Kidney stones are hard masses that form in the kidneys from concentrated minerals and salts in the urine. These masses, also known as renal calculi, develop when the urine contains too many crystal-forming substances and not enough fluid to dilute them. Once formed, a kidney stone may either remain in the kidney without symptoms or travel down the urinary tract, causing severe pain. Whether a stone resolves naturally depends heavily on its physical size and chemical composition.
Spontaneous Passage: The Role of Stone Size
The size of the stone is the primary factor determining the likelihood and timeframe for spontaneous passage through the urinary tract. Stones smaller than 4 to 5 millimeters in diameter have a high chance of passing without medical intervention, with success rates often cited between 68% and 98%.
A stone of 2 millimeters or less may pass in about eight days, while a stone in the 2 to 4-millimeter range often takes closer to 12 days. Even stones in the 4 to 6-millimeter range have a reasonable chance of spontaneous passage, though the success rate drops considerably to around 33% to 65%. Passage for this group can extend to four to six weeks or longer.
The stone’s location in the urinary tract also affects passage; stones located closer to the bladder (distal ureter) pass more easily and quickly than those higher up (proximal ureter). For stones greater than 6 millimeters, the chance of spontaneous passage drops below 10%, making intervention more likely. Medical expulsive therapy, often involving alpha-blockers like tamsulosin, may be prescribed to relax the ureter muscles and facilitate the passage of smaller stones.
Stone Types That Can Dissolve
True dissolution, where the stone is chemically broken down, is only possible for a minority of kidney stones. The majority are composed of calcium oxalate, which is chemically resistant to dissolution through common medical treatments. Most patients rely on the stone passing physically or requiring a procedure to break it apart.
Uric acid stones, which make up about 9% of all kidney stones, are the most responsive to chemical dissolution. Uric acid is a byproduct of purine metabolism and forms stones in acidic urine environments, typically with a pH below 5.5. Treatment involves urinary alkalinization, where medication like alkaline citrate salts or sodium bicarbonate is used to raise the urine pH until the uric acid stone material becomes soluble.
Struvite stones, often called infection stones, make up about 10% of cases and are caused by certain bacteria that produce the enzyme urease. These stones, composed of magnesium ammonium phosphate, can be dissolved in theory, but management is complicated by the underlying, persistent urinary tract infection. Struvite stones can grow very large, sometimes filling the entire kidney collection system, and often require surgical management despite their potential for chemical breakdown.
When Intervention Becomes Necessary
While many stones pass naturally, medical intervention is necessary when spontaneous resolution is unlikely or the stone poses a direct threat to health. The size threshold is a major indicator; stones larger than 10 millimeters rarely pass and almost always require active removal. A stone causing severe pain that cannot be managed with standard analgesics is another indication for intervention.
A complete blockage of the ureter causes urine to back up and swell the kidney, a condition known as hydronephrosis, which necessitates treatment to prevent long-term kidney damage. The most urgent scenario is an obstructing stone accompanied by signs of infection, such as fever or chills, which is a medical emergency requiring immediate drainage and treatment.
If a stone is causing persistent symptoms or has not progressed after four to six weeks of observation, active treatment is recommended. The two primary non-surgical and minimally invasive interventions are Extracorporeal Shock Wave Lithotripsy (ESWL) and Ureteroscopy (URS). ESWL uses focused sound waves from outside the body to break the stone into smaller pieces that are then passed naturally. Ureteroscopy involves passing a small scope through the urinary tract to directly remove the stone or break it up with a laser.