The number of joints and the time course during which a joint disorder develops guide the approach to differential diagnosis. Acute and severe monarthritis, especially one that presents along with fever, may represent septic arthritis, which is a rheumatologic emergency. The promptness of the diagnosis and treatment of a potentially septic process is likely to profoundly alter the outcome.

Alternatively, a single abnormal inflamed joint that persists beyond 2 months presents a different diagnostic challenge, with infection being a much less likely
disease process. In each case, one must view the overall clinical presentation, including factors such as extra-articular visceral involvement, constitutional signs and symptoms, severity of illness, limitation of function, potential foci of infection, skin lesions, hyperuricemia, and history of trauma or bleeding disorders.

Often, an aggressive initial approach is indicated, including joint aspiration with synovial fluid analysis and occasionally, in more refractory or unexplained situations, referral for synovial biopsy or arthroscopy. Therapy will vary significantly depending on the presumptive diagnosis. Specific therapy for each disease is discussed in later chapters. Tables 14-1 and 14-2 summarize the diagnostic approach to this group of disorders.

Infectious arthritis has an abrupt onset and a marked systemic inflammatory response. Because prompt therapy is required to prevent irretrievable joint damage, it is important to diagnose bacterial infection promptly. One may be deceived, however, in a partially treated patient on oral antibiotics or in an immunosuppressed patient in whom the inflammatory response is dampened by steroid therapy. A prudent approach includes careful examination for associated infectious foci and clinical clues (e.g., cutaneous pustules with neisserial infection), prompt joint aspiration and synovial fluid analysis, synovial fluid culture and Gram stain, and empiric antibiotics (depending on age and epidemiology). A viral process is typically polyarticular and may be accompanied by rash or other viral signs and symptoms. Lyme arthritis, caused by a Borrelia spirochete in endemic areas, can present acutely in recurring episodes of knee monarthritis; but more often it presents early in the course of the disease as migratory polyarthralgias, is associated with a characteristic rash or tick bite, and has a less “toxic” presentation (see Chapter 47).

Hemarthrosis is defined through the aspiration of bloody joint fluid.

Periarticular syndromes. Any of the tissues surrounding the joint can be involved in an inflammatory or traumatic process. A careful musculoskeletal examination can distinguish between tendinitis, bursitis, overuse syndromes, and surrounding cellulitis. Erythema nodosum is often seen with drug reaction, inflammatory bowel disease, or acute sarcoidosis. It often causes a periarthritis about the ankles and can result in an associated joint effusion. Osteomyelitis or neoplasia should be considered
with focal bone pain. Severe nighttime periarticular pain in a child is uncommon with juvenile arthritis and should always suggest leukemia.