Volume 51, Number 3

Myopathic Diseases

Robert L. Wortmann, MD
Department of Internal Medicine
University of Oklahoma College of Medicine
Tulsa, OK

Differential Diagnosis:
Idiopathic Inflammatory Myopathies

The idiopathic inflammatory myopathies (IIM) are a heterogeneous group of disorders characterized by symmetric proximal muscle weakness and elevated serum levels of enzymes derived from skeletal muscle. These include creatine phosphokinase (CPK), aldolase SGOT, SGPT, and LDH. In addition EMG, MRI, and muscle histology show changes indicative of nonsupparative inflammation (3,4). Today, this group includes the specific diagnoses of polymyositis, dermatomyositis, juvenile dermatomyositis, myositis associated with another connective tissue disease, myositis associated with a malignancy and inclusion body myositis. Patients with an IIM can be further categorized by the presence or absence of a circulating myositis-specific autoantibody (MSA, Table 1).

The IIM are rare diseases with estimates of incidence ranging from 0.5 to 8.4 cases per million. The age at onset has a bimodal distribution, with peaks between 10 and 15 years and again between 45 and 60 years. The age of onset for myositis with another connective tissue disease is similar to that for the associated condition. Both malignancy-associated myositis and inclusion body myositis are more common after age 50. Women are affected twice as often as men.

Polymyositis in Adults. Weakness of the pelvic and shoulder girdle muscles is the cardinal feature of polymyositis in adults. Weakness of neck flexors can occur, but ocular and facial muscles are spared. Symptoms usually begin insidiously with no precipitating event. Dysphagia may develop secondary to esophageal dysfunction or cricopharyngeal muscles. Pharyngeal muscle weakness may cause dysphonia and difficulty swallowing. Myalgias and arthalgias are not uncommon, but frank synovitis is unusual. Additional findings may include Raynaud’s phenomenon or periorbital edema. Velcro-like crackles may be heard on chest ausculatation from pulmonary fibrosis. Aspiration pneumonia may complicate the disease course in patients with swallowing difficulties. Cardiac involvement is usually limited to asymptomatic EKG abnormalities, although cardiomyopathy can occur.

In polymyositis, muscle fibers are found to be in varying stages of necrosis and regeneration. The inflammatory cell infiltrate is predominantly focal and endomysial with T lymphocytes, especially CD8+ cytotoxic cells, surrounding and invading initially non-necrotic fibers (5).

Dermatomyositis in Adults. The clinical features of dermatomyositis in adults include all of those described for polymyositis plus cutaneous involvement. Gotron’s papules -- symmetric lacey pink or violaceous raised or macular areas found on the dorsal aspect of interphalangeal joints, elbows, patellae, and malleoli -- are considered pathognomonic. Other skin changes include heliotrope discoloration of the eyelids; macular erythema of the posterior shoulders and neck (shawl sign), anterior neck and upper chest (V sign), face, and forehead; and dystrophic cuticles with periungual telangiectasias or abnormal nail fold capillaries. Muscle histology usually differs from that of polymyositis. The inflammatory infiltrate is perivascular in location and is composed of B and CD4+ lymphocytes. Capillary plugging and perifascicular atrophy are also observed (5).

Juvenile Dermatomyositis. The usual IIM in children, termed juvenile dermatomyositis (JDM), has characteristic features. Although the rash, muscles involved and muscle histology are similar, JDM differs from the adult form because of the coexistence of vasculitis, ectopic calcification, joint contractures, and lipodystrophy.

The features of an IIM may dominate the clinical picture in some patients with scleroderma, systemic lupus erythematosus, mixed connective tissue disease, and Sjögren’s syndrome. In vascultic syndromes, however, weakness is more commonly related to arteritis and nerve involvement than to inflammation in muscle.

IIM with an Associated Malignancy. The true incidence of this relationship is not clear, although it may be more common with dermatomyositis (6). Myositis has been found associated with malignancy in all age groups but is quite rare in children. It appears that the sites and types of malignancy that occur are those most expected for the age and gender of the patient. Ovarian cancer may prove the exception because it is over-represented in women with dermatomyositis.

Inclusion Body Myositis (IBM). IBM mainly affects older individuals. Onset is truly insidious with symptoms often having been present for more than 5 years before diagnosis. Clinically and histolologically, IBM maybe identical to polymyositis, although differences are clear in more than half the patients. Weakness may be focal, distal, or asymmetric, and it may be accompanied by diminished deep-tendon reflexes. EMG may reveal neurogenic or mixed neurogenic and myopathic changes. Disease progression is usually slow and steady in some, while it seems to plateau in others, leaving them with fixed weakness and atrophy of the involved musculature. The characteristic histologic change in IBM is the presence of intracellular rimmed vacuoles. The vacuolated fibers are now recognized to contain abnormal deposits of amyloid proteins, similar to those found in the brain in Alzheimer’s disease (7).

Fifty percent of patients with an IIM have a circulating autoantibody found almost exclusively in these diseases. Because of this specificity, these have been termed myositis-specific autoantibodies (MSAs). The presence of a particular antibody appears to identify a relatively homogeneous group of patients with regards to clinical features and prognosis (Table 1).