**3.2.2 Urethral atresia, stenosis, or hypoplasia**

Although PUV is the most common cause of congenital lower urinary tract obstruction postnatally, detailed postmortem analysis of fetuses with megacystis and hyperechogenic kidneys showed that isolated severe lower urinary tract obstruction before 25 weeks' gestation was as likely to be due to urethral atresia or stenosis as PUV (Robyr et al., 2005). Urethral atresia may arise in association with complex collections of other genitourinary and/or gastrointestinal anomalies. Moreover, urethral atresia may be a cause of bladder outlet obstruction in females whereas PUV is not. Urethral atresia is incompatible with life unless an alternative connection between the bladder and the amniotic sac is present. Prenatal surgical decompression has been performed to relieve this obstruction, although a spontaneous fistula or patent urachus may also provide the necessary bladder drainage

Although less common than UPJ or UVJ obstructions, congenital obstructions can arise elsewhere within the kidney or along the course of the ureter. Examples include infundibular or infundibulopelvic stenosis, which may be idiopathic or associated with Beckwith-Wiedemann or Bardet-Biedl syndrome; anomalous ureteric position, such as a

There are several congenital anomalies that result in chronic lower urinary tract obstruction, the most familiar being posterior urethral valves (PUV). Other inborn causes of lower urinary tract obstruction include urethral atresia, stenosis or hypoplasia; anterior urethral valves; urethral diverticula; and cloacal anomalies. Congenital lower urinary tract obstructions put both kidneys at risk for abnormalities in fetal renal development and impaired renal function, and may be associated with oligohydramnios and pulmonary hypoplasia. Congenital lower urinary tract obstructions may also result in bