Septic arthritis, also known as infectious arthritis or joint infection, 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.
Septic arthritis is usually caused by bacteria, but may be caused by viral, mycobacterial, and fungal pathogens as well. 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.
Signs and symptoms
Septic arthritis can cause pain with any movement of the affected joint. Therefore, those affected by septic arthritis will often refuse to use the extremity and prefer to hold joint rigidly. Other common signs and symptoms are joint swelling, redness, and warmth.
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 gonorrhoeae-the most common cause of septic arthritis in young, sexually active 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 person with rapid onset of joint pain. Usually only one joint is affected however in seeding arthritis, several joints can be affected at the same time; this is especially the case when the infection is caused by staphylococcus or gonococcus bacteria.
The diagnosis of septic arthritis is based on clinical assessment and should prompt arthrocentesis. The diagnosis of septic arthritis 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 included in Kocher criteria.
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.
A lactate level in the synovial fluid of greater than 10 mmol/l makes the diagnosis very likely.
Connie’s comments: My 81 yr old mother has septic arthritis. She eats with her bare hands and had worked as a caregiver. Lately, she has less salt in her diet and get nervous all the time. She had been sleeping only 4 hrs at a time. And so I encouraged her to ensure good hygiene and add celery, pickles and sushi wrappers in her diet. And to sleep more and be more calm.