General pathogenesis factors
The development of nephrolithiasis is based on an imbalance between stone-forming substances and protective mechanisms. The main triggering factor is supersaturation of urine with salts. This process is promoted by a decrease in urine output, shifts in acid-base balance (changes in pH), and the presence of bacterial flora.
Normally, salt precipitation is prevented by crystallization inhibitors: citrate, pyrophosphate, diphosphate, glycosaminoglycans, and a specific glycoprotein, nephrocalcin. Their deficiency sharply increases the risk of crystallization. However, it is important to understand that the presence of metabolic disorders (e.g., hypercalciuria) does not guarantee the development of the disease, just as the absence of obvious risk factors does not exclude lithiasis.
Classification of calculi
All stones form on an organic framework of mucoprotein, onto which specific salts are deposited. There are four main groups:
- Calcium stones (about 75%) — composed of pure calcium oxalate or its mixture with calcium phosphate.
- Struvite stones (about 15%) — formed by magnesium ammonium phosphate.
- Uric acid stones (about 6%) — composed of uric acid salts.
- Cystine stones (1–2%) — formed from the amino acid cystine.
Etiology of specific stone types
- Calcium stones form due to various metabolic shifts. In 55% of patients, isolated hypercalciuria is detected without an elevation in blood calcium levels. Less commonly (about 5%), the cause is hypercalcemia associated with hyperparathyroidism, diffuse bone diseases, or sarcoidosis. Hyperoxaluria plays an important role: it can be primary (congenital) or enteral (due to excessive calcium absorption caused by small bowel diseases or in vegetarians). Decreased citrate levels (hypocitraturia) in acidosis or chronic diarrhea also promote lithiasis. A specific variant is hyperuricosuric calcium nephrolithiasis, where uric acid crystals in the collecting ducts serve as a nucleus coated with calcium oxalate.
- Struvite (infectious) stones are associated with urea-splitting bacteria (e.g., Proteus and certain staphylococci). Urea is transformed into ammonium, urine becomes alkaline, which triggers the precipitation of magnesium ammonium phosphate. Such calculi tend to grow rapidly and reach large sizes.
- Uric acid stones form in an acidic environment (pH 5.5) where uric acid is insoluble. Although they are associated with hyperuricemia (gout, leukemias), more than half of patients have completely normal blood uric acid levels. An important diagnostic detail: uric acid stones are radiolucent (not visible on plain radiographs), unlike radiopaque calcium stones.
- Cystine stones are the result of a genetically determined defect in the transport of cystine and other amino acids in the renal tubules.
Morphological picture
The favored localization of stones is the renal pelvis, calyces, and urinary bladder. Macroscopically, calculi vary significantly depending on composition and the duration of the process.
Multiple small formations 2–3 mm in diameter with a smooth or spiky surface are often found in the renal pelvis. With prolonged progressive salt excretion, staghorn calculi are formed — massive branching structures that completely mirror the anatomical shape of the pelvicalyceal system, resembling deer antlers.
Briefly: answers to topical questions
Which bacteria contribute to the formation of struvite stones?
The formation of struvite (infectious) stones is promoted by specific bacteria capable of splitting urea in the urinary tract. These microorganisms include:
- Proteus (Proteus) — a urea-splitting bacterium.
- Staphylococci (Staphylococcus) — certain species of these microorganisms.
These bacteria convert urea into ammonium, which causes urine alkalinization and subsequent precipitation of magnesium ammonium phosphate, leading to the formation of large calculi.
What complications develop during urinary tract obstruction by a calculus?
Obstruction of the urinary tract by a calculus can lead to the following complications:
- Hydronephrosis, including with renal atrophy.
- Chronic pyelonephritis.
- Nephrosclerosis.
- Renal abscesses.
- Renal colic.
- Anuria — in case of ureteral obstruction by a stone.
At what urine pH values do various types of urinary stones form?
The formation of various types of urinary stones directly depends on the urine reaction (pH).
| Urine reaction | Types of forming stones |
|---|---|
| Acidic environment (incl. pH = 5.5) | Calcium oxalate, uric acid, and cystine stones |
| Alkaline environment | Calcium phosphate and carbonate stones, triple phosphate (struvite) |
Uric acid stones form because uric acid is insoluble in an acidic environment, whereas struvites often have an infectious genesis and precipitate upon alkalinization.
What classical theories of urolithiasis pathogenesis exist in pathological anatomy?
Pathological anatomy distinguishes two main competing theories of urolithiasis pathogenesis that explain which stone components are primary.
| Theory | Mechanism essence |
|---|---|
| Crystallization theory | Salt crystallization is primary, while organic components (fibrin, collagen, cellular detritus) are incorporated into the stone secondarily |
| Colloidal theory | The organic matrix of the stone forms primarily, upon which salts subsequently crystallize |
Both theories are based on the fact that calculi always consist of two components: organic and mineral.
To remember radiolucent stones: Uric acid stones elude X-rays (they are not visible on imaging, unlike dense calcium stones).
Class and exam questions
Does the presence of risk factors always lead to stone formation?
No, the presence of hypercalciuria or hyperoxaluria does not guarantee the development of lithiasis. Likewise, the absence of obvious risk factors does not exclude the possibility of stone formation.
Why do urinary tract infections often lead to the formation of large stones?
Certain bacteria (e.g., Proteus) split urea into ammonium, which sharply alkalinizes the urine. In an alkaline environment, magnesium ammonium phosphate rapidly precipitates, forming large struvite calculi.
Do patients with uric acid stones necessarily have gout?
No, in more than 50% of patients with uric acid stones, blood uric acid levels (hyperuricemia) remain within the normal range. The main factor in their formation is a persistently acidic urine reaction.
What else to study on this topic
- Mechanisms of development of isolated hypercalciuria without concomitant hypercalcemia.
- Primary and secondary (enteral) hyperoxaluria: differences in pathogenesis.
- The role of uric acid in the formation of calcium stones (hyperuricosuric calcium nephrolithiasis).
- Pathogenesis of hydronephrosis in prolonged urinary tract obstruction.
- Functions of crystallization inhibitors (nephrocalcin, citrate, glycosaminoglycans).
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