|Classification and external resources|
Septic arthritis is the purulent invasion of a joint by an infectious agent which produces arthritis. People with artificial joints are more at risk than the general population but have slightly different symptoms, are infected with different organisms and require different treatment. Septic arthritis is considered a medical emergency. If untreated, it may destroy the joint in a period of days. The infection may also spread to other parts of the body.
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 arthritis occur occasionally. A broader term is "infectious arthritis", which describes arthritis caused by any infectious organism. Viruses can cause arthritis, but it can be hard to determine if the arthritis is directly due to the virus or if the arthritis is reactive.
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.
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 bovine tb).
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, or from an unknown focus
- dissemination from an acute osteomyelitic focus,
- dissemination from adjacent soft tissue infection,
- entry via penetrating trauma
- entry via iatrogenic means.
Bacteria that are commonly found to cause septic arthritis are:
- Staphylococcus aureus - the most common cause in adults
- Streptococci - the second most common cause 
- Haemophilus influenzae - was the most common cause in children but is now uncommon in areas where Haemophilus vaccination is practiced
- Neisseria gonorrhoea - in young adults, multiple macules or vesicles seen over the trunk are a pathognomonic feature. 
- Escherichia coli - in the elderly, IV drug users and the seriously ill
- M. tuberculosis, Salmonella spp. and Brucella spp. - cause septic spinal arthritis 
Septic arthritis should be considered whenever one is assessing a patient with rapid onset of 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. Pain can be significant with any movement, therefore, patient will refuse to use extremity and prefers to hold joint rigidly. May have associated swelling, redness & warmth, often absent for deep joints such as hips & shoulders. In the case of gonorrhea the knee or wrist may be chronically affected. The pain may be chronic and the physician may inject steroids to reduce symptoms. Weeks later increased pain, redness and swelling- signs of inflammation- appear leading to drainage by needle puncture. Then the gram stain and cultures are typical of a Neisserian infection.
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.
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). The ESR and CRP are almost always raised on admission, CRP being faster in diagnostics. In the joint aspirate, the typical white blood cell count in septic arthritis is over 50,000-100,000 cells per 10-6/l (50,000-100,000 cell/mm3). However, septic synovial fluid can have white blood cell counts as low as a few thousand in the early stages and, therefore, differentiation of septic arthritis from aseptic inflammatory arthritis is not always possible based on cell counts alone.
The Gram stain can rule in the diagnosis of septic arthritis, however cannot exclude it. X-rays may not be helpful early, but may show subtle increase in joint space tissue swelling. Ultrasound may reveal joint effusion.
The diagnosis of septic arthritis is based on clinical assessment and should prompt arthrocentesis. Imaging can sometimes be used to aid in the diagnosis of septic arthritis.
Native X-ray of the joint is neither sensitive nor specific. Ultrasound can detect joint-swelling. MRI findings 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 67% and 98% respectively.
Therapy is usually with intravenous antibiotics, analgesia and washout/aspiration of the joint to dryness. Among pediatric patients with an acute hematogenous septic arthritis a short total course of 10 days of antimicrobials is sufficient in uncomplicated cases.
In infection of a prosthetic joint, a biofilm is often created on the surface of the prosthesis which is resistant to antibiotics. Surgical debridement or arthrotomy is usually indicated in these cases. A replacement prosthesis is usually not inserted at the time of removal to allow antibiotics to clear infection of the region.
Patients in whom surgery is contraindicated may trial long-term antibiotic therapy.
Close follow up with physical exam & labs must be done to make sure patient remain afebrile, pain resolved, improved range of motion and normalized lab values.
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