2 and a half-year-old, Tim, is brought into the emergency department by his mother. He has been refusing to walk for four days.
A limping child with a fever has a wide differential diagnosis.
Septic arthritis is a medical emergency.
Urgent referral to orthopaedics and begin antibiotic therapy within 4 hours if a delay to theatres.
Knee pain can be referred from the hip.
Don’t forget about NAI, the fever might be a red herring.
Every hospital will do things a bit differently, check your local protocol.
What would you focus on in the history?
- Trauma? – Toddler’s fracture, contusions, non-accidental injury
- Coryzal symptoms? – transient synovitis
- Fever? – Septic arthritis/osteomyelitis/malignancy/arthritis
- Red flag symptoms – night pain, weight loss, easy bruising, easy bleeding – neoplasm, or haemarthrosis
- Leg length discrepancy – ?arthritis ?hemihypertrophy
He is complaining of pain in his left leg. He has a temperature of 38.7 C. He has had a coryzal illness for a few days, mum is unsure when it started.
Term baby, no previous hospital attendances. Fit and thriving. No recent weight loss, loss of appetite or changes in behaviour. He began cruising furniture at about 9 months of age.
Immunisations are up to date. No regular medications. Mum has not given any paracetamol or ibuprofen. No allergies.
What would you focus on during the examination?
Remember that knee pain can be referred from the hip, therefore look for range of movement in all joints.
Well looking, playful. Cap refill <2 seconds centrally and peripherally. No lymphadenopathy. Heart and chest sounds normal. No hepatomegaly or splenomegaly. No rashes or bruises were seen.
Right leg – hip knee ankle and toes all have a full range of movement and look normal.
Left leg hip – decreased range of movement on external rotation, due to pain
Knee – full range of movement
Ankle – full range of movement
How would you structure your differential diagnosis now?
Need to rule out septic arthritis and osteomyelitis.
Could still be transient synovitis – this is a diagnosis of exclusion as septic arthritis is a medical emergency.
How would you structure your investigations?
Check your local hospital guidelines.
The baseline blood tests would include FBC, ESR, and CRP. Consider clotting, LDH and blood film if there is any suspicion of malignancy. A blood culture would also be appropriate as the child might have septic arthritis or osteomyelitis. A full blood count alone is not sufficient to exclude septic arthritis and osteomyelitis. An ESR and CRP will give you a better idea of the inflammatory process.
What imaging tests would you order? Of which joint?
A plain film hip x-ray would be helpful in ruling out a fracture, though this is unlikely with the history.
During the early period of septic arthritis or osteomyelitis, there are frequently no x-ray changes. If there were any bony tenderness in the femur you should image the whole bone. This gives you a baseline for future imaging. Changes are visible by day 10 – 21 of the symptoms.
An ultrasound scan of the joint or bone involved is helpful to assess for effusions and collections.
The best imaging modality for osteomyelitis, particularly if it is thought to be multifocal is MRI, however, this needs sedation.
The next best scan would be a technetium radionuclide bone scan. This does not need sedation, and changes can be seen from ~48 hours of symptoms.
Why is septic arthritis an emergency?
Septic arthritis and osteomyelitis are often caused by the haematogenous spread of pathogens, therefore if untreated they could lead to septic shock. Local infection can lead to joint destruction, joint fusion, fistula formation or chronic infection. If the growth plate is damaged it can lead to asymmetry of the limbs.
The child needs urgent orthopaedic assessment with potential arthroscopy/arthrotomy with a washout.
In a case series in Sydney, the most common organism was S. aureus (85% with 1 case of MRSA). Other organisms included Streptococci (12%) and Yersinia species.
There is some variation depending on the age of the child. In the under 3 year olds Kingella kingae causes a significant disease burden (14% of cases).
Prior to the successful rollout of immunisation to Haemophilus influenza B, (HiB), it caused a significant number of cases of osteomyelitis and septic arthritis. In subpopulations the epidemiology is different. For example, children with sickle cell disease are at increased risk of Salmonella osteomyelitis.
Mycobacteria can also cause osteomyelitis and septic arthritis.
What organisms cause septic arthritis and osteomyelitis?
There is a different epidemiological pattern depending on age and geography. Generally speaking, S. aureus is by far the most common infective organism.
What antibiotic would you choose?
Treatment would be guided by your local epidemiology. Anti-staphylococcal agents such as flucloxacillin should be part of the treatment. There is ongoing controversy as to the length of IV treatment, so again it is best to follow local guidelines.
In Melbourne, for example, they use flucloxacillin 50mg/kg IV 6 hourly post wash out. In the US, empirical antibiotic cover would include naficillin and cefotaxime.
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