Chemical composition of urinary stones

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

Stones of the urinary tract are insoluble components of urine of different chemical composition. The formation of insoluble formations occurs according to the scheme: supersaturated solution (noncrystalline form) → formation of small crystals (nucleation process) → the appearance of large crystals and even their aggregates (crystal growth and conglomeration).

The formation of small crystals is facilitated by the so-called epitaxial induction, based on the similarity of the shape of the constituents of the crystallized solution, regardless of their chemical composition. For example, crystals of uric acid, oxalate and calcium phosphate, having a similar shape, with mutual influence facilitate the process of occurrence of stones. In addition to compounds that facilitate the process of the formation of crystals (promoters), there are substances that interfere with this process (inhibitors). These include pyrophosphates, ATP, citrate, glycosaminoglycans (especially heparin, hyaluronic acid and dermatan sulfate).

In the study of urinary stones, first of all, their magnitude, color, surface properties, hardness, and the type of transverse cuts are noted. Most often, the following types of stones are identified.

• Oksalatovye stones (from oxalic calcium), they account for up to 75% of the cases of stones formed by calcium salts. They are either small and smooth, or of large size (up to several centimeters) and have a large-scale surface. In the latter case, they have a complex chemical composition, with the oxalates forming only the surface layers. Compared with other stones, they are the hardest. The most common cause of oxalate stones is an increased release of calcium in the urine, which may be due to increased calcium resorption in the intestine, a violation of its filtration and renal resorption 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 oxalate in the body can be formed with an overdose of vitamin C (more than 3-4 g / day). Crystals of calcium oxalate can also form in patients with gout (induction is caused by sodium urate crystals). Excess formation of oxalates in the body due to congenital insufficiency of enzymes that catalyze deamination of glycine and thereby lead to an increase in the content of oxalates in the blood is observed extremely rarely.
• Urate stones (from uric acid salts and uric acid), they account for up to 10% of cases of urolithiasis. The size and shape of their very different. Stones of the bladder can have a size from a pea to a goose egg. In the kidney they can fill the entire renal pelvis. The color of urate stones is usually grayish-yellow, yellow-brown or reddish-brown, the surface is sometimes smooth, often rough or fine-grained. They are very hard and cut with difficulty. On the cross-section, small, differently colored concentric layers are visible. The causes of urate stones are different: excessive formation of uric acid in the body, increased intake of purines with food, with gout, especially in cases where therapeutic substances are prescribed substances that prevent the reverse resorption of uric acid in the renal tubules. The appearance of stones is facilitated by acid pH values of urine and its small amount. There are 4 types of uric acid urolithiasis.
  • Idiopathic, in which the concentration of uric acid in serum and urine is normal in patients, but urine pH is constantly lowered; this type includes patients with chronic diarrhea, ileostomy, and also receiving drugs that acidify urine.
  • Hyperuricemic, in patients with gout, myeloproliferative diseases and Lesch-Nyen syndrome. Approximately 25% of patients with gout symptoms have uric acid stones, and 25% of patients with uric acid stones suffer from 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.
  • With chronic dehydration. Concentrated acid urine is characteristic for patients with chronic diarrhea, ileostomy, inflammatory bowel disease or with 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 triphosphate). Crystals of calcium phosphates 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 crystal cut surface. Usually, they form around a small mochex stone or foreign body. The reasons for their occurrence are in many respects the same as those of urate stones.
• Cystine stones are rarely detected, in 1-2% of cases of urolithiasis. Cystine stones can reach a significant size, their color is white or yellowish, the surface is smooth or rough, the consistency is soft, like wax, the surface of the saw seems to be crystalline. Cystine stones appear with a congenital disruption of cystine resorption in the cells of the proximal tubules of the kidneys. Along with cystine, the resorption of lysine, arginine and ornithine is disrupted. Cystine is the least soluble amino acid among all of these, so its excessive amount in the urine is accompanied by the formation of hexagonal crystals (a diagnostic sign of cystinuria).
• Infectious (struvite) stones show relatively frequently, in 15-20% of cases of urolithiasis (in women it is 2 times more often than in men). Struvite stone consists mainly of ammonium and magnesium phosphate, their formation indicates the presence at the time of the study or the pre-existing infection caused by bacteria that cleave urea (most often - Proteus, Pseudomonas, Klebsiella ). Enzymatic cleavage of urea by urease leads to an increase in the concentration of bicarbonates and ammonium, which contributes to an increase in urine pH above 7. In alkaline reaction, urine is supersaturated with magnesium, ammonium, phosphates, which leads to the formation of stones. Struvite stones are formed only with alkaline urine reaction (pH more than 7). Approximately 60-90% of coral stones are struvite. Establishment of the chemical composition of urinary stones allows the attending physician to orientate 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 urinary content of sulphates and uric acid. High concentrations of sulphate and uric acid can promote the formation of oxalate stones. Sulphates cause acidosis, which reduces the content of citrate in the urine. The intake of calcium preparations, recommended for the prevention and treatment of osteoporosis, can lead to hypercalciuria. The high content of oxalates in food enhances the crystalluria of calcium oxalates. All this must be taken into account when selecting a diet, since only the right diet contributes to the restoration of metabolism.

Stones of uric acid differ from all other stones of the urinary tract in that they can dissolve 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 release of uric acid from it. When 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 the urine to the acid side (in the presence of urates, the pH of urine is 4.6-5.8), and since in such patients the amount of citrates in the urine is reduced, this promotes crystallization of uric acid. It must be remembered that a sharp shift of urine pH to the main side leads to the precipitation of salts of phosphates, which, enveloping the urates, make their dissolution difficult.

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, strawberry, grapefruit, oranges, cocoa, cranberry juice, raspberry juice, tea). In addition to dietary restrictions, magnesium salts are prescribed that bind oxalates in the intestine and limit their absorption.

With phosphaturia and phosphate stones, urine has a basic reaction. To change the main reaction of urine in acidic prescribe drugs ammonium chloride, ammonium citrate, methionine, etc. (under the control of urine pH).

Many patients can prevent the development of cystine stones and even dissolve them. To reduce the concentration of cystine should drink 3-4 liters of fluid per day. In addition, urine needs to be alkaline, since cystine is better soluble in alkaline urine. If cystine stones are formed or increase in size, despite the intake of large amounts of fluid and alkaline therapy, drugs should be prescribed that bind cystine and form a more soluble cysteine (penicillamine, etc.).

To prevent 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 discontinuation of therapy, the bacteria again enter the urine and cause a relapse of the disease. Patients with non-curable infectious processes in the urinary tract are prescribed urease inhibitors that block the corresponding enzyme of bacteria, which leads to acidification of urine and dissolution of stones.

[1], [2], [3], [4], [5], [6]