Kidney stones are small, hard mineral deposits that form within the urinary tract when substances like calcium, oxalate, and uric acid become highly concentrated in the urine. This common condition causes intense discomfort when these formations attempt to move through the narrow urinary passages. Whether these stones can dissolve depends entirely on their chemical make-up.
Stone Composition and the Potential for Dissolution
Calcium stones, including calcium oxalate and calcium phosphate, are the most common type, making up about 80% of all kidney stones. These calcium-based formations are chemically stable and cannot be dissolved by simply increasing fluid intake or taking oral medications once they have fully formed.
Uric acid stones are a notable exception due to their responsiveness to medical therapy. These stones form when the urine is persistently acidic, causing the uric acid to precipitate into an insoluble crystalline form.
The medical strategy for dissolving uric acid stones is known as urinary alkalinization, which involves using medications such as potassium citrate or sodium bicarbonate. The goal is to maintain a urinary pH level optimally between 6.5 and 7.0, a range where uric acid becomes highly soluble. When this therapeutic pH is consistently achieved, existing uric acid stones can gradually shrink and completely dissolve over a period of weeks to months.
Other less common stone types include struvite and cysteine stones. Struvite stones, also called infection stones, are typically related to chronic urinary tract infections and require aggressive intervention to clear both the stone and the underlying infection. Cysteine stones are caused by a rare genetic condition and are notoriously resistant to dissolution, often necessitating a combination of high-volume hydration and specific medications to prevent new growth.
Factors Determining Whether Stones Pass Naturally
For the majority of stones that do not dissolve, the alternative is spontaneous passage through the urinary system. The two most significant predictors of whether a stone will pass naturally are its size and its location within the ureter, the tube connecting the kidney to the bladder.
Stones measuring 4 millimeters or less in diameter have the highest chance of passing spontaneously, with success rates often exceeding 80% to 90%. As the size increases, the likelihood of passage decreases sharply; stones between 5 millimeters and 10 millimeters may pass only about 40% to 60% of the time, and stones larger than 10 millimeters rarely pass without intervention.
The stone’s position also determines its speed and likelihood of passage. Stones located in the distal ureter, the segment closest to the bladder, pass much more easily than those lodged in the proximal ureter, which is nearer to the kidney. The tightest point in the ureter is the ureterovesical junction, meaning any stone that reaches this point has successfully navigated most of the passage.
Patients attempting natural passage are often advised to increase their fluid intake significantly to maintain a high urine output, which helps push the stone along. Physicians may also prescribe alpha-blocker medications, such as tamsulosin, to facilitate the process. These drugs work by relaxing the smooth muscles in the wall of the ureter, making the passage wider and easier for the stone to travel through.
Medical Treatments When Stones Cannot Clear
When a stone is too large to pass, causes uncontrolled pain, or creates a blockage that threatens kidney function, medical intervention becomes necessary. One non-invasive option is Extracorporeal Shock Wave Lithotripsy (ESWL), which uses high-energy sound waves generated outside the body. These waves are precisely focused on the stone, causing it to fragment into tiny pieces that can then be passed naturally in the urine.
Another common method is Ureteroscopy, a minimally invasive procedure performed under anesthesia. A thin, flexible scope is inserted through the urethra and bladder and guided up the ureter to the stone. Once the stone is visualized, it can either be grasped and removed with a tiny basket or broken into smaller, passable fragments using a laser fiber. This technique is highly effective and allows for immediate stone removal.
For very large or complex stones, a procedure called Percutaneous Nephrolithotomy (PCNL) is typically required. This is a surgical technique involving a small incision in the patient’s back to create a direct tract into the kidney. A specialized scope is then used to remove the stone, either whole or by fragmenting it, offering the highest stone-free rates for the largest stone burdens.