18th January 2024

Septic Arthritis

Background

The acute hot joint is a common clinical presentation with a wide differential diagnosis, of which septic arthritis confers the highest morbidity and mortality. The British Society for Rheumatology (BSR) guidelines on management of the hot swollen joint state that:

“Patients with a short history of a hot, swollen and tender joint (or joints) with restriction of movement should be regarded as having septic arthritis until proven otherwise”.

Septic arthritis is defined as joint space infection which can occur in either a native or prosthetic joint. The most commonly implicated organism is Staphylococcus aureus. Other causes of bacterial septic arthritis include Streptococcus pneumonia, Salmonella species (associated with sickle cell disease), Pseudomonas aeruginosa (associated with IV drug use and puncture wounds) and Neisseria gonorrhoea (in those with concomitant gonococcal genitourinary infection).

Risk factors for development of septic arthritis include:

• Prosthetic joints
• Prior joint damage, such as from RA/gout
• Diabetes mellitus
• Immunodeficiency
• Intravenous drug use
• Recent steroid injections or joint surgery
• Bacterial infections at another site (haematogenous spread)
• Previous joint infection
• Older age

Diagnosis

It is vital that a suspected septic joint is investigated and managed as early as possible, as untreated infection quickly leads to joint destruction and can be fatal.

Signs and symptoms

The typical case is of an acute red, hot, swollen joint in a systemically unwell patient with a fever – however, it is important to note that in many cases, the patient may not appear systemically unwell.

Symptoms include pain and swelling, usually in one joint – most commonly the knee (can be polyarticular in a minority of patients). The patient will typically have an inability to weight bear and limited range of motion in the affected joint. Infections in other joints can lead to a variety of symptoms, e.g. sacroiliac joint infections may present as buttock or hip pain, and sternoclavicular joint infections may present with chest pain.

Investigations

A joint aspiration should be prioritised as it ideally needs to be performed prior to antibiotic administration to obtain the highest yield results. The synovial fluid would typically be cloudy in appearance in the presence of infection and should be examined for:

• Culture & Sensitivity
• Gram stain
• White Cell Count (typically >50,000/mm3)
• Polarising crystal microscopy

Other important investigations include: FBC, CRP and blood cultures. U&E and LFTs should be checked, as this may impact on choice of antibiotics. If Neisseria gonorrhoea is suspected, then cultures of extended sites (i.e. rectal, cervical, urethral or pharyngeal) for gonococcal infection will be required as this organism is poorly cultured in synovial fluid.

Plain radiographs are usually of no diagnostic benefit in septic arthritis but should be obtained if there is suspicion of injury or fracture. They may show chondrocalcinosis in pseudogout or osteomyelitis if the infection has progressed further into the bone, necessitating a longer treatment course.

Differential diagnoses

• Crystal arthropathy – this includes gout and pseudogout, which are common causes of an acute hot joint. Their clinical features are extremely difficult to differentiate from septic arthritis as the patient may even have a fever/raised inflammatory markers. Joint aspiration is vital in these cases, showing uric acid crystals (negatively birefringent) in gout; and calcium pyrophosphate crystals (positively birefringent) in pseudogout.
• Reactive arthritis – this is often confused with gonococcal septic arthritis but is a different condition entirely. Reactive arthritis is an inflammatory process usually triggered by a systemic genitourinary or gastrointestinal infection. In reactive arthritis, the joint aspirate is sterile. It may present in a mono- or poly-articular manner.
• Injury – this is important to consider prior to a joint aspiration, particularly if the patient is unable to give an accurate history, e.g. due to young age or confusion. If in doubt, perform a radiograph prior to aspiration.
• Lyme arthritis caused by infection with Borrelia burgdorferi – this is not a true septic arthritis as it is a systemic infection which results in joint inflammation. It usually causes polyarthralgia and joint inflammation many months after the original infection.

Management

Septic arthritis may be managed by the rheumatology or orthopaedic teams according to local trust policies. However, in the case of prosthetic joint infections, the orthopaedics team ought to be involved.

The mainstay of treatment is a prolonged course of antibiotics, initially intravenous for at least two weeks, followed by 2-4 weeks of oral antibiotics. The most common organisms are covered by Flucloxacillin, and therefore, this is usually used as empirical treatment (Clindamycin if penicillin allergic). In some cases, a surgical joint washout will be needed.

References

• Coakley G, on behalf of the British Society for Rheumatology Standards G and AWG, Mathews C, on behalf of the British Society for Rheumatology Standards G and AWG, Field M, on behalf of the British Society for Rheumatology Standards G and AWG, et al. BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology [Internet]. 2006 Aug 1;45(8):1039–41. Available from: https://academic.oup.com/rheumatology/article/45/8/1039/1784962
• Momodu II, Savaliya V. Septic Arthritis [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538176/