Differential Diagnosis for a 13-year-old Female with Left Knee Arthritis
Single Most Likely Diagnosis
- Juvenile Idiopathic Arthritis (JIA): This is the most common cause of arthritis in children and adolescents. The presentation of a warm, red, and swollen knee with elevated CRP is consistent with JIA, particularly the oligoarticular subtype which often affects large joints like the knee.
Other Likely Diagnoses
- Septic Arthritis: Although the patient does not have systemic symptoms like fever, septic arthritis is a possible cause of a warm, red, and swollen joint. It's essential to consider this diagnosis due to its potential for rapid joint destruction if not promptly treated.
- Lyme Arthritis: If the patient lives in an area endemic for Lyme disease, this could be a consideration, especially if there's a history of tick exposure. Lyme arthritis often presents with intermittent knee swelling and can have a similar presentation to JIA.
- Traumatic Synovitis: Recent trauma to the knee could lead to synovial inflammation, presenting similarly to arthritis. However, the absence of a reported injury does not rule out minor or forgotten incidents.
Do Not Miss Diagnoses
- Osteomyelitis or Bone Infection: Although less common, infections of the bone adjacent to the joint can present with similar symptoms to septic arthritis. Missing this diagnosis could lead to severe consequences, including bone destruction and long-term disability.
- Malignancy (e.g., Osteosarcoma, Leukemia): While rare, malignancies can initially present with joint or bone pain and swelling. It's crucial to consider these diagnoses to ensure timely intervention if present.
Rare Diagnoses
- Rheumatic Fever: This condition, associated with streptococcal infections, can cause arthritis but is less common and typically presents with migratory polyarthritis.
- Sarcoidosis: A systemic disease that can cause arthritis, but it's rare in children and usually accompanied by other systemic symptoms.
- Hemophilia or Other Bleeding Disorders: These conditions can cause hemarthrosis (bleeding into the joint), leading to arthritis-like symptoms, but they are rare and typically have a known family history or previous episodes.
Next steps should include a thorough history and physical examination, laboratory tests (e.g., complete blood count, blood cultures, Lyme titers if endemic), and possibly imaging studies (e.g., X-ray, ultrasound, or MRI) to evaluate the joint and rule out other causes. Referral to a pediatric rheumatologist may also be considered for further evaluation and management.