🩺 Hot Swollen Joint

A single acutely hot, swollen, and painful joint (monoarthritis) is a medical emergency because septic arthritis can cause rapid, irreversible joint destruction and systemic sepsis. While crystal arthropathies like gout and pseudogout are more common, they cannot be reliably distinguished from infection by clinical examination alone. In the UK, standard practice involves urgent aspiration of the joint fluid for culture and microscopy. Prompt diagnosis and treatment with appropriate antibiotics or anti-inflammatories are essential for joint preservation.

Inquire about the speed of onset; gout often reaches peak intensity within 24 hours, whereas septic arthritis may be slightly more gradual but still acute. Ask about previous episodes (suggesting crystal arthropathy), recent trauma, or joint surgery (increasing risk of infection). Screen for systemic symptoms like fever and rigors. Identify risk factors such as diabetes, immunocompromised status, IV drug use, or pre-existing joint disease (e.g., Rheumatoid Arthritis). A history of recent sexual activity or urethral discharge might point toward disseminated gonococcal infection.

Assess the joint for the four cardinal signs of inflammation: rubor (redness), calor (heat), tumor (swelling), and dolor (pain). Compare the affected joint with the contralateral side. Evaluate the range of motion, noting that in septic arthritis, both active and passive movements are severely restricted and painful. Check for a joint effusion (e.g., patellar tap or bulge sign in the knee). Ensure you examine the skin for any overlying portals of entry, such as cellulitis or trauma, and check for systemic signs like tachycardia or fever.

Septic Arthritis, Gout, Pseudogout (CPPD), Reactive Arthritis, Haemarthrosis, Rhumatoid Arthritis flare, Cellulitis (mimic).

Fever/rigors, inability to bear weight, severe pain on passive motion, and a prosthetic joint (high risk of biofilm infection).

The gold standard is urgent joint aspiration for synovial fluid analysis (Gram stain, culture, and crystal microscopy). Blood tests should include FBC, CRP, and ESR (typically raised), along with blood cultures if the patient is febrile. Serum urate may be normal during an acute attack of gout, so it is not a reliable exclusion tool. Plain X-rays are often normal in the early stages but can show soft tissue swelling or pre-existing arthritic changes. Ultrasound can be used to confirm an effusion and guide aspiration in difficult joints like the hip.

If the patient is febrile and has a single hot joint, it is septic arthritis until proven otherwise—aspirate 'before' starting antibiotics if the patient is stable. Gout typically affects the first MTP joint (podagra) but can present in the knee; the presence of crystals does not 100% rule out concurrent infection. Never start allopurinol during an acute attack of gout, as it can worsen the inflammation. Aspirated fluid that is 'frankly purulent' is highly suggestive of sepsis regardless of initial Gram stain results.

The 'Hot Swollen Joint' is a classic emergency in the MLA content map under 'Musculoskeletal' and 'Acute Medicine'. The critical skill is knowing how to manage suspected septic arthritis (aspirate, then treat) and the contraindications of starting allopurinol acutely. Discrimination between cellulitis and arthritis is also frequently examined.