Silent ureteral stones are an interesting phenomenon and have been reported only in two publications thus far [7, 8]. Before the first description of silent ureteral stones established as a primary diagnosis, asymptomatic ureteral calculi had been studied as residual fragments following the treatment of (primarily symptomatic) ureteral stones [12, 13].
The aim of the present study was to identify patients with the primary diagnosis of silent ureteral stones, and investigate excretory function and the morphology of the affected renal units in order to determine why the stones caused no symptoms. MAG-3 scans were used to study excretory function and the grade of obstruction prior to stone treatment [9]. Interestingly, the population of 14 patients with silent ureteral stones exhibited different types of obstruction, grades of hydronephrosis, stone sizes, and kidney function. According to these findings, the absence of symptoms related to a ureteral stone does not mean that the respective renal units are not obstructed. Furthermore, the absence of symptoms is not correlated with the grade of hydronephrosis or kidney function.
The high variability of renal obstruction and impairment of function in this subgroup of patients with silent ureteral stones is well correlated with studies concerning primarily symptomatic ureteral stones [14].
In contrast to silent ureteral stones, Kelleher et al. found a clear correlation between stone size (>5 mm), the presence of obstruction, and impairment of renal function in acute calculus obstruction [15]. In another study of acute obstructive urinary calculi, Gandolpho et al. established a significant association between renal impairment and stone size (1.1 to 2.0 cm) in nearly 70% of patients [16].
Sfakianakis et al. found an acute obstruction in 56.5% of patients presenting with renal colic on MAG-3 scans in the emergency setting [10]. Thus, pain is obviously not directly related with obstruction.
Irving et al. studied renal function on the basis of MAG-3 scans in 54 patients with symptomatic ureteral calculi measuring > 4 mm in size; the stones had been treated conservatively [17]. Irving et al. found that 28% of their patients had “silent loss of function” during follow-up, as established on follow-up scans. In this respect, primarily asymptomatic ureteral stones may be regarded as a form of “chronic obstruction.”
Biancani et al. were the first to study physiologic changes caused by chronic obstruction in an animal model in 1976 [18]. Acute obstruction of the ureter led to a rapid increase in intraluminal pressure and dilation of the diameter of the ureter. Subsequently intraluminal pressure declined, whereas deformation of the urinary tract persisted. Primarily silent ureteral stones are found in a very diverse patient population. At the time of presentation, renal units may be obstructed or not; hydronephrosis may be present or not; and renal function may be impaired or not. In patients with just two dysfunctional renal units at the time of presentation, obstruction appears to diminish to some degree in most cases.
Most recently, Marchini et al. were the second group of scientists who studied primarily silent ureteral stones [8]. In a highly selected cohort of 506 patients with ureteral calculi, silent stones were found in 5.3% (27 patients). The study was focused on preservation of kidney function. Patients were investigated with DMSA scans after treatment of the stones. In nine of the 27 patients, DMSA scans were performed before and after treatment. The authors found impaired kidney function in patients with silent ureteral stones; mean postoperative function on the DMSA scan was 31%. DMSA scans and serum creatinine levels revealed no difference in kidney function before and after treatment.
With regard to secondary signs of obstruction on ultrasound, IVU, or CT and actual obstruction on renal scintigraphy, German et al. showed that morphological changes are not directly correlated with the degree of obstruction – even in patients with renal colic [19]. Only 34% of patients with anatomical signs of obstruction had complete obstruction on renal scintigraphy, whereas 24% of patients with renal colic had partial obstruction and no anatomical signs of obstruction on CT. These findings concur with those obtained in the present study: presumably chronic silent ureteral stones are associated with different degrees of hydronephrosis, renal function, and degree of obstruction on MAG-3 scans.
Eisner et al. found that stones located in the proximal ureter were associated with a greater degree of ureteral dilation compared to those located in the distal ureter [20]. In the present study focused of silent ureteral stones, the degree of hydronephrosis was also higher in proximal ureteral stones (88% hydronephrosis in proximal stones versus 60% in distal stones); however, this finding was not statistically significant. The only parameter that correlated with the grade of obstruction was stone size (p = 0.02). This relationship was first reported by Kelleher et al. in symptomatic obstruction [15]. Furthermore, stone size has been established as an important factor in the planning and outcome of treatment [21, 22].
In conclusion, silent ureteral stones are clinically relevant. Silent obstruction may lead to irreversible renal function impairment [12].
Limitations
The major limitation of the present study is the relatively small number of investigated patients. Ureteral stones with no symptoms constitute a rare subgroup of the nephrolithiasis population – even in high-volume stone centers [7]. Larger samples of patients will be needed to obtain statistically significant results. On the other hand, patients with silent ureteral stones might just be an entirely heterogeneous group with different outcomes, depending on the degree and duration of obstruction.
Examination of kidney function and obstruction in patients with silent ureteral stones represents a snapshot of renal units at the time of presentation. As these patients have never experienced subjective symptoms, the chronological history of stone passage or formation – and its impact on excretory function of the kidney over time – cannot be studied at the time of presentation.
