A 13-year-old girl presents with pain in the right flank and mid-abdomen. Several family members have had kidney stones. Her urinalysis shows blood but no signs of infection. A pregnancy test is negative. Abdominal x-ray reveals bilateral stones (Figure 70-1). A CT shows a right ureteral stone and a non-obstructing stone in the left kidney (Figures 70-2 and 70-3). She successfully passes and catches the symptomatic stone. Stone analysis shows calcium oxalate. A metabolic workup shows idiopathic hypercalciuria as the cause of her stones.

A kidney stone is a solid mass that forms when minerals crystallize and collect in the urinary tract. Kidney stones can cause pain and hematuria, and may lead to complications such as urinary tract obstruction and infection.

Kidney stone, nephrolithiasis, renal calculus, renal stone, urinary tract stone, ureterolithiasis, urolithiasis.

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  The prevalence of kidney stones among children and adults in the US is increasing.¹ Part of this increase may be due to improvements in imaging techniques.^2,3 Although pediatric data are incomplete, children appear to be about 1/10 as likely to develop stones as adults.

  
  
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  Kidney stones affect children of all ages but are most prevalent among adolescents, who are also more likely than younger children to present with symptomatic ureteral stones.^3,4

  
  
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  Boys and girls overall have similar rates of stone formation, although prevalence varies by age, type of stone, and geographic region.^5,6

  
  
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  African-American children have a lower rate of kidney stones than white children.^4,7

  
  
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  Genetic, metabolic, and anatomic disorders are the main causes of stones in children. Children are at higher risk of recurrent stones and subsequent renal dysfunction than adults for whom environmental and dietary causes are more common.⁴ Obesity and weight gain increase the risk of stone formation in adults;⁸ the effects of pediatric obesity are being studied.

  
  
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  Calcium oxalate and calcium phosphate stones are the most common in children, occurring in about 90 percent of cases. Struvite (magnesium ammonium phosphate) stones occur in about 5 percent of cases and are becoming less common. About 2 percent of pediatric stones are cystine stones. Uric acid stones are less common in children than adults, occurring in less than 1 percent of cases. Medication stones and other types of stones are rare.⁶

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  Kidney stones form when there is supersaturation of otherwise soluble materials, usually from increased excretion of these compounds or from dehydration. Idiopathic hypercalciuria is the most common abnormality found in adolescents. Low urinary citrate can increase the risk of calcium stone formation because citrate in the urine binds calcium and impedes stone formation in several other ways. Urine pH can affect stone formation: calcium phosphate and cystine stones form in more alkaline urine (pH >7), while uric acid stones form in acidic urine (pH <5.5).

  
  
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  Struvite stones are caused by infection with urea-splitting bacteria, mainly Proteus.

  
  
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  Cystine stones occur in patients with an inherited defect of dibasic amino acid transport. These stones can be pure cystine or cystine mixed with calcium oxalate.

  
  
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  Uric acid stones form in patients with hyperuricemia from gout, myeloproliferative disorders, chemotherapy, or Lesch-Nyhan syndrome. Acidic urine due to a ketogenic diet or chronic diarrhea can increase the risk of uric acid stones.

  
  
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  Struvite, cystine, and uric acid stones can grow large, filling the renal pelvis and extending into the calyces to form staghorn calculi (Figure 70-4).

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  Many kidney stones occur in patients with no known risk factors. However, a 24-hour urine collection often shows a low urine volume from inadequate fluid intake.

  
  
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  The risk of forming calcium stones has been shown in adults to be increased by obesity,⁸ diabetes mellitus, metabolic syndrome, and by diets high in animal protein, salt, and oxalate-containing foods; these factors are being studied in children.

  
  
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  Contrary to popular belief, calcium in the diet does not lead to calcium stones; in fact, dietary calcium can help prevent calcium oxalate stones by trapping oxalate in the GI tract.

  
  
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  Patients with neurogenic bladder, anatomic anomalies causing poor urinary drainage, or long-term catheters are at risk for Proteus urinary tract infections and struvite stones.

  
  
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  Cystine stones are found in families with an autosomal recessive defect in a protein that transports dibasic amino acids in the kidneys, increasing the concentration of insoluble cystine in the urine. All homozygotes and some heterozygotes are at risk for cystinuria and stones.

  
  
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  Uric acid stones are most likely in patients who have hyperuricemia along with acidic urine from chronic diarrhea, diabetes mellitus, or a ketogenic diet.