Inclusion Body Myositis

Inclusion body myositis (IBM) is the commonest acquired myopathy over age 50, with slowly progressive asymmetric weakness of quadriceps and finger flexors.

• Prevalence 5 to 15 per 100,000 (higher in older adults); men more than women.

• Mixed inflammatory and degenerative pathology: endomysial inflammation, rimmed vacuoles, protein aggregates (p62, TDP-43), mitochondrial changes.

• Anti-cN1A (NT5C1A) antibodies in 33 to 70%.

• Resistant to immunosuppression; progression over years leads to loss of ambulation and dysphagia.

• Slowly progressive painless weakness (over years), with early quadriceps weakness (falls, knee buckling) and finger flexor/wrist flexor weakness (difficulty with grip, buttons).

• Asymmetric involvement, atrophy of forearm and thigh.

• Dysphagia in 40 to 70%, facial weakness mild.

• No rash; CK mildly raised.

• CK normal to 10 times upper limit (usually under 1500 U/L).

• EMG: myopathic changes with mixed neurogenic features. MRI muscle: fatty replacement of quadriceps, flexor digitorum profundus.

• Muscle biopsy: endomysial inflammatory cells, rimmed vacuoles, congophilic amyloid inclusions, COX negative fibres.

• Anti-cN1A antibody; videofluoroscopy for dysphagia.

• Exclude polymyositis, necrotising myopathy, motor neuron disease, limb girdle dystrophies.

• No disease modifying therapy: corticosteroids, methotrexate, IVIG trials fail to halt progression, so avoid prolonged immunosuppression.

• Exercise and physiotherapy (resistance and aerobic) safely improve function; occupational therapy, walking aids and orthoses, home adaptation.

• Dysphagia: speech therapy, diet modification, cricopharyngeal myotomy or botulinum toxin injection for cricopharyngeal dysfunction, IVIG 2 g/kg short trial, PEG feeding if aspiration.

• Fall prevention, bone health, nutrition.

• Emerging: sirolimus, arimoclomol (negative), bimagrumab, and rapamycin trials.

• Review every 6 months for functional decline and swallowing.