Objective: To elucidate the clinical and radiological features of Pneumocystis pneumonia (PCP) in patients with rheumatoid arthritis (RA), compared with methotrexate (MTX) pneumonitis in RA and Pneumocystis pneumonia in acquired immunodeficiency syndrome (AIDS). Subjects and Methods: Retrospective analysis of 14 PCP cases in RA (RA-PCP), 10 MTX pneumonitis cases in RA (MTX-P) and 11 PCP cases in AIDS (AIDS-PCP) from 9 centers in the Kanto area in the last 6 years. Results: Compared with AIDS-PCP, both RA-PCP and MTX-P developed more rapidly, showing higher serum CRP and lower plasma β-D-glucan levels, and more severe oxygenation impairment. In most of the RA-PCP cases, a high dose of corticosteroid was administered as adjunctive therapy, resulting in a favorable outcome. The mortality was 14% in RA-PCP, 0% in AIDS-PCP and 0% in MTX-P cases. In RA-PCP patients the CD4 cell count showed only mild suppression, not reaching the predisposing level for PCP in HIV infection, suggesting that there are risk factors for RA-PCP other than immunosuppression.,Radiologic analysis revealed some characteristic patterns of each disease. In MTX-P, diffuse homogeneous ground glass opacity (GGO) with sharp demarcation by interlobular septa (type A GGO) was found in 70%, while in AIDS-PCP diffuse, homogeneous or nonhomogeneous GGO without interlobular septal boundaries (type B GGO) was predominant (91%). In RA-PCP, type A GGO was found in 6 cases and type B GGO in 5 cases, showing the complex nature of this disease. Conclusion: RA-PCP differed considerably from AIDS-PCP clinically and radiologically. Clinically it occurred without severe immunosuppression, and showed characteristic aspects, with more intense inflammation and less parasite burden. Radiologically it mimicked MTX-P in some cases sharing the conspicuous CT features of MTX-P, rendering the distinction of these two disorders difficult.
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