New findings add to questions about existence of gouty nephropathy

Relation to CKD still unclear

In a related editorial, Federica Piani, MD, and Richard J. Johnson, MD, of the division of renal diseases and hypertension at the University of Colorado at Denver, Aurora, explained that gout was considered by some clinicians to be a cause of CKD in a time before urate-lowering therapies, because as many as 25% of patients with gout went on to experience kidney failure and about half experienced lower kidney function.

The association between gout and CKD was thought to be attributable to “frequent deposition of urate crystals in the tubular lumens and interstitium in the outer medulla of these patients,” but the concept was later challenged because “the crystals were generally found focally and did not readily explain the kidney damage.”

But even as interest in rheumatology moved away from the concept of gouty nephropathy to how serum uric acid impacts CKD, “the possibility that urate crystal deposition in the kidney could also be contributing to the kidney injury was never ruled out,” according to Dr. Piani and Dr. Johnson.

Kidney biopsies can sometimes miss urate crystals because the crystals dissolve if alcohol fixation is not used and because the biopsy site is often in the renal cortex, the authors noted. Recent research has identified that dual-energy CT scans can distinguish between calcium deposits and urate crystals better than ultrasound, and previous research from Thomas Bardin, MD, and colleagues in two patients noted a correlation between dual-energy CT scan findings of urate crystals and hyperechoic medulla findings on renal ultrasound.

The results by Dr. Bardin and associates, they said, “have reawakened the entity of urate microcrystalline nephropathy as a possible cause of CKD.”

Robert Terkeltaub, MD, professor of medicine at the University of California, San Diego, and section chief of Rheumatology at the San Diego VA Medical Center, said in an interview that he also believes the findings by Dr. Bardin and associates are real. He cited a study by Isabelle Ayoub, MD, and colleagues in Clinical Nephrology from 2016 that evaluated kidney biopsies in Germany and found medullary tophi were more likely to be present in patients with CKD than without.

“Chronic gouty nephropathy did not disappear. It still exists,” said Dr. Terkeltaub, who was not involved in the study by Dr. Bardin and colleagues.

The prospect that, if “you look hard enough, you’re going to see urate crystals and a pattern that’s attributed in the renal medulla” in patients with untreated gout is “very provocative, and interesting, and clinically relevant, and merits more investigation,” noted Dr. Terkeltaub, who is also president of the Gout, Hyperuricemia and Crystal-Associated Disease Network.

If verified, the results have important implications for patients with gout and its relationship to CKD, Dr. Terkeltaub said, but they raise “more questions than answers.

“I think it’s a really good wake-up call to start looking, doing good detective work here, and looking especially in people who have gout as opposed to just people with chronic kidney disease,” he said.

The authors reported no relevant conflicts of interest. Dr. Johnson, who coauthored an accompanying editorial, reported having equity in XORTX Therapeutics, serving as a consultant for Horizon Pharma, and having equity in Colorado Research Partners LLC. Dr. Terkeltaub reported receiving a research grant from AstraZeneca in the field of hyperuricemia and consultancies with AstraZeneca, Horizon, Sobi, Selecta Biosciences.