Differential Diagnosis for Rapid Onset Bilateral Leg Pain in a 3-Year-Old

Single Most Likely Diagnosis

• Viral myositis or muscle strain: This is a common condition in children, often resulting from viral infections or minor trauma, leading to muscle inflammation and pain. The rapid onset and bilateral nature of the pain could be consistent with a viral etiology affecting the muscles.

Other Likely Diagnoses

• Growing pains: Although typically more common in older children, growing pains can occur in younger children and are characterized by recurrent, often bilateral leg pain, usually in the evenings or at night.
• Trauma: Accidental injuries, such as falls, can cause bilateral leg pain, especially if the child has fallen onto both legs or has been involved in a more significant trauma.
• Infection (e.g., osteomyelitis, septic arthritis): While less common, infections of the bone or joints can cause severe, acute pain and should be considered, especially if there are signs of infection like fever or redness.

Do Not Miss Diagnoses

• Child abuse: It's crucial to consider non-accidental trauma, especially if the history provided does not match the injury pattern or if there are other signs suggestive of abuse.
• Cauda equina syndrome or spinal cord injury: Although rare, these conditions can present with bilateral leg pain and are neurological emergencies requiring immediate intervention.
• Neuroblastoma or other malignancies: While rare, certain cancers can cause bone or nerve pain and must be considered in the differential diagnosis to ensure timely diagnosis and treatment.

Rare Diagnoses

• Leukemia: Can cause bone pain due to marrow infiltration, though it would typically be accompanied by other systemic symptoms.
• Henoch-Schönlein purpura (HSP): A form of vasculitis that can cause leg pain, purpura, and sometimes abdominal pain or renal involvement.
• Fabry disease: A genetic disorder that can cause episodes of acute, severe pain in the hands and feet, though it's very rare and usually diagnosed later in life.

Each of these diagnoses requires careful consideration of the child's overall clinical presentation, history, and additional diagnostic tests as necessary to determine the cause of the bilateral leg pain.