroids (for suspected PM) AND statins (for cardiovascular risk) who develops worsening weakness may have drug-induced myopathy layered on top of IBM, confounding the picture.

Steroid Myopathy vs. Worsening PM/IBM

This is a classic clinical trap. A patient with presumed PM is treated with high-dose steroids. They initially improve (inflammatory component dampened), then plateau or worsen. Is the disease relapsing, or is the steroid causing its own myopathy? Key differentiator: in steroid myopathy, CK is normal (steroids don't cause muscle necrosis, just atrophy). In active myositis relapse, CK rises again. In IBM, CK was never very high to begin with.

Condition	Mechanism	Features	Why Not IBM
Hypothyroidism ^[1]^[3]	Impaired muscle energy metabolism + ↓protein synthesis	Proximal weakness, myalgia, ↑CK, delayed relaxation phase of reflexes	Check TFT; responds to thyroid replacement; weakness symmetric and purely proximal
Cushing's syndrome ^[3]	Glucocorticoid-induced type II fibre atrophy (same mechanism as steroid myopathy)	Proximal weakness, Cushingoid features	Clinical features of Cushing's; 24h urine cortisol / overnight dexamethasone suppression test
Electrolyte disturbance: hypokalemia, hypocalcaemia, hypophosphataemia ^[3]	Abnormal membrane potential / excitation-contraction coupling	Acute or subacute weakness, often diffuse	Check electrolytes; weakness resolves with correction

C. Inherited Myopathies / Muscular Dystrophies

D. Non-Myopathic Mimics (Is It Even a Myopathy?)

These conditions affect different levels of the neuraxis but can superficially resemble IBM:

ALS can cause progressive weakness in the same age group (> 50, M > F)
Both proximal and distal weakness can occur
Key differences from IBM:

Feature	IBM	ALS
UMN signs	Absent	Present (hyperreflexia, Babinski, spasticity)
LMN signs	Atrophy, reduced reflexes	Fasciculations (hallmark), atrophy
Sensory	Normal	Normal
Bulbar	Dysphagia (late)	Dysarthria + dysphagia (can be early)
CK	Mild elevation	Mild elevation possible
Progression	Slow (years)	Rapid (months)
EMG	Myopathic	Neurogenic (widespread denervation)

The presence of fasciculations strongly favours ALS. Fasciculations are absent in IBM.
ALS has both UMN AND LMN signs (mixed picture); IBM is purely LMN/myopathic with no UMN signs.

Distinguished from myositis by presence of facial muscle weakness, normal muscle enzymes, characteristic EMG changes and AChR antibodies ^[3]:

Feature	IBM	MG
Weakness pattern	Fixed, progressive	Fatigable, fluctuating (worse at end of day)
Ocular involvement	None	Ptosis, diplopia (hallmark)
Muscle wasting	Prominent	Absent
CK	Mild elevation	Normal
Reflexes	Reduced	Normal
EMG	Myopathic	Decremental response on repetitive nerve stimulation (RNS)
Antibodies	Anti-cN1A	Anti-AChR (85%) or anti-MuSK

Why MG ≠ IBM: The hallmark of MG is fatigability — weakness worsens with repetitive use and recovers with rest. IBM weakness is fixed and does not fluctuate. MG affects ocular muscles (ptosis, diplopia); IBM does not. MG does not cause muscle wasting; IBM causes prominent wasting.

E. Infectious and Other Causes

The key "anchor points" that distinguish IBM from its differentials are:

Distinguishing Feature	What It Excludes
Age > 50, male	DM/PM (younger, female-predominant)
Finger flexor weakness	PM, DM, IMNM, endocrine myopathy (all purely proximal)
Asymmetry	PM, DM, IMNM (symmetric)
CK ≤ 10× ULN	PM, DM, IMNM (much higher CK)
No skin rash	DM
No myotonia	Myotonic dystrophy
No fasciculations	ALS/MND
No fatigability	MG, LEMS
No sensory loss	Neuropathies (CIDP, diabetic), myelopathy
No UMN signs	ALS/MND, myelopathy
Failed steroid therapy	PM, DM, IMNM (all respond to immunosuppression)
Rimmed vacuoles + congophilic inclusions on biopsy	All other myopathies

High Yield Summary — DDx of IBM

Most commonly misdiagnosed as PM — look for the red flags: male > 50, distal weakness, asymmetry, low CK, steroid non-response
DM is excluded by absence of skin manifestations (heliotrope, Gottron's, V sign, shawl sign)
Drug-induced myopathy (statins, glucocorticoids, colchicine) must always be excluded by drug history ^[1]^[3]
Endocrine causes (hypothyroidism, Cushing's) excluded by TFT and cortisol assessment ^[1]^[3]
ALS/MND excluded by absence of UMN signs and fasciculations
MG excluded by absence of fatigability, ocular involvement, and normal CK
Myotonic dystrophy excluded by absence of myotonia, younger age, multisystem features
Hereditary IBM (GNE myopathy) is a different disease — younger onset, quadriceps-sparing, no inflammation
Muscle biopsy is definitive: rimmed vacuoles, CD8+ T cells invading non-necrotic fibres, congophilic inclusions ^[5]

Active Recall - Differential Diagnosis of IBM

References

^[1] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (slide: Differential Diagnosis of Myopathy) ^[2] Senior notes: Maksim Medicine Notes.pdf (p.318, Idiopathic inflammatory myopathies) ^[3] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1757, Differential diagnosis of DM/PM; p.1116, Differential diagnosis of myopathy) ^[4] Senior notes: Maksim Medicine Notes.pdf (p.318, Clinical features and diagnostic criteria) ^[5] Lecture slides: GC 056. Generalized muscle weakness.pdf (slide: Inclusion Body Myositis) ^[6] Senior notes: Ryan Ho Neurology.pdf (p.194, Inflammatory Myopathies — malignancy association) ^[7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1760, Muscle biopsy findings) ^[8] Senior notes: Block A - Dermatology PBL 2.pdf (p.6-7, Dermatomyositis cases and antibody-malignancy associations) ^[9] Lecture slides: GC_Interactive tutorial (Rheum case 2) student copy.pdf (p.6, anti-Jo1 and anti-synthetase syndrome) ^[10] Senior notes: Ryan Ho Neurology.pdf (p.191, Diseases of Muscles — inherited and acquired causes table) ^[11] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.145, Myotonic Dystrophy) ^[12] Senior notes: Block A - Rheumatology Interactive Tutorial.pdf (p.2, cervical myelopathy as DDx)

Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Inclusion Body Myositis

2. Diagnostic Criteria

These are the most widely used IBM-specific criteria. They define three levels of diagnostic certainty:

Category	Requirements
Clinicopathologically defined IBM	Clinical features (duration > 12 months, age > 45, CK ≤ 15× ULN, weakness of knee extension ≥ hip flexion AND/OR finger flexion > shoulder abduction) PLUS pathological features (endomysial inflammatory infiltrate + rimmed vacuoles + protein accumulation or 15–18 nm filaments)
Clinically defined IBM	Clinical features as above PLUS biopsy showing one of: (a) inflammatory infiltrate invading non-necrotic fibres, or (b) rimmed vacuoles, or (c) ↑MHC-I expression — but not all three together
Probable IBM	Clinical features are typical BUT biopsy is inconclusive (e.g., inflammation only, no rimmed vacuoles)

Key clinical criteria details (ENMC 2011):

Duration of weakness: > 12 months
Age of onset: > 45 years
CK ≤ 15× ULN (usually < 10× ULN; recall from GC lecture: CK normal or up to 10-fold above ULN ^[5])
Weakness pattern meeting at least one of:
- Knee extension weakness ≥ hip flexion weakness (i.e., quadriceps more or equally affected as hip flexors — unusual for PM/DM where hip flexors are usually weaker than knee extensors)
- Finger flexion weakness > shoulder abduction weakness (the hallmark distal-more-than-proximal upper limb pattern)

4. Investigation Modalities

Why we check it: CK (creatine kinase) is an enzyme found inside muscle cells. When the sarcolemma (muscle cell membrane) is damaged, CK leaks into the blood. The degree of CK elevation correlates roughly with the degree of ongoing muscle necrosis — not with weakness or disease severity.

CK normal or up to 10-fold above ULN in IBM ^[5]

Context	CK Level	Interpretation
Normal	< 200 IU/L	—
IBM	Normal to ≤ 10× ULN ^[5] (rarely up to 15×)	Modest elevation because the degenerative process (amyloid, vacuoles) does not cause massive necrosis like PM/DM
PM/DM	Usually > 10× ULN, often 10–50× ^[7]	Active inflammatory muscle fibre necrosis
IMNM	> 50–100× ULN possible	Massive necrosis with minimal inflammation
Statin myopathy	Variable; rhabdomyolysis → > 10,000 ^[10]	Direct myotoxicity
Steroid myopathy	Normal	Type II fibre atrophy without necrosis — no CK leak

CK values: rarely normal (~5%), usually > 10× ULN, may even be > 50–100× ULN in severe cases — this describes PM/DM, not IBM ^[7]

Key principle: In IBM, the CK is modest because the disease is slowly progressive with gradual fibre loss rather than acute waves of necrosis. A very high CK (> 15× ULN) should prompt you to reconsider PM, DM, or IMNM.

Other muscle enzymes:

LDH, AST, ALT, aldolase may also be elevated ^[14] — AST and ALT elevation from muscle damage can be mistaken for liver disease if the clinician doesn't check CK

B. Autoantibodies

Checking MSAs is important to exclude other IIMs:

Autoantibodies: Anti-Mi2, anti-NXP2, anti-TIF1, anti-Jo1 (associated with interstitial lung disease) ^[15]

Myositis-specific antibodies (only in myositis): anti-synthetase (anti-Jo1), anti-SRP, anti-Mi2, anti-MDA5 ^[7]

Antibody	Associated Condition	Clinical Association
Anti-Jo-1 (most common anti-synthetase)	Anti-synthetase syndrome	ILD, mechanic's hands, Raynaud's, arthritis ^[9]^[14]
Anti-Mi-2	Classic DM	Good prognosis, skin-predominant
Anti-TIF1-γ	DM	High association with malignancy ^[8]
Anti-NXP-2	DM	High association with malignancy ^[8]
Anti-MDA-5	Clinically amyopathic DM (CADM) ^[4]	Rapidly progressive ILD
Anti-SRP	IMNM	Severe necrotising myopathy, poor prognosis
Anti-HMGCR	IMNM (statin-associated)	Persists after statin withdrawal

If any of these are positive, the diagnosis is NOT IBM — it is the corresponding IIM subtype.

C. Electromyography (EMG) and Nerve Conduction Studies (NCS)

Why we do it: EMG is the single most useful electrophysiological test for distinguishing myopathic from neuropathic causes of weakness ^[14]^[13].

EMG: most useful in distinguishing myopathic causes of weakness from neuropathic cause ^[14]

IBM has a uniquely mixed electrophysiological pattern:

Feature	Typical Myopathic Pattern	Typical Neurogenic Pattern	IBM Pattern
Spontaneous activity	Fibrillation potentials, positive sharp waves	Fibrillation potentials	Present (fibrillation, positive sharp waves)
Motor unit potentials (MUPs)	Low amplitude, short duration, polyphasic ^[4]^[14]	Large amplitude, long duration, polyphasic	BOTH: short-duration myopathic MUPs AND long-duration neurogenic MUPs
Recruitment	Early recruitment (more MUPs fire to compensate for weak individual fibres)	Reduced recruitment	Mixed
Complex repetitive discharges	May be present	Rare	Often present

Why is the EMG mixed in IBM? This is because IBM has two pathological processes:

Inflammatory/necrotic process → ongoing muscle fibre loss → myopathic features (small MUPs)
Chronic denervation-like changes from long-standing fibre loss → surviving motor units undergo collateral reinnervation → neurogenic features (large MUPs)

This mixed myopathic + neurogenic pattern is characteristic of IBM and helps distinguish it from PM (which is purely myopathic). It can, however, lead to confusion with MND if the examiner is not aware of IBM as a possibility.

