Objective: Oligoarthritis is the most common manifestation of late Lyme disease in children. Considerable overlap can occur in the clinical presentation of Lyme arthritis and acute septic arthritis. Early differentiation is critical, given the disparate therapeutic implications; Lyme arthritis is treated with outpatient oral antibiotics, while septic arthritis requires hospitalization, IV antibiotics, and, often, surgical drainage. We wanted to identify MRI features that may distinguish Lyme arthritis from septic arthritis in children.
Materials and methods: Knee MR images in 11 children with Lyme arthritis and 7 with septic arthritis, with a mean age 10.6 years old and 11.7 years old, respectively, were reviewed by a radiologist blinded to the final diagnosis. Joint effusion size, synovial thickness, adenopathy, subcutaneous, marrow, and muscle edema on MRI; and clinical parameters including age, sex, fever, WBC, erythrocyte sedimentation rate, C-reactive protein, and joint fluid WBC in the two patient groups were compared using univariate and multivariate analyses.
Results: Subcutaneous edema was seen in all septic arthritis patients but in only one of 11 patients with Lyme arthritis (p < 0.01). Myositis and adenopathy were present in all Lyme arthritis patients and two of seven patients with septic arthritis (both p < 0.01). No significant difference was present in synovial thickness, marrow edema, or joint fluid size. There were no statistically significant differences in the clinical parameters assessed.
Conclusion: Our results identified three MRI features, specifically, myositis, adenopathy, and lack of subcutaneous edema, that strongly suggest the diagnosis of Lyme arthritis rather than septic arthritis in children with acute inflammation of the knee. Awareness of these characteristic MRI features may avoid unnecessary invasive procedures and cost.