Chronic Nonbacterial Osteomyelitis in a Young Child: A Case Report of a Diagnostic Challenge Mimicking Malignancy.

Chronic nonbacterial osteomyelitis (CNO) is a rare, autoinflammatory bone disorder that primarily affects children and adolescents. It typically presents with localized bone pain and lacks systemic symptoms, making diagnosis difficult. Radiologic features often mimic malignant or infectious etiologies, contributing to delays in diagnosis and the use of unnecessary invasive procedures. We report the case of a 5-year-old female patient with a history of sickle cell trait who presented with intermittent left arm pain. The initial physical examination was unremarkable, and symptoms were attributed to benign musculoskeletal causes. However, persistent symptoms over a five-month period prompted further evaluation. Radiographs revealed a permeative lesion in the humerus with areas of sclerosis and lucency, along with a multilayered periosteal reaction. Magnetic resonance imaging (MRI) showed an enhancing intraosseous lesion with cortical breakthrough and periosteal edema, raising concerns for malignancy, including osteosarcoma and Ewing sarcoma. Laboratory studies showed elevated erythrocyte sedimentation rate (ESR) and mildly increased C-reactive protein (CRP), but other results were unremarkable. Given the imaging findings and concerning differential diagnoses, a computed tomography (CT)-guided bone biopsy was performed. Histopathology confirmed chronic inflammatory changes consistent with CNO, and no evidence of neoplasia or infection was found. Whole-body magnetic resonance imaging revealed no other lesions. The patient was referred to pediatric rheumatology and began treatment with methotrexate, selected due to the lesion size and risk of pathologic fracture. The patient showed marked clinical improvement and returned to baseline function within six months. CNO, also referred to as chronic recurrent multifocal osteomyelitis in its multifocal form, remains underrecognized due to its vague symptoms and radiologic similarity to more ominous pathologies. Biopsy often plays a critical role in excluding malignancy. CNO can be unifocal or multifocal and is associated with other autoimmune conditions such as inflammatory bowel disease, psoriasis, and juvenile idiopathic arthritis. Given the patient's family history of ulcerative colitis, long-term monitoring is warranted. The condition predominantly affects female individuals and typically presents between the ages of 7 and 12. Treatment focuses on anti-inflammatory and immunomodulatory therapies. While nonsteroidal anti-inflammatory drugs are first-line agents, second-line therapies such as methotrexate, tumor necrosis factor inhibitors, corticosteroids, and bisphosphonates may be needed in more severe or refractory cases. CNO is a rare but clinically significant pediatric condition that can mimic malignancy and cause considerable diagnostic anxiety. Awareness among clinicians, particularly regarding atypical presentations such as unifocal disease in very young children, is essential to avoid unnecessary procedures and initiate early effective treatment. A multidisciplinary approach is often required for accurate diagnosis and optimal management.