Calcium stones are the most common type of stones found in the kidney and ureter. There are six main causes of calcium stone.
Hypercalciuria refers to the excretion of too much calcium in the urine. There are several mechanisms whereby calcium is excreted in urine in larger than normal amounts.
The primary abnormality in absorptive hypercalciuria is too much absorption of calcium by the intestine. This leads to an increase in the concentration of calcium in the bloodstream which leads to an increased load of calcium excreted in the urine. There are two types of absorptive hypercalciuria.
Type I--Urinary calcium is increased while a person is on a high calcium diet, but urinary calcium is also increased while a person is on a calcium restricted (low calcium) diet. This is the more severe form.
Type II--Urinary calcium is increased only while a person consumes a diet high in calcium.
The primary abnormality in renal hypercalciuria is the kidney has difficulty reabsorbing (taking back) the calcium that has been filtered into the urine. Since the kidney is not able to reclaim the filtered calcium, there is a fall in the level of calcium in the bloodstream. This in turn stimulates a gland known as the parathyroid gland, whose job it is to maintain the proper level of calcium in the bloodstream. To do this, the parathyroid gland tells the bones to release more calcium into the bloodstream, and the parathyroid gland tells the bowels to absorb more calcium from the food. This all leads to an increase in calcium levels in the bloodstream, but now, even a larger amount of calcium is being excreted in the urine. This cycle continues, and makes it easier to form calcium stones in the urinary tract.
Resorptive hypercalciuria is caused by primary hyperparathyroidism. An overactive parathyroid gland causes release of calcium from bones and an increase in calcium absorbed by the intestines. This leads to an increase in calcium levels in the bloodstream, which causes a larger amount of calcium being sent to the kidneys. The urine becomes overloaded with calcium, leading to calcium stone formation in the urinary tract.
Hyperuricosuria is another mechanism that leads to the formation of calcium stones in the urinary tract. In 10% of patients with calcium stones, the only abnormality found is too much uric acid in the urine. This may be due to eating too many purine-rich foods (meat, fish, and poultry) or to over-production of uric acid by the body. If the urine is acidic and is supersaturated with uric acid, the uric acid will not be able to remain dissolved in the urine. It will precipitate out of solution and form either uric acid crystals or monosodium urate crystals. Calcium oxalate crystals already in the urine adhere fairly easily to uric acid crystals and monosodium urate crystals and grow into a calcium oxalate stone.
The term gouty diathesis refers to the formation of uric acid stones or calcium oxalate stones in a patient who suffers from gout. In patients with gouty diathesis, the urine is unusually acidic. Uric acid does not dissolve well in acidic urine, and therefore comes out of solution and forms uric acid crystals. The stones found in these patients can be uric acid, calcium oxalate, or calcium phosphate stones.
Hyperoxaluria refers to an excess amount of oxalate in the urine. This is usually due to too much oxalate being absorbed by the intestine. Patients who suffer from inflammatory bowel disease or who have had portions of their small bowel surgically removed do not have enough healthy small bowel remaining for normal absorption of nutrients. For example, the fat in the small bowel is not absorbed well, and this excess of fat remaining unabsorbed in the small bowel binds up the calcium also sitting in the bowel. Now there is less calcium available to bind the oxalate in the small intestine. Therefore, more oxalate is sitting free, unbound, and this excess of free oxalate is absorbed into the body. The body has to get rid of this excess amount of oxalate, and more oxalate is excreted in the urine, leading to calcium oxalate stones. Other causes of hyperoxaluria include excess Vitamin C intake (Vitamin C is converted into oxalate in the body) and low calcium intake.
Hypocitraturia refers to a lower than normal amount of citrate in the urine. Citrate is a natural inhibitor of calcium oxalate stone formation. When there is a low amount of citrate in the urine, calcium oxalate stones form more easily. Hypocitraturia is seen in patients with renal tubular acidosis (a disease in which the kidneys have difficulty excreting the acid that has been produced by the body), patients with low levels of potassium, patients who frequently partake in strenuous physical exercise, and patients who consume a large amount of animal protein in the diet.
This term refers to abnormally low amounts of magnesium in the urine. Like citrate, magnesium is an inhibitor of calcium stones. Too little magnesium in the urine can lead to calcium stone formation.