Septic Arthritis

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What is it?

Septic arthritis, also called infectious or bacterial arthritis, is an infection in a joint.  It is not as common as some of the other types of arthritis, however it needs to be diagnosed and treated quickly because it can destroy joints in a short period of time.It may also become life threatening if the infection spills over into the blood stream causing septicaemia. Thankfully this is an uncommon occurrence due to the widespread use of antibiotics.

Septic arthritis occurs most often from a bacterial infection travelling in your blood stream (bacteraemia). This may be from an infection elsewhere in the body that has now spread, however bacteria can also enter the bloodstream directly via the nose and the gut. It may also occur following direct injury, such as in an accident, especially where there is a cut or a wound that allows germs to enter the joint directly. It can also occur from surgery or a joint injection to a particular joint although this is uncommon. Additional risk factors include age (older than 80 years), being an intravenous drug user, having diabetes or rheumatoid arthritis, having an artificial joint and recent joint surgery. If your immune system is low from conditions such as AIDS or from medications such as chemotherapy you are also more at risk.

Septic arthritis can affect any joint however the knee and the hip are the most commonly infected joints. The joint becomes hot, red, swollen, painful and difficult to move, and there is often a fever. More than one joint may be affected especially if the cause is bacteraemia where the germs can spread from one area to another. The symptoms usually develop quickly over hours to a few days.

The acute form of septic arthritis is usually caused by bacteria, such as staphylococcus, streptococcus pneumoniae, and group B streptococcus and sometimes by the organisms that cause gonorrhoea and Lyme disease. The rare chronic form may be due to the organism that causes tuberculosis or the yeast Candida albicans. In the latter cases the symptoms may develop more slowly.

What tests are used?
The following are common tests used to diagnose septic arthritis:  

  • Blood culture  – often in septic arthritis some of the infection in the joint will also be present in the blood stream. This allows us to find out what bug is causing the infection to know how to treat it. It can also check for the presence of infections in the bloodstream that may have been the cause of the septic arthritis, again allowing us to know which germ is the cause.
  • Full blood count (FBC) or inflammatory markers such as CRP/ESR – in an infection or severely inflammed joint the white cell count (the cells which fight infection) may be raised. CRP and ESR may also be raised which are signs of inflammation. It does not tell us however where the infection is or what bug may be causing it.
  • Culture of joint fluid or synovial fluid analysis – this is where a sample of fluid is withdrawn from the affected joint with a small needle. It is then examined to check for the presence of infection. This allows us to confirm the presence of an infection directly in the joint and also tells us what germ is causing the infection.
  • X-ray of joint(s)– this is not helpful in diagnosing a septic arthritis in the early stages as it is often normal. However it can help rule out other causes of joint pain and swelling.

What treatments exist?
Treatment is with antibiotics. The exact antibiotic used may depend on the bacteria, which can be identified from growing the bacteria in a laboratory from the blood culture or from the culture of the joint fluid. As treatment must be started immediately, a standard antibiotic that would cover the usual germs that cause a septic arthritis is used initially. Then, the antibiotics can be adjusted depending on the results of the laboratory antibiotic susceptibility tests, which show what bug has caused the infection and what antibiotics have been found to work against that bug. The antibiotics are usually given via a vein at first (intravenously) and then changed to tablets once the infection has started to clear. Usually the oral antibiotics have to be continued for several weeks to ensure the infection has gone completely.

In some cases, the build-up of synovial fluid in the joint that occurs because of the infection requires aspiration (using a needle and suction to remove the liquid); in more severe cases, surgery may be needed to drain the fluid. This is to prevent further damage to the joint from the fluid build-up. Surgery may also be needed if the infection is in an artificial joint. This is because the joint usually has to be removed and replaced to clear the infection.

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