Introduction to Septic Arthritis

A medical emergency, septic (infectious) arthritis is caused by bacterial invasion of a joint, resulting in inflammation of the synovial lining. If the organisms enter the joint cavity, effusion and pyogenesis follow, with eventual destruction of bone and cartilage.
Septic arthritis can lead to ankylosis and even fatal septicemia. However, prompt antibiotic therapy and joint aspiration or drainage cures most patients.

Pathophysiology
In most cases of septic arthritis, bacteria spread from a primary site of infection, usually in adjacent bone or soft tissue, through the bloodstream to the joint.
Common infecting organisms include four strains of gram-positive cocci—Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, and Streptococcus viridans—and two strains of gram-negative cocci—Neisseria gonorrhoeae and Haemophilus influenzae. Various gram-negative bacilli—Escherichia coli, Salmonella, and Pseudomonas, for example—also cause infection.
Anaerobic organisms such as gram-positive cocci usually infect adults and children older than age 2. H. influenzae most often infects children younger than age 2.

Risk factors
Various factors can predispose a person to septic arthritis. Any concurrent bacterial infection (of the genitourinary or the upper respiratory tract, for example) or serious chronic illness (such as cancer, renal failure, rheumatoid arthritis, systemic lupus erythematosus, diabetes, or cirrhosis) heightens susceptibility. Consequently, alcoholics and elderly people run a higher risk of developing septic arthritis.
Of course, susceptibility increases with diseases that depress the autoimmune system or with prior immunosuppressant therapy. I.V. drug abuse (by heroin addicts, for example) can also cause septic arthritis.
Other predisposing factors include recent articular trauma, joint surgery, intra-articular injections, and local joint abnormalities.

Signs and symptoms
Acute septic arthritis begins abruptly, causing intense pain, inflammation, and swelling of the affected joint, with low-grade fever. Although it usually affects a single, large joint, it can affect any joint, including the spine and small peripheral joints.

Diagnosis
Identifying the causative organism in a Gram stain or culture of synovial fluid or a biopsy of synovial membrane confirms septic arthritis. Joint fluid analysis shows gross pus or watery, cloudy fluid of decreased viscosity, usually with 50,000/µl or more white blood cells (WBCs), primarily neutrophils.
When synovial fluid culture is negative, a positive blood culture may confirm the diagnosis. Synovial fluid glucose is typically low compared with a simultaneous 6-hour postprandial blood glucose test.

Other diagnostic measures include the following:

• X-rays can show typical changes as early as 1 week after initial infection— distention of joint capsules, for example, followed by narrowing of joint space (indicating cartilage damage) and erosions of bone (joint destruction).
• Radioisotope joint scan for less accessible joints (such as spinal articulations) may help detect infection or inflammation but isn’t itself diagnostic.
• C-reactive protein may be elevated, as well as WBC count, with many polymorphonuclear cells; erythrocyte sedimentation rate is increased.
• Two sets of positive culture and Gram stain smears of skin exudates, sputum, urethral discharge, stools, urine, or nasopharyngeal smear confirm septic arthritis.
• Lactic assay can distinguish septic from nonseptic arthritis.

Treatment
Antibiotic therapy should begin promptly; it may be modified when sensitivity results become available. Medication selection requires drug sensitivity studies of the infecting organism. Bioassays or bactericidal assays of synovial fluid and bioassays of blood may confirm clearing of the infection.

Other measures
Treatment of septic arthritis requires monitoring of progress through frequent analysis of joint fluid cultures, synovial fluid WBC counts, and glucose determinations.
The patient should be encouraged to rest and to keep the affected area immobile. The affected joint can be immobilized with a splint or traction. Warm compresses and elevation of the extremity help control pain.
Needle aspiration (arthrocentesis) to remove grossly purulent joint fluid should be repeated daily until fluid appears normal. If excessive fluid is aspirated or the WBC count remains elevated, open surgical drainage (usually arthrotomy with lavage of the joint) may be necessary for resistant infection or chronic septic arthritis.

Surgery
Late reconstructive surgery is warranted only for severe joint damage and only after all signs of active infection have disappeared, which usually takes several months. In some cases, the recommended procedure may be arthroplasty or joint fusion.
Prosthetic replacement remains controversial; it may exacerbate the infection. However, it has helped patients with damaged femoral heads or ace-tabula.