ESSENTIALS OF DIAGNOSIS
Bilateral proximal muscle weakness.
Dermatomyositis: characteristic cutaneous manifestations (Gottron papules, heliotrope rash); increased risk of malignancy.
Elevated creatine kinase, myositis-specific antibodies, diagnostic muscle biopsy.
Mimics include inclusion body myositis and statin-induced immune-mediated necrotizing myopathy.
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 quite distinct. They affect persons of any age group, but the peak incidence is in the fifth and sixth decades of life. Women are affected twice as commonly as men, and the diseases (particularly polymyositis) also occur more often among blacks than whites. 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 scleroderma, systemic lupus erythematosus, mixed connective tissue disease, and Sjögren syndrome.
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 as well as 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 PMR, pain and tenderness of affected muscles occur in one-fourth of cases, but these are rarely the chief complaints. About one-fourth of patients have dysphagia. In contrast to scleroderma, which affects the smooth muscle of the lower esophagus and can cause a “sticking” sensation below the sternum, polymyositis or dermatomyositis involves the striated muscles of the upper pharynx and can make initiation of swallowing difficult. Muscle atrophy and contractures occur as late complications of advanced disease. Clinically significant myocarditis is uncommon even though there is often creatine kinase-MB elevation. Patients who are bed-bound from myositis should be screened for respiratory muscle weakness that can be severe enough to cause CO2 retention and can progress to require mechanical ventilation.
The characteristic rash of dermatomyositis is dusky red and may appear in a malar distribution mimicking the classic rash of SLE. Facial erythema beyond the malar distribution is also characteristic of dermatomyositis. Erythema also occurs over other areas of the face, neck, shoulders, and ...