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Many mechanisms provoke acute joint symptoms: degradation and degeneration of articular cartilage (osteoarthritis), deposition of immune complexes or immune system–related phenomena (rheumatoid arthritis, rheumatic fever and possibly, a component of gonococcal arthritis), crystal-induced inflammation (gout and pseudogout), seronegative spondyloarthropathies (ankylosing spondylitis [see chapter 282, "Systemic Rheumatic Diseases"] and reactive arthritis [postinfectious with HLA-B27 susceptibility]), and bacterial invasion (gonococcal and nongonococcal septic arthritis, including Lyme arthritis) or viral invasion (viral arthritis). These processes impact joint capsules and surfaces, resulting in a cascade of reactive and inflammatory events. Septic arthritis is invasion of a joint by an infectious agent with organism proliferation and associated inflammation; bacterial arthritis is a subset of septic arthritis. Under ideal conditions, the infectious agent is recoverable from the joint fluid in septic arthritis, but in clinical practice, this is often not the case. This chapter reviews the common causes and treatments of acute nontraumatic joint pain. Joint injuries are discussed in section 22, "Injuries to Bones and Joints," and disorders due to repetitive use syndromes are discussed in section 23, "Musculoskeletal Disorders," by anatomic site.
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CLINICAL APPROACH TO ACUTE JOINT PAIN
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Septic arthritis is the most important consideration in the evaluation of a swollen, warm, and painful joint. Urgent treatment may prevent both joint destruction and mortality (11% with treatment).1,2 The diagnosis of septic arthritis is clinical and is supported by diagnostic tests.1,2 No single diagnostic parameter is sufficiently sensitive to screen patients for septic arthritis including synovial WBC counts.3
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CLINICAL FEATURES AND RISK FACTORS
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Risk factors (Table 284-1),3,4 the number of joints involved (Table 284-2), and the migratory pattern (Table 284-3), if one exists, aid in the differential diagnosis. Approximately 85% of patients with nongonococcal septic arthritis present with a single joint infected; Staphylococcus aureus and Streptococcus pneumoniae are more likely to infect two or more joints simultaniously.5,6,7,8 Septic arthritis involving more than one joint can occur in rheumatoid arthritis (50%), immunocompromise, gout, diabetes, and/or renal disease; the morality rate is significantly higher in patients with polyarticular septic arthritis (11% vs 30%).5,7 Recent joint surgery and cellulitis overlying a prosthetic hip or knee are the only findings on history or physical examination that significantly alter (both increase) the probability of nongonococcal septic arthritis.3
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