Inflammatory inclusion body myositis

**Definition:** Inclusion body myositis (IBM) is the most common idiopathic inflammatory myopathy after age 50. It typically presents with chronic insidious proximal leg and/or distal arm asymmetric muscle weakness leading to recurrent falls and loss of dexterity. Creatine kinase is up to 15 times elevated in IBM and needle electromyography mostly shows a chronic irritative myopathy. Muscle histopathology demonstrates endomysial inflammatory exudates surrounding and invading non-necrotic muscle fibres often times accompanied by rimmed vacuoles and protein deposits. Despite inflammatory muscle pathology, it is likely that IBM has a prominent degenerative component as supported by refractoriness to immunosuppressive therapy.

**Exclusions:** - Myasthenia gravis or certain specified neuromuscular junction disorders