Septic Arthritis

Pyogenic septic arthritis is a medical emergency also known as infectious arthritis. It occurs when bacteria invade a joint and cause inflammation of the synovial lining. If the organisms enter the joint cavity, effusion and pyogenesis follow, with eventual bone and cartilage destruction.

The disorder usually affects a single joint. It most often develops in a large joint but can strike any joint, including the spine and small peripheral joints. Migratory polyarthritis sometimes precedes localized joint inflammation.

Septic arthritis can lead to ankylosis and fatal septicemia, but prompt antibiotic therapy and aspiration or drainage of the joint cure most patients.

Causes

Septic arthritis occurs when some type of infecting organism, most often bacteria, reaches a joint. Bacteria can get into a joint through the bloodstream, or through surgery, an injection, or injury that directly contaminates the joint. The cause of septic arthritis in babies and young children is usually staphylococci, hemophilus influenzae, and gram-negative bacilli. In adults and older children, septic arthritis is more commonly caused by gonococci, staphylococci, and streptococci. Mycobacteria, which causes tuberculosis, and the bacteria that causes Lyme disease can also cause septic arthritis. Intravenous drug users and people with diseases that weaken the immune system, such as HIV, are more likely to have septic arthritis caused by gram-negative bacteria. The staphylococcus organism can also be introduced to a joint during arthroscopic surgery and prosthetic joint surgery.

Signs and Symptoms

Signs and symptoms of septic arthritis include:

• Fever
• Shaking chills
• Severe pain in the affected joint, especially when you move that joint
• Swelling of the affected joint
• Warmth in the area of the affected joint

People taking medications for other types of arthritis may not feel severe pain with septic arthritis, since those medications may mask the pain and fever.

The joints of your arms and legs are most commonly affected by septic arthritis. In rare cases other joints, such as those in your back, neck and head, may be affected.

Diagnostic tests

Arthrocentesis allows the collection of a synovial fluid specimen.

Synovial fluid analysis shows gross pus or watery, cloudy fluid of decreased viscosity, typically with 50,000/µl or more white blood cells (WBCs) containing primarily neutrophils. It may also show a lower glucose level than a simultaneous 6-hour postprandial blood glucose level.

Gram stain or culture of the fluid - or a biopsy of the synovial membrane - confirms the diagnosis and identifies the causative organism.

Blood cultures may be positive and confirm the diagnosis even when the synovial culture is negative.

X-rays may be normal for several weeks and usually don't aid the diagnosis; however, radiographic changes may appear as early as a week after infection. These can include distention of the joint capsule, narrowing of the joint space (indicating cartilage damage), and erosion of bone (joint destruction).

Radioisotope joint scan may be used for less accessible joints such as spinal articulations and may help detect infection or inflammation. However, the test by itself isn't diagnostic. Joint bone scans are invariably positive but are useful only in occult sepsis (as in vertebral osteomyelitis).

Countercurrent immunoelectrophoresis measures bacterial antigens in body fluids and helps to guide treatment.

WBC count may be elevated, with many polymorphonuclear cells.

Erythrocyte sedimentation rate is increased. C-reactive protein may be elevated.

Lactic assay can be used to distinguish septic from, nonseptic arthritis.

Treatment

Parenteral antibiotic therapy should begin right away. Treatment may be modified as needed when sensitivity studies of the infecting organism become available. Penicillin G is effective against infections caused by S. aureus. S. pyogenes. S. pneumoniae. S. viridans. and N. gonorrhoeae. A penicillinase-resistant penicillin such as nafcillin is recommended for penicillin G­resistant strains of S. aureus, ampicillin for influenza and gentamicin for gram-negative bacilli.

Treatment for septic arthritis requires monitoring of progress through frequent analysis of joint fluid cultures, synovial fluid leukocyte counts, and glucose determinations. Bioassays or bactericidal assays of synovial fluid and bioassays of blood may confirm clearing of the infection.

Codeine or propoxyphene can be given for pain, if needed. (Aspirin misleadingly reduces swelling and may mask fever, hindering accurate monitoring of progress.)

The joint may be immobilized with a splint or put into traction until the patient can tolerate movement. As the infection resolves, exercise is added to the treatment regimen to restore strength and mobility.

Needle aspiration (arthrocentesis) to remove grossly purulent joint fluid may be repeated daily until the fluid appears normal. If cultures remain positive or the WBC count remains elevated, the patient may need arthroscopic surgical drainage to remove resistant infection. (Septic arthritis of the hip requires open surgical drainage.)

Reconstructive surgery is warranted only for severe joint damage and only after all signs of active infection disappear. This usually takes several months. The patient is likely to need arthroplasty or joint fusion. Prosthetic replacement is controversial because it may exacerbate the infection; it has been used successfully when the femoral head or acetabulum has sustained damage.

Prevention:

Prophylactic (preventive) antibiotics may be helpful for high-risk people.