Classification and Pathology

The inflammatory myopathies are a heterogeneous group of disorders that share a common pathologic feature: inflammatory cells in muscles. They comprise three major categories of muscle disease, polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM), conditions that are clinically, histologically, and pathogenetically distinct. Inflammatory myopathies are often classified into two major types (Table C20-1), a primary and secondary (associated with systemic or other identifiable disorders).

Table C20-1 General Classification of Inflammatory Myopathies

Polymyositis and dermatomyositis are organ-specific autoimmune disorders in which the skeletal muscles and skin (in DM only) are the primary target(s). In IBM, there is evidence that the disorder is primarily degenerative while the immune mechanisms are secondary.

Pathologically, the inflammatory myopathies are characterized by mononuclear cell inflammation, segmental muscle fiber necrosis, and muscle fiber regeneration. The pathologic findings in PM and DM have both common and diverse features. Both also are different from the findings in inclusion body myositis. Table C20-2 shows the major differences among these three primary inflammatory myopathies.

Table C20-2 Pathologic and Treatment Response Differences Between Polymyositis, Dermatomyositis, and Inclusion Body Myositis

Clinical Features

Polymyositis and dermatomyositis affect patients of all ages, with a predilection to women. In the United States, these disorders are twice as common among blacks than whites (incidence rate of 0.77 versus 0.32 per 100000, respectively). Most patients present with muscle weakness that develops subacutely over weeks to months, and affects predominantly the proximal pelvic and shoulder girdle muscles, including the neck flexors. Dysphagia and myalgia/muscle tenderness are common, each occurring in one-third of patients. Extramuscular manifestations are not uncommon. They include involvement of the lungs (interstitial lung disease) and the heart (cardiomegaly, congestive heart failure, and conduction defects), and manifestations of diffuse necrotizing vasculitis (especially in juvenile DM).

In DM only, there is an associated skin rash that can be the presenting symptom. Two classic rashes are characteristic. The first is a “heliotrope rash,” an erythematous, violaceous (hence the name) rash over the malar and periorbital areas that may extend to involve other sun-exposed areas, such as the dorsum of the hands, knees, elbows, or forehead. The second rash is Gottron papules, an erythematous papular rash over the knuckles of the fingers. Subcutaneous calcinosis complicates up to half of children with juvenile DM and may be present in chronic DM. These lesions are most often seen in the buttocks, thighs, knuckles, and elbows (Figure C20-1). They may be painful, ulcerate through the skin, or get infected.

Figure C20-1 Calcinosis in the popliteal fossa of a patient with a long history of dermatomyositis.

In contrast to PM and DM, IBM affects patients over the age of 50, has a male predominance, and is more common in the white than black population. The onset is insidious and it progresses slowly, evolving over years. It is the most common inflammatory myopathy in patients over the age of 50. Clinically, IBM has a unique muscle involvement that easily distinguishes it from the other inflammatory myopathies. There is usually asymmetrical involvement of the finger flexors and wrist flexors in the upper extremities, and the knee extensors and ankle dorsiflexors in the lower extremities (Figure C20-2). Dysphagia is common and afflicts up to 60% of patients as the disease progresses.

Figure C20-2 Bilateral asymmetrical hand weakness and atrophy (A) with severe bilateral asymmetrical quadriceps weakness, necessitating an assisted knee brace (B), in a 67-year-old man with inclusion body myositis.

Up to a quarter of patients with DM/PM have an associated connective tissue disease. The “overlap syndrome” links PM and DM with other connective tissue disease such as scleroderma, systemic lupus erythematosus, Sjögren syndrome, rheumatoid arthritis, and mixed connective tissue disease.

Laboratory features include an elevated serum CK in 90% of patients (at least 5 to 10 times normal values). LDH, ALT, and AST may be elevated, which can lead to the erroneous diagnosis of liver disease. In these situations, the ALT/AST ratio is useful. In hepatocellular disease, the ratio is greater than 1 while in myopathies, the ratio should be reversed (i.e., ALT/AST < 1). Also, measuring serum GGT activity is helpful in excluding concomitant hepatic disease, since this enzyme highly specific for hepatocellular disease and has a low level or is absent in muscle. Serum aldolase may also be elevated, but the utility of this enzyme in the diagnosis and follow-up of myopathies is of limited value for two reasons. First, aldolase is less sensitive or specific than CK since it is present in lower amounts in skeletal muscle. Second, serum aldolase is elevated in primary muscle as well as liver disease. ESR is normal or mildly elevated. Autoantibodies, such as ANA, SSA, or SSB, are positive in overlap syndromes. Anti-Jo-1 antibody is the most prevalent in DM/PM, occurring in 20% of patients and in 50 to 75% of patients with associated interstitial lung disease.

The incidence of malignancy in patients with DM and PM older than 40 years of age is higher than that expected for the general population, suggesting that DM and PM may be paraneoplastic. The increased risk in patients older than 40 years of age is 6-fold for DM and 2-fold for PM compared with that of the general population. The neoplasms are variable, but most are reported as carcinoma of the breast, ovary, and lung and gastrointestinal tract.

The diagnoses of DM and PM are confirmed based on the combination of clinical, laboratory, electrophysiologic, and pathologic findings. In 1975, Bohan and Peter proposed a classification that has been used since, with some revisions, in confirming the diagnoses of PM and DM. Based on these criteria, the confidence limits in the diagnosis of PM or DM range from definite to probable to possible (Table C20-3). The differential diagnoses of PM include IBM, polymyalgia rheumatica, metabolic myopathies (such as acid maltase deficiency), and limb girdle muscular dystrophy. In dermatomyositis, the presence of typical skin rash combined with muscle weakness often is diagnostic.

Table C20-3 Criteria and Confidence Limits in the Diagnosis of Polymyositis and Dermatomyositis

Criteria
1. Clinical	Predominantly proximal (limb girdle and neck flexor muscles), usually symmetrical, muscle weakness progressing over weeks or months, with or without myalgia, dysphagia or respiratory muscle involvement
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