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Abdominal masses are frequent in the neonate, and nearly two-thirds are renal in origin. Most are benign, representing hydronephrosis, multicystic dysplastic kidney (MCDK), polycystic kidney disease, or congenital mesoblastic nephroma. Less commonly, a flank mass may be caused by a malignant renal lesion, such as a Wilms tumor. The initial diagnosis may be made on prenatal ultrasonography (US), found in an asymptomatic infant on physical palpation, or discovered as a consequence of postnatal imaging performed for hypertension, hematuria, urinary sepsis, or difficult feeding in the new infant.1 Fetal ultrasonography may demonstrate not only the renal mass but also associated findings, such as oligohydramnios, polyhydramnios, renal cysts, hydronephrosis, and bladder wall thickness, which may aid in developing a more specific diagnosis in the antenatal period.2 Early recognition of renal masses may allow timely treatment of congenital renal anomalies that would otherwise be missed because of their asymptomatic nature.

The postnatal examination will be the first opportunity to palpate an abdominal mass and evaluate for any associated genitourinary or systemic anomalies. Examination of the external genitalia and lumbosacral region is imperative because malformations of these organ systems are increased in the setting of renal and urinary anomalies. A postnatal abdominal US should be performed to confirm the renal origin of the mass, define whether it is solid or cystic, and better delineate the collecting system anatomy. More specific postnatal imaging, such as voiding cystourethrogram (VCUG), nuclear medicine renal scans, and magnetic resonance imaging (MRI) will be guided by the suspected diagnosis.

Cystic Masses

Cystic masses of the neonate may include hydronephrosis, MCDK, and autosomal dominant and autosomal recessive polycystic kidney disease. The most common cause of a renal mass in the newborn is hydronephrosis. This may be caused by ureteropelvic junction (UPJ) obstruction, primary megaureter, vesicoureteral reflux (VUR), ureterovesical junction obstruction, or bladder outlet obstruction. Follow-up imaging with repeat renal bladder US and a VCUG should be performed to delineate the etiology of the hydronephrosis. The recommended timing of the first postnatal US has been a controversial subject. It has been posited that an US performed within the initial 24-48 hours of life may underestimate the degree of hydronephrosis because of relative dehydration3 (Figure 100-1).

FIGURE 100-1

A, Newborn with left antenatal hydronephrosis imaged with ultrasound on day one of life with minimal hydronephrosis (Society for Fetal Urology Grade 1).B, Same newborn with left antenatal hydronephrosis imaged with ultrasound at one week of life with moderate hydronephrosis (Society for Fetal Urology Grade 3).

Multicystic dysplastic kidney is the second-most-common cause for a cystic renal mass in the newborn. More than 50% of cases are detected antenatally. It is typically a unilateral cystic renal mass, more commonly found in male ...

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