Single joint arthritis
A child presenting with a single swollen joint requires careful assessment, as there are several potential causes. The key causes to consider are:
- Infection (including reactive arthritis) – bacterial and mycobacterial
- Trauma – including non-accidental injury
- Inflammatory causes – such as oligoarticular Juvenile Idiopathic Arthritis or arthritis associated with inflammatory bowel disease
- Haemarthrosis – due to trauma or an underlying bleeding disorder
- Malignancy
In the absence of trauma or infection, Juvenile Idiopathic Arthritis (JIA) is the most likely diagnosis. In contrast to adults presenting with a single swollen joint, children rarely develop gout.
The most important aspect of assessment is the clinical picture and the presence or absence of red flags, particularly when considering serious causes such as infection, malignancy, or non-accidental injury. Careful clinical assessment should guide the use of investigations, which are used to support or exclude suspected diagnoses rather than replace clinical judgement.
Children with septic arthritis typically appear unwell, are febrile, and have severe pain with joint movement. The affected limb is often held still, and the condition usually involves a single large joint.
Reactive arthritis may present as a single swollen joint (or occasionally multiple joints) and typically follows a preceding infection. In younger children this is usually gastrointestinal (for example Salmonella, Shigella, Campylobacter or Yersinia). In older children and adolescents it may follow sexually transmitted infection such as chlamydia or gonorrhoea, and safeguarding considerations may therefore arise.
Lyme disease, a reactive arthritis caused by Borrelia burgdorferi transmitted by ticks, should be considered where there is arthritis with a history of travel to an endemic area. A history of tick bite or the characteristic erythema migrans rash may not always be present.
Mycobacterial infection should be considered in children with chronic joint swelling, particularly if they are immunosuppressed due to disease or treatment.
Multisystem inflammatory conditions such as systemic lupus erythematosus are unlikely to present with an isolated swollen joint, although arthritis associated with inflammatory bowel disease may occur before or after gastrointestinal symptoms.
