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Outcomes of Active Surveillance for Asymptomatic Renal Stones
Shuo-chieh Wu, MD1, Amanda R. Swanton, MD1, Kevin J. Krughoff, MD1, Benjamin M. Dropkin, MD2, Vernon M. Pais, Jr., MD1.
1Dartmouth-Hitchcock Medical Center, Lebanon, NH, 2Vanderbilt University Medical Center, Nashville, TN.

A significant proportion of patients with asymptomatic nephrolithiasis pursue active surveillance with periodic imaging reassessment. While on surveillance, some patients will become symptomatic, but it is difficult to predict which patients will develop symptoms and over what time course symptoms will arise. We report the extended follow up outcomes of a cohort of patients with planned surveillance of asymptomatic renal stones.
Material and Methods
Medical records of patients with asymptomatic non-obstructing renal calculi who had declined initial surgical intervention and opted for active surveillance between 2008-2018 were retrospectively reviewed. Baseline demographics, stone size, number and location were collected. Cox proportional-hazards model was used to identify predictors of renal colic and to analyze clinical outcomes based on initial stone burden. Kaplan-Meier curve and log-rank test were used to examine event-free survival.
A total of 110 patients (Male=60, Female=50) with 160 stones were followed for 71 35 months (median=85). Stone size of the cohort was 7.0 4.2 mm (median=6). Symptoms developed in 41.8% of patients, comprising 35.6% of stones in the entire cohort by 31 24 months (median=26). BMI ≥ 30 was associated with higher likelihood of symptom development (HR 1.91, CI 1.02-3.57, Table 1) and lower 5-year symptom free survival (50% vs 71%, Figure 1). Age, gender, stone size, location, prior history of stones, and multiple renal stones were not associated with likelihood of symptom development. Clinical outcomes of the 160 stones are as follows: 12.5% triggered ER visit, 16.9% developed hydronephrosis, 1.9% developed silent hydronephrosis, 9.4% passed spontaneously, 20.6% underwent surgery for symptoms, and 18.8% underwent elective surgery without symptoms (Table 2). On univariate Cox analysis of the association between stone characteristics and clinical outcomes, non-lower pole stones had higher likelihood for spontaneous passage compared to lower pole stones (HR 4.63, CI 1.15-20.35). Stone size greater than or equal to 10mm was associated with more surgical intervention (HR 2.11, CI 1.21-3.55) and lower 5-year surgery free survival (40% vs 66%, Figure 2).
Over the course of 7-year median follow up for asymptomatic renal stones, most patients remain asymptomatic; a minority will spontaneously pass or require surgical intervention for symptoms. Those with higher BMI are more likely to develop symptoms, and stones ≥10mm are associated with an increased odds of ultimately undergoing surgical intervention. Active surveillance is a viable option for asymptomatic renal stones, and these data may aid in patient counseling.

Table 1. Aassociation between baseline patient characteristic and symptom developement.
Became symptomatic Remained AsymptomaticHR
All Patients46 (41.82%)64 (58.18%)(95% CI)
F5021 (42.00%)29 (58.00%)
M6025 (41.67%)35 (58.33%)
< 658235 (42.68%)47 (57.32%)
≥ 652811 (39.29%)17 (60.71%)
< 306222 (35.48%)40 (64.52%)
≥ 304523 (51.11%)22 (48.89%)1.91 (1.02-3.57)
Stone size
< 10mm8335 (42.17%)48 (57.83%)
≥ 10mm2711 (40.74%)16 (59.26%)
Stone location
Lower pole5823 (39.66%)35 (60.34%)
Non-lower pole5223 (44.23%)29 (55.77%)
History of stone
No166 (37.50%)10 (62.50%)
Yes9440 (42.55%)54 (57.45%)
Multiple stones
No3415 (44.12%)19 (55.88%)
Yes7631 (40.79%)45 (59.21%)

Table 2. Percentage breakdown of asymptomatic stones by clinical outcomes and time to event.
Became symptomatic Remained Asymptomatic
All Stones57 (35.63%)103 (64.38%)
ER Visit
No14037 (23.13%)103 (64.38%)
Yes2020 (12.50%)0 (0.00%)
Time (month)37 32 (28)n/a
No13030 (18.75%)100 (62.50%)
Yes3027 (16.88%)3 (1.88%)
Time (month)38 26 (34)36 17 (30)
Spontaneous passage
No14542 (26.25%)103 (64.38%)
Yes1515 (9.38%)0 (0.00%)
Time (month)23 17 (23)n/a
Stone growth > 50%
No11447 (26.25%)67 (41.88%)
Yes4610 (6.25%)36 (22.50%)
Time (month)37 41 (16)47 30 (43)
Surgical Intervention
No9724 (15%)73 (45.63%)
Yes6333 (20.63%)30 (18.75%)
Time (month)32 22 (29)34 29 (24)

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