We report an unusual case in which misinterpretation of clinical or serologic findings led to overdiagnosis and overtreatment of Lyme borreliosis.

In October 1992, a 46-year-old man developed multiple, slowly enlarging annular erythematous lesions on his legs. These were accompanied by fatigue, headache, low-grade fever, myalgias, and arthralgias. Because of a history of insect bites, an enzyme-linked immunosorbent assay (ELISA) for Lyme antibody was done. The test result was negative, and no laboratory test abnormalities were found. In December, he was admitted to a neurology department because of paresis of his right median nerve; cutaneous lesions and systemic symptoms persisted unchanged. Laboratory findings were normal again except for the presence of positive IgM antibody to Borrelia burgdorferi at low titer. The diagnosis of Lyme disease was suggested, and treatment with ceftriaxone (2 g/d for 15 days) was begun. Paresis and skin lesions resolved within a few days, but the patient's general condition did not significantly improve.

In February 1993, we saw him for the first time because his skin condition and constitutional symptoms had worsened. Physical examination showed annular erythematous-urticarial lesions and erythematous-purpuric papules on his thighs and legs. Histopathologic examination showed leukocytoclastic vasculitis of the dermal vessels with some extravasation of the erythrocytes. The patient had a faint IgG antibody response to the 31 KD antigen of B. burgdorferi, but indirect immunofluorescence and ELISA results were negative. Routine laboratory tests, chest radiographs, an electrocardiogram, and an echograph of the abdomen were normal. Finally, because of his disabling fatigue, echocardiography was done and showed the presence of a left atrial myxoma. A few days after surgical excision of the tumor, systemic symptoms and the skin lesions disappeared and have not recurred.

Atrial myxoma and Lyme disease should be regarded as "great simulators" because they can produce many clinical symptoms [2, 3]. Cardiac myxomas are frequently misdiagnosed as endocarditis, connective tissue disease, fever of unknown origin, or other neoplasms because they can appear with noncardiac manifestations or various skin lesions [4, 5]. This report also confirms that clinical findings and serologic tests suggestive of Lyme disease, coupled with an uncertain history of tick bites, must be regarded with caution before making a diagnosis.