Septic Arthritis

Joint inflammation resulting from bacterial or fungal infection. Septic arthritis most commonly affects joints in the arms and legs, often the knees in adults and the hips in children.

Septic arthritis is often caused by an infection elsewhere in the body. Artificial joint implants, recent joint injury, recent joint arthroscopy or other surgery, and intravenous drug use can all expose joints to infection. Chronic conditions such as diabetes and rheumatoid arthritis, as well as medicines that affect the immune system, can render the body less able to fight off infections and can raise the risk of septic arthritis. However, people already undergoing treatment for arthritis when they develop septic arthritis may have less severe symptoms. The symptoms of septic arthritis, which can occur rather suddenly, include joint swelling, intense pain, and fever. In some cases, people with septic arthritis can’t move the limb with the infected joint.

Doctors may suspect septic arthritis based on the rapid onset of symptoms and may use a variety of tests to diagnose it. Drawing fluid from the joint can serve both as a way of diagnosing septic arthritis, and as a way of treating it. Whereas normal synovial (joint) fluid is thick and colorless, infected joint fluid may have a color or different consistency. The fluid may also be analyzed for the presence of white blood cells or microorganisms, either of which could indicate infection. The bacterium most commonly responsible for septic arthritis is Staphylococcus aureus (staph). Various fungi and other bacteria can also infect the synovial fluid, including the bacteria that cause gonorrhea and streptococci.

Blood samples may also be tested for the presence of C-reactive protein (CRP), which can point toward inflammation due to infection. X-rays may also be used to help confirm or rule out septic arthritis or to assess damage from the infection.

Antibiotics are used to treat the underlying infection. Pain may be alleviated by such measures as keeping the joint still, raising the joint, and using cool compresses. Passive range-of-motion exercises may help keep the affected joint mobile. If the joint fluid builds up quickly as a result of the infection, the joint may need to be drained with a needle, though surgery may be required to remove the excess fluid. Unfortunately, even after treatment, people who have had septic arthritis may continue to have pain and decreased range of motion.

This column is written by Robert S. Dinsmoor, a medical writer and editor based in Massachusetts.

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