Electromyography shows low amplitude motor-unit potentials with occasional fibrillation potentials, compatible with inflammatory myopathy — this describes the standard IIM pattern seen in the GC rheumatology interactive tutorial ^[9]

Why we do it: MRI is sensitive (but non-specific) for detecting muscle inflammation and helps guide biopsy site selection.

MRI: sensitive but non-specific; findings include patchy ↑T2W indicating inflammation, oedema ^[7]

MRI Findings in IBM

Sequence	Finding	Interpretation
T1-weighted	Fatty infiltration in affected muscles (quadriceps, medial gastrocnemius, finger flexors)	Indicates chronic, irreversible muscle damage and fibrosis
T2-weighted / STIR	High signal (oedema) in affected muscles	Indicates active inflammation/oedema
Pattern	Selective involvement — quadriceps (especially vastus lateralis/medialis), medial > lateral gastrocnemius, forearm flexors	The pattern itself is diagnostically helpful

Characteristic MRI pattern in IBM:

Quadriceps: diffuse fatty replacement, often asymmetric — vastus lateralis and medialis more affected than rectus femoris
Medial gastrocnemius preferentially affected (lateral gastrocnemius relatively spared)
Forearm flexor compartment affected more than extensor compartment
Relative preservation of posterior thigh muscles (hamstrings) — contrast with PM/DM where inflammation is more diffuse

MRI vs. biopsy guidance: MRI helps identify muscles with active inflammation (T2 bright) that are still worth biopsying — you want to biopsy a muscle that is inflamed but not yet replaced by fat. A muscle that is fully fatty on T1 is "burnt out" and biopsy will show only end-stage changes ^[7].

E. Muscle Biopsy — The Gold Standard

Muscle biopsy should be done on weak but not atrophied muscle, guided by P/E, EMG ± MRI ^[7]

This is the definitive investigation. There is no substitute.

Inflammatory infiltrates (CD8+ T cells, macrophages) invading non-necrotic muscle fibres, rimmed vacuoles, congophilic inclusions (amyloidogenic proteins) ^[5]

Finding	Stain/Method	Description	Significance
1. Endomysial inflammatory infiltrate	H&E, immunohistochemistry	CD8+ cytotoxic T cells + macrophages surrounding and invading non-necrotic muscle fibres (partial invasion)	The inflammatory component — similar to PM but with the critical addition of invading non-necrotic fibres
2. Rimmed vacuoles	Modified Gomori trichrome (MGT)	Vacuoles within muscle fibres with a "rim" of basophilic granular material	The degenerative component — represent failed autophagic vacuoles filled with membranous debris
3. Congophilic inclusions	Congo red stain with polarised light (apple-green birefringence)	Amyloid-like protein deposits containing Aβ, phosphorylated tau, TDP-43, p62	Hallmark of the proteinopathy/degenerative process — analogous to Alzheimer pathology in muscle
4. MHC class I upregulation	Immunohistochemistry	Diffuse sarcolemmal MHC-I expression on muscle fibres	Normally muscle does not express MHC-I; upregulation makes fibres visible to CD8+ T cells
5. COX-negative fibres	COX/SDH stain	Fibres that lack cytochrome c oxidase staining	Indicates mitochondrial dysfunction — mitochondrial DNA deletions
6. 15–18 nm tubulofilaments	Electron microscopy	Filamentous inclusions in cytoplasm or nucleus	Historically considered pathognomonic; now known to contain amyloid-related proteins

Feature	IBM	PM	DM	IMNM
Inflammatory pattern	Endomysial (CD8+ T cells invade non-necrotic fibres)	Endomysial (CD8+ T cells invade fibres) ^[7]	Perimysial/perivascular (B cells, CD4+ T cells, plasmacytoid DCs) ^[14]	Minimal or absent inflammation
Rimmed vacuoles	Present (pathognomonic for IBM among IIMs)	Absent	Absent	Absent
Congophilic inclusions	Present	Absent	Absent	Absent
Necrosis	Mild	Moderate	Moderate	Prominent (hallmark)
Perifascicular atrophy	Absent	Absent	Present (hallmark of DM) ^[14]	Absent
MHC-I upregulation	Diffuse	Diffuse	Perifascicular	Variable
Complement on capillaries	Absent	Absent	Present (MAC deposition)	Absent
COX-negative fibres	Present	Rare	Rare	Rare

High-Yield — Biopsy Findings That Distinguish IBM from PM

The biopsy of IBM can look superficially similar to PM (both have CD8+ T cell endomysial infiltration). The distinguishing features are: (1) rimmed vacuoles — present only in IBM, (2) congophilic inclusions — present only in IBM, (3) COX-negative fibres — much more common in IBM, (4) mixed fibre sizes with grouped atrophy — more prominent in IBM. If the biopsy shows endomysial inflammation WITHOUT rimmed vacuoles, the diagnosis may be "probable IBM" if the clinical phenotype fits, but the specific diagnosis is less certain.

These are done not to diagnose IBM per se, but to exclude secondary causes and assess for comorbidities:

Investigation	Purpose	Expected in IBM
CBC	Baseline; exclude anaemia of chronic disease	Usually normal
L/RFT	Baseline renal/liver function; note AST/ALT may be elevated from muscle (not liver)	Normal; AST/ALT may be mildly elevated
TFT	R/o thyroid myopathy ^[7]^[13]	Normal (hypothyroidism excluded)
ESR/CRP	Inflammatory markers	Usually normal or mildly elevated (unlike PM/DM where ESR/CRP are often significantly raised)
Electrolytes (K, Ca, PO4, Na)	R/o electrolyte-induced myopathy ^[3]	Normal
Glucose/HbA1c	Exclude diabetes (may cause neuropathy mimicking weakness)	Normal or incidental DM
Vitamin B12	Exclude B12 deficiency myelopathy/neuropathy	Normal
ECG ± Echo	Detect cardiac muscle involvement ^[10]	Usually normal (IBM does not significantly affect heart)
CXR	Screen for ILD (relevant for DM/PM, not typically IBM)	Normal

