A 54-year-old man presents to the emergency department with excruciating pain in his right great toe that began suddenly last night. He describes the pain as 10 out of 10 in intensity, noting that even the weight of a bedsheet is unbearable. He recently started a high-protein and low-carbohydrate diet and attended a bachelor party two days ago where he consumed several steaks and a significant amount of beer. His medical history includes hypertension and chronic kidney disease stage 3. On physical examination, the first metatarsophalangeal (MTP) joint is exquisitely tender, erythematous, and warm. Arthrocentesis is performed.
Which of the following is the most likely finding on compensated polarized light microscopy of the synovial fluid?
The correct answer is:
A) Negatively birefringent, needle-shaped crystals
This patient is experiencing an acute attack of gouty arthritis, which is caused by the deposition of monosodium urate crystals in the joint space. Acute flares are often triggered by dietary excess of purine-rich foods like red meat and seafood, alcohol consumption which competes with uric acid for excretion in the kidney, and dehydration. The gold standard for diagnosis is synovial fluid analysis showing needle-shaped crystals which demonstrate strongly negative birefringence under polarized light (the crystals appear yellow when parallel to the slow-axis compensator and blue when perpendicular).
Answer choice B: Positively birefringent, rhomboid-shaped crystals, is incorrect. This describes pseudogout or calcium pyroprophosphate deposition disease. These crystals appear blue when parallel to the axis. Pseudogout more commonly affects larger joints like the knee or wrist.
Answer choice C: Rhomboid crystals with stair-step appearance, is incorrect. This refers to cholesterol crystals, which can sometimes be seen in chronic inflammatory joint effusions like rheumatoid arthritis but are not the cause of acute podagra.
Answer choice D: Weakly positively birefringent, needle-shaped, is incorrect. This does not describe any common crystalline arthropathy. Monosodium urate crystals are always strongly negatively birefringent.
Answer choice E: Yellow-green crystals under non-polarized light, is incorrect. Crystals are generally translucent or poorly visualized under regular light. Polarized light is necessary to determine their birefringent properties for a definitive diagnosis.
Key Learning Point
Gout is an inflammatory arthritis triggered by hyperuricemia. Acute flares present as a rapid-onset, severely painful monoarthritis, often affecting the 1st MTP joint. Diagnosis is confirmed by the presence of negatively birefringent, needle-shaped monosodium urate crystals in the synovial fluid.
