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Septic Arthritis

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Overview

Any hot, swollen, acutely painful joint with restriction of movement is septic arthritis until proven otherwise, even in the absence of fever and irrespective of blood test and microbiology results.

Delayed diagnoses can lead to permanent joint damage and the resulting disability and mortality rates can be as high as 50%.

The most commonly affected joints are the knee, hip, shoulder, ankle, elbow, and wrist.

Axial joints such as the sternoclavicular or sacroiliac joint are more commonly affected in patients with a history of intravenous drug use.

Causes

  • The most common cause of septic arthritis is haematogenous spread
  • Staphylococcus aureus is the most frequent pathogen responsible
  • In young adults who are sexually activated, Neisseria gonorrhoea is the most common organism found. These patients have disseminated gonococcal infection.
  • Haematogenous spread from distal bacterial infections can cause septic arthritis

Risk Factors

  • Underlying joint disease and joint damage e.g. osteoarthritis/rheumatoid arthritis
  • Prosthetic joints
  • Joint surgery
  • Age >80yrs
  • Immunosuppression
  • Intravenous drug use
  • HIV
  • Diabetes mellitus
  • Alcohol misuse
  • Skin infections
  • Exposure to ticks

Presentation

Septic arthritis typically presents with a single swollen joint with pain on active or passive movement. Symptoms emerge over 1-2 weeks:

  • Hot, swollen, painful, and restricted joint
  • Fever is present in most patients
  • Patients may hold joints in positions that maximise joint space:
    • Fully extended knee
    • Hip abducted, flexed, and externally rotated
  • Presentation may be insidious in tuberculosis or if the joint is prosthetic

Signs on examination

  • Septic joints may be held in a position of maximum joint volume
  • Joint effusions may be present
  • Passive and active movement are limited and very painful
  • Patients may not be able to weight-bear/walk

Investigations

If a patient has a prosthetic joint, do not aspirate the joint and immediately refer to orthopaedics.

  • Synovial fluid aspiration in all patients (except prosthetic joints), ideally before starting antibiotics but do not delay treatment. The aspirate is assessed for:
    • Synovial fluid microscopy and Gram stain:
      • To screen for crystals in gout/pseudogout or identify microorganisms
    • Synovial fluid culture and sensitivities:
      • May reveal organism type and sensitivity to antibiotic therapy
    • Synovial fluid white cell count:
      • A white blood cell count >50,000 per mm3 and a polymorphonuclear cell count >90% are correlated with septic arthritis, but can also be present in crystal arthritis
  • Blood cultures, ideally before starting antibiotic therapy:
    • Do not delay treatment arranging a blood culture
    • Haematogenous spread of infection is the most common cause of septic arthritis
    • A negative result doesn’t exclude the diagnosis of septic arthritis
  • Full blood count (FBC):
    • May show leukocytosis
  • C-reactive protein (CRP)/erythrocyte sedimentation rate (ESR):
    • Non-specific markers of inflammation that may be elevated
  • Other baseline tests (e.g. urea and electrolytes, liver function tests)

Management

  • If prosthetic joint involvement, immediately refer to orthopaedics to consider surgery
  • Immediate needle aspiration and blood cultures before giving antibiotics
  • IV antibiotics that cover Gram-positive cocci according to local hospital policy
    • Flucloxacillin can be given
    • Clindamycin can be given if the patient is allergic to penicillin

Complications

  • Joint destruction
  • Osteomyelitis
  • Sepsis

Prognosis

  • Delayed or inadequate treatment can lead to permanent joint destruction and subsequent disability
  • Mortality rates can range from 10-20%

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