Investigation Tier	Tests	Purpose
Tier 1: Blood tests	CK, CBC, L/RFT, TFT, ESR/CRP, electrolytes, glucose	Confirm myopathy, exclude secondary causes
Tier 2: Autoantibodies	Anti-cN1A; MSA panel (anti-Jo1, anti-Mi2, anti-SRP, anti-HMGCR, anti-MDA5, anti-TIF1-γ, anti-NXP-2); MAAs (anti-Ro, anti-La, ANA)	Support IBM diagnosis; exclude other IIMs and overlap CTD
Tier 3: Electrophysiology	EMG + NCS	Confirm myopathic process; mixed myopathic + neurogenic pattern suggests IBM; NCS normal excludes neuropathy
Tier 4: Imaging	MRI of affected muscle groups	Identify pattern of involvement; guide biopsy site; distinguish inflammation (T2 bright) from irreversible fatty replacement (T1 bright)
Tier 5: Definitive	Muscle biopsy (open preferred)	Rimmed vacuoles + CD8+ T cell invasion of non-necrotic fibres + congophilic inclusions → clinicopathologically defined IBM
Tier 6: Functional	Swallowing assessment (SLT ± videofluoroscopy)	Assess dysphagia severity and aspiration risk

High Yield Summary — Diagnosis of IBM

Diagnostic criteria: ENMC 2011 criteria define clinicopathologically defined, clinically defined, and probable IBM based on clinical phenotype + biopsy
Bohan-Peter criteria (1975) are for PM/DM, NOT IBM — IBM does not fit these criteria
CK: normal or up to 10-fold above ULN ^[5] — much lower than PM/DM
EMG: mixed myopathic + neurogenic pattern (unique to IBM among IIMs)
Muscle biopsy is the gold standard — rimmed vacuoles, CD8+ T cells invading non-necrotic fibres, congophilic inclusions ^[5]; biopsy from weak but not atrophied muscle ^[7]
Anti-cN1A: sensitivity 33–76%, specificity ~90% — supportive but not required
MSAs should be NEGATIVE in IBM — if positive, consider DM, PM, IMNM, or anti-synthetase syndrome instead
No malignancy screening needed (unlike DM/PM)
MRI pattern: selective fatty replacement of quadriceps + medial gastrocnemius + forearm flexors
Immunotherapy not beneficial ^[5] — treatment failure itself is a diagnostic clue

Active Recall - IBM Diagnosis and Investigations

References

^[3] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1116, Differential diagnosis of myopathies table) ^[4] Senior notes: Maksim Medicine Notes.pdf (p.318–320, Diagnostic criteria and classification) ^[5] Lecture slides: GC 056. Generalized muscle weakness.pdf (slide: Inclusion Body Myositis) ^[6] Senior notes: Ryan Ho Neurology.pdf (p.194, Inflammatory Myopathies — malignancy association) ^[7] Senior notes: Ryan Ho Rheumatology.pdf (p.92, Diagnosis of PM/DM) ^[8] Senior notes: Block A - Dermatology PBL 2.pdf (p.7, anti-TIF1-γ and anti-NXP-2 malignancy association) ^[9] Lecture slides: GC_Interactive tutorial (Rheum case 2) student copy.pdf (p.1 and p.6) ^[10] Senior notes: Ryan Ho Neurology.pdf (p.191–192, Investigations for muscle diseases) ^[13] Senior notes: Ryan Ho Fundamentals.pdf (p.336, Generalised Weakness differential and investigation table) ^[14] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1758–1760, Diagnostic criteria, biochemical tests, muscle biopsy) ^[15] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (p.40, Dermatomyositis investigations) ^[16] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.709, NCS and EMG in inflammatory myopathy)

Management Algorithm and Treatment Modalities for Inclusion Body Myositis

3. Non-Pharmacological Management (The Cornerstone)

Since no drug has proven efficacy in IBM, supportive and rehabilitative care is the mainstay of management. This is not "giving up" — it is providing the interventions that actually improve the patient's quality of life and functional independence.

The occupational therapist (OT) is essential for maintaining independence:

Problem	Adaptive Solution
Weak finger flexors → difficulty gripping	Built-up handles on utensils, button hooks, key turners, electric jar openers
Quadriceps weakness → difficulty rising from chairs	Raised toilet seats, chair risers, grab rails
Foot drop → tripping	Ankle-foot orthosis (AFO) to maintain dorsiflexion during gait
Falls risk	Home modifications: remove loose rugs, install handrails, adequate lighting
Progressive mobility loss	Walking stick → rollator walker → wheelchair (progressive as needed)

4. Pharmacological Management

Let's systematically address each drug that works in PM/DM and explain why it fails in IBM:

Drug	Mechanism	Role in PM/DM	Evidence in IBM
Oral prednisolone ^[17]	Broad immunosuppression: inhibits T cell activation, reduces pro-inflammatory cytokines, stabilises cell membranes	Initial treatment for PM/DM ^[17]	Multiple trials show no sustained benefit. May transiently improve CK levels (dampening inflammation) but does NOT improve strength. Causes steroid myopathy and metabolic complications
Azathioprine	Purine antimetabolite → inhibits lymphocyte proliferation	Steroid-sparing agent ^[17]^[18]	No benefit in IBM
Mycophenolate mofetil (MMF)	Inhibits inosine monophosphate dehydrogenase → blocks de novo purine synthesis in lymphocytes	Steroid-sparing agent ^[17]	No benefit in IBM
Methotrexate (MTX)	Folate antagonist → inhibits dihydrofolate reductase → suppresses T cell function	Steroid-sparing agent ^[18]	No benefit in IBM; one small trial showed possible transient slowing but not replicated
Cyclophosphamide	Alkylating agent → crosslinks DNA → kills dividing lymphocytes	Severe/refractory PM/DM ^[7]	No benefit in IBM; significant toxicity
Rituximab (anti-CD20)	Depletes B lymphocytes	Refractory PM/DM ^[17]	Pilot trials in IBM showed no significant benefit — likely because B cells are not the primary driver (CD8+ T cells and degeneration are)

Management of PM/DM myositis: high dose steroids + immunosuppressants (azathioprine, MTX, cyclophosphamide); IVIG for refractory disease; anti-CD20 (still experimental) ^[7] — This treatment paradigm does NOT apply to IBM.

