Kidney stones are stones formed in the renal tubules or renal collecting system. Some kidney stones can be passed out through the urinary tract or bladder, resulting in ureteral or bladder stones. The clinical manifestations of kidney stones vary greatly, with mild cases being asymptomatic, typical cases presenting with pain and hematuria, and some cases showing symptoms of urinary tract infection. Severe cases can lead to urinary tract obstruction and renal function impairment. Kidney stones can be classified into calcium stones, infectious stones, uric acid stones, and cystine stones.
1. Calcium Stones
Many physiological or metabolic disorders can lead to the formation of calcium stones. These disorders serve as the basis for the classification of calcium stones and include hypercalciuria, hyperuricosuria, hyperoxaluria, renal acidification defects, and impaired renal excretion of stone formation inhibitors. Although each disorder is not the sole cause, it can promote stone formation.
Hypercalciuria is likely the most common abnormality among patients with calcium stones, and it can lead to stone formation by causing urinary calcium salt supersaturation. Oxalate is the second most common component of kidney stones, but most calcium oxalate stone patients do not have abnormalities in oxalate metabolism. Persistent hyperoxaluria can be seen in primary hyperoxaluria, vitamin B6 deficiency, excessive intake of vitamin C, enhanced absorption of oxalate in food, and other conditions.
2. Non-Calcium Stones
(1) Uric Acid Stones
Uric acid stones often form in urine with a pH lower than the pKa of uric acid, especially when there is an absolute increase in uric acid. This leads to an increase in the amount of free uric acid in the urine. Uric acid stones are common in primary gout and can be accompanied by low urine pH and high uric acid levels. They can also occur in various situations where there is excessive production of uric acid. Most uric acid stone patients do not have clinical manifestations of gout.
(2) Cystine Stones
Cystine kidney stones are only seen in patients with hereditary cystinuria. Other types of amino aciduria do not excrete enough cystine to form stones. The production of cystinuria is due to the disorder in the handling of lysine, arginine, ornithine, and cysteine by the kidneys and intestines. In a small portion of cystinuria patients, stone formation occurs as a result of excessive renal excretion of cystine and its low solubility.
