1. Have a differential diagnosis for nontraumatic joint pain, based on clinical presentation.
2. Become familiar with the most common diagnostic tests for the above conditions, and have a rationale when ordering these tests.
This 45-year-old male presents with the sudden onset of monoarticular joint pain. The lirst diagnosis that needs to be excluded is an infected joint. A
joint becomes septic by blood inoculation, by contiguous infection (such as from bone or soft tissue), or from direct inoculation from trauma or surgery. Exclusion of an infectious etiology is paramount as cartilage can be destroyed within the first 24 hours of infection. In this case, the patient's history and clinical scenario do not favor an infectious cause, although it cannot be excluded by history and physical examination alone.
There arc several additional pieces of information that guide the diagnosis in this case. Most gout attacks occur between the ages of 30 and 50 years in men. with a later onset in postmenopausal women (50 to 70 years of age). The patient's recent increase in alcohol consumption can be considered an exacerbating factor. Other factors that may also increase the risk of a gout attack include trauma, surgery, or a large meal that induces hyperuricemia. Finally, the patient's history of taking a thiazide diuretic is also important, as these drugs may induce hyperuricemia.
The examination of a joint aspirate is essential for the diagnosis. The gross appearance of fluid is not very specific, as both a septic aspirate and a heavily condensed crystal-induced arthritis may have a thick, yellowish/chalky appearance. To diagnose crystal-induced arthritis, polarizing microscopy has to reveal monosodium urate (MSU) crystals, which will look like needles and have a strong negative birefringence. Other crystals that may be seen are:
• Calcium pyrophosphate dihydrate—rod shaped, rhomboid, weakly positive birefringence
• Calcium hydroxyapatite—seen by electron microscopy, cytoplasmic inclusions that are nonbirefringent.
• Calcium oxalate—bipyramidal appearance, strongly positive birefrin gence; seen mostly in end-stage renal disease patients
In crystal-induced arthritis, the white blood cell count of the joint aspirate is on average 2,000 to 60.000/|iL. with less than 90% neutrophils, while a septic-joint will have an average of 100,000 white blood cells/|iL (25,000-250,000 cells) with a >90% neutrophil count. Aspirate that has been determined to be crystal-induced must also be cultured so as to rule out a coexisting infection.
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