Mycobacterium-associated lobular panniculitis, mimicking a rheumatoid nodule in a patient with rheumatoid arthritis

Mycobacterium-associated lobular panniculitis can mimic a rheumatoid nodule and has been seldom reported in rheumatoid arthritis (RA). We describe a 69-year-old woman with RA who presented initially with fever and an indurated skin lesion on the right thigh. Lobular panniculitis was diagnosed after biopsy and was then treated with prednisolone. After this therapy, pulmonary infiltration developed and was later shown by transbronchial biopsy to be caused by Mycobacterium tuberculosis. The panniculitis skin lesion became smaller after prednisolone therapy and was further improved after antituberculosis drugs were added. Reexamination of the previously biopsied skin tissue disclosed acid-fast bacilli. Reactivation or new infection of M. tuberculosis is a current important issue in RA patients, especially after treatment with disease-modifying anti rheumatic drugs or antitumor necrosis factor agents. Mycobacterium-associated lobular panniculitis should be included in the differential diagnosis of indurated skin disorder in RA patients, and acid-fast staining or polymerase chain reaction examination of tuberculosis should be performed routinely on biopsied skin tissue.