Inclusion body myositis
It is the most common acquired inflammatory muscle disorder in adults older than 50 years. Asymmetric proximal and/or distal muscle atrophy and weakness are typically seen. Classically, this first is noted in the quadriceps muscles, with weakness in knee extension more than flexion, resulting in difficulty with walking, climbing, and rising from a seated position. This may eventually predispose the patient to falls. Distally, asymmetric finger flexor and wrist weakness interfere with gripping. The biceps, triceps, facial muscles, and muscles of deglutition are commonly involved. Rarely, respiratory insufficiency may occur if the diaphragm is involved.
Men are more commonly affected than women. While most cases occur sporadically, rare familial occurrences have been reported. There is a genetic association with HLA-DRB1. A subgroup of individuals will be positive for cytosolic 5'-nucleotidase 1A (cN1A) autoantibodies. While this autoantibody is fairly specific for inclusion body myositis, it may occur in other autoimmune diseases.
There is no effective treatment. Management consists largely in easing symptoms, exercise therapy, use of mobility devices, and medications. Immunosuppressive medications such as corticosteroids, methotrexate, azathioprine, and intravenous immunoglobulin (IVIG) may be tried. These medications do not alter the long-term natural course of the disease.
Typically, life expectancy is not directly affected. However, those with cN1A antibodies may have a higher mortality risk. Significant morbidity does occur. As muscle weakness progresses, most individuals lose the ability to ambulate and require a wheelchair, especially after extended periods of disease duration. Dysphagia may become severe enough to interfere with nutritional needs, requiring alternative feeding methods. As respiratory muscle function declines, the risk for pneumonia may increase.
G72.41 – Inclusion body myositis [IBM]
72315009 – Inclusion Body Myositis
Differential Diagnosis & Pitfalls
- Polymyositis – Finger flexors are not usually involved; muscle biopsy findings will distinguish.
- Dermatomyositis – Check for classic rashes; muscle biopsy findings will distinguish.
- Antisynthetase syndrome – Check for autoantibodies; muscle biopsy findings will distinguish.
- Autoimmune necrotizing myopathies – Muscle biopsy findings will distinguish; check for history of recent statin use.
- Drug-induced myopathy – Review recently used medications.
- Osteoarthritis – Muscles not typically weak, and x-ray of involved bones shows degenerative changes.
- Inflammatory arthritis – Muscles not typically weak; check for joint inflammation.
- Systemic lupus erythematosus – Check for classic features, eg, malar rash.
- Autoimmune overlap syndrome – Check for features of other autoimmune diseases.
- Systemic sclerosis – Check for skin thickening, Raynaud phenomenon, and gastroesophageal reflux disease (GERD).
- Polymyalgia rheumatica – Shoulder and hip girdle pain and stiffness but not weakness; rapid response to corticosteroid treatment.
- Hypothyroidism – Check thyroid stimulating hormone, and check for classic features.
- Myasthenia gravis – Muscle strength worsens with fatigability; check electromyogram (EMG) / nerve conduction studies (NCS).
- Muscular dystrophies and hereditable myopathies – Check family history, genetic testing; check for distal muscle weakness.
- Pyomyositis – Check for recent infection, including HIV.
- Amyotrophic lateral sclerosis – Check for distal muscle weakness, hyperreflexia.
- Diabetic amyotrophy – Check for history of poorly controlled diabetes and diabetic neuropathy.