Chemical composition of urinary stones

In healthy people, urinary stones are not found in the urine.

Urinary tract stones are insoluble components of urine of various chemical compositions. The formation of insoluble formations occurs according to the scheme: supersaturated solution (non-crystalline form) → formation of small crystals (nucleation process) → formation of large crystals and even their aggregates (crystal growth and their conglomeration).

The formation of small crystals is facilitated by the so-called epitaxial induction, based on the similarity of the shape of the components of the crystallized solution, regardless of their chemical composition. For example, crystals of uric acid, calcium oxalate and phosphate, which have a similar shape, facilitate the process of stone formation when they interact with each other. In addition to compounds that facilitate the process of crystal formation (promoters), there are substances that hinder this process (inhibitors). These include pyrophosphates, ATP, citrate, glycosaminoglycans (especially heparin, hyaluronic acid and dermatan sulfate).

When examining urinary stones, their size is noted first, followed by color, surface properties, hardness, and type of cross-sections. The following types of stones are most often identified.

• Oxalate stones (from calcium oxalate) account for up to 75% of cases of stones formed by calcium salts. They are either small and smooth, or large (up to several centimeters) and have a large warty surface. In the latter case, they have a complex chemical composition, with oxalates forming only the surface layers. Compared to other stones, they are the hardest. The most common cause of oxalate stones is increased excretion of calcium in the urine, which may be due to increased resorption of calcium in the intestine, impaired filtration and resorption in the kidneys, or unrecognized hyperparathyroidism. In these cases, against the background of hypercalciuria, increased intake of oxalates with food creates additional favorable conditions for the formation of stones. An increased amount of oxalates in the body can form with an overdose of vitamin C (more than 3-4 g / day). Calcium oxalate crystals can also form in patients with gout (induced by sodium urate crystals). Excessive formation of oxalates in the body due to congenital deficiency of enzymes that catalyze the deamination of glycine and thus lead to an increase in the content of oxalates in the blood is observed extremely rarely.
• Urate stones (from urate salts and uric acid), they account for up to 10% of cases of urolithiasis. Their size and shape are very different. Bladder stones can be from a pea to a goose egg in size. In the kidney, they can fill the entire renal pelvis. The color of urate stones is usually grayish-yellow, yellow-brown or red-brown, the surface is sometimes smooth, but more often rough or finely warted. They are very hard and difficult to cut. In the cross section, small concentric layers of different colors are visible. The causes of urate stones are different: excessive formation of uric acid in the body, increased intake of purines with food, gout, especially in cases where substances that prevent the reverse resorption of uric acid in the renal tubules are prescribed for therapeutic purposes. The occurrence of stones is facilitated by acidic pH values of urine and its small amount. There are 4 types of uric acid urolithiasis.
  • Idiopathic, in which patients have normal serum and urine uric acid concentrations, but persistently low urine pH; this type also includes patients with chronic diarrhea, ileostomies, and those receiving medications that acidify urine.
  • Hyperuricemic, in patients with gout, myeloproliferative disorders, and Lesch-Nyen syndrome. Approximately 25% of patients with gout symptoms have uric acid stones, and 25% of patients with uric acid stones have gout. If the daily excretion of uric acid in a patient with gout exceeds 1100 mg, the incidence of urolithiasis is 50%. In addition, an increase in the concentration of uric acid in the blood and urine is possible in patients receiving chemotherapy for neoplasms.
  • In chronic dehydration. Concentrated acidic urine is typical for patients with chronic diarrhea, ileostomies, inflammatory bowel disease or increased sweating.

