Almost any joint disorder may begin as monoarthritis, or inflammation of a single joint; however, the primary concern is always infectious arthritis, because it may lead to joint destruction and resultant severe morbidity. For that reason, acute monoarthritis should be considered a medical emergency and investigated and treated aggressively.
Monoarthritis may be a result of infection (e.g., bacterial, fungal, Lyme disease, tuberculosis) or crystal-induced arthritis (e.g., pseudogout and gout); less often, it may be the presentation of a systemic disease typically associated with polyarticular disease, such as rheumatoid arthritis or systemic lupus erythematosus. It may also be a result of noninflammatory causes such as trauma or osteoarthritis.
Accurate diagnosis starts with a good history and physical examination supplemented by additional diagnostic testing, such as synovial fluid analysis, radiography, and occasionally synovial biopsy. A history of episodes of arthritis suggests crystalline disease or other noninfectious arthropathies. Patients with crystal-induced arthritis may give a history of recurrent, self-limited episodes. Precipitation of an attack by surgery or some other stress can occur with both crystalline disorders, but gout is far more common than is pseudogout. The clinical course can provide some clues to the etiology: septic arthritis usually worsens unless treated; osteoarthritis worsens with physical activity.
The location of joint involvement may be helpful. Gout most commonly involves the first MTP joint (podagra), ankle, midfoot, or knee. Pseudogout most commonly affects the large joints, such as the knee; it may also affect the wrist or the first MTP joint (hence, the name pseudogout). In gonococcal arthritis, there are often migratory arthralgias and tenosynovitis, often involving the wrist and hands, associated with pustular skin lesions, before progressing to a purulent monoarthritis or oligoarthritis. Nongonococcal causes of septic arthritis often involve large weight-bearing joints, such as the knee and hip.
The basic approach in physical examination is to differentiate arthritis from inflammatory conditions adjacent to the joint, such as cellulitis or bursitis. True arthritis is characterized by swelling and redness around the joint, and painful limitation of motion in all planes, during active and passive motion, joint movement that is not limited by passive motion suggests a soft tissue disorder such as bursitis rather than an arthritis.
Diagnostic arthrocentesis usually is necessary when evaluating an acute monoarthritis and is always essential when infection is suspected. Synovial fluid analysis helps to differentiate between inflammatory and noninflammatory causes of arthritis. Fluid analysis typically includes gross examination. cell count and differential. Gram stain and culture, and crystal analysis. Table 21-1 gives the typical results that can help one distinguish between noninflammatory conditions such as osteoarthritis, inflammatory arthritis such as crystalline disease, and septic arthritis, which most often is a bacterial infection.
Normal joints contain a small amount of fluid that is essentially acellular. Noninflammatory effusions should have a while blood cell count < 1000-2000/mm3 with less than 25-50% polymorphonuclear (PMN) cells. If the fluid is inflammatory, the joint should be considered infected until proven otherwise, especially if the patient is febrile.
Crystal analysis requires the use of a polarizing light microscope. Monosodium urate crystals, the cause of gout, are needle-shaped, typically intracellular within a PMN cell, and are negatively birefringent, appearing yellow under the polarizing microscope. Calcium pyrophosphate dehydrate (CPPD) crystals, the cause of pseudogout. are short and rhomboid, and are weakly positively birefringent. appearing blue under polarized light. Even if crystals are seen, infection must be excluded when the synovial fluid is inflammatory! Crystals and infection may coexist in the same joint, and chronic arthritis or previous joint damage, such as occurs in gout, may predispose that joint to hematogenous infection.
In septic arthritis. Gram stain and culture of the synovial fluid is positive in 60-80% of cases. False-negative results may be related to prior antibiotic use or fastidious microorganisms. For example, in gonococcal arthritis, joint fluid cultures typically are negative, whereas cultures of blood or the pustular skin lesions may be positive. Sometimes, the diagnosis rests upon demonstration of gonococcal infection in another site, such as urethritis, with the typical arthritis-dermatitis syndrome. Synovial biopsy may be required when the cause of monoarthritis remains unclear. It is usually necessary to diagnose arthritis caused by tuberculosis or hemochromatosis.
Plain radiographs usually are unremarkable in cases of inflammatory arthritis: the typical finding is soft tissue swelling. Chondrocalcinosis or linear calcium deposition in joint cartilage suggests pseudogout. They are often found when evaluating for fracture in patients with a history of trauma.
Generally, patients require initiation of treatment before all test results are available. When septic arthritis is suspected, the clinician should culture the joint fluid and start antibiotic therapy: the antibiotic choice should be initially based on the Gram stain and, when available, the culture results. If the Gram stain is negative, the clinical picture should dictate antimicrobial selection. For example, if the patient has the typical presentation of gonococcal arthritis.
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