What are the histopathological findings on a muscle biopsy in inclusion body myositis?

Histopathological Findings in Inclusion Body Myositis

The muscle biopsy in inclusion body myositis characteristically shows rimmed vacuoles (the most specific finding), endomysial inflammatory infiltrates with CD8+ T cells invading non-necrotic muscle fibers, groups of atrophic fibers, and filamentous inclusions on electron microscopy. 1

Core Diagnostic Features

The histopathological triad that defines IBM includes:

• Rimmed vacuoles are the pathognomonic feature, appearing bluish on H&E staining and reddish on modified Gomori-Trichrome stains, present in 100% of confirmed cases 1, 2, 3
• Endomysial inflammatory infiltrates consisting predominantly of CD8+ cytotoxic T cells and macrophages that surround and invade non-necrotic muscle fibers expressing MHC class I, present in 89-92% of cases 1, 2, 3
• Groups of atrophic fibers occurring in small clusters, present in 92-96% of cases 2, 3

Additional Histological Characteristics

Beyond the diagnostic triad, IBM biopsies demonstrate:

• Eosinophilic cytoplasmic inclusions visible on light microscopy 4
• Congophilic amyloid deposits reflecting protein aggregation 1, 5
• Phosphorylated tau proteins accumulation 1
• Filamentous inclusions (15-18 nm diameter) detected on electron microscopy, typically located near vacuoles, present in all confirmed cases when adequate tissue is examined 2, 3

Critical Diagnostic Considerations

The EULAR/ACR classification criteria assign rimmed vacuoles the highest point value (3.1 points) among all histopathological features, reflecting their specificity for IBM. 1

Important caveats for interpretation:

• Any given biopsy may lack these histopathological abnormalities, particularly early in disease, making clinical examination crucial for diagnosis 4, 6
• A minimum of three vacuolated fibers must be scrutinized on electron microscopy to detect filamentous inclusions with confidence 2
• Endomysial infiltration alone (without vacuoles) only scores 1.7 points and is insufficient for diagnosis 1

Distinguishing IBM from Other Inflammatory Myopathies

IBM differs from other inflammatory myopathies by:

• Presence of rimmed vacuoles (absent in polymyositis, dermatomyositis, and IMNM) 7
• Absence of perifascicular atrophy (which characterizes dermatomyositis) 7
• Prominent inflammatory infiltrates (minimal or absent in IMNM) 7
• Degenerative features alongside inflammation, leading to IBM being increasingly considered a degenerative myopathy with inflammatory features rather than purely inflammatory 1

Immunohistochemical Pattern

The inflammatory infiltrate composition is distinctive:

• CD8+ T cells predominate in the endomysial region, actively invading muscle fibers 1
• Macrophages are present alongside T cells 1
• MHC class I expression is upregulated on muscle fibers 1