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BACKGROUND: Management strategies for asymptomatic non-obstructing renal stones include elective surgical treatments and active surveillance. We sought to document the natural history of such stones followed with active surveillance to better inform shared decision making.
METHODS: We retrospectively identified all patients with documented asymptomatic non-obstructing renal stones seen by a single surgeon between June 25, 2008 and December 28, 2010 who elected active surveillance of their stone(s) with routine follow up imaging. We collected the following data: stone characteristics (size, location, and date and modality of first radiographic visualization for the largest non-obstructing stone present in each kidney); patient characteristics (age, BMI at inclusion, gender, and history of prior stones); and stone-related events (elective stone removal, stone-growth, spontaneous passage, development of renal colic, development of silent obstruction, emergency department visits, and surgical intervention for pain).
RESULTS:
A total of 160 stones (84 left and 76 right) with an average size of 7.0 + 4.2 mm were identified among 110 patients (60 males and 50 females). Stones were initially identified using CT scan (79, 49.4%), ultrasound (78, 48.8%), or KUB (3, 1.8%). The patients had an average age of 56 + 14 years and an average BMI of 30 + 10. Eighty-seven percent of patients had a prior history of renal or ureteral stones and 76% had multiple stones on first imaging. No renal units were lost during follow up.

	Remained Asymptomatic			Became Symptomatic
N (%)	115 (72%)			45 (28%)
Outcome	Followed without Treatment	Followed and then Treated at Patient’s Request	Silent Obstruction Prompted Treatment	Did Not Require Surgery	Required Surgery
N (%)	94 (59%)	18 (11%)	3 (2%)	18 (11%)	27 (17%)
Average time to treatment	-	19 + 12 months (median 13 months)	37 + 17 months (median 32 months)	-	24 + 15 months (median 22 months)
Average time to last follow up	39 + 19 months (median 37 months)	-	-	42 + 17 months (median 44 months)	-
Growth >50% of size on first visualization	22 (14%)	5 (3%)	1 (1%)	3 (2%)	2 (1%)
Average time to growth >50% of size on first visualization	26 + 18 months (median 20 months)	19 + 12 months (median 12 months)	31 months	15 + 2 months (median 15 months)	12 + 1 months (median 12 months)
Spontaneous passage	-	-	-	12 (8%)	-
Average time to spontaneous passage	-	-	-	24 + 16 months (median 23 months)	-
ER visit for pain		-	-	6 (4%)	4 (2%)
Average time to ER visit for pain	-	-	-	22 + 17 months (median 25 months)	17 + 18 months (median 15 months)
Developed hydronephrosis	-	-	3	-	19
Average time to hydronephrosis	-	-	36 + 17 months (median 30 months)	-	26 + 18 months (median 27 months)

CONCLUSIONS: Among our cohort of 160 asymptomatic non-obstructing renal stones managed with active surveillance, about 60% remained asymptomatic through an average follow up of over 3 years while less than 30% caused renal colic and less than 20% were operated on for pain. Three stones led to silent obstructions and were then treated prior to the development of permanent renal injury. This data can help patients and providers make informed decisions regarding the management of asymptomatic non-obstructing renal stones.