Pathogenesis. Microorganisms first traverse the highly vascular synovial membrane and subsynovial tissue, where they propagate. This incites an acute inflammatory process characterized histologically by infiltration with polymorphonuclear cells and later with lymphocytes and mononuclear cells, tissue proliferation, and neovascularization. If the inflammatory process spreads into the joint cavity, a purulent exudate develops in the joint space, producing a septic joint. Enzymes released from the bacteria and leukocytes rapidly destroy articular cartilages and bone, causing severe structural damage and joint dysfunction; ankylosis may result.

Clinical Manifestations. Septic arthritis presents as joint pain, with swelling that is evident on examination of peripheral joints. Initially, patients may not have fever, rigors, or even leukocytosis. The presence or absence of these symptoms and laboratory findings thus may not distinguish infection from crystal-induced arthritis. A source for the infection is often not found. Gram-negative infections, other than gonococcal, tend to occur in patients with underlying factors such as malignancy, diabetes, immunosuppression, or gram-negative infection elsewhere.

Diagnosis/Treatment. Prompt identification and treatment of infectious arthritis is essential to prevent irreversible joint damage and mortality. Infectious arthritis should be suspected in all cases of acute articular inflammation in one or two joints, particularly in young adults and children who are far less likely to experience gout or pseudogout. The causative organism should be determined immediately by examination of all mucosal areas, repeated blood cultures, and, especially, analysis of smears and culture of fluid from the inflamed joint. A complete joint examination should be undertaken, including the spine, and alternative disease processes such as psoriasis, reactive arthritis, and inflammatory bowel disease should be sought by history and examination. If a septic joint is suspected, and the causative infectious agent has not yet been identified, initial therapy should be instituted covering staphylococci (including methicillin-resistant staphylococci [MRSA]) and gonococci or gram-negative organism if the clinical scenario suggests these are reasonably likely. Infected joint fluid, especially if purulent, should be aspirated daily until the infection is controlled (or infection excluded). As soon as the causative agent is isolated and the antibiotic susceptibilities identified, parenteral antimicrobial therapy should be tailored to culture results and continued until the infection is cured. Septic joints that respond slowly to antibiotic therapy may require arthroscopic or open drainage, and this should be considered early if the joint is not amenable to frequent drainage, the patient is immunosuppressed, there was a delay in diagnosis, or the fluid is extremely thick and adequate drainage is not possible with arthrocentesis. Patients generally require post-treatment rehabilitation.