A previously healthy two-year-old boy presents GP with a two-day history of fever and irritability. He is refusing to weight bear, although there is no recent history of injury. The child is fully immunised. He is pyrexial with a temperature of 38.7⁰C, tachycardic with a heart rate of 160bpm, and his respiratory rate is 30 breaths per minute. He keeps his left knee flexed and is reluctant to move it. Further examination reveals his knee is swollen, warm and painful. Systemic examination is unremarkable. An urgent referral to orthopaedics with suspected septic arthritis is made.
The child has a raised white cell count and elevated CRP. X-ray of the knee is unremarkable. He has a joint aspiration which reveals increased white cell count and positive gram stain.
The problem
Septic arthritis is a microbial infection in a joint space. Haematogenous spread of bacteria into the synovium is the most common route of acquisition. Other aetiologies include septic arthritis following a penetrating trauma or adjacent osteomyelitis.
All age groups are affected, but in children, septic arthritis is most common in those under three years.
The most common organism is Staphylococcus aureus. Other pathogens include Streptococcus species, Pseudomonas aeruginosa, Neisseria meningitidis, Escherichia coli, Klebsiella and Enterobacter species. Gonococcal septic arthritis should be considered in sexually active teenagers or adults, or newborns who may acquire it through vertical transmission.1 In the non-immunised child, consider Haemophilus influenzae type B.
Early diagnosis and treatment is essential to prevent permanent joint damage.
Features
The classic presentation in a child is a short history of fever with a hot, swollen and tender joint with limited movement, though absence of fever at presentation does not exclude the diagnosis. The child is usually unwell.
Typically one joint is involved and in most cases this is a lower extremity joint, especially the knee or hip. The elbow is the most common upper extremity joint to be infected. Neonates are more likely to have infection in multiple joints.
In patients with an infected hip joint there may be no erythema or swelling due to the deep location of the joint, but typically the patient is reluctant to weight bear. Children often orient an affected joint to minimise the pain – for example the hip is flexed, abducted, and externally rotated, the knee, ankle, and elbow are partially flexed, whereas the shoulder is adducted and internally rotated. Psuedoparalysis of the affected limb is seen in neonates and younger children.
Investigation
Joint aspiration and blood culture should be performed promptly. Typically, these are done in hospital.
- The synovial fluid analysis should include cell count and differential white cell count, gram stain and culture.
- Blood analysis often shows elevated white cell count, but a normal value does not rule out septic arthritis. CRP and ESR are more sensitive markers.
Other investigations may include:
- Plain X-ray – often normal in septic arthritis but can exclude other causes of joint pain.2
- Ultrasonography – the modality of choice to reveal hip effusions and to guide needle aspiration.
- Bone scan – may be helpful if multifocal disease is suspected. It also helps to detect associated osteomyelitis.
Diagnosis
A painful hot swollen joint – or joints – should be treated as septic arthritis until proven otherwise. Diagnosis is established by a combination of clinical findings and results of synovial fluid analysis. A low index of suspicion is required.
Management
- Septic arthritis is a medical emergency and requires urgent referral to orthopaedics.
- Joint aspiration and blood cultures are required, followed by early treatment.
- IV antibiotics are then changed to oral once clinically indicated. Antibiotics are generally continued for three to six weeks.
Dr Liza McCann is a consultant paediatric rheumatologist, Dr Raja Syahanee is a specialist paediatric registrar in rheumatology and Dr Thomas Morgan is an SHO in rheumatology at Alder Hey Children’s Hospital, Liverpool
Alder Hey is one of Europe’s busiest children’s hospitals providing care for over 275,000 patients each year. Alder Hey has a broad range of hospital and community services for direct referral from primary care. The Trust is the designated national centre for head and face surgery and a Centre of Excellence for children with cancer, spinal and brain disease. Alder Hey has been chosen to be a national centre for heart surgery, a respiratory ECMO surgery centre and one of just four specialist centres to provide surgery for drug-resistant epilepsy. For more information visit: alderhey.nhs.uk
References
1 Rice PA. Gonococcal arthritis (disseminated gonococcal infection). Infect Dis Clin North Am. Dec 2005;19(4):853-61
2 Baskett A, Hosking J and Aickin R. Hip radiography for the investigation of nontraumatic, short duration hip pain presenting to a children’s emergency department. Pediatr Emerg Care 2009; 25:78-82
