Hypogammaglobulinemic patient with polyarthritis mimicking rheumatoid arthritis finally diagnosed as septic arthritis caused by Mycoplasma hominis.

Hypogammaglobulinemia is a reduction or absence of immunoglobulin, which may be congenital or associated with immunosuppressive therapy. In addition to infectious diseases, autoimmune diseases have also been reported in patients with hypogammaglobulinemia. A 26-year-old man with hypogammaglobulinemia had multiple joint pain and swelling with erosive changes in the proximal interphalangeal joint of the right middle finger on X-ray film, mimicking rheumatoid arthritis (RA). As polyarthritis remained after immunoglobulin replacement therapy and there was no finding indicating any infection at that time, a diagnosis of RA was made. Prednisolone and etanercept were started. However, his polyarthritis did not improve and he developed meningitis and massive brain ischemia. Finally, a diagnosis of disseminated Mycoplasma hominis infection was made. The differential diagnosis of polyarthritis in patients with hypogammaglobulinemia should strictly exclude Mycoplasma infection by culture with special media or longer anaerobic culture, and molecular methods for mycoplasma.