20-12: Immune-Mediated Inflammatory Myopathies

ESSENTIALS OF DIAGNOSIS

GENERAL CONSIDERATIONS

Immune-mediated inflammatory myopathies include polymyositis, dermatomyositis, myositis resulting from a rheumatic disease or overlap syndrome, inclusion body myositis (IBM), and immune-mediated necrotizing myopathy. These disorders are characterized by progressive muscle weakness, and all but IBM demonstrate an inflammatory infiltrate in muscle tissue.

Polymyositis and dermatomyositis are systemic disorders of unknown cause whose principal manifestation is muscle weakness. Although their clinical presentations (aside from the presence of certain skin findings in dermatomyositis, some of which are pathognomonic) and treatments are similar, the two diseases are pathologically distinct. They affect persons of any age group, but the peak incidence is in the fifth and sixth decades of life and women are affected twice as commonly as men. There is an increased risk of malignancy, especially in dermatomyositis. Indeed, up to one patient in four with dermatomyositis has an occult malignancy. Malignancies may be evident at the time of presentation with the muscle disease but may not be detected until months afterward in some cases. The malignancies most commonly associated with dermatomyositis are lung, ovarian, breast, colorectal, cervical, bladder, nasopharyngeal, esophageal, pancreatic, and renal cancer. Patients may have skin disease without overt muscle involvement, a condition termed dermatomyositis sine myositis; these patients can have aggressive interstitial lung disease. Myositis may also overlap with other connective tissue diseases, especially systemic sclerosis, SLE, mixed connective tissue disease, and Sjögren syndrome.

IBM affects older men and is characterized by more distal weakness in the upper extremities and is generally less symmetric. Immune-mediated necrotizing myopathies include those associated with the signal recognition particle or with anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (anti-HMGCR) autoantibodies in the setting of statin use. This latter entity is distinct from the more common statin-induced myopathy, which occurs in about 1% of statin users. Unlike anti-HMGCR necrotizing myopathy, statin-induced myopathy generally resolves within 6 months of drug discontinuation.

CLINICAL FINDINGS

A. Symptoms and Signs

Polymyositis may begin abruptly, but the usual presentation is one of progressive muscle weakness over weeks to months. The weakness chiefly involves proximal muscle groups of the upper and lower extremities and the neck. Leg weakness (eg, difficulty in rising from a chair or climbing stairs) typically precedes arm symptoms. In contrast to myasthenia gravis, polymyositis and dermatomyositis do not cause facial or ocular muscle weakness. In contrast to polymyalgia rheumatica (PMR), pain and tenderness of affected muscles occur in only one-fourth of cases, and these are rarely the chief complaints. About one-fourth of patients have dysphagia. In contrast to systemic sclerosis, which affects the smooth muscle of the lower esophagus and can cause a “sticking” sensation below ...