Single swollen joint

A single swollen joint has several potential causes.

Key causes include:

• Infection (and reactive arthritis) - bacterial, mycobacterial (including TB) and rheumatic fever.
• Trauma (including non-accidental injury). 
• Inflammatory (Oligoarticular onset Juvenile Idiopathic Arthritis or arthritis related to inflammatory bowel disease).
• Haemarthrosis (trauma or bleeding disorder).
• Pigmented villonodular synovitis or synovial hemangioma.
• Malignancy.

In the absence of trauma or infection, Juvenile Idiopathic Arthritis (JIA) is the most likely diagnosis; in contrast to adults with a single swollen joint, children rarely get gout although this has been described in older adolescents in certain at risk populations (e.g., Maori and Pacific Island origins, and those with sickle cell disease with hyperuricemia). 

Clinical assessment and presence or absence of red flags are most important when considering the differential diagnosis. Careful use of investigations will invariably confirm or refute potential causes. 

The onset, progression and the presence of extra-articular features including fever, red eyes, urethritis, and history of preceding diarrhoea or sore throat and cardiac murmur, often guides the clinician to the correct diagnosis. Travel history is also important (considering Lyme disease).

Children with septic arthritis appear unwell, are febrile and the affected joint and limb is held still due to severe pain with joint movement. Septic arthritis usually occurs in large joints, generally involving a single joint.

Rheumatic fever can often present with or without fever.  Although the arthritis is usually of a migratory pattern (i.e., it moves from joint to joint) with prompt response to NSAIDs, it can present with a single swollen joint.  The arthritis can present as a very tender joint. Differentiation from a septic joint may be difficult. This diagnosis should be considered in all those presenting with arthritis with evidence of streptococcal infection and especially in high risk (Maori and Pacific), school aged populations. 

Further Reading: Septic arthritis and acute rheumatic fever in children: the diagnostic value of serological inflammatory markers – Mistry et al. J Pediatr Orthop 2015;35:318-322

Reactive arthritis is usually monoarticular or less commonly oligoarticular (up to 4 joints) and commonly follows bacterial infection in the gut (Salmonella, Shigella, Campylobacter, Yersinia). In older children and adolescents, it is considered to be a sexually acquired infection (Chlamydia, gonorrhoea). The potential for sexual abuse needs to be considered with sexually acquired infection in a child or adolescent. 

Lyme disease (a reactive arthritis due to Borrelia burgdorferi, an infection transmitted by ticks) is suggested by the presence of an oligoarthritis, after travel to an endemic area. The classic history of a tick bite, or rash (erythema chronicum migrans) may not be prominent.

Mycobacterial disease (including Tuberculosis - TB) must be considered in the context of the unwell child with joint pain or swelling whose family are from an endemic area; they might appear well or have systemic symptoms (night sweats, poor growth or weight loss). TB should be considered in the context of chronic joint pain in otherwise well looking children with families from endemic areas. TB is detected by clinical suspicion, Mantoux or positive TB quantiFERON® gold, followed by synovial biopsy. TB should be considered in the context of immunosuppression (through disease or treatment). Co-existent Human Immunodeficiency Virus (HIV) infection is common and must be excluded.

Multisystem disease (such as Juvenile Systemic Lupus Erythematosus) is unlikely to present with a single swollen joint.

Inflammatory bowel disease associated arthritis may precede or follow the gastrointestinal features.