4.1.6 Urate nephropathy

Definition: Acute or chronic nephropathy due to urate salt deposition

Two forms must be differentiated:

• Acute uric acid nephropathy
• Chronic urate nephropathy

Uric acid salts are dissolved in case of formalin fixation. In case of suspicion,  absolute alcohol should be used as fixative. A variety of special stains allows the specific staining of uric acid salts. After alcohol fixation the monosodium urate crystals are birefringent and have a distinctive needle shaped appearance.

In acute uric acid nephropathy, due to a massive increase in uric acid in the serum, mostly in case of chemotherapy of malignant tumors, the collecting ducts are filled with amorphous and crystalline urate salts (mainly ammonium urate). The urate precipates may cause acute obstruction of the collecting ducts. Tubular epithelial cell damage is minor.

In chronic urate nephropathy (gouty kidney) monosodium urate crystals and amorphous deposits (“tophus”) are found in the medulla i.e. in and around distal tubules and collecting ducts. Cortical deposits are exceptional. Initially granular amorphous deposits are present in the interstial space, followed by needle shaped aggregates. These deposits are surrounded by giant cells and / or histiocytes. The histiocytes may form a crown, so that confusion with epithelial cells is possible. In the outer periphery of these foci a rim of granulation tissue may follow. In late stages the tophi are transformed into scars.

 Large tophi may destroy adjacent tubules, which leads to focal tubular atrophy and fibrosis also in the cortex. Renal tophi may be found in patients without clinical gout and any other manifestation of gout. Renal tophi will probably never cause terminal renal insufficiency.