This pathology review focuses on distinguishing between gout (monosodium urate deposition) and pseudogout (calcium pyrophosphate deposition disease, CPPD) within a skin nodule sample, likely originating from the foot. The initial observation strongly suggests gout, characterized by pale, feathery nodules composed of histiocytes surrounding faintly visible needle-shaped crystal deposits, which are typically washed out during standard H&E processing. However, the presence of distinct, large, rhomboid-shaped crystals that stain purple and exhibit birefringence under polarized light—hallmarks of pseudogout—is noted elsewhere in the specimen. The speaker expresses surprise at finding pseudogout in a superficial foot sample, as it is more commonly observed in deep joints like the knee or spine. The core diagnostic challenge presented is that the sample appears to contain features characteristic of both conditions simultaneously, leading the speaker to suspect a rare co-occurrence of gout and pseudogout in the same patient.
Differentiating Gout and Pseudogout in a Tissue Biopsy
0:00 Pale Nodules: The sample exhibits pale, feathery nodules surrounded by histiocytes, which is consistent with the presentation of gout (monosodium urate crystals).
0:01 Crystal Dissolution: Needle-shaped urate crystals typical of gout are often washed away during standard formalin fixation and H&E staining; fresh smears or alcohol fixation are the best methods to confirm them.
2:14 Purple, Rhomboid Crystals: Other areas show dark purple, diamond, square, or rhomboid-shaped crystals.
2:44 Crystal Morphology Difference: These rhomboid crystals are larger and chunkier than gout crystals and are easily visible on H&E slides, often staining purple (characteristic of calcium).
3:59 Birefringence: Under polarized light, these rhomboid crystals glow and are birefringent, confirming they are not urate crystals.
4:23 Diagnosis: Pseudogout: The purple, rhomboid, birefringent crystals are characteristic of pseudogout (calcium pyrophosphate deposition disease, CPPD).
4:36 Location Anomaly: While pseudogout is usually found in deep joints (knee, spine), this specimen is a nodule from the foot, suggesting superficial involvement.
5:05 Suspected Co-occurrence: The slide shows features perfect for gout in one area and features perfect for pseudogout in another, leading to the suspicion that the patient may have both conditions present.
5:50 Rare Foot Presentation: The speaker notes that finding pseudogout near the skin surface in the hands or feet is rare.