• Hyperuricosuric without hyperuricemia, observed in patients receiving uricosuric drugs (salicylates, thiazides, probenecid) or eating foods rich in purines (meat, sardines).
• Phosphate stones (from calcium phosphate and triple phosphate). Calcium phosphate crystals are rarely detected, in about 5% of cases. They can reach a significant size, their color is yellowish-white or gray, the surface is rough, as if covered with sand, the consistency is soft, quite brittle, the cut surface is crystalline. They usually form around a small uric acid stone or foreign body. The reasons for their occurrence are largely the same as for urate stones.
• Cystine stones are rare, occurring in 1-2% of cases of urolithiasis. Cystine stones can be quite large, their color is white or yellowish, the surface is smooth or rough, the consistency is soft, like wax, the cut surface seems crystalline. Cystine stones appear with a congenital disorder of cystine resorption in the cells of the proximal tubules of the kidneys. Along with cystine, the resorption of lysine, arginine and ornithine is impaired. Cystine is the least soluble amino acid of all listed, therefore its excess in the urine is accompanied by the formation of hexagonal crystals (a diagnostic sign of cystinuria).
• Infectious (struvite) stones are found relatively often, in 15-20% of cases of urolithiasis (in women 2 times more often than in men). Struvite stones consist mainly of ammonium and magnesium phosphate, their formation indicates the presence at the time of the study or a previously existing infection caused by bacteria that break down urea (most often - Proteus, Pseudomonas, Klebsiella ). Enzymatic breakdown of urea by ureases leads to an increase in the concentration of bicarbonates and ammonium, which contributes to an increase in urine pH above 7. With an alkaline reaction, urine is supersaturated with magnesium, ammonium, phosphates, which leads to the formation of stones. Struvite stones are formed only with an alkaline urine reaction (pH over 7). Approximately 60-90% of coral stones are struvite. Determining the chemical composition of urinary stones allows the attending physician to orient himself in selecting a diet for a patient with urolithiasis. High protein intake with food (1-1.5 g/kg per day) can increase the content of sulfates and uric acid in the urine. High concentrations of sulfates and uric acid can contribute to the formation of oxalate stones. Sulfates cause acidosis, which reduces the content of citrate in the urine. Taking calcium supplements, recommended for the prevention and treatment of osteoporosis, can lead to hypercalciuria. High oxalate content in food increases calcium oxalate crystalluria. All this must be taken into account when selecting a diet, since only proper nutrition helps restore metabolism.

Uric acid stones differ from all other urinary tract stones in that they can be dissolved with the appropriate diet and the use of therapeutic agents. The objectives of treatment are to increase the pH of urine, increase its volume and reduce the excretion of uric acid with it. In uraturia, the patient is recommended to exclude products that promote the formation of uric acid (brains, kidneys, liver, meat broths). In addition, it is necessary to strictly limit the consumption of meat, fish, vegetable fats, which shift the pH of urine to the acidic side (in the presence of urates, the pH of urine is 4.6-5.8), and since such patients have a reduced amount of citrates in the urine, this contributes to the crystallization of uric acid. It is necessary to remember that a sharp shift in the pH of urine to the basic side leads to the precipitation of phosphate salts, which, enveloping urates, hinder their dissolution.

With oxalate stones, it is necessary to limit the intake of foods with a high content of oxalic acid salts (carrots, green beans, spinach, tomatoes, sweet potatoes, rhubarb root, strawberries, grapefruit, oranges, cocoa, cranberry juice, raspberry juice, tea). In addition to dietary restrictions, magnesium salts are prescribed, which bind oxalates in the intestines and limit their absorption.

In phosphaturia and phosphate stones, urine has a basic reaction. To change the basic reaction of urine to acidic, ammonium chloride, ammonium citrate, methionine, etc. are prescribed (under the control of urine pH).

In many patients, cystine stones can be prevented from developing and even dissolved. To reduce cystine concentrations, drink 3-4 liters of fluid per day. In addition, urine should be alkalized, since cystine dissolves better in alkaline urine. If cystine stones form or increase in size despite drinking large amounts of fluid and alkalizing therapy, drugs that bind cystine and form more soluble cysteine (penicillamine, etc.) should be prescribed.

To prevent the formation and growth of struvite stones, rational therapy of urinary tract infections is necessary. It should be noted that bacteria are present on the surface of the stone and can remain there even after the completion of the course of antibiotic therapy and the disappearance of the pathogen in the urine. After stopping the therapy, the bacteria again enter the urine and cause a relapse of the disease. Patients with intractable infectious processes in the urinary tract are prescribed urease inhibitors, which block the corresponding bacterial enzyme, which leads to acidification of the urine and dissolution of stones.

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