Multicystic dysplastic kidney (MCDK) is the second most common cause of a flank mass in the newborn (Kaplan, 1998). MCDK was first described by Schwartz in a 7-month-old child in 1929 as a “bunch of grapes” replacing the kidney with associated ureteral atresia. Spence (1955) subsequently described MCDK and distinguished it from other cystic renal diseases.

MCDK is an extreme form of dysplasia associated with an atretic ureter. There is oftena lack of normal reniform shape, with multiple large cysts and little stroma, giving the “bunch of grapes” appearance. MCDK is typically unilateral, but bilateral MCDKs have been reported in stillborn infants with oligohydramnios and Potter syndrome facies. In unilateral MCDK, the contralateral kidney is normal but at high risk for other abnormalities (Greene et al., 1971; DeKlerk et al., 1977). The contralateral kidney may be affected by ureteropelvic junction obstruction, vesicoureteral reflux, or obstructive megaureter.

MCDK often presents as an asymptomatic abdominal mass in the neonate (Langino and Martin, 1958). However, currently approximately 85% to 100% of MCDKs are detected by prenatal sonographic examination (Manzoni and Caldamone, 1998; Kuwertz-Broeking et al., 2004). MCDK was often discovered during the course of investigation of other associated congenital anomalies. In particular, esophageal atresia and cardiac anomalies may be associated with MCDK. Other associated nongenitourinary anomalies include anencephaly, hydrocephalus, spina bifida, cleft palate, microphthalmia, duodenal stenosis, tracheoesophageal fistula, and imperforate anus. The pathogenesis of this disorder is poorly understood but is thought to involve failure of the ureteric bud to integrate and branch appropriately into the metanephron during the development of the kidneys (Kitagawa et al., 2000).

The incidence of MCDK is estimated at between 1 in 1000 and 1 in 4300 livebirths (Sanders, 1996; Kaplan, 1998). The incidence of bilateral MCDK is estimated at 1 in 10,000 livebirths (Resnick and Vernier, 1981). The male-to-female ratio is 2:1 in contralateral MCDK (Sanders and Hartman, 1984). No estimates are available for the prenatal incidence of MCDK, but it may be more common than the incidence figures above would indicate. It has been also suggested that MCDK is the major cause of apparent unilateral renal agenesis in adults (Aslam and Watson, 2006) as a result of progressive involution.

The sonographic features of MCDK include a cystic paraspinal flank mass. The cysts are usually of various sizes, are distributed along the periphery, do not appear to communicate with each other, and are absent of normal renal sinus echos and normal renal parenchyma (Stuck et al., 1982) (Figure 78-1). The large cysts often distort the kidney shape and no normal renal parenchymal tissue is seen (Mahony, 1994). The size and shape of an MCDK may change significantly during the course of gestation (Hashimoto et al., 1986). The cysts may regress or enlarge and then, subsequently, regress in size. Once the condition is identified, the kidney opposite to the MCDK becomes of paramount importance because of the 40% incidence of contralateral anomalies (Kleiner et al., 1986). Preservation of normal amniotic fluid volume suggests normal renal function in the contralateral kidney. However, oligohydramnios and absence of bladder filling suggest a lethal fetal renal disease, which occurs in 30% of fetuses with MCDK. In two-thirds of the cases of oligohydramnios there is bilateral MCDK, and in the remaining third there is renal agenesis.

The development of oligohydramnios in bilateral MCDK may occur as early as 12 weeks of gestation (Stiller et al., 1988). A first trimester ureteropelvic junction obstruction may be observed to progress during the second trimester to MCDK. In approximately 10% of cases of MCDK, the contralateral kidney shows signs of hydronephrosis, often from obstruction at the ureteropelvic junction (Mahony, 1994).

The main differential diagnosis of MCDK includes ureteropelvic junction obstruction, adult or infantile forms of polycystic kidney disease, trisomy 13, and Meckel–Gruber syndrome (D’Alton et al., 1986). Perhaps the most difficult distinction is between ureteropelvic junction obstruction (see Chapter 81) and the hydronephrotic form of MCDK (Rizzo et al., 1987). The sonographic detection of renal parenchymal cysts that communicate with the renal pelvis suggests the diagnosis of ureteropelvic junction obstruction (Romero et al., 1988). In adult polycystic kidney disease (see Chapter 79), the cysts are randomly distributed, in contrast to MCDK, in which the cysts tend to be at the periphery (Sanders, 1996). In Meckel–Gruber syndrome, the cysts are small, uniform, and scattered throughout the kidney. In contrast to MCDK, the kidneys in infantile polycystic kidney disease are very large and echogenic and discrete cysts are not visible (see Chapter 79). In trisomy 13, the kidney is brightly echogenic and has small cysts scattered in the renal parenchyma. The appearance of MCDK should be distinguished from Wilms’ tumor or hamartoma, which may have areas of cystic necrosis (Aslam and Watson, 2006).

The natural history of MCDK is becoming better understood, based on studies of serial sonographic examinations of fetuses with MCDK. Among 64 fetuses diagnosed prenatally with MCDK at the Hospital for Sick Children in London, 33% were diagnosed at 18 to 20 weeks of gestation, 44% between 20 and 30 weeks of gestation, and 33% between 30 and 40 weeks of gestation (Manzoni and Caldamone, 1998). These authors noted that 36% of sonographic examinations were reported as normal on the 18- to 20-week scans, which may indicate that MCDK is easier to diagnose in the latter half of gestation.