Where are nonobstructing kidney stones typically located?

Location of Nonobstructing Kidney Stones

Nonobstructing kidney stones are most commonly located in the renal calyces (upper, middle, or lower pole) and the renal pelvis, with lower pole stones being particularly prevalent due to gravitational effects and anatomical factors that impair drainage. 1

Anatomical Distribution

Calyceal Locations

• Lower pole calyces are the most common site for nonobstructing stones to form and remain, as gravity causes stone fragments to migrate downward and anatomical factors impede their clearance 2, 3
• Upper and middle pole calyces also harbor nonobstructing stones, though these locations are associated with higher rates of spontaneous passage (14.5% vs 2.9% for lower pole) and symptom development (40.6% vs 24.3%) compared to lower pole stones 2
• The renal pelvis itself can contain nonobstructing stones, particularly when they are smaller or when pelvicalyceal anatomy allows stones to remain mobile without causing obstruction 4

Why Lower Pole Stones Persist

Several anatomical factors explain why lower pole stones remain nonobstructing yet persistent 3, 5:

• Acute infundibulopelvic angle (IPA): A narrower angle between the lower pole infundibulum and renal pelvis impairs stone clearance 3, 5
• Long infundibular length: Greater distance from the lower calyx to the renal pelvis reduces spontaneous passage 3, 5
• Narrow infundibular width: Tight infundibular diameter mechanically prevents stone migration 3, 5

Clinical Implications for Management

Size and Location Considerations

• According to EAU guidelines, active surveillance is acceptable for asymptomatic, nonobstructing calyceal stones up to 15 mm 1
• Lower pole stones are significantly less likely to cause symptoms or pass spontaneously compared to upper/middle pole stones, making them ideal candidates for observation 2
• Despite being nonobstructing initially, approximately 28% of these stones will eventually cause symptoms during follow-up, with an average observation period exceeding 3 years 2

Critical Pitfall: Silent Obstruction

• A small but important subset (2-3%) of initially nonobstructing stones can cause painless silent obstruction, necessitating intervention after an average of 37 months 2
• This underscores the importance of regular follow-up imaging (as recommended by AUA/ES and EAU guidelines) to prevent renal loss from unrecognized obstruction 1, 2

Imaging Follow-Up Strategy

• Ultrasound is the primary diagnostic tool for monitoring nonobstructing stones, with 95% sensitivity and 100% specificity for detecting hydronephrosis, though it has lower sensitivity (45%) for detecting the stones themselves 1, 6
• Non-contrast CT remains the gold standard when precise stone characterization is needed, with 93-96% sensitivity 1, 7
• KUB radiography has limited utility, detecting only 8% of stones <5mm but improving to 78% detection for stones >5mm, making it useful primarily for tracking known radio-opaque stones over time 1, 7

Treatment Decision Algorithm

For stones in the renal pelvis or upper/middle calyx <20mm: Flexible ureteroscopy (fURS) and shock wave lithotripsy (SWL) are first-line treatments 1

For lower pole stones: Treatment selection depends on size 1:

• <10mm: fURS or SWL are primary options
• 10-20mm: fURS and percutaneous nephrolithotomy (PCNL) are preferred, as SWL has poor clearance rates due to unfavorable lower pole anatomy 1, 3
• >20mm: PCNL is first-line regardless of location 1

The anatomical factors that allow stones to remain nonobstructing in the lower pole (acute IPA, long infundibulum, narrow width) are the same factors that make them resistant to SWL clearance, favoring endoscopic approaches for symptomatic lower pole stones 3, 4.