Septic arthritis

Septic arthritis is the purulent invasion of a joint by an infectious agent which produces arthritis.^[1]

Terminology

The term "suppurative arthritis" is a near synonym for septic arthritis. ("Suppurative" refers to the production of pus, without necessarily implying sepsis.)

ICD-10 uses the term "pyogenic arthritis". Pyogenic also refers to the production of pus.

Reactive arthritis refers to arthritis caused by an immune consequence of an infection, but not directly attributable to the infection itself.

The usual etiology of septic arthritis is bacterial, but viral, mycobacterial, and fungal^[2] arthritis occur occasionally. A broader term is "infectious arthritis", which describes arthritis caused by any infectious organism.^[3] Viruses can cause arthritis,^[4] but it can be hard to determine if the arthritis is directly due to the virus or if the arthritis is reactive.^[5]

Septic/suppurative arthritis and "bacterial arthritis" are sometimes considered equivalent, but there are exceptions. For example, Borrelia burgdorferi can cause infectious arthritis, but is not associated with suppurative arthritis.^[5]

Etiology

Bacteria are carried by the bloodstream from an infectious focus elsewhere, introduced by a skin lesion that penetrates the joint, or by extension from adjacent tissue (e.g. bone or bursae).

Micro-organisms must reach the synovial membrane of a joint. This can happen in any of the following ways:

• dissemination of pathogens via the blood, from abscesses or wound infections,
• dissemination from an acute osteomyelitic focus,
• dissemination from adjacent soft tissue infection,
• entry via penetrating trauma
• entry via iatrogenic means.^[6]

Bacteria that are commonly found to cause septic arthritis are:

• Staphylococcus aureus - the most common cause in adults
• Streptococci - the second most common cause ^[7]
• Haemophilus influenzae - was the most common cause in children but is now uncommon in areas where Haemophilus vaccination is practised^[8]
• Neisseria gonorrhoea - in young adults (now thought rare in Western Europe) ^[7]
• Escherichia coli - in the elderly, IV drug users and the seriously ill
• M. tuberculosis, Salmonella spp. and Brucella spp. - cause septic spinal arthritis ^[6]

In bacterial infection, Pseudomonas aeruginosa has been found to infect joints, especially in children who have sustained a puncture wound. This bacteria also causes endocarditis.^[9]

Diagnosis

Septic arthritis should be considered whenever one is assessing a patient with joint pain. Usually only one joint is affected (monoarthritis) however in seeding arthritis, several joints can be affected simultaneously; this is especially the case when the infection is caused by staphylococcus or gonococcus bacteria.

The diagnosis of septic arthritic can be difficult as no test is able to completely rule out the possibility.

A number of factors should increase one's suspicion of the presence of an infection. In children these are: fever > 38.5 C, non-weight-bearing, serum WBCs > 12 x 10^9, ESR > 40 mm/hr, CRP > 20 mg/dL, a previous visit for the same.^[10]

Diagnosis is by aspiration (giving a turbid, non-viscous fluid), Gram stain and culture of fluid from the joint, as well as tell-tale signs in laboratory testing (such as a highly elevated neutrophils (approx. 90%), ESR or CRP). A proportion of patients with septic arthritis have little in the way of fever or raised ESR, although the CRP is usually raised ^[11]

The Gram stain can rule in the diagnosis of septic arthritis however cannot exclude it.^[12]

Treatment

Therapy is usually with intravenous antibiotics, analgesia and washout/aspiration of the joint to dryness.

Radiologic Findings

Traditionally, the diagnosis of septic arthritis was based on clinical assessment and prompt arthrocentesis. However, the clinical picture may be obscured by multiple confounding factors and a paucity of specific findings especially for the deep joints, ie. the hip or shoulder. Imaging can be used to confirm the diagnosis of septic arthritis and more importantly, imaging findings suggestive of septic arthritis can direct the clinician to a diagnosis that may not have been considered.

Plain film findings of septic arthritis include: joint effusion, soft tissue swelling, periarticular osteoporosis, loss of joint space, marginal and central erosions and bone ankylosis. CT is more sensitive than plain films for the detection of early bone destruction and effusion.

The role of MRI in the diagnosis of septic arthritis has been increasing in recent years in an effort to detect this entity earlier. Findings are usually evident within 24 hours following the onset of infection and include: synovial enhancement, perisynovial edema and joint effusion. Signal abnormalities in the bone marrow can indicate a concomitant osteomyelitis. The sensitivity and specificity of MRI for the detection of septic arthritis has been reported to be 100% and 77% respectively.

See also

• Arthritis
• Human biting injuries
• Transient synovitis
• Arthritis Care

References