Arthur Kavanaugh, MD


March 18, 2002


I have a patient with myositis and panniculitis. A muscle biopsy was normal (but was followed by pyogenic infection). Other test results are as follows: creatine phosphokinase (CPK) slightly elevated; lactic dehydrogenase slightly elevated; C-reactive protein normal; systemic lupus erythematosus (SLE) negative; antinuclear antibodies (ANA) negative; thyrotropin-releasing hormone slightly elevated; triiodothyronine and thyroxine normal. There is no evidence of paraneoplastic disease. What is the possible differential diagnosis?

Response from Arthur Kavanaugh, MD

The differential diagnosis of myositis is extensive and includes a variety of neuromuscular, metabolic, endocrinologic, toxic, and infectious etiologies. In this case, perhaps the first question should be, on what basis was the diagnosis of myositis made?

While muscle biopsy can be falsely negative (which can be explained by sampling error related to the inhomogeneous involvement of muscle in inflammatory myositis), the "slight" elevation of CPK is also puzzling. Has an electromyelogram been performed? In the hands of an experienced operator this can be a powerful diagnostic test; not only can it define inflammatory myositis, but if the etiology of the symptoms is not related to myositis, important information regarding potential other etiologies can often be detected. Has an aldolase test been performed? It is perhaps more specific for muscle injury.

The presence of panniculitis is intriguing. While inflammatory myositis may occur as part of an overlap condition with other autoimmune diseases, the most common of these are SLE and scleroderma, neither of which is typically associated with panniculitis. As implied in the question, consideration of thyroid abnormalities is always key in patients with suspected myositis; in this case, however, such abnormalities seem to have been excluded. The absence of ANA does not necessarily exclude inflammatory myositis, as ANA are seen in only 50% to 75% of cases.[1]


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