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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.
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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.
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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.
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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.
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Types of Urinary
Tract Stones
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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.
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Hypercalciuria is found in as many as 4% healthy children...