Inclusion Body Myositis

A progressive inflammatory myopathy of unknown cause characterized by typical rimmed vacuoles and intracellular amyloid deposits in muscle.

This disorder, which resembles chronic polymyositis, usually occurs in patients > 50; it affects men 3 times as often as women. It is the most common inflammatory myopathy among elderly persons.

The cause is unknown. Autosomal recessive and autosomal dominant inheritance accounts for some cases.

Amyloid deposits and cytoplasmic and intranuclear filamentous inclusions of unknown origin are characteristic. The inherited forms are called inclusion body myopathies, because they do not display the inflammatory infiltrates that characterize inclusion body myositis.

Symptoms, Signs, and Diagnosis

Proximal and distal muscle weakness usually progresses gradually without pain; it is often asymmetric. Weakness of wrist and finger flexors and of knee extensors occurs early and is usually more severe than weakness of other muscles. This pattern of weakness can mislead physicians to suspect motor neuron disease or another neuropathic disorder.

Unlike in polymyositis and dermatomyositis, muscle atrophy develops relatively early in inclusion body myositis. Facial muscles are involved in about one third of cases, but extraocular muscles are spared. Rarely, spinal, respiratory, and abdominal muscles are affected. One recessive form spares the quadriceps femoris muscle, a finding that is atypical. Dysphagia occurs in about 40% of patients. Tendon reflexes are normal or increased and become reduced or absent only with marked muscle wasting. Usually, patients remain ambulatory for many years after symptoms begin.

Diagnosis is based on the clinical findings and elevated CK levels (up to 10-fold above normal). Diagnosis is confirmed by muscle biopsy results, which reveal eosinophilic cytoplasmic inclusion bodies composed of 15- to 18-nm filaments.

Prognosis and Treatment

Prognosis is guarded. No clearly effective treatment is available. The clinical course is slowly progressive over many years. Respiratory compromise due to aspiration pneumonitis is often the cause of death.

Immunosuppressive therapy is usually ineffective. Occasionally, prednisone 1 to 1.5 mg/kg/day po leads to improvement, as does IV immune globulin. Therapeutic trials of -interferon are ongoing. Supportive care, bracing for occasional footdrop or severe wristdrop, and aquatic physical therapy are helpful.