High-Yield — Steroids in IBM

Do not start corticosteroids for IBM. If the patient was already on steroids from a prior misdiagnosis of PM, taper and discontinue them. Steroid myopathy (Type II fibre atrophy → proximal weakness) will worsen the patient's weakness. The only reason CK may improve on steroids is because the inflammatory component is suppressed — but this does not translate to functional improvement. Immunotherapy not beneficial. ^[5]

Intravenous immunoglobulin (IVIG) is the only immunomodulatory agent that has shown any (very limited) potential benefit in IBM, specifically for dysphagia:

Dose: 2 g/kg divided over 2–5 days, repeated monthly
Mechanism: IVIG modulates the immune system through multiple pathways — Fc receptor blockade, anti-idiotypic antibody effects, complement inhibition, and cytokine modulation
Evidence in IBM:
- A randomised controlled trial showed no improvement in muscle strength with IVIG
- However, a subset of patients showed modest improvement in swallowing function
- A secondary analysis suggested possible benefit in the first 1–2 years of disease
Current recommendation: IVIG may be considered as a time-limited trial (3–6 months) in IBM patients with significant dysphagia causing nutritional compromise or aspiration risk. If no improvement in swallowing function is demonstrated objectively (by videofluoroscopy or SLT assessment), discontinue.

Indication	IVIG in IBM
Muscle weakness	Not recommended — no proven benefit
Dysphagia	May be considered as a time-limited trial
Fall prevention	Not recommended

Contraindications to IVIG (general):

IgA deficiency (risk of anaphylaxis due to anti-IgA antibodies)
Renal impairment (IVIG can cause osmotic nephropathy)
Known hypersensitivity to IVIG components
Uncontrolled congestive heart failure (volume overload)
Recent thromboembolic event (IVIG increases thrombosis risk)

These are not standard of care but represent the current research landscape:

Agent	Mechanism	Status
Sirolimus (rapamycin)	mTOR inhibitor → enhances autophagy → theoretically clears protein aggregates (targeting the degenerative component)	Phase 2/3 trials; the RAPAMI trial (2024) showed possible slowing of disease progression but results were mixed. Considered the most promising current candidate
Arimoclomol	Heat shock protein co-inducer → promotes protein refolding → reduces protein aggregation	Phase 2/3 trial (2023) failed to meet primary endpoint
Natalizumab	Anti-α4 integrin → blocks lymphocyte migration into muscle tissue	Small pilot studies; no clear benefit
Anti-cN1A antibody-directed therapies	Target the specific autoantibody pathway	Very early research stage
Gene therapy	Addressing the protein aggregation pathway at the genetic level	Preclinical

Why mTOR Inhibitors Make Pathophysiological Sense in IBM

Rapamycin (sirolimus) inhibits mTOR (mechanistic target of rapamycin). When mTOR is inhibited, the cell upregulates autophagy — the cellular "garbage disposal" system that clears misfolded proteins and damaged organelles. In IBM, autophagy is impaired, leading to accumulation of rimmed vacuoles and amyloid-like protein aggregates. By enhancing autophagy, rapamycin theoretically addresses the degenerative component of IBM — which is precisely the component that immunosuppressants cannot touch. This is why it is the most logical drug target in IBM from first principles.

5. Management of Specific Complications

Stage	Intervention
Early (mild quadriceps weakness, occasional falls)	Physiotherapy, balance training, ankle-foot orthosis for foot drop, home hazard assessment
Intermediate (frequent falls, difficulty with stairs)	Walking aids (stick → rollator), stair rail, avoidance of high-fall-risk activities
Late (unable to walk independently)	Wheelchair (manual → powered), home modifications (ramps, stairlift), carer support

Fall prevention is critical — falls in elderly patients with IBM lead to fractures (hip, wrist, vertebral), head injuries, and loss of confidence that accelerates functional decline
Osteoporosis screening and treatment (DEXA scan, vitamin D, calcium, bisphosphonates if indicated) — especially important if the patient was previously on steroids from a PM misdiagnosis

Aspect	IBM	PM/DM
First-line	Non-pharmacological: physiotherapy, OT, exercise, adaptive devices	Oral prednisolone (1 mg/kg/day) ^[17]^[18]
Steroid-sparing	Not applicable	Azathioprine, MMF, MTX ^[17]^[18]
Severe/refractory	IVIG trial for dysphagia only	IV methylprednisolone, IVIG, rituximab ^[17]
Dysphagia	SLT, diet modification, cricopharyngeal myotomy/Botox, PEG	SLT, steroids (often improves with immunosuppression)
Immunosuppression	NOT beneficial ^[5]	Cornerstone of treatment
Exercise	STRONGLY recommended	Recommended (safe, but less critical since drugs improve strength)
Malignancy screen	Not required	Required (DM 5×, PM 2× risk) ^[6]
Prognosis	Slowly progressive, life expectancy not significantly affected ^[5]	Variable; response to steroid: overlap myositis > DM > PM ^[7]

Parameter	Frequency	Purpose
Muscle strength testing (manual muscle testing, grip strength dynamometry)	Every 3–6 months	Track disease progression
Functional assessments (6-minute walk test, timed up-and-go, IBM Functional Rating Scale)	Every 3–6 months	Quantify functional decline
CK	Every 6–12 months	Track disease activity (though CK does not closely correlate with IBM severity)
Swallowing assessment (SLT ± videofluoroscopy)	Every 6–12 months, or sooner if symptoms change	Detect worsening dysphagia before aspiration event
FVC	Annually (or more frequently if symptoms suggest respiratory involvement)	Detect diaphragmatic weakness
Body weight and nutritional status	Every visit	Detect malnutrition from dysphagia
Bone density (DEXA)	Baseline, especially if previously on steroids	Fracture risk assessment
Psychological assessment	As needed	Depression screening

