Inclusion body myositis (IBM) is the most common acquired idiopathic myopathy after the age of 50. The idiopathic inflammatory myopathies include polymyositis (PM), dermatomyositis (DM), autoimmune necrotizing myopathy, and IBM. They are associated with inflammatory involvement of muscle fibers and recurrent weakness. Although IBM is classified among the inflammatory myopathies, its underlying pathophysiology has not yet been delineated, and antiinflammatory medications do not improve weakness.

IBM

• Insidious onset of slowly progressive proximal leg and distal arm weakness.³
• Time to diagnosis from symptom onset often is delayed up to 5 yr. ⁴
• Functional loss of strength in the legs most often precedes arm weakness.²
• The cardinal clinical features include early weakness and atrophy of quadriceps muscles (difficulty climbing stairs, rising from chairs, and getting out of cars) along with wrist and finger flexor muscles (difficulty grasping, opening jars, and turning doorknobs). Ankle dorsiflexion weakness also may be prominent leading to foot drop and tripping.²
• Muscle weakness may be asymmetric.²
• Dysphagia and/or mild facial weakness are present in up to half of cases. Dysphagia may be the presenting symptom.^2,3,5
• Although sensory symptoms are usually lacking, one third have evidence of peripheral neuropathy on physical examination and/or electrodiagnostic testing.⁶
• Individuals with IBM have another autoimmune condition such as Sjögren or systemic lupus erythematosus up to 15% of the time.²

The pathogenesis of IBM is not known. Inflammatory, degenerative, viral, and prion etiologies have been postulated, but none substantiated.

• Polymyositis (Table E1)⁷
• Dermatomyositis
• Amyotrophic lateral sclerosis
• Late-onset muscular dystrophies
• Acid maltase deficiency

Thorough neurologic examination with emphasis on the motor examination. Nerve conduction studies should be performed to exclude other causes and electromyography performed to document a myopathy.

The diagnosis of definite IBM requires the following features on muscle biopsy (Fig. E1): (1) Inflammation, (2) inflammatory cells invading healthy muscle fibers, (3) vacuoles, and (4) amyloid deposits by Congo red staining, TDP-43 sarcoplasmic staining, or tubulofilaments on electron microscopy.²

• Creatine kinase level (mildly elevated early in disease but <10× normal levels; creatine kinase levels may normalize as the disease progresses).
• Anti-cN1A autoantibodies are found in up to 90% to 95% of patients with IBM, but also may be positive, up to 10% of the time in individuals with other neuromuscular diseases.³
• CBC and coagulation studies should be drawn in anticipation of the muscle biopsy.

MRI may reveal atrophy and signal abnormalities in volar forearm muscle groups and quadriceps atrophy with relative preservation of the rectus femoris muscle.

• Physical therapy and occupational therapy with the use of assistive mobility devices such as canes, walkers, and wheelchairs are the mainstay of therapy. Occasionally, knee orthoses or ankle-foot orthoses may improve and prolong ambulation.

• Experts have not found clinically significant improvement in functional strength with any pharmacologic therapy. Clinical trials of corticosteroids, methotrexate, intravenous immunoglobulin, anti-T-lymphocyte globulin, etanercept, interferon β-1a, and oxandrolone have all failed to demonstrate functional improvements in limb strength.²
• IBM is generally refractory to therapy.²

Some patients choose to self-treat with creatine supplementation, coenzyme Q10, or lithium. There is no evidence supporting these treatments.

• Patients with suspected IBM should be referred to a neurologist with subspecialty expertise in neuromuscular medicine.
• Physical therapy and occupational therapy consultations help the patient optimize ambulation and fine motor tasks, respectively.
• Speech therapy consultations can assist with symptomatic dysphagia.

Life expectancy is not significantly altered in this late-onset, slowly progressive disorder. On average, individuals require use of assistive device for ambulation 7 to 10 years from symptom onset, and a wheelchair 13 to 15 years from onset.³

• In contrast to PM and DM, muscle weakness in IBM generally affects both distal and proximal muscles. Although typically symmetric, muscle distribution may be asymmetric.
• A key to diagnosis rests in finding weakness of wrist and/or finger flexors (especially the deep finger flexors at the distal interphalangeal joints) on examination.

Patient information and support groups can be found at https://www.ninds.nih.gov/Disorders/All-Disorders/Inclusion-Body-Myositis-Information-Page and https://www.myositis.org/.

Inflammatory Myopathies (Related Key Topic)