Classification

Pathophysiology

In children, stone disease is less common than it is in adults. Stones are not common in children, but the incidence of stones in children does seem to be increasing.

Although there are a number of contributing factors, the central concept in urolithiasis is urinary supersaturation. When a solute is added to a solvent, it dissolves until a certain concentration is reached, at which point the solution is saturated. Beyond this point, the solute may form crystals in the solution, and those crystals may aggregate. Supersaturation occurs when this point is surpassed and crystal precipitation occurs in the urine in the form of nucleation, the basis of urinary stones. Crystallization and aggregation must also occur for stones to form. These processes are influenced by the presence of inhibitors and promoters. Citrate, magnesium, pyrophosphate, glycosaminoglycans, nephrocalcin, and Tamm-Horsfall proteins are inhibitors of crystallization and aggregation. Bacterial infections and anatomic abnormalities such as obstruction or stasis may encourage crystal aggregation and retention, thus increasing the risk of clinically significant urolithiasis.

u Basically stones form because there is too much of the ingredients of the stone and not enough water in the urine. This can occur either because there is an abnormally high amount of stone-forming material in the urine, or the urine is too concentrated because of dehydration.

Renal, urologic, endocrine, and metabolic disorders may lead to the development of crystallized material in the urinary system.

Multiple studies of urolithiasis in children have shown metabolic abnormalities in up to 92% of patients, with hypercalciuria and hypocitraturia the most common.

The prevalence of infection-related stones is especially high in children younger than 6 years. Approximately 12% of children with urolithiasis have no identifiable risk factor (idiopathic).

Urinary stone disease can, therefore, be classified as metabolic, anatomic, infectious, or idiopathic, on the basis of underlying factors.

Stones can also be classified on the basis of their location in the upper urinary tract (kidneys and ureters) or lower urinary tract (bladder and urethra) and as symptomatic or asymptomatic. These classifications may influence the decision-making process with regard to treatment options.

Stones are most often classified into groups based on their chemical components. Materials that produce stones in the urinary tract of children include (with the approximate frequency):

1. calcium with phosphate or oxalate – 57%

2. magnesium ammonium phosphate (struvite) – 24%

3. uric acid – 8%

4. cysteine – 6%

5. combinations of the preceding items (mixed) – 2%

6. purine derivatives

7. drugs or their metabolites (eg, phenytoin, triamterene).

· endemic - 2%

· mixed - 2%

· and other types - 1%