Pediatric urolithiasis: Update and practical pointers

Urolithiasis occurrence is increasing in adults in the United States, with nearly 1 in 11 adults having a stone some time in their life (J Urol 2014; 192:316-24). Recurrence rates in adults appear to show that about 50% of patients who suffer from stone disease will generally experience another stone event within 10 years of their first stone event. Women appear to have an increasing incidence of urolithiasis so that their risk of stone formation is almost equal to the prevalence of stone formation in men.

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In the 1950s through the 1970s, the stone incidence in children was only about 1%-2% of the incidence in adults. Now, stone occurrence in children appears to have increased to almost 10%. In the U.S., the rate of stone incidence in adolescents went from 18 per 100,000 in 1999 to 57 per 100,000 in 2008. Adolescent girls (12-18 years old) appeared to have a higher rate of occurrence than the other groups studied, although the overall sex distribution in all age ranges was about the same. Hospital admission rates for urolithiasis increased to 1 in 685 admissions in the 2002 to 2007 time period.

This article provides an update on the risk factors for stone disease and its presentation in children, and offers practical tips on its evaluation, treatment, and prevention.

Risk factors

Pediatric stone disease has different geographic and racial prevalence rates. Stones are very common in the Middle East, Pakistan, India, and Southeast Asia. Children in developing countries tend to have more bladder calculi than calculi elsewhere in the urinary tract. Bladder stone composition in these children consists predominantly of ammonium acid, uric acid, and urate, likely due to the relatively low availability of dietary phosphate in these countries (Pediatr Clin N Am 2012; 59:881-96).

Children of African descent worldwide rarely have stones, while in the U.S., Caucasian children are more likely to suffer from urolithiasis, especially if they are from the Southeast U.S. Stones are more likely to be found in the kidneys and ureters than in the bladder in American children.

Previously, most children who developed kidney stones also had anatomic abnormalities that increased their likelihood to develop stones, such as obstruction of the ureter or renal pelvis, exstrophy, or static drainage with horseshoe kidney or megaureter. Now, between 40% and 50% of children with urolithiasis have metabolic abnormalities identified, while only 30% of stones are associated with genitourinary abnormalities. Most likely, the children with anatomic abnormalities and urolithiasis have concomitant metabolic risk factors.

The most common abnormalities found have been hypercalcuria and hypocitraturia. Other metabolic problems seen but less frequently in children are hyperoxaluria, cystinuria, and hyperuricosuria. The most common stones found in the U.S. are calcium oxalate (40%-65% of all stones), calcium phosphate (14%-30%), magnesium ammonium phosphate (struvite, 10%-20%), cystine (5%-10%), and uric acid (only 1%-4%). In children, increased uric acid in urine promotes calcium oxalate stone formation, whereas uric acid stones are more commonly seen in adults.

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Stones form when there is a supersaturation of these minerals. Promotion of crystallization through low total urine volume, increased concentrations of stone-forming ions, and decreased concentrations of inhibitors of crystallization have been implicated in the increased rate of stones in children. Children notoriously fail to have adequate fluid intake, particularly water intake. They also are more likely to drink beverages that increase their risk of stone formation, such as dark, caffeinated, sugary sodas.

In the past, by contrast, high milk intake helped children maintain adequate calcium intake to prevent stones. Appropriate calcium intake decreases the absorption of oxalate in the intestines, preventing increased oxalate excretion by kidneys that potentiates stone formation.

Increased sodium intake by children in the U.S. is also rising above recommended dietary allowances. This increase in body sodium increases the excretion of urinary calcium, which promotes supersaturation of calcium and subsequent stone formation (Pediatr Clin N Am 2012; 59:881-96; JAMAPediatr 2015; 169:964-70).

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Increased obesity in children may also be putting them at risk for stone formation, as is being seen in adults. Obesity causes a decrease in urine pH and increased excretion of sodium, phosphorous, and oxalate, increasing the rate of stone formation. While the rate of childhood obesity tripled from 1980 to 2002, obesity as a cause of increased stone rate in children has not been as well founded as it has been in adults.

There is some speculation that increases in stone occurrence seen in adolescent girls may be hormonally related, as their estrogen increases with the onset of puberty. It has been found that hormone replacement therapy in postmenopausal women causes a decrease in calcium excretion and an increase in citrate secretion, but postmenopausal women have an increased stone rate compared with premenopausal women, possibly due to increased calcium oxalate supersaturation. Thus, increased estrogen levels in adolescent girls and postmenopausal women on supplements may explain the higher stone prevalence in these groups.

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Other at-risk groups include children with seizure disorders who are placed on a ketogenic diet to prevent seizures. In this population, anti-seizure medications (topiramate and zonisamide) are known to potentiate stone formation by increasing hypocitraturia. Additionally, these children frequently are fluid restricted or cannot adequately hydrate themselves to offset citrate loss.

Stone presentation in older children is similar to adult stone presentation: flank pain, abdominal pain, nausea, and vomiting are common. Younger children do not always present this way. In fact, only 10%-14% of younger children present with typical acute renal colic symptoms. They tend to have vague symptoms and less localized pain but may present with hematuria or urinary tract infection.

Treatment and prevention

Treatment of children with renal stones is actually similar to that of adults. Many children will pass similarly sized stones as adults. Fifty percent of children will pass stones 4-5 mm in size regardless of the child’s size. Increased hydration and pain management with either nonsteroidal anti-inflammatory drugs (NSAIDs) or, rarely, narcotics help children manage their symptoms while passing a stone.

Alpha-adrenergic blockade (tamsulosin) has been used in children as well. The use of stone expulsion treatment in passing distal ureteral stones appears to have success similar to that seen in adults and the medications are well tolerated by children, although current studies are limited.

Long-term prevention includes increased fluid, particularly water intake, up to 2-2.5 liters/day in adolescents or more in cystine stone formers. Specific treatments depend on obtaining urine and serum testing for causative factors in stone formation. This is somewhat difficult in the non-toilet-trained child, where obtaining a 24-hour urine collection would require an indwelling catheter and bag collection. Spot urine tests for causative factors have been shown to be helpful in determining the specific metabolic abnormality that the child may have and may help direct preventive treatment.

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Decreasing high animal protein intake and sodium intake are important in any dietary recommendations for stone prevention, as is appropriate intake of potassium, calcium, and magnesium, which are protective against oxalate composite stone formation. Potassium citrate (2-4 mEq/kg/day) is also a mainstay in preventive treatment in children at risk for stone formation due to underlying metabolic abnormalities (Pediatr Clin N Am 2012; 59:881-96; JAMA Pediatr 2015; 169:964-70).

Radiologic evaluation for urolithiasis in children is predicated on ALARA (as low as reasonably achievable) techniques. Adult patients are generally evaluated with high-radiation testing with computerized tomography for diagnosis, treatment, and surveillance. These high-radiation techniques are rarely needed in children unless there is some uncertainty as to diagnosis or conflicting findings on low-radiation techniques that are generally used.

It is more common to obtain an abdominal flat plate (KUB) and ultrasound imaging of the kidneys and bladder, as these tests will find most stones because the majority of stones in children are radiopaque. Furthermore, most large stones in the kidney or ureter near the bladder that may need interventional treatment will be found on these studies (JAMAPediatr; 169:964-70). Smaller stones, as mentioned, tend to pass spontaneously and may be inferred if not directly seen by the presence of hydronephrosis or hydroureteronephrosis in ultrasonography (figures 1, 2, and 3).

Surgical techniques

Although many stones will pass spontaneously, surgical techniques used in removing stones in adults have been used successfully in children. This includes ureteroscopy with or without lithotripsy, percutaneous nephrolithotomy with or without lithotripsy, and extracorporeal shock wave lithotripsy (ESWL) (JAMA Pediatr 2015; 169:964-70). Open and minimally invasive procedures can also be considered but are currently less often used in both children and adults.

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Although ESWL is less likely used in adults due to poor stone-free rates, renal stones in children, including fairly large stones, tend to break up nicely in children with reasonable stone-free rates. So this technique can still be reasonably considered for appropriate stone management in children (figures 4 and 5).

Summary

Urolithiasis is becoming much more prevalent in children of all ages and needs to be treated aggressively with prevention. Increased hydration, limited salt intake, and improved dietary intake as well as the previously discussed spot urine or 24-hour urine collection for stone risk as an appropriate evaluation of metabolic risks are important steps in prevention. Limited use of ionizing radiation is also imperative, as children may have more lifetime risk of radiation exposure. Most children will not need surgical intervention for their stone disease, but the techniques used in adults are equally applicable in children.