High Yield Summary — IBM Management

Immunotherapy not beneficial ^[5] — this is the single most important management fact for exams
Steroids should NOT be started; if already on steroids (from PM misdiagnosis), taper and discontinue
Non-pharmacological management is the cornerstone: physiotherapy, exercise, OT, adaptive devices, falls prevention
IVIG: the only agent with any evidence — limited to a time-limited trial for dysphagia only
Dysphagia management is critical: SLT → diet modification → cricopharyngeal myotomy/Botox → PEG tube
Sirolimus (rapamycin) is the most promising investigational agent — works by enhancing autophagy to clear protein aggregates (targeting the degenerative component)
Falls prevention and osteoporosis management are important given the elderly population and falls risk
Slowly progressive, life expectancy not significantly affected ^[5] — focus on quality of life and functional independence
No malignancy screen needed (unlike PM/DM)
The management of PM/DM (high dose steroids + steroid-sparing agents + IVIG for refractory disease ^[7]^[17]) is the direct opposite of IBM management — this contrast is a high-yield exam point

Active Recall - IBM Management

References

^[5] Lecture slides: GC 056. Generalized muscle weakness.pdf (slide: Inclusion Body Myositis) ^[6] Senior notes: Ryan Ho Neurology.pdf (p.194, Inflammatory Myopathies — malignancy association) ^[7] Senior notes: Ryan Ho Rheumatology.pdf (p.92, Management of PM/DM) ^[17] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (p.41, Dermatomyositis — Management) ^[18] Senior notes: Maksim Medicine Notes.pdf (p.319, Management of IIM)

Complications of Inclusion Body Myositis

1. Dysphagia and Aspiration Pneumonia — The Most Serious Complication

Falls in IBM are a direct consequence of the characteristic pattern of muscle weakness:

Muscle Group	Weakness Effect	Fall Mechanism
Quadriceps ^[5]	Cannot maintain knee extension → knee buckles under load	Forward falls — the knee gives way while walking or standing
Ankle dorsiflexors ^[5]	Cannot clear the foot during the swing phase of gait (foot drop)	Tripping — the toe catches on uneven surfaces, curbs, carpet edges
Hip flexors (later)	Cannot adequately lift the leg to step	Stumbling on stairs, inability to step over obstacles
Combined proximal + distal weakness	Impaired balance, inability to recover from perturbation	Loss of balance in any direction with inability to self-correct

3. Progressive Disability and Loss of Independence

Stage	Approximate Timeline from Symptom Onset	Functional Impact
Early	0–5 years	Difficulty with stairs, rising from chairs, frequent tripping. Still ambulant and largely independent
Intermediate	5–10 years	Requires walking aids (cane, rollator). Difficulty with fine motor tasks (grip weakness). May need help with some ADLs
Late	10–15+ years	Wheelchair dependence. Significant hand disability (cannot grip, write, type). Dependent for most ADLs. Dysphagia requiring diet modification or PEG

4. Malnutrition and Weight Loss

5. Respiratory Complications (Uncommon but Important)

This is a unique and important category of complications specific to IBM's clinical context.

Pathophysiology

Because IBM is frequently misdiagnosed as PM (average 5–8 years diagnostic delay), patients are often treated with prolonged high-dose corticosteroids and immunosuppressive agents that provide no benefit but cause significant harm:

Iatrogenic Complication	Drug	Mechanism	Impact on IBM Patient
Steroid myopathy	Prednisolone	Type II fibre atrophy from catabolic protein breakdown	Worsens proximal weakness — the very symptom you are trying to treat. Creates a diagnostic paradox: the patient worsens, the clinician increases the steroid dose, the patient worsens further
Osteoporosis	Prednisolone	Inhibits osteoblast function + promotes osteoclast activity + reduces calcium absorption + reduces gonadal hormones	Increases fracture risk in a patient who is already falling frequently → hip fractures, vertebral crush fractures
Diabetes mellitus	Prednisolone	Increases hepatic gluconeogenesis + causes insulin resistance	New-onset DM or worsened glycaemic control in an elderly population already at risk
Infections	Prednisolone, azathioprine, MTX, cyclophosphamide	Immunosuppression → increased susceptibility to opportunistic infections	Pneumonia (including PJP), herpes zoster, fungal infections — adding morbidity to an already debilitated patient
Cushingoid features	Prednisolone	Redistribution of fat (central obesity, moon face, buffalo hump)	Weight gain increases mechanical load on weak quadriceps → faster functional decline
GI complications	Prednisolone, NSAIDs	Peptic ulceration, GI bleeding
Hepatotoxicity	Methotrexate, azathioprine	Direct hepatic toxicity
Bone marrow suppression	Azathioprine, MTX, cyclophosphamide	Cytotoxic effect on haematopoietic precursors	Anaemia, leucopaenia (↑infection risk), thrombocytopaenia

The Harm of Treating IBM Like PM

The iatrogenic complications of inappropriate immunosuppression in IBM can be more disabling than the disease itself. Steroid myopathy directly worsens the patient's weakness. Steroid-induced osteoporosis + IBM-related falls = fractures. Immunosuppression-related infections in a frail elderly patient can be fatal. Recognising IBM early and avoiding unnecessary immunosuppression is one of the most impactful interventions a clinician can make.

