The acute monoarthritis is inflammation (arthritis) of one joint at a time..The causes of acute monoarthritis are bacterial infection, crystal-induced arthritis, and trauma. Septic arthritis is a major concern and must be ruled out. Arthrocentesis is the most important diagnostic test.
Initial Clinical Evaluation of Acute Monoarthritis
The history and physical examination should determine whether the process is acute (onset over hours to days), involves the joint rather than surrounding tissues or bone, and is truly monarticular.
The most common causes of acute monoarthritis are infection, crystal-induced arthritis, and trauma. In cases of suspected trauma, it is important to ascertain whether the reported trauma is sufficiently severe to account for the joint findings. The foremost concern in evaluating a patient with acute pain and swelling in a single joint that is not clearly due to trauma is the possibility of a joint space infection.
Laboratory Evaluation of Acute Monoarthritis
Arthrocentesis is indicated for all cases of unexplained acute monoarthritis. Synovial fluid should be sent for culture (bacterial, mycobacterial, and fungal), WBC count, and Gram stain and examined for crystals by polarized light microscopy. Determining whether the synovial fluid is inflammatory, noninflammatory, or bloody guides the initial differential diagnosis.
Polarized light microscopy is a sensitive test for urate crystals. Calcium pyrophosphate dihydrate crystals are somewhat more difficult to visualize because of their weaker birefringence, but their detection should not present difficulties for the experienced clinician. Occasionally, infection and microcrystalline disease coexist; therefore, the finding of crystals in the synovial fluid does not exclude the possibility of infection.
Properly performed cultures of synovial fluid are a sensitive test for nongonococcal septic arthritis (positive in up to 90% of cases). However, synovial fluid cultures are positive in only 20-50% of cases of gonococcal arthritis. The diagnosis often depends on identifying Neisseria gonorrhoeae on culture from the pharynx, urethra, cervix, or rectum (in aggregate, positive in 80-90%) or, in some cases, on the patient’s response to appropriate antibiotic therapy.
Routine laboratory tests (eg, complete blood cell count, serum electrolytes and creatinine, and urinalysis) can provide helpful ancillary information. Blood cultures should be obtained if septic arthritis is suspected.
Imaging Studies of Acute Monoarthritis
Radiographs can demonstrate fractures in cases of trauma but usually contribute little to the diagnosis of acute nontraumatic monoarthritis.
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