Introduction

Septic arthritis is a medical emergency. The use of antibiotics has virtually eliminated mortality but permanent joint damage may result if appropriate antibiotics are not administered promptly (within 24–48 h of onset).

Septic arthritis is an uncommon cause (less than 5%) of acute inflammatory monoarthritis. It is important to distinguish these patients from the majority of patients with monoarthritis to initiate prompt appropriate antibiotic therapy.

Some bacterial infections such as salmonella food poisoning or gonorrhoea can cause a type of arthritis that is due to an immune response to infection rather than active infection of the joint with live intra-articular organisms. This reactive arthritis is distinct from septic arthritis and can be differentiated on clinical grounds.

Pathogenesis of septic arthritis

Aetiology of septic arthritis

The most common pathogens are Gram-positive organisms. Infection with Gram-negative organisms is uncommon, but more likely in high-risk patients. Infections in intravenous drug users are more likely to be polymicrobial and may involve anaerobes.

Pathogens include:

• Staphylococcus aureus including strains of MRSA
• Streptococcus spp.
• Gram-negative organisms including Haemophilus influenzae type B
• Mycobacterium tuberculosis
• Anaerobes
• Neisseria gonorrhoeae.
  

Organisms identified as causes of septic arthritis.

Diagnosis of septic arthritis

Clinical

Septic arthritis most frequently affects the knee in adults or the hip in children. Hip infections are more difficult to treat due largely to the difficulty of undertaking needle aspiration, and open surgical drainage is usually required.

Septic arthritis in the shoulder is often indicative of serious underlying disease, such as cancer.

Although the normal presentation is as monoarthritis, in about 9% of cases the infection is polyarticular.

Septic arthritis is of sudden onset, and there may be a history of recent infection, overseas travel, or intravenous drug use. It is useful to compare the affected joint with the contralateral joint.

Patients typically present with:

• Acute pain in the affected joint
• Restricted movement
• Swelling and erythema around the joint
• Signs of systemic infection.

The infection can damage the affected joint very quickly (in a few hours or days).

Rapid diagnosis and initiation of appropriate antibiotic therapy are essential if septic arthritis is suspected.

Microbiology

The key to diagnosis is immediate joint aspiration with proper examination of the synovial fluid.

• The fluid and any pus obtained must be sent for Gram stain, culture and analysis for the presence of crystals. In septic arthritis the synovial fluid is turbid, yellow, and has reduced viscosity.
• The synovial fluid white blood cell (WBC) count is elevated to 50–200 × 10⁹/L and polymorphonuclear cells make up more than 90% of the WBC. 
• The neutrophil count rises rapidly in septic arthritis, but rarely increases with such speed in other forms of inflammatory arthritis. 
• Synovial fluid glucose is decreased in about half of cases.

Other microbiological investigations include:

• Blood culture (50% are positive)
• Urine culture
• Throat swab
• Swab of any skin lesions
• Serology for Borrelia burgdorferi/Treponema pallidum/Brucella spp. (if suspected).

Laboratory tests

Obtain a blood sample for:

• Full blood count
• Acute phase reactants (CRP 15 ≥ mg/L and ESR is elevated in 95% of patients with septic arthritis)
• Biochemical analysis (urate, urea and creatinine)
• Rheumatoid factor (optional)
• Antinuclear antibodies.

Imaging

Gonococcal arthritis

• Arthritis in gonorrhoea is often reactive arthritis due to immune mechanisms rather than infection per se.
• Reactive and septic arthritis can be differentiated on clinical signs and symptoms.

Gonococcal septic arthritis is rare in the UK but is the most common cause of monoarthritis in the USA. It is most common in younger people and the incidence is three to four times higher in women (especially during menstruation or pregnancy) than men.

Management of septic arthritis

Septic arthritis is a medical emergency. Unless appropriate treatment is initiated within 24–48 h, permanent joint damage may result. Empirical antibiotic treatment should be started as soon as joint aspiration samples have been taken in suspected cases. Do not wait until the microbiological test results are available.

Most cases will respond to antibiotics after the first aspiration for synovial fluid samples but effective drainage is essential and repeat aspiration or surgical drainage may be required. Early surgical drainage is indicated for infections of the hip and some shoulder infections.

The joint should be maintained in an optimal functional position but strict immobilisation is not normally required.

Antibiotic rationale

Because Staphylococcus aureus is the most common pathogen, initial treatment should normally be targeted against Staphylococcus aureus, and unless there are reasons to suspect MRSA, high-dose flucloxacillin is the foundation of antibiotic treatment.

If there is a risk of MRSA consult your microbiologist.

The choice of additional antibiotics (including fusidic acid, aminoglycosides, quinolones or rifampicin) depends on the severity of the infection and the presence of any risk factors. In high-risk patients or if there is a history of recent trauma, then Gram-negative organisms or anaerobes are more likely and this should be reflected in the choice of antibiotic therapy.

If unusual pathogens are isolated, consult your microbiologist to determine the most appropriate treatment.

There is no benefit to intra-articular injection of antibiotics since most antibiotics penetrate joint fluid well, and intra-articular injection may cause a chemical synovitis.

Aminoglycosides do not achieve high concentrations in joints and do not function well at the low pH inside a joint. However, aminoglycosides such as gentamicin can still be a valuable addition because of their synergistic action, which is achieved even at sub-therapeutic concentrations and enhances the rate of bacterial killing against Staphylococcus spp. and other multiply resistant organisms. In addition aminoglycosides provide a broader spectrum of systemic cover.

Empirical therapy

Because of the potential for permanent joint damage if sub-optimal treatment is given, treatment should be discussed with your microbiologist.

The following are suitable empirical antibiotic regimens:

• Non high-risk patients:
• High-dose, IV flucloxacillin plus benzylpenicillin (plus either fusidic acid or rifampicin depending on the severity of infection)
• If the patient is in a high-risk group, has pre-existing rheumatoid arthritis, or Gram-negative organisms were identified on Gram stain, treatment can be modified as follows:
  • High-dose, IV flucloxacillin plus either an aminoglycoside such as gentamicinor a quinolone such as ciprofloxacin (plus either fusidic acid or rifampicin depending on severity)
  or
  • A second-generation cephalosporin such as cefuroxime (plus either fusidic acid or rifampicin depending on severity).
• In penicillin-allergic patients:
  • Clindamycin plus a quinolone such as ciprofloxacin
  or
• Vancomycin plus a quinolone such as ciprofloxacin.
• If MRSA is suspected, consult your microbiologist, as treatment will depend on local antibiotic susceptibilities. The following general principles usually apply:
  • IV vancomycin should be used instead of flucloxacillin
  • Gentamicin or a quinolone such as ciprofloxacin can be added to vancomycin subject to local policies and the advice of your microbiologist.
• If clinical signs indicate gonococcal septic arthritis, suitable treatments include:
  • High-dose IV second- or third-generation cephalosporin such as cefuroxime or cefotaxime
or
• A quinolone such as ciprofloxacin plus flucloxacillin.