31st January 2024

Differential diagnoses

Septic arthritis

Septic arthritis is a potentially life-threatening condition. It should be suspected in anyone who is systemically unwell with or without fever and has an acutely hot, swollen, painful joint. The sepsis 6 guidelines should be followed, and local guidelines of antibiotic choice used. Once clinically suspected, septic arthritis should be investigated and treated urgently, as there is a ~10% mortality rate in septic arthritis. 

Treatment with intravenous antibiotics and fluids should commence within 1 hour of clinical suspicion. Culture sample should be taken in the same way as for treatment of sepsis; however, importantly, joint aspiration samples should be obtained using an antiseptic non-touch technique. If confirmed, discussion with the Orthopaedics team is often needed for consideration of joint washout. If there is a suspected joint prosthesis infection, discussion with the orthopaedic team is needed promptly, and joint aspiration should be performed by them. 

Joints are commonly infected with bacteria, but viral and fungal infections should be considered in the differential diagnoses as treatment will differ. Microbiology advice should be sought if there is doubt.

Delayed or poorly treated septic arthritis can result in reduced joint functioning and possibly joint loss, and therefore, long-term disability. 

Haemarthrosis

Haemarthrosis is due to bleeding into a joint. It typically results following trauma and affects 1 joint- therefore, it can present as a painful, swollen single joint. A clear history is important to diagnose the condition, as well as clinical examination and imaging. Atraumatic haemarthrosis can occur in people with bleeding disorders, for example, haemophilia. A definitive diagnosis will involve joint aspiration- where clotting or bleeding disorders are of concern, this should be done by Orthopaedic colleagues and should be discussed with the Haematology team as consideration of blood products may be needed to avoid life-threatening bleeding.

Reactive arthritis

Reactive arthritis is a seronegative arthritis resulting from an inflammatory response to an extra-articular infection (rather than intra-articular). It commonly presents with urogenital infection (such as Chlamydia) or gastrointestinal infection (such as Campylobacter, Shigella, Salmonella and Yersinia). There is a delay in joint manifestation following an acute infection, and aspiration of the joint will not yield a micro-organism, unlike in septic arthritis. There is a typical triad of symptoms- conjunctivitis, urethritis and arthritis, although other manifestations can also be present. It is also common for only 2 parts of the triad to be present in reactive arthritis. 

Reactive arthritis can present as a monoarthritis or a polyarthritis. It is typically asymmetrical, and a distant site of infection can be determined on thorough history taking and systems examination, which can include examination of the external genitalia. A detailed but sensitive sexual history is important here.

Monoarticular manifestation of inflammatory arthritis

Inflammatory arthritis, such as rheumatoid arthritis, psoriatic arthritis, and other connective tissue diseases with joint manifestations, can all result in monoarticular pathology. However, inflammatory arthritis is usually polyarticular, and some are typically symmetrical in pattern. The type of joint involved can also vary, from small joints to large joints.

The pathophysiology of arthritis is similar in crystal arthropathy- an influx of inflammatory cells from the circulation to the joint space and synovium, triggering the inflammatory reaction. However, inflammatory arthritis typically follows a more chronic course and has different extra-articular manifestations. The pain and loss of function of the joints involved are associated with disuse of the joint and improvement on movement- therefore, time of day and exacerbating factors are important to ask about. The treatment options for inflammatory arthritis also differ with consideration of disease-modifying anti-rheumatoid drugs, DMARDs, and immunosuppressive therapy.

Traumatic synovitis

Traumatic synovitis is an inflammatory process occurring after a traumatic insult. The inflammatory reaction occurs as part of the healing process. Traumatic synovitis is associated with osteoarthritis.

References

• Taylor-Robinson D, Keat A. Septic and aseptic arthritis: a continuum? Best Practice & Research Clinical Rheumatology. 1999 Mar;13(1):179–92.
• Nigrovic P. UpToDate [Internet]. www.uptodate.com. 2022 [cited 2022 May 7]. Available from: https://www.uptodate.com/contents/overview-of-hemarthrosis
• Yu D, Tubergen A. UpToDate [Internet]. www.uptodate.com. 2021 [cited 2022 May 16]. Available from: https://www.uptodate.com/contents/reactive-arthritis
• Stavropoulos PG, Soura E, Kanelleas A, Katsambas A, Antoniou C. Reactive Arthritis. Journal of the European Academy of Dermatology and Venereology. 2014 Sep 8;29(3):415–24.
• Smolen J. UpToDate [Internet]. www.uptodate.com. 2020 [cited 2022 May 16]. Available from: https://www.uptodate.com/contents/undifferentiated-early-inflammatory-arthritis-in-adults
• Liao L, Zhang S, Zhao L, Chang X, Han L, Huang J, et al. Acute Synovitis after Trauma Precedes and is Associated with Osteoarthritis Onset and Progression. International Journal of Biological Sciences. 2020;16(6):970–80.