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Inclusion Body Myositis (IBM)

Definition

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Classification of Inflammatory Myopathies

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Epidemiology

• Most common inflammatory myopathy in adults over 50
• Prevalence: ~50/million (rises with age)
• Male > Female (3:1)
• Predominantly affects Caucasians
• Sporadic IBM (sIBM) is the common form; hereditary IBM (hIBM) is rare

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Etiology and Pathogenesis

1. Inflammatory / Autoimmune Component

• CD8+ cytotoxic T-lymphocytes invade non-necrotic muscle fibers (same as PM)
• T cells are clonally expanded and antigen-driven
• MHC class I upregulation on muscle fibers
• Autoantibody: Anti-cN1A (anti-cytosolic 5'-nucleotidase 1A) — present in ~33–76% of IBM patients; relatively specific

2. Degenerative Component

• Abnormal protein accumulation within muscle fibers (similar to neurodegeneration)
• Accumulation of: β-amyloid precursor protein (β-APP), phosphorylated tau, TDP-43, p62, ubiquitin
• Endoplasmic reticulum stress → unfolded protein response
• Mitochondrial dysfunction with cytochrome oxidase (COX)-negative fibers
• Autophagic vacuole formation → rimmed vacuoles

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Pathology (Muscle Biopsy)

Light Microscopy (H&E Stain):

• Rimmed vacuoles — sarcoplasmic vacuoles lined by granular material (most characteristic)
• Endomysial inflammation — CD8+ T cells invading non-necrotic fibers
• Ragged red fibers (modified Gomori trichrome)
• Eosinophilic cytoplasmic or intranuclear inclusions

Histochemical Stains:

• COX (Cytochrome Oxidase) stain → increased COX-negative (pale) fibers
• p62 / TDP-43 immunostaining → cytoplasmic inclusions stain positive (highly specific)
• Congo red stain → amyloid deposits within vacuoles

Electron Microscopy:

• Tubulofilamentous inclusions: 15–21 nm diameter — intranuclear or cytoplasmic — pathognomonic
• Paired helical filaments

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Clinical Features

Onset

• Insidious onset over months to years
• Often misdiagnosed as PM for years before correct diagnosis

Pattern of Weakness — Selective and Characteristic:

Key exam point: IBM causes asymmetric, distal + proximal weakness, with selective involvement of finger flexors and quadriceps — this pattern is virtually diagnostic.

Other Features:

• Atrophy of forearm flexors and quadriceps
• Hyporeflexia (reduced deep tendon reflexes)
• Mild or no muscle pain
• Falls are common due to quadriceps weakness
• No skin rash, no Raynaud's, no arthritis
• Rare systemic involvement (cardiac, pulmonary less common than PM/DM)

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Investigations

1. Serum Creatine Kinase (CK)

• Mildly elevated — typically < 12× upper limit of normal (often only 2–5× ULN)
• May be normal in up to 20% of cases
• (Contrast: PM/DM — markedly elevated)

2. Autoantibodies

• Anti-cN1A (anti-NT5C1A) — present in ~33–76%; associated with more severe dysphagia
• MSA (myositis-specific antibodies like anti-Jo1) are typically absent

3. Electromyography (EMG)

• Mixed pattern (unique to IBM):
  • Myopathic features: short-duration, low-amplitude, polyphasic potentials
  • Neuropathic features: long-duration potentials, fibrillations
• This mixed pattern often leads to misdiagnosis as motor neuron disease

4. Muscle Biopsy

5. MRI of Muscles

• Early involvement of vastus lateralis and vastus medialis with relative sparing of rectus femoris — characteristic pattern on thigh MRI (Harrison's, 21st ed., p. 12782)
• MRI is more sensitive than EMG or clinical exam in early disease
• Helps guide biopsy site selection

6. Other Labs

• ESR, CRP: mildly elevated or normal
• ANA: may be weakly positive
• CBC, metabolic panel: to exclude systemic causes

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Diagnostic Criteria

European Neuromuscular Centre (ENMC) 2011 Criteria — Three levels:

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Differential Diagnosis

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Treatment

Immunosuppressive Therapy

Critical exam point: IBM is largely refractory to all immunosuppressive therapies — unlike PM and DM.

Supportive / Rehabilitative (Most Important)

• Physiotherapy — resistance exercises to slow functional decline; eccentric strengthening of quadriceps
• Occupational therapy — adaptive devices (grip aids, jar openers)
• Speech therapy + dysphagia management — modified diet, cricopharyngeal myotomy or dilation for severe dysphagia
• Orthoses — ankle-foot orthoses (AFO) for foot drop
• Fall prevention — walking aids, home modifications

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Prognosis

• Slowly but relentlessly progressive — most patients require a walking aid within 10–15 years
• No spontaneous remissions
• Major morbidity:
  • Loss of ambulation (wheelchair dependence)
  • Aspiration pneumonia from dysphagia (leading cause of death)
  • Respiratory failure (late)
• Life expectancy minimally reduced if complications (aspiration, falls) are managed
• No increased risk of malignancy (unlike DM)

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Summary: High-Yield Exam Points

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