This negative list is important for exam purposes because it highlights the key differences between IBM and DM/PM:

Complication	Seen in DM/PM?	Seen in IBM?	Why?
Interstitial lung disease (ILD)	Yes — especially with anti-Jo-1 (anti-synthetase syndrome) and anti-MDA-5 ^[9]^[18]	No	IBM does not produce the autoantibodies or vascular pathology that drive ILD
Myocarditis / cardiac conduction defects	Yes — myocarditis leading to conduction abnormalities and fatal arrhythmia ^[19]	No	IBM does not target cardiac muscle
Malignancy association	Yes — DM 5× risk, PM 2× risk ^[6]; NPC in this locality	No	IBM is not a paraneoplastic process
Calcinosis cutis	Yes — especially juvenile DM	No	No calcium deposition pathway in IBM
Skin manifestations	Yes — heliotrope rash, Gottron's papules, shawl sign, V sign	No	IBM is a pure myopathy with no skin component
Raynaud's phenomenon	Yes — in overlap myositis and anti-synthetase syndrome	No	No vascular inflammatory component in IBM

Complication	Mechanism	Clinical Significance	Prevention/Management
Aspiration pneumonia (most important)	Pharyngeal + cricopharyngeal muscle weakness → aspiration	Leading cause of IBM-related death	SLT assessment, diet modification, cricopharyngeal intervention, PEG tube
Falls and fractures	Quadriceps weakness → knee buckling; dorsiflexor weakness → tripping	Hip fractures (high mortality in elderly), vertebral fractures, head injuries	Physiotherapy, AFO, walking aids, home modifications, osteoporosis treatment
Progressive disability	Relentless muscle loss (inflammation + degeneration) with no effective treatment	Wheelchair dependence (~10–15 years); hand disability; head drop	Exercise, OT, adaptive devices, psychological support
Malnutrition	Dysphagia → reduced intake; muscle catabolism → lean mass loss	Weight loss, frailty, micronutrient deficiency	Dietitian, oral supplements, PEG tube
Respiratory failure (uncommon)	Late diaphragmatic weakness; recurrent aspiration	Nocturnal hypoventilation, chronic aspiration-related lung disease	FVC monitoring, NIV/BiPAP, vaccination
Depression/anxiety	Loss of independence, chronic progressive disease with no cure	Reduced quality of life, social isolation	Psychological support, patient groups, antidepressants if indicated
Iatrogenic complications	Misdiagnosis as PM → inappropriate steroids/immunosuppression	Steroid myopathy, osteoporosis + fractures, infections, DM, Cushing's	Correct diagnosis of IBM; discontinue unnecessary immunosuppression

High Yield Summary — Complications of IBM

Aspiration pneumonia from dysphagia is the leading cause of death in IBM — pharyngeal and cricopharyngeal muscle involvement causes aspiration
Falls are extremely common due to quadriceps weakness (knee buckling) and dorsiflexor weakness (tripping) → hip fractures, head injuries
Progressive disability is inevitable — most patients need walking aids by 5–10 years and wheelchair by 10–15 years
IBM does NOT cause ILD, myocarditis, or malignancy — these are complications of DM/PM, not IBM
Life expectancy is not significantly affected ^[5] — disability, not mortality, is the primary burden
Iatrogenic harm from misdiagnosis (steroids for "PM") is a major preventable complication — steroid myopathy worsens weakness, steroid osteoporosis + falls = fractures
Malnutrition from dysphagia-related reduced oral intake compounds frailty

Active Recall - IBM Complications

References

^[5] Lecture slides: GC 056. Generalized muscle weakness.pdf (slide: Inclusion Body Myositis) ^[6] Senior notes: Ryan Ho Neurology.pdf (p.194, Inflammatory Myopathies — malignancy association) ^[7] Senior notes: Ryan Ho Rheumatology.pdf (p.92, Management — supportive for late bulbar S/S) ^[9] Lecture slides: GC_Interactive tutorial (Rheum case 2) student copy.pdf (p.6, anti-Jo1 and ILD) ^[18] Senior notes: Maksim Medicine Notes.pdf (p.319–321, Autoantibodies and complications) ^[19] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1762–1764, Complications of DM/PM)

High Yield Summary

Inclusion Body Myositis (IBM) — Key Points for Exams:

Commonest acquired inflammatory myopathy in patients ≥ 50 years, M > F ^[1]
Very rare in Hong Kong ^[2]
Characteristic pattern: quadriceps, volar forearm (finger flexors), ankle dorsiflexors → axial muscles ^[1]
Often asymmetric (unique among IIMs)
CK normal or up to 10-fold above ULN ^[1] — much lower than PM/DM
Dual pathogenesis: inflammatory (CD8+ T cells) + degenerative (rimmed vacuoles, amyloid deposits)
Biopsy: inflammatory infiltrates (CD8+ T cells, macrophages) invading non-necrotic muscle fibres, rimmed vacuoles, congophilic inclusions (amyloidogenic proteins) ^[1]
Autoantibody: anti-cytosolic 5′-nucleotidase 1A (cN1A) ^[1]
Immunotherapy not beneficial ^[1] — the single most important management fact
Slowly progressive, life expectancy not significantly affected ^[1] — but significant disability over years
No association with malignancy (unlike DM 5× and PM 2×)
Dysphagia in 40-80% → aspiration pneumonia is the leading cause of IBM-related death

High Yield Summary — DDx of IBM

Most commonly misdiagnosed as PM — look for the red flags: male > 50, distal weakness, asymmetry, low CK, steroid non-response
DM is excluded by absence of skin manifestations (heliotrope, Gottron's, V sign, shawl sign)
Drug-induced myopathy (statins, glucocorticoids, colchicine) must always be excluded by drug history ^[1]^[3]
Endocrine causes (hypothyroidism, Cushing's) excluded by TFT and cortisol assessment ^[1]^[3]
ALS/MND excluded by absence of UMN signs and fasciculations
MG excluded by absence of fatigability, ocular involvement, and normal CK
Myotonic dystrophy excluded by absence of myotonia, younger age, multisystem features
Hereditary IBM (GNE myopathy) is a different disease — younger onset, quadriceps-sparing, no inflammation
Muscle biopsy is definitive: rimmed vacuoles, CD8+ T cells invading non-necrotic fibres, congophilic inclusions ^[5]

High Yield Summary — Diagnosis of IBM

Diagnostic criteria: ENMC 2011 criteria define clinicopathologically defined, clinically defined, and probable IBM based on clinical phenotype + biopsy
Bohan-Peter criteria (1975) are for PM/DM, NOT IBM — IBM does not fit these criteria
CK: normal or up to 10-fold above ULN ^[5] — much lower than PM/DM
EMG: mixed myopathic + neurogenic pattern (unique to IBM among IIMs)
Muscle biopsy is the gold standard — rimmed vacuoles, CD8+ T cells invading non-necrotic fibres, congophilic inclusions ^[5]; biopsy from weak but not atrophied muscle ^[7]
Anti-cN1A: sensitivity 33–76%, specificity ~90% — supportive but not required
MSAs should be NEGATIVE in IBM — if positive, consider DM, PM, IMNM, or anti-synthetase syndrome instead
No malignancy screening needed (unlike DM/PM)
MRI pattern: selective fatty replacement of quadriceps + medial gastrocnemius + forearm flexors
Immunotherapy not beneficial ^[5] — treatment failure itself is a diagnostic clue

High Yield Summary — IBM Management

Immunotherapy not beneficial ^[5] — this is the single most important management fact for exams
Steroids should NOT be started; if already on steroids (from PM misdiagnosis), taper and discontinue
Non-pharmacological management is the cornerstone: physiotherapy, exercise, OT, adaptive devices, falls prevention
IVIG: the only agent with any evidence — limited to a time-limited trial for dysphagia only
Dysphagia management is critical: SLT → diet modification → cricopharyngeal myotomy/Botox → PEG tube
Sirolimus (rapamycin) is the most promising investigational agent — works by enhancing autophagy to clear protein aggregates (targeting the degenerative component)
Falls prevention and osteoporosis management are important given the elderly population and falls risk
Slowly progressive, life expectancy not significantly affected ^[5] — focus on quality of life and functional independence
No malignancy screen needed (unlike PM/DM)
The management of PM/DM (high dose steroids + steroid-sparing agents + IVIG for refractory disease ^[7]^[17]) is the direct opposite of IBM management — this contrast is a high-yield exam point

High Yield Summary — Complications of IBM

Aspiration pneumonia from dysphagia is the leading cause of death in IBM — pharyngeal and cricopharyngeal muscle involvement causes aspiration
Falls are extremely common due to quadriceps weakness (knee buckling) and dorsiflexor weakness (tripping) → hip fractures, head injuries
Progressive disability is inevitable — most patients need walking aids by 5–10 years and wheelchair by 10–15 years
IBM does NOT cause ILD, myocarditis, or malignancy — these are complications of DM/PM, not IBM
Life expectancy is not significantly affected ^[5] — disability, not mortality, is the primary burden
Iatrogenic harm from misdiagnosis (steroids for "PM") is a major preventable complication — steroid myopathy worsens weakness, steroid osteoporosis + falls = fractures
Malnutrition from dysphagia-related reduced oral intake compounds frailty

Inclusion Body Myositis

1. Definition

2. Epidemiology

Incidence and Prevalence

Demographics

Risk Factors

3. Anatomy and Function

Relevant Muscle Groups

Why This Pattern Matters

Normal Muscle Physiology (Brief Review)

4. Aetiology and Pathophysiology

A. The "Two-Hit" Model

B. Inflammatory Component (Autoimmune)

C. Degenerative Component

D. Pathogenesis Summary Table

5. Classification

A. Within the Idiopathic Inflammatory Myopathies (IIMs)

B. IBM vs Hereditary Inclusion Body Myopathy (hIBM)

C. Clinicopathological Classification of IBM

6. Clinical Features

Overview

A. Symptoms (with Pathophysiological Basis)

i. Proximal Lower Limb Weakness (Quadriceps)

ii. Distal Upper Limb Weakness (Finger Flexors)

iii. Distal Lower Limb Weakness (Ankle Dorsiflexors)

iv. Dysphagia

v. Falls and Functional Decline

vi. What IBM Does NOT Typically Cause

B. Signs (with Pathophysiological Basis)

i. Pattern of Weakness

ii. Muscle Atrophy and Wasting

iii. Deep Tendon Reflexes

iv. Specific Examination Findings

v. Negative Findings (What You Will NOT Find)

C. Natural History and Prognosis

7. Associations and Comorbidities

A. Autoimmune Associations

B. No Malignancy Association

C. No Interstitial Lung Disease (ILD)

D. No Cardiac Involvement

Active Recall - Inclusion Body Myositis

References

Approach to the Differential

A. Mimics Within the Idiopathic Inflammatory Myopathies (IIMs)

1. Polymyositis (PM) — The Most Common Misdiagnosis

2. Dermatomyositis (DM)

3. Immune-Mediated Necrotizing Myopathy (IMNM)

4. Overlap Myositis / Anti-Synthetase Syndrome

B. Non-Inflammatory Myopathies

5. Drug-Induced Myopathy

6. Endocrine Myopathies

7. Metabolic Myopathies

C. Inherited Myopathies / Muscular Dystrophies

8. Myotonic Dystrophy Type 1 (DM1)

9. Facioscapulohumeral Dystrophy (FSHD)

10. Oculopharyngeal Muscular Dystrophy (OPMD)

11. Limb-Girdle Muscular Dystrophy (LGMD)

D. Non-Myopathic Mimics (Is It Even a Myopathy?)

12. Motor Neuron Disease / Amyotrophic Lateral Sclerosis (ALS)

13. Myasthenia Gravis (MG)

14. Lambert-Eaton Myasthenic Syndrome (LEMS)

15. Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

16. Cervical Myelopathy

E. Infectious and Other Causes

17. Infectious Myositis

18. Critical Illness Myopathy

Summary: Approach to Differentiating IBM

Active Recall - Differential Diagnosis of IBM

1. Overview of Diagnostic Approach

2. Diagnostic Criteria

A. Bohan and Peter Criteria (1975) — For PM/DM, NOT for IBM

B. 2011 European Neuromuscular Centre (ENMC) Diagnostic Criteria for IBM

C. 2013 Lloyd et al. Research Criteria (Simplified Clinically Defined IBM)

D. 2017 EULAR/ACR Classification Criteria for IIM

3. Diagnostic Algorithm

4. Investigation Modalities

A. Serum Creatine Kinase (CK)

B. Autoantibodies

i. IBM-Specific: Anti-cN1A

ii. Myositis-Specific Antibodies (MSAs) — Should Be Negative in IBM