Kidney stones are dense mineral deposits that form inside the kidneys. These structures develop when compounds in the urine become highly concentrated and crystallize. The most common type of kidney stone, accounting for up to 80% of all cases, is the calcium oxalate stone. This composition presents a unique challenge, leading many to wonder about the possibility of dissolving them.
Composition and Formation of Calcium Oxalate Stones
Calcium oxalate stones are a hard, crystalline structure composed of calcium ions and oxalate ions bound together. These stones primarily exist in two forms: calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD), with the monohydrate form being the harder and more common constituent of larger stones. The formation process begins when urine becomes supersaturated, meaning the concentration of calcium and oxalate exceeds the amount that can remain dissolved.
Supersaturation leads to nucleation, where the dissolved ions come together to form a solid crystal structure. These crystal nuclei then grow and aggregate, often in the renal tubules, developing into a stone large enough to cause obstruction. The stability and insoluble nature of the calcium oxalate crystal lattice make these stones resistant to natural or chemical breakdown.
Answering the Dissolution Question
Calcium oxalate stones cannot be dissolved using standard medications due to their inherent chemical properties. The crystalline structure of calcium oxalate is highly stable and does not readily break down when exposed to oral medications or changes in urine acidity. This stability contrasts sharply with other stone types, such as uric acid stones, which are chemically soluble.
Uric acid stones can often be dissolved through a process called chemolysis, which involves making the urine more alkaline using oral medications like potassium citrate. This change in pH increases the solubility of uric acid, causing the stone to shrink and eventually dissolve entirely. Unfortunately, this alkalization strategy has little effect on the insoluble calcium oxalate structure, which remains solid across the typical range of urinary pH.
One area of emerging research has focused on compounds chemically similar to citrate, such as hydroxycitrate (HCA), which has shown promise in laboratory settings. Studies using atomic force microscopy have demonstrated that HCA can potentially bond with the crystal surface, inducing a strain that leads to the release of calcium and oxalate ions and causes the crystals to shrink. While this is a promising scientific detail, it is not yet a standard or widely available medical therapy.
Standard Medical Removal Procedures
Since chemical dissolution is not a viable option, stones that are too large to pass naturally or are causing severe symptoms must be removed or fragmented. Several procedures are used depending on the stone’s size and location.
Shock Wave Lithotripsy (SWL)
SWL is a non-invasive procedure that uses high-energy sound waves directed from outside the body to break the stone into tiny pieces. These small fragments can then pass through the urinary tract, though SWL is often less effective on the hard calcium oxalate monohydrate stones.
Ureteroscopy (URS)
URS involves inserting a small, flexible telescope through the urethra and bladder up to the ureter or kidney. The surgeon can directly visualize the stone and use a laser fiber to fragment it into pieces or remove them with a basket. URS is effective for stones located anywhere in the ureter or kidney.
Percutaneous Nephrolithotomy (PCNL)
For very large or complex stones, PCNL is the preferred treatment. This minimally invasive surgery requires a small incision in the back to create a direct tract into the kidney. A specialized instrument is used to remove the stone whole or break it up and suction the fragments out, offering the highest stone-free rates for large stone burdens.
Strategies for Preventing Recurrence
For patients who have experienced a calcium oxalate stone, the focus shifts to prevention, as the recurrence rate can be high without intervention. Increasing fluid intake is the most important factor, as it dilutes the concentration of stone-forming minerals in the urine. The goal is to produce a high volume of urine, typically aiming for at least 2.5 liters of urine output daily.
Dietary modifications center on managing the balance of calcium and oxalate in the diet. Patients are advised to limit foods with very high oxalate content, because intestinal oxalate is a direct contributor to stone formation. These foods include:
- Spinach
- Rhubarb
- Almonds
- Chocolate
Paradoxically, severely restricting dietary calcium is discouraged, as calcium consumed with meals binds to oxalate in the gut, preventing its absorption and subsequent excretion in the urine.
Reducing the intake of sodium and animal protein also helps, as both can increase the amount of calcium excreted in the urine and lower the levels of protective citrate. Long-term medical management is employed to control urine chemistry. Prescription medications, such as potassium citrate, are used to raise citrate levels and inhibit crystal growth, while thiazide diuretics may be prescribed to reduce the amount of calcium released into the urine.