Abdominal X-ray demonstrating metallic beads within the bladder
Renal, endocrine, and metabolic disorders can lead to the development of crystallized material in the urinary system ultimately causing stones (See Chapter 8). Stones are classified based on their chemical components. Most stones in children contain calcium, most commonly calcium oxalate, calcium phosphate, or a combination thereof. Uric acid, magnesium ammonium phosphate (struvite), and cystine are some of the lesser common causes of stones in the pediatric population. Struvite stones are associated with infection and are much less common in the pediatric population than they were a decade ago. Cystine stones form in patients with cystinuria, an autosomal recessive defect that impairs renal reabsorption of the amino acids cystine, ornithine, lysine, and arginine. Elevated urinary cystine leads to the formation of cystine stones.
Inborn errors of metabolism can cause significant but variable stone production in pediatric patients. Patients with the rare diagnosis of primary hyperoxaluria form calcium oxalate monohydrate stones while those with xanthinuria produce xanthine stones (which, interestingly, are not able to be seen on a radiograph – they are radiolucent). Another stone-forming syndrome is Lesch-Nyhan disease (juvenile gout). These patients have a deficiency of an enzyme called hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and produce uric acid stones.
As we have explored, the formation of urolithiasis is a multifactorial process influenced by the patient’s metabolic background/predisposition along with environmental conditions/exposures (See Chapter 9).
Stones form via super-saturation of the urine and crystallization of stone-forming (lithogenic) components, which is followed by growth of the crystals. A nidus for crystal precipitation (i.e., the urothelial surface properties that affect crystal retention) occurs when the microscopic crystalline structure of one crystal is similar to another crystal and the second crystal grows on the first.
Important risk factors for pediatric urolithiasis include habitually low urine volumes and high urine content of stone-forming components (e.g. calcium, uric acid and oxalate). Urine pH also affects stone formation – uric acid and cystine are less soluble in acidic urine while struvite and calcium phosphate are less soluble in alkaline urine.
In addition to lithogenic components, urine also contains substances that are inhibitory to stone formation, including citrate, magnesium, glycosaminoglycans, and osteopontin. Citrate is the only inhibitor that can be increased in the urine, having therapeutic applications.
More than 50 % of children diagnosed with stones have a metabolic risk factor. The most common risk factor is hypercalciuria, followed by hypocitraturia, hyperuricosuria, and hyperoxaluria, respectively.
Dietary influences are hypothesized to be a large contributor to the increase in the incidence of urolithiasis in the pediatric population. Inadequate fluid intake leading to low urine volume is the most significant cause coupled with an increased salt intake, which increases calcium excretion in the urine.
Congenital anomalies of the urinary tract causing obstruction and stasis include ureteropelvic or ureterovesical junction obstruction and sometimes horseshoe kidney. The stasis of urine can predispose the child to stone formation.
Urinary tract infections with urea-splitting microorganisms are a decreasing cause of stones (namely struvite as discussed earlier) in children as the diagnosis and management has greatly improved over the past decade.
Exposure to particular drugs including topiramate, phenytoin, loop diuretics, acetazolamide, and high dose vitamin C and D increase calcium stone formation. Excessive consumption of calcium (milk alkali syndrome) as well as melamine-contaminated powdered formula (seen in China in 2008) have also been shown to be risk factors.
The classic symptoms of renal colic are less common in younger children, who often present with irritability, abdominal pain and/or hematuria. Older children and adolescents present with more localizing symptomatology, as in adults. Urolithiasis should always be on the physician’s radar when evaluating a child with abdominal pain.
During the acute episode, assessment with urinalysis, urine culture, blood work and imaging are necessary. In the pediatric patient, radiation exposure should always be limited. Evaluation with kidney and bladder ultrasound is the first line imaging choice to assess the urinary tract in children. This modality will demonstrate any evidence of swelling of the collecting system and/or ureter (hydroureteronephrosis, hydroureter, or hydronephrosis) indicating an acute obstruction. Urine flow from the ureters into the bladder can also be evaluated with ultrasonography, which will show ureteral jets as urine enters the bladder from each ureter in an unobstructed system. Urolithiasis may be visualized utilizing ultrasonography, although helical computed tomography (CT) without intravenous or oral contrast, called a spiral CT scan, is a much more sensitive study compared to ultrasound. There is no radiation exposure with ultrasound, as compared to CT. When CT is necessary, the imaging protocol should always be designed to decrease the amount of radiation exposure while ensuring adequate imaging quality, a technique known as ALARA (“As Low As Reasonably Achievable”). Abdominal and pelvic radiograph can be limited for detecting calculi in the pediatric patient due to overlying bowel gas.
There are many options for the treatment of urinary tract stones in the pediatric patient. The choice for management is determined based on the characteristics of the stone, the chance for spontaneous passage, potential complications of intervention versus observation, and the chance for stone-free success.
Watchful waiting or observation can be used to manage asymptomatic or small calculi in the kidney, ureter, or bladder. Periodic imaging with renal and bladder ultrasound to ensure that the stone(s) are not enlarging is important. Additionally, small ureteral stones may be observed as they pass in the acute phase. Adequate hydration and pain control may necessitate an inpatient hospital admission. Medical expulsive therapy utilizing alpha-blockers such as tamsulosin dilate the distal ureter and can facilitate passage of the stone into the bladder. Persistent pain, intractable nausea and emesis, and/or fever prompt intervention for an obstructing ureteral calculus. This intervention in the acute setting is a drainage procedure to allow the kidney to be unobstructed – using a ureteral stent placed endoscopically under anesthesia, or a percutaneous nephrostomy tube.