Arthritis of the small joints associated with Mycoplasma pneumoniae in a 2-year-old patient.

Mycoplasma pneumoniae (MP)is a pathogen responsible for upper and lower respiratory tract infections. Arthritis, although uncommon, can occur as an extrapulmonary complication of MP infections in children. Diagnosis typically involves serological testing and nucleic acid amplification tests (NAATs), such as the loop-mediated isothermal amplification (LAMP) test, which is widely used in Japan because of its speed, sensitivity, and specificity. Here, we present a case of arthritis linked to an MP infection in a young child.A 2-year-old girl presented with a persistent fever and swelling of the proximal interphalangeal joints in both hands. Laboratory investigations showed elevated levels of C-reactive protein (CRP) and serum matrix metalloproteinase-3 (MMP-3). Musculoskeletal ultrasonography revealed synovial thickening and increased blood flow in the affected joints. Although the LAMP test for MP using throat swab specimens was negative, the MP antibody titer measured with the particle agglutination test was 1 : 640, indicating MP infection. Treatment with clarithromycin led to defervescence and normalization of both CRP and MMP-3 levels.This case suggests that arthritis may be a manifestation of an MP infection in young children. In pediatric cases of fever of unknown origin and arthritis, MP infection should be included in the differential diagnosis.