Kidney Stones - Complications

Causes

The key process in the development of kidney stones is supersaturation.

• Salts, including calcium oxalate, uric acid, cystine, or xanthine, are carried in urine.
• These salts can become extremely concentrated if the volume of urine is significantly reduced or if abnormally high amounts of crystal-forming salts are present.
• When concentration levels reach the point at which the salts no longer dissolve, the salts form crystals.

Different factors may be involved in either reducing urine volume or increasing the levels of the salts.

Deficiencies in Protective Factors. Normally, urine contains substances (magnesium, citrate, pyrophosphate, various proteins, enzymes) that may protect against stone formation. These substances:

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• Allow salt in the urine to be at higher-than-normal concentrations without forming crystals
• Prevent crystal formation
• Coat the crystals and prevent them from sticking to the surface of kidney tubes

Deficiencies in these protective substances can cause stones.

Changes in the Acidity of the Urine. Changes in the acid balance of urine can affect stone formation.

• Uric acid and cystine stones thrive in acidic urine.
• Calcium phosphate and struvite stones thrive in alkaline.

Factors that Bind Crystals to the Kidney Tubules. Researchers are studying the cells lining the kidney tubules in order to understand how and why early crystals bind to the tubes long enough to form stones. Under investigation are elevated levels of substances that either cause crystals to stick to the tubes or deficiencies in those that prevent them from sticking.

Causes of Calcium Stones

In general, calcium stones form when there is an imbalance in the urine substances that promote and block the formation of stones. Often, the cause of calcium stones is not known. This condition is called idiopathic nephrolithiasis. Research suggests that abnormalities in metabolism (i.e., digestion and intestinal absorption of calcium or oxalate) are responsible for nearly all stones. Genetic factors may play a role in about half of these cases. A number of medical conditions and drugs can also affect digestion and intestinal absorption.

Excess Calcium in the Urine (Hypercalciuria). About 70% of calcium-containing stones are caused by hypercalciuria, a condition in which there is too much calcium in the urine. A number of conditions may produce hypercalciuria. Many are due to genetic factors, but most cases are idiopathic (due to unknown causes).