Inclusion Body Myositis

Unknown cause

Begins insidiously, usually after middle age

Progressive proximal weakness of first the lower and then the upper extremities, and affecting facial and pharyngeal muscles

Weakness often begins in the quadriceps femoris in the lower limbs and the forearm flexors in the upper limbs

Distal weakness is usually mild

Serum creatine kinase levels may be normal or increased

Muscle biopsy confirms diagnosis

Anticytosolic 5′-nucleotidase 1A antibodies

• Detected in one-third of cases

• May be associated with a more severe phenotype

Corticosteroid and immunosuppressive therapy is sometimes offered but usually ineffective

Intravenous immunoglobulin (IVIG) therapy is not recommended