Crossed fused ectopia with multicystic renal dysplasia

Crossed renal ectopia is the second most common fusion anomaly of the kidney after horse-shoe kidney. In 90% of patients, the crossed kidney fuses with the normal kidney. Multicystic dysplasia of the crossed (ectopic) kidney is relatively rare. Only a few isolated cases of crossed fused ectopia with multicystic dysplastic ectopic kidney have been reported. Though this is an unusual combination of rare renal anomalies, the radiologic features are characteristic enough to allow accurate preoperative diagnosis.  The entity can be suspected on antenatal ultrasonography (USG) and confirmed by postnatal USG and magnetic resonance urography (MRU). MRU is particularly useful to detect associated urinary tract abnormalities. We present one such rare case in an infant who was found to have a congenital renal anomaly on routine second trimester antenatal ultrasound.

Renal ectopia is a common congenital malformation. Crossed ectopia occurs when a kidney lies on the contralateral side. It is second only to horse-shoe kidney as the most common congenital malformation [1],  and has an incidence of 1 in 7000 autopsies [2]. Most crossed ectopic kidneys fuse with the ipsilateral kidney resulting in crossed fused ectopia. Multicystic dysplastic kidney is a common cause of abdominal mass in infancy [3]. Histologically such kidneys show primitive metanephric structures [4]. Association of crossed fused ectopia with multicystic renal dysplasia is rare and presents an interesting appearance on imaging. We report one such case of multicystic dysplasia in an ectopic left kidney.

A five-month-old asymptomatic male infant presented for follow up of an abnormality that was recorded on antenatal ultrasound performed at 20 weeks of gestation. The antenatal ultrasound had shown the absence of a kidney in the left renal fossa with an abnormal multicystic structure related to the lower pole of the right kidney, which was structurally normal. The amniotic fluid volume and remainder of the ultrasound scan were reported as normal. The postnatal ultrasound scan showed a similar abnormality (figure 1a and b).

A presumptive diagnosis of crossed renal ectopia with dysplastic ectopic kidney was made. Blood urea and serum creatinine were normal. A DMSA scan revealed two areas of renal tissue on the right side. No renal tissue was found elsewhere in the abdomen or pelvis. A renal dynamic scan revealed mild right hydronephrosis with a non-functioning ectopic kidney. A micturating cystourethrogram (MCU) was performed to rule out lower urinary tract abnormalities. It revealed a small incidental diverticulum near the left vesicoureteric junction. No vesicoureteric reflux was noted and the urinary stream was normal. Magnetic resonance urography (MRU) was done to depict the anatomy of the entire urinary tract. MRU confirmed absence of the left kidney with a multicystic mass associated with the lower pole of the right kidney which was normal in position, shape and size with minimal hydronephrosis of the orthotopic right kidney. The cysts were non-communicating (figure 2).

The ureters could not be visualised. A diagnosis of crossed fused multicystic dysplastic ectopic left kidney was made. The infant is well, and continues to be followed up.

Horseshoe kidney is the most common congenital renal anomaly. and crossed renal ectopia is the second most common, with a reported incidence of 1 in 7000 autopsies. It is reported to be more common in males and in most cases; the left kidney is crossed to the right [5]. The crossed kidney has an aberrant vascular supply [5], which can arise from either side of the lower abdominal aorta or from the common or external iliac arteries. During embryogenesis, normal renal development and position relies on interaction between the ureteric buds and nephrogenic cords. Crossed renal ectopia results when there is fusion of both the ureteric buds with only one nephrogenic cord [6].

Multicystic renal dysplasia is defined as a disturbed differentiation of nephrogenic tissue with the persistence of structures inappropriate to the age of the patient [7]. Multicystic renal dysplasia is characterised by the presence of cysts of varying sizes, pelvicalyceal and ureteric atresia and a non-functioning kidney.

Few cases of multicysticystic dysplasia of a crossed kidney (MCKD) have been previously reported [8-11]. Embryogenesis of this entity involves early in utero fusion of the two kidneys and variation in axial deviation of the fused renal elements. MCDK is believed to occur due to early ureteric obstruction caused by renal fusion. The underlying mechanism of ureteric obstruction may include pressure by the orthotopic kidney, abnormal vasculature, ureteric ectopia or ectopic ureterocele [12].

This rare and interesting entity should be considered in an infant who presents with a flank swelling caused by a multicystic mass caudal to the orthotopic kidney which may show variable degrees of hydronephrosis with absence of the contralateral kidney. Nussbaum et al [13] described the classical findings of multicystic dysplasia in crossed renal ectopy: a variable sized multicystic mass located adjacent to the lower pole of a hydronephrotic malrotated orthotopic kidney; ureteric displacement with or without dilatation; absence of the contralateral kidney and its renal artery. Hydronephrosis of an orthotopic kidney is frequently associated with this malformation and the most common cause for this is ureteric narrowing at the pelviureteric  junction. It occurs secondary to pressure caused by the caudal dysplastic kidney and the resultant ischaemia. The course of the ureter is altered; it may lie anterior to the mass, behind it or it may be deviated laterally.

Most of the features of MCDK are amenable to ultrasound visualisation, so a preoperative diagnosis can be made confidently based on ultrasound features. The characteristic ultrasound appearance of MCDK is the presence of multiple randomly distributed non-communicating cysts of variable sizes with the absence of peripheral parenchyma as well as an echogenic sinus complex. However, severe hydronephrosis can mimic the uncommon hydronephrotic form of MCDK [14]. With the advent of MRU, similar features may be shown exquisitely and with a more complete evaluation of both upper and lower urinary tract. The degree of hydronephrosis, presence of a ureter, and its course, are all useful information provided by MRU, without exposing infants to radiation from intravenous urography (IVU) or computed tomographic urography (CTU). A renogram is required to assess the function of the crossed kidney and a micturating cystourethrogram is indicated to identify abnormalities of the lower urinary tract.

Most multicystic kidneys undergo involution over the first few years of life and may disappear altogether [11, 15]. A number of studies have shown that MCDK can be reliably followed by ultrasonography. The majority of studies that followed children with MCDK with serial ultrasonography have demonstrated a high percentage of regression of the dysplastic kidney. This has led to a change in the management of multicystic kidneys [11, 15]. A low incidence of complications and a high incidence of spontaneous regression support conservative management in most cases (as was decided in our patient). Nephrectomy is reserved for masses that do not regress.

In conclusion, MCDK in an ectopic kidney should be suspected when antenatal USG shows a multicystic mass associated with a kidney and the absence of a contralateral kidney. The diagnosis can be made accurately with ultrasonography in most cases.. Mild and asymptomatic cases are followed conservatively with serial USG and those with severe disease are managed surgically.

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