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Pathology - Crystal Arthropathies
I. Definition & Pathogenesis
A. Gout
Microscopic examination of joint fluid is crucial for differentiating gout and pseudogout:
I. Definition & Pathogenesis
- Crystal Arthropathies: A family of joint diseases stemming from crystal deposition within the joints.
- Pathogenic Mechanism: Crystals trigger an inflammatory response. Neutrophils attempt to phagocytose (engulf) the crystals, leading to degranulation and the release of damaging enzymes. This enzymatic action causes joint damage.
A. Gout
- Etiology: Deposition of urate crystals in joints. Primarily caused by hyperuricemia (high uric acid levels in the blood), often due to impaired renal (kidney) excretion of urate.
- Acute Gout: Presents as an acutely painful, swollen, and red joint. While any joint can be affected, the first metatarsophalangeal joint (base of the big toe) is highly characteristic.
- Chronic Tophaceous Gout: In individuals with persistently high urate levels, large urate deposits (tophi) can form in the skin and around joints.
- Etiology: Deposition of calcium pyrophosphate crystals in joints. Pyrophosphate is a byproduct of nucleotide triphosphate hydrolysis within cartilage chondrocytes.
- Pathogenesis: Shedding of these crystals into the joint space initiates an acute arthritis closely resembling gout.
- Clinical Presentation: Typically affects older women, commonly involving the knee and wrist joints.
Microscopic examination of joint fluid is crucial for differentiating gout and pseudogout:
- Joint Fluid: Contains neutrophils and crystals.
- Urate Crystals (Gout): Needle-shaped; exhibit negative birefringence under polarized light.
- Pyrophosphate Crystals (Pseudogout): Rhomboid or rod-shaped; exhibit positive birefringence under polarized light. The difference in birefringence is key for distinguishing between the two conditions.
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