Atraumatic Swollen Knee in a Child: An Approach

The atraumatic swollen knee in the paediatric patient can be a diagnostic challenge. Our goal is to diagnose serious conditions such as septic arthritis and osteomyelitis, whilst avoiding unnecessary arthrocentesis. This is an approach.

Differential Diagnosis of an Acutely Atraumatic Swollen Knee:

1. Infectious
   1. Septic Arthritis
      1. Staphylococcus species can occur in all age groups
      2. Group B streptococcus is more common in < 3mo, whereas Group A is more common in all other age groups
      3. Haemophilus species occur in < 12 yo
      4. Neisseria gonorrhoeae occur in < 3 mo and > 12 yo
      5. Other infectious causes include:
         1. <3mo: Escherichia coli, other G-ve bacilli, Haemophilus species
         2. >3mo - < 12yo: Streptococcus pneumoniae, Salmonella species, Kingella kingae
         3. >12-18yo: Streptococcus pneumonia, Kingella cinge
   2. Osteomyelitis
   3. Pyomyositis
   4. Septic Bursitis
   5. (Lyme arthritis)- varies according to geographical location
2. Inflammatory
   1. Juvenile arthritis
   2. Acute Rheumatic Fever
   3. SLE. arthritis
   4. IBD arthritis
   5. Henoch-Schönlein purpura
   6. Post infection arthritis
   7. Gout (rare except in renal failure and genetic urate metabolism conditions)
   8. Popliteal Cyst
3. Other
   1. Occult trauma
   2. Overuse injury
   3. Haemarthrosis (Haemophilia or other bleeding disorders)
   4. Malignancy

History and Examination Pearls

History

Symptom duration, severity of pain, range of motion restriction, ability to bear weight and involvement of other joints, and systemic symptoms ie.,fever.

Multiple joint involvement suggests rheumatologic causes,

Fever is more common (not always present) with infectious causes.

A sexual history in older children may identify risk for gonococcal arthritis.

A  history of tick bite could be helpful.

Examination

 Anterior swelling without joint involvement suggests prepatellar bursitis,

Swelling mostly in the popliteal fossa could be a Baker’s cyst.

Note the presence and extent of erythema

The range of motion and associated pain on movement are important to examine as septic arthritis tends to cause severe pain during any range of motion and the range of motion is limited. 

Asses the ability to bear weight and gait.