Kidney stones are hard mineral and salt deposits that form within the kidneys when there is a high concentration of these substances in the urine. While small stones may exit the body unnoticed, larger ones can become lodged, causing severe pain and potential complications. The duration a stone remains in the kidney or urinary tract is highly unpredictable and depends on factors related to the stone itself and the patient’s anatomy. Professional medical diagnosis is necessary for an accurate assessment and management plan.
Stone Size and Location: Primary Factors in Retention
The dimensions of a kidney stone are the primary determinants of whether it will be retained or is likely to pass spontaneously. Stones with a diameter less than 5 millimeters (mm) have a high probability of spontaneous passage, often around 75% or greater. Stones measuring between 5 mm and 10 mm are less likely to pass on their own, with spontaneous passage rates dropping to between 25% and 50%. Stones larger than 10 mm are unlikely to pass naturally and almost always require medical intervention.
The stone’s original location within the kidney also influences its ability to move into the ureter. Stones situated in the renal pelvis have a more direct path. Stones located in the lower pole calyx, the bottom-most chamber of the kidney, face a challenging exit due to gravity and narrow angles. Once a stone enters the ureter, its location affects passage time, with stones closer to the bladder (distal ureter) being more likely to pass than those near the kidney (proximal ureter).
The Timeline of Watchful Waiting
For stones that are small and not causing severe symptoms, physicians often recommend “expectant management,” or watchful waiting, to allow the stone to pass naturally. This approach relies on the body’s natural processes, sometimes aided by medication, to expel the stone without the need for an invasive procedure. The timeline for this monitoring period is dictated by the stone’s size, with most small stones passing within a few weeks of the onset of symptoms.
A stone smaller than 4 mm may pass spontaneously within an average of 31 days, and most stones of this size will do so within six weeks. For stones in the 4 mm to 6 mm range, the passage time can extend to an average of 45 days. If the stone remains in the kidney and is not causing any symptoms or obstruction, it may be monitored indefinitely with periodic imaging.
When a stone enters the ureter and causes symptoms, the duration of expectant management is more strictly limited. If a ureteral stone has not moved or passed after four to six weeks of observation, or if the patient’s symptoms become unmanageable, an active medical intervention is typically recommended. Medications, such as alpha-blockers, may be prescribed to relax the muscles of the ureter, which can modestly increase the chance and speed of stone passage. Staying well-hydrated is also a standard recommendation to increase urine flow and help flush the stone.
Signs That Retention Requires Immediate Intervention
While many stones pass on their own, the retention of a stone can lead to severe complications that require urgent medical attention. The most significant immediate concern is complete urinary tract obstruction, where the stone completely blocks the flow of urine from the kidney to the bladder. This blockage causes the kidney to swell, a condition known as hydronephrosis, which can impair kidney function.
Another sign that a retained stone demands immediate intervention is uncontrolled pain, often described as renal colic, which is unresponsive to standard pain medication. This intense, fluctuating pain typically occurs as the ureter spasms in an attempt to push the stone down. The presence of a stone combined with a concurrent infection is a medical emergency. Symptoms like fever, chills, and cloudy or foul-smelling urine signal pyelonephritis or sepsis, requiring immediate drainage of the obstructed kidney to prevent life-threatening complications.
Medical Options for Retained Stones
When a stone is retained for too long, is too large, or causes serious symptoms, several medical procedures are available to remove or fragment it.
Extracorporeal Shock Wave Lithotripsy (ESWL)
This common, non-invasive option uses focused sound waves generated from outside the body to break the stone into tiny pieces. These small fragments can then be passed naturally in the urine. This procedure is generally recommended for stones less than 2 centimeters in diameter.
Ureteroscopy (URS)
For stones located in the ureter or those resistant to ESWL, Ureteroscopy is a highly effective, minimally invasive treatment. A thin, flexible tube called a ureteroscope is inserted through the urethra and bladder, up into the ureter or kidney. The stone can then be removed intact with a small basket or fragmented using a laser, a process known as laser lithotripsy.
Percutaneous Nephrolithotomy (PCNL)
PCNL is typically reserved for very large or complex stones, often those greater than 2 centimeters, or stones that have failed other treatments. This surgical procedure involves making a small incision in the back to create a tract directly into the kidney. A specialized tube and camera, called a nephroscope, are used to visualize the stone, which is either removed whole or broken down and extracted through the incision.