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Figure 7 A six-month-old girl with left MCDK. (A) Coronal T2-weighted image showing a large multicystic structure in the left renal fossa with no visible normal parenchyma. A normal right kidney is present. (B) Coronal postcontrast, excretory phase MIP image showing a normally functioning right kidney with no functioning renal parenchyma on the left. Abbreviations: MCDK, multicystic dysplastic kidney; MIP, maximum intensity projection.

kidney is usually first detected at antenatal sonography (in more than 80% of patients). A specific diagnosis of MCDK is now most commonly made noninvasively in the newborn period based on the characteristic imaging findings on US combined with renal diuretic or cortical scintigraphy. Together, these two imaging tests demonstrate that the kidney is nonfunctioning and has been replaced by multiple noncom-municating cysts (31,57,62,63). On US, the cysts in MCDK are usually noted to be of different sizes, with the largest cyst rarely located centrally. This feature distinguishes MCDK from severe pelvocaliectasis. When the cystic areas are produced by renal collecting system dilatation, the largest centrally located "cyst" actually represents the dilated renal pelvis. The pelvis can be seen to be surrounded by smaller, radially arranged dilated calyces. The lack of a visible mantle of parenchyma around the MCDK cysts can also be used to differentiate this entity from pelvocaliectasis, where at least a thin rim of renal parenchyma is usually identified.

Patients with MCDK who are not diagnosed in the neonatal period present either later in childhood or as an adult with a palpable abdominal mass or have the diagnosis made as an incidental finding during imaging that is performed for an unrelated reason. On CT or MRI, the appearance is similar to that previously described for US. The affected kidney is readily visible, again being replaced by multiple cysts of different sizes that are separated by a small amount of abnormal-appearing soft tissue.

Several less common variants of MCDK are worth mentioning. In the hydro-nephrotic variant of MCDK (60), also referred to as pelvo-infundibular atresia (56), the kidney is nonfunctioning and contains one or more smaller peripheral cysts with a large central cyst. This appearance is due to the fact that in the hydronephrotic variant, the large central cyst does, in fact, represent the central portion of a dysmor-phic and dilated collecting system. In the segmental MCDK, only one moiety of a partially or completely duplicated collecting system is affected (64,65). When the multicystic dysplastic segment of the kidney involutes, recognition that there is a duplex collecting system is difficult in the absence of a tell-tale ureterocele or ectopic ureter (65). Bilateral multicystic renal dysplasia is incompatible with life and is accompanied by severe oligohydramnios, pulmonary hypoplasia, and other characteristics of Potter syndrome (36,37,53).

The tendency for prenatally diagnosed MCDK to involute, both before and after birth, has been well documented. Fluid in the cysts is frequently absorbed, and the cysts collapse (62,63,66). Eventually, there is only a tiny, solid remnant. Because the kidney is not functioning, it will not be visible on either urography or renal scintigraphy, and the small amount of residual dysplastic parenchyma is often undetectable on US. In this situation, confusion with unilateral renal agenesis is likely if the patient's first imaging examination is performed only after the cystic kidney has involuted (55). In circumstances in which differentiation between these disorders is clinically valuable, MRI is the most reliable imaging modality for demonstrating the small dysplastic remnant of an otherwise occult involuted MCDK (13,31,67).

Associated Anomalies in Renal Agenesis and MCDK

MRI and MRU can be of particular value in the evaluation of children of both genders who have renal agenesis or MCDK, because these studies can also detect the various lower genitourinary anomalies that are occasionally associated with these entities (4,67,68).

In girls, both unilateral renal agenesis and MCDK are occasionally associated with uterovaginal anomalies. Although a wide spectrum of anatomic variations have been reported in this situation, the classical description is of uterovaginal duplication, with unilateral vaginal obstruction and hemihydrocolpos on the same side as the absent or involuted kidney (68). An ectopic ureter to the obstructed uterovaginal unit is occasionally present, in which case there is also usually a small renal remnant, either in the renal fossa or elsewhere in the abdomen or pelvis (4,67,68).

In boys, both unilateral renal agenesis and MCDK can be associated with an ipsilateral seminal vesicle cyst, often with an atretic, ectopic ureter (Fig. 8) (69). Occasionally, the ipsilateral vas deferens and testes are absent.

Duplications and Other Anomalies of the Renal Collecting System and Ureter

Duplication anomalies of the renal collecting system and ureter are quite common (1 in 160 live births), with a strong familial association (36,37,54,70,71). Partial duplications, including bifid collecting systems in which the two ureters join before inserting into the bladder at a single orifice, are more common than are complete duplications in which the duplicated ureters have separate distal orifices. In complete duplications, the upper pole ureter inserts medial and distal to the lower pole ureter.

Most duplications of the renal collecting system and ureters, either partial or complete, are not complicated by collecting system or ureteral dilatation, reflux,

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