Thrombi Within the Urinary Tract May Serve as a Nidus for Rapid Stone Recurrence: A Report of Two Cases

Abstract

Background: Nephrolithiasis is increasingly becoming one of the most prevalent and costly urologic conditions in the United States. The most common type of kidney stone in humans is calcium oxalate, accounting for 75% of idiopathic stones in first-time stone formers. Stone formation is typically a gradual process; however, certain factors can accelerate stone development and recurrence.

Case Presentation: We present two cases of adult white men who were found to have rapidly recurrent symptomatic kidney stones that were ultimately determined to be comprised of an outer mineral shell with an inner core of blood clot. Both patients had a history of nephrolithiasis and recent hematuria. Urine supersaturation values at time of presentation supported formation of kidney stones.

Conclusion: Thrombi within the urinary tract can serve as a nidus for formation of multiple types of kidney stones, including calcium oxalate and uric acid stones. Stones arising from such a nidus may exhibit unusually rapid growth.

Introduction and Background

Urolithiasis is a debilitating and painful disease that affects 10% of the U.S. population.¹ Prevalence is increasing globally, likely due to modern dietary practices, lifestyle factors, and climate change.¹ Approximately 75% of idiopathic kidney stones in first-time stone formers are comprised of calcium oxalate, whereas other minerals such as struvite and uric acid are found in a minority of stones. Rarely, kidney stones can be formed by a variety of other substances such as cystine or drug metabolites.

The pathophysiology of kidney stone formation is a complex and multifactorial process that generally requires high levels of crystal supersaturation (SS) within the urine. The presence of a nidus can expedite stone growth. Pre-existing stones are the most common source of further mineral overgrowth; however, there are reports of stone overgrowth on foreign objects, including ureteral stents, sutures, staples, or exposed mesh. The presence of a foreign body may expedite stone formation and recurrence in the appropriately supersaturated urinary environment.

Although hematuria is a common consequence of stone passage and treatment, mineral overgrowth on a urinary tract thrombus has not previously been reported in humans. In this study, we report two cases where rapid stone recurrence occurred through mineral overgrowth on thrombi shortly after treatment and passage of ureteral stones.

Presentation of Case

Patient 1 is an 84-year-old man with history of diabetes and nephrolithiasis who initially presented for treatment of a 9 mm stone located at the left ureteropelvic junction. This stone was treated with holmium laser lithotripsy and all pieces were extracted from the urinary tract. Stone analysis revealed calcium oxalate composition (90% monohydrate, 10% dihydrate). Follow-up CT performed 2 months later demonstrated a new 10 mm stone in a similar location (Fig. 1). Although the initial calcium oxalate stone had a homogenous appearance and a mean HU of 1131, the new stone had a distinct and uniquely different appearance on CT scan with a more radiopaque outer shell (140–190 HU) surrounding a relatively radiolucent core with corresponding radiodensity of <100 HU (40–70 HU). At time of surgical treatment, the stone had an endoscopic appearance of an outer mineralized shell with an inner core of coagulated gelatinous blood.

FIG. 1.

A 10 mm stone at the left ureteropelvic junction, characterized by a relatively radiolucent inner core.

Stone analysis of the mineral shell demonstrated composition of 80% uric acid and 20% calcium oxalate monohydrate. A 24-hour urine study demonstrated high SS of CaOx at 6.51, high SS of uric acid at 2.63, and an acidic urine pH of 5.158.

Patient 2 is a 52-year-old man with history of combined kidney-pancreas transplant and nephrolithiasis who initially presented with a 7 mm obstructing stone in the ureter of his transplant kidney, located in the left lower quadrant. A ureteral stent was placed in the acute setting and the patient was scheduled to undergo definitive stone treatment at a later date. Approximately one month after stent placement, the patient reported passing the stone; follow-up CT confirmed stone absence. His stent was removed uneventfully in the clinic although he did experience gross hematuria for several days after stent removal that resolved. Approximately 2 months later, he presented with recurrent gross hematuria.

Work-up revealed a new 12 mm stone within the transplant ureter that had a dense outer shell (160–190 HU) surrounding a relatively radiolucent core with HU <100 (Fig. 2). The patient had a percutaneous nephrostomy tube placed and shortly thereafter underwent antegrade ureteroscopy with holmium laser lithotripsy. Endoscopically, the stone was notably brittle with a hard outer shell and a soft inner core, consistent with the appearance of a calcified gelatinous thrombus (Fig. 3). Stone analysis of the mineral shell demonstrated calcium oxalate monohydrate. Twenty-four-hour urine study demonstrated high SS of CaOx at 6.79, high SS of uric acid at 2.08, and low urine pH of 5.221.

FIG. 2.

A 12 mm stone within the left lower quadrant transplant ureter, characterized by a relatively radiolucent inner core.

FIG. 3.

Intraoperative images obtained during removal of the stone from Patient 2. (A) The stone's mineral shell is apparent upon initial observation of stone. (B) An inner thrombus is evident upon initiation of laser fragmentation.

Discussion

The stones observed in these patients are notable for several reasons. First, they formed quite rapidly and in the immediate aftermath of a stone intervention. More importantly, they shared an atypical CT appearance consistent with a radiopaque shell surrounding a radiolucent core that was demonstrated to be thrombus upon endoscopic stone treatment. Furthermore, they occurred in the contexts of hematuria and highly supersaturated urine. These cases demonstrate that thrombi have the potential serve as a nidus for rapid formation of multiple types of stones, including calcium oxalate and uric acid stones, in the appropriate urinary microenvironment.

Despite the high prevalence of hematuria in the presence of a kidney stone, as well as the known ability for blood clots to calcify, extensive review of the literature revealed no previous reports of mineralized thrombi in the urinary tract of humans. There have, however, been case reports of solidified blood calculi in the urinary tract of cats.^2,3 A review by Westropp et al. analyzed 49 urinary tract stones from cats that were composed of >99% dried solidified blood.² Among this population, none of these stones were visible on radiography and all were removed either surgically or at time of necropsy. It is unclear how many of these cats had a history of urolithiasis.

Because of the rarity of the condition, the method by which a thrombus can serve as a nidus for stone formation is uncertain. In a procrystallization supersaturated urinary environment, it is possible that the surface of the thrombi may have merely provided a convenient surface for nucleation. In addition, there is limited evidence that some urinary glycoproteins, such as urinary prothrombin fragment 1 (UPF1), may have dual roles in the prevention of stone disease and coagulation.⁴

Conclusions

These cases illustrate two examples of thrombi within the urinary tract acting as an atypical nidus for rapid kidney stone recurrence. The provider should have increased suspicion for this unusual process in a recurrent stone former who presents with a rapidly growing stone after a period of hematuria.

Abbreviations Used

CT

computed tomography

HU

Hounsfield units

SS

supersaturation

UPF1

urinary prothrombin fragment 1

Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Cite this article as: Alford AV, Mocol M, Borofsky MS (2020) Thrombi within the urinary tract may serve as a nidus for rapid stone recurrence: a report of two cases, Journal of Endourology Case Reports 6:4, 425–427, DOI: 10.1089/cren.2020.0175.