Characteristics of Pyomyositis at a Pediatric Hospital in Osaka, Japan.

Introduction Pyomyositis, also known as tropical pyomyositis, has traditionally been considered more prevalent in tropical regions. The number of pediatric pyomyositis cases has been increasing in several temperate countries. In this study, we examined the characteristics of pyomyositis in a single pediatric hospital in Japan, a temperate country. Methods We analyzed 108 patients diagnosed with pyogenic arthritis, pyogenic osteomyelitis, and pyomyositis who were followed up at our hospital over a 32-year period. Data were collected on sex, age of onset, affected site, causative organism, and year of occurrence. The characteristics of pyomyositis were identified through comparison with pyogenic osteomyelitis and pyogenic arthritis. Results The number of patients with pyogenic osteomyelitis, pyogenic arthritis, and pyomyositis was 44, 51, and 13, respectively. In pyogenic arthritis, the hip joint was the most commonly affected site (24 of 51 patients, 47.1%), where the femur was the most frequently involved site in osteomyelitis (14 of 44 patients, 31.8%). Among 13 patients with pyomyositis, 12 (92.3%) had muscle involvement around the hip joint, with the obturator muscles being the most commonly affected. Bacterial organisms were identified by venous blood or pus culture in 11 pyomyositis patients, with methicillin-sensitive  (MSSA) detected in five cases. During the first 16 years of the study period, 43 patients were diagnosed with pyogenic arthritis or osteomyelitis, while only two cases of pyomyositis were recorded. In contrast, over the most recent 16 years, pyogenic arthritis and osteomyelitis cases increased to 53, while pyomyositis cases rose to 10. Conclusions At our institution, pediatric pyomyositis primarily affected the periarticular muscles of the hip. Although a rising trend in cases was observed, the difference between the earlier and later periods was not statistically significant (p=0.069). These findings emphasize the importance of including pyomyositis in the differential diagnosis of pyogenic hip infections and recommend early MRI evaluation prior to joint aspiration.