Stone Formation

Approximately 75% of all stones contain calcium as a major constituent, and 60% are composed of calcium oxalate. Most “spontaneous” stones are composed of calcium, oxalate, or phosphate crystals; others are due to uric acid, cystine, ammonium crystals, or phosphate crystals, or a combination of these substances (Table 541-1). The risk of stone formation increases in the presence of increasing concentrations of these crystals and is reduced with increasing concentrations of inhibitors. Renal calculi develop from crystals that form on the calyx and aggregate to form a calculus. Bladder calculi may be stones that formed in the kidney and traveled down the ureter, or they can form primarily in the bladder.

Stone formation depends on four factors: matrix, precipitation-crystallization, epitaxy, and the absence of inhibitors of stone formation in the urine. Matrix is a mixture of protein, non-amino sugars, glucosamine, water, and organic ash that makes up 2-9% of the dry weight of urinary stones and is arranged within the stones in organized concentric laminations. Precipitation-crystallization refers to supersaturation of the urine with specific ions composing the crystal. Crystals aggregate by chemical and electrical forces. Increasing the saturation of urine with respect to the ions increases the rate of nucleation, crystal growth, and aggregation and increases the likelihood of stone formation and growth. Epitaxy refers to the aggregation of crystals of different composition but similar lattice structure, thus forming stones of a heterogeneous nature. The lattice structures of calcium oxalate and monosodium urate have similar structures, and calcium oxalate crystals can aggregate on a nucleus of monosodium urate crystals. Urine also contains inhibitors of stone formation, including citrate, diphosphonate, and magnesium ion.

Clinical Manifestations

Children with urolithiasis usually have gross or microscopic hematuria. If the calculus is in the renal pelvis, calyx, or ureter and causes obstruction, then severe abdominal or flank pain (renal colic) occurs. Typically the pain radiates anteriorly to the scrotum or labia. Often the pain is intermittent, corresponding to periods of obstruction of urine flow, which increases the pressure in the collecting system. If the calculus is in the distal ureter, the child can have irritative symptoms of dysuria, urgency, and frequency. If the stone passes into the bladder, the child usually is asymptomatic. If the stone is in the urethra, dysuria and difficulty voiding can result, particularly in boys. Some children pass small amounts of gravel-like material. Stones can also be asymptomatic.

Diagnosis

Approximately 90% of urinary calculi are calcified to some degree and consequently are radiopaque on a plain abdominal film. However, many calculi are only a few millimeters in diameter and are difficult to see, particularly if they are in the ureter. Struvite (magnesium ammonium phosphate) stones are radiopaque. Cystine, xanthine, and uric acid calculi may be radiolucent but often are slightly opacified. Some children have nephrocalcinosis, which is calcification of the renal tissue itself. Nephrocalcinosis is seen most commonly in premature neonates receiving furosemide, which causes hypercalciuria, and in children with medullary sponge kidney.

In a child with suspected renal colic, there are multiple imaging options. The most accurate study is an unenhanced spiral CT scan of the abdomen and pelvis (Fig. 541-1). This study takes only a few minutes to perform, has 96% sensitivity and specificity in delineating the number and location of calculi, and demonstrates whether the involved kidney is hydronephrotic. However, the radiation exposure is high. An alternative is to obtain a plain radiograph of the abdomen and pelvis plus a renal ultrasonogram. These studies can demonstrate hydronephrosis and possibly the calculus on the radiograph; however, the calculus is not visualized on sonography unless it is adjacent to the bladder. Consequently, the clinician needs to carefully balance the risks of CT imaging against the lower sensitivity of the plain abdominal film plus sonography.

Figure 541-1 Noncontrast CT scan of the mid-abdomen in a male infant with cystinuria shows a left-sided calculus at the ureteropelvic junction with proximal hydronephrosis.

In 2008 the Society for Pediatric Radiology initiated the Imaging Gently initiative to educate providers on the risks of radiologic imaging in children and to encourage the use of limited imaging in children, particularly those with suspected urolithiasis (http://www.pedrad.org/associations/5364/ig/). This approach was also advocated by the National Quality Forum. In a child with an already-diagnosed calculus, serial plain x-rays or renal ultrasonography can be used to follow the status of the calculus, such as whether it has grown or diminished in size or has moved. If a child has a renal pelvic calculus, a ureteropelvic junction obstruction should be suspected. In some cases, it can be difficult to determine whether hydronephrosis in such a child is secondary to an obstructing stone or the ureteropelvic junction obstruction, or both.

Any material that resembles a calculus should be sent for analysis by a laboratory that specializes in identifying the components of urinary calculi.

Metabolic Evaluation

A metabolic evaluation for the most common predisposing factors should be undertaken in all children with urolithiasis, bearing in mind that structural, infectious, and metabolic factors often coexist. This evaluation should not be undertaken in a child who is in the process of passing a stone, because the altered diet and hydration status, as well as the effect of obstruction on the kidney, can alter the results of the study. The basic laboratory studies required are listed in Table 541-2, and the normal values for 24-hr urine collections are shown in Table 541-3. In children with hypercalciuria, further studies of calcium excretion with dietary calcium restriction and calcium loading are necessary.