Chapter 475

Urolithiasis is the term used to describe the presence of a stone or a calculus anywhere in the urinary tract. Nephrolithiasis is the term generally used when calculi are found in the kidneys; it should not be confused with nephrocalcinosis, which is the deposition of calcium in the tubulointerstitial regions of the kidneys. The prevalence of urolithiasis varies widely, depending on geographic locations. Although uncommon in some countries, it remains an important childhood diagnosis because it is often associated with morbidity and high rates of recurrence. A thorough evaluation should be done to identify specific metabolic defects or factors predisposing to stone formation.

### Epidemiology

The true incidence of stones in children is unknown. Many children probably remain undiagnosed, and no recent epidemiologic studies have been published in North America. Earlier reports showed nephrolithiasis responsible for 1:1000 to 1:7600 pediatric hospital admissions in the United States.1,2 Stones are generally more common in white than African American and Asian children and in males.3 In North America, most stones are found in the kidneys. Bladder stones occur in less than 10% of affected children and are usually related to urologic abnormalities.

### Pathophysiology

The process of stone formation begins with the crystallization of stone-forming solutes, especially calcium, oxalate, and uric acid. These aggregate with other crystals and adhere to the renal tubule cells, with growth of large crystals that can detach and obstruct the urinary tract.4 The crystallization and aggregation of solutes depend on urinary solute concentration, urinary pH and presence of inhibitors in the urine. Increased solute concentration (resulting from increased urinary excretion or low urinary volume) and low levels of natural inhibitors of stone formation, which include citrate, magnesium, and pyrophosphate, predispose to stone formation.

### Etiology

Metabolic or anatomic predisposing factors are identifiable in the majority of patients. In a study from Argentina, 84.4% of their 90 patients with kidney stones were found to have a metabolic risk factor defined as hypercalciuria, hyperoxaluria, hypocitraturia, hypomagnesuria, or hyperuricosuria. The most common metabolic risk factors were idiopathic hypercalciuria in 40% (alone or in combination) and hypocitraturia in 37.8% (alone or in combination).5 In a study from Canada, 67% of affected patients had identifiable anatomic or metabolic predisposing factors for stone formation.6 Infection is often associated with nephrolithiasis and is causative in struvite calculi.

### Types of Urinary Tract Stones

Calcium phosphate and calcium oxalate are the most common stones and can result from hypercalciuria, hyperoxaluria, hypocitraturia, in combination, or alone, as discussed later in this section. Hyperuricosuria may also contribute to formation of calcium stones by providing a nidus for stone formation, or may form urate stones. Less common types of stones include struvite stones and cystine stones.

#### Hypercalciuria

Hypercalciuria is found in as many as 4% healthy children...

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