Inclusion Body Myositis

Serum creatine kinase (CK) is mildly to, at most, moderately elevated (<10 times normal). Electromyography (EMG) discloses fibrillation potentials and a mixed population of motor unit potentials (MUPs): low-amplitude myopathic potentials with reduced recruitment, as seen in DM and PM, and enlarged MUPs more typical of chronic neurogenic disorders, reflecting the very long-standing IBM course, and opportunity for muscle fibers to undergo splitting and hypertrophy. Magnetic resonance imaging (MRI) muscle signal changes occasionally aid in identifying a biopsy site.

Muscle biopsy demonstrates distinctive muscle fiber changes, including vacuoles containing basophilic granules (rimmed vacuoles), small angular atrophic fibers groups with eosinophilic inclusions, and endomysial mononuclear inflammatory infiltrate similar to PM. Electron microscopy is essential for defining morphologic findings characteristic of IBM: 18-nm filamentous inclusions within muscle nuclei and cytoplasm. Some filamentous inclusions bind to antibodies directed against beta-amyloid.

IBM can often be mistaken as polymyositis clinically, when the weakness is proximal and symmetric, accompanied by a modest CK elevation, and has similar EMG abnormalities. It is differentiated by muscle biopsy. Toxic (drug-induced) myopathies may show similar vacuoles on muscle biopsy but typically have a subacute course. Hereditary IBMs present similarly with slowly progressive weakness and muscle biopsy demonstrating a vacuolar myopathy with inclusions. However, the family history and sparse inflammation noted in hereditary IBM distinguish it from sporadic IBM. Often, IBM patients with predominant distal and/or asymmetric weakness are confused with motor neuron disorders and less commonly peripheral neuropathies.

In contrast to other inflammatory myopathies responding robustly to corticosteroids and other immunosuppressive agents, IBM is refractory to immunotherapy. Weakness is gradually progressive; after 10 to 30 years, most patients will require assistance with activities of daily living, a walker for ambulation, and ultimately, a motorized wheelchair or scooter. Supportive measures for dysphagia and physical therapy help to improve quality of life.

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