Asymptomatic stones can take many forms. Those causing silent obstruction require immediate attention and will not be considered here. Non-obstructing renal stones lacking discernable signs and symptoms will also be considered separately from post-therapeutic residual fragments in this chapter.

Though overall rates are unknown, routinely screened cohorts (i.e., colon cancer, living kidney donors) have shown rates of 7–9.8 % of incidental stones on CT imaging. In those with incidental stones, 13.5 % had a pre-existing diagnosis of kidney stones. Subsequent stone related events were experienced by 45 % of those with incidental stones, 1.3 ± 1.1 years after their initial imaging.

The natural history of asymptomatic stones has been described by six studies [1–6]. Five studies retrospectively assessed those who were diagnosed with incidental stones, and attempted to characterize the risk factors for subsequent stone related events (pain, spontaneous passage, surgical intervention, or increase in size). Ranging from 50 to 340 patients, follow-up over 3–4 years identified 30–70 % of patients eventually developed symptoms, a third of whom required emergency room attention. However, over half of all patients who develop symptoms appear to pass their stones spontaneously, and only 12–35 % go on to require surgical intervention. Kaplan Meyer curves estimate the 5-year risk of developing future symptoms as 45–48 % [2, 4].

Pelvic, followed by upper pole locations are most likely to progress to symptomatic stones. Larger stones (>15 mm), in addition to a history of multiple stones and recurrent stone disease have also been shown to increase the rates of progression to symptoms. Data from prospectively followed patients show lower pole stones may increase in size more readily than other locations, though they tend to have lower rates of stone related events, and require intervention less often (10 % at 4 years) than other locations. In terms of urinary indices, one study found stone growth is linked to hyperuricosuria, though stone composition was not be shown to affect growth kinetics [2].

Fragments left after surgical intervention appear to have a divergent natural history. Though conceptually similar, several studies have described their unique clinical course after various lithotripsy modalities. After SWL, residual fragments ≤4 mm have been prospectively shown to result in symptomatic episodes or require intervention in 70 % of patients by 5 years. This is notably higher than 5-year estimates of progression (to stone related events) in primary asymptomatic stones. After PCNL, 70 % of patients with fragments >2 mm develop a stone related event, whereas even fragments ≤2 mm cause symptoms or require medical attention in 43 % of patients. Though the 5-year probability of a fragment related event is 48 % and similar to incidental stones, significantly higher rates of secondary procedures (up to 53 %) are required for patients with known residual stones.

No consensus exists as to how these patients should be followed once observation is deemed appropriate. In reviewing published study designs and practice patterns, follow-up strategies range in frequency (3–12 months), utilization of imaging modalities (X-ray, IVP, ultrasound, low dose CT), and the inclusion of lab tests (serum creatinine, urinalysis, and/or urine cultures). With increasing concerns of the over radiation exposure and medical costs, X-ray (± ultrasound) tends to be adequate in most patients with longer durations (i.e., 12 months) suggested in stable disease.