Paediatric septic arthritis with no risk factors

A four-year-old New Zealand European boy presented to an urgent care facility at 9 pm with an inability to walk on his right leg. His mother reported that the previous day he had injured himself after jumping off rocks at daycare, though the event was unwitnessed. Initially, he managed with regular pain relief, but on the day of presentation his pain worsened and he refused to weight-bear. He indicated pain around the right knee. There was no history of fever, viral illness, or recent infection. His past medical history included eczema and anaphylaxis to eggs, with full immunisations up to date. Medications included paracetamol, ibuprofen, Ventolin, and Flixotide.

On examination, his temperature was 37.7°C, heart rate 140 bpm, respiratory rate 22, and oxygen saturation 98%. He looked well but was unable to weight-bear. There was normal perfusion and sensation in the right foot, with no swelling or bony tenderness in the foot, ankle, tibia, or thigh. The right knee had a full range of motion without swelling, effusion, or erythema. The hip, however, was significantly painful on range-of-motion testing. The clinical findings seemed inconsistent with the initial history of trauma. Although fracture needed to be excluded, septic arthritis of the hip was a key consideration, particularly given the tachycardia which could be due to pain or infection. X-rays of the tibia, femur, and hip showed no evidence of fracture. The child was referred to the local paediatric emergency department for further assessment, inflammatory markers, and ultrasound to exclude hip septic arthritis.

At hospital, initial blood tests revealed a CRP of 7 and normal white cell count with neutrophils of 9.7. An MRI the following day showed moderate right hip joint effusion, and aspiration confirmed frank pus, establishing the diagnosis of septic arthritis. The child was treated with two weeks of intravenous antibiotics followed by four weeks of oral therapy.

Learning Nuggets

*The initial history suggested a possible injury, but no confirmed mechanism was identified. This highlights the importance of obtaining a clear and detailed history to avoid misdiagnosis.

*Despite appearing well, the patient had significant tachycardia (140 bpm), which could be attributed to pain but also an underlying infectious process.

*The severity of pain seemed inconsistent with a simple transient synovitis. The patient did not adopt the typical posture (hip flexed, externally rotated, abducted) of hip septic arthritis¹ but he had an abnormal/painful range of motion of the hip and was unable to weight-bear, necessitating exclusion of septic arthritis.

*Knee pain can be referred pain from hip pathology.

*Septic arthritis requires urgent recognition, as delayed diagnosis can lead to systemic sepsis, joint damage, and disability.^1,5 Transient synovitis is more common, making differentiation challenging.¹

*Common criteria for septic arthritis include: temperature >38.5°C, WCC >12 , ESR >40 and CRP>20.^1,2 In this case both WCC and CRP were normal at initial ED assessment.

*A 2015 study suggested that in primary care, absence of fever suggests transient synovitis, with initiation of regular NSAIDs and next-day review being appropriate.¹ However, this case highlights that absence of fever does not exclude septic arthritis.

*A 2023 systematic review found no consensus on diagnostic algorithms for septic arthritis and concluded that absence of fever and normal inflammatory markers cannot rule it out.⁴

*Joint aspiration is the gold standard for diagnosing septic arthritis.⁴ Ultrasound, is highly sensitive for detecting hip joint effusion⁵ and is more reliable than plain radiographs or MRI.⁵

*Fever is often high in MRSA cases but may be mild or absent in Kingella kingae infections.^2,3

*Kingella kingae can also cause endocarditis and discitis, necessitating broader clinical assessment.³ Several studies have noted the rise in Kingella kingae as a cause of septic arthritis in relation to a decrease in Haemophilus influenza infections.⁴ Nevertheless, Staphylococcus aureus remains the most common pathogen, responsible for 61% of cases.⁴

*Paediatric septic arthritis is usually hematogenous.¹ Though this child had no recent infection, respiratory tract infections can precede septic arthritis by several weeks.⁵

*Septic arthritis incidence varies by ethnicity in New Zealand: 1:16,000 in NZ Europeans, 1:8,760 in Pasifica, and 1:4,300 in Māori.⁶ This child was NZ European.

*When the clinical presentation is not so clear we should maintain a high index of suspicion especially in Maori and Pacifica children. Māori and Pasifica children also have higher rheumatic fever risk which can be a cause of monoarthritis.³

*Salmonella typhi should be considered in patients with sickle cell disease.⁵

References

1. Practitioners TRAC of general. Septic arthritis in children. Australian Family Physician. Available from: https://www.racgp.org.au/afp/2015/april/septic-arthritis-in-children
2. Pääkkönen M. Septic arthritis in children: diagnosis and treatment. Pediatric Health Med Ther. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5774603/
3. Starship Clinical Guidelines. Osteomyelitis. Available from: https://starship.org.nz/guidelines/osteomyelitis/
4. Nannini A, Giorgino R, Bianco Prevot L, Bobba A, Curci D, Cecchinato R, et al. Septic arthritis in the pediatric hip joint: a systematic review of diagnosis, management, and outcomes. Front Pediatr. Available from: https://www.frontiersin.org/articles/10.3389/fped.2023.1311862/full
5. Donders CM, Spaans AJ, Wering H van, Bergen CJ van. Developments in diagnosis and treatment of paediatric septic arthritis. World J Orthop. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8891656/
6. Hunter S, Baker JF. Ten-year retrospective review of paediatric septic arthritis in a New Zealand centre. International Orthopaedics (SICOT). Available from: https://link.springer.com/article/10.1007/s00264-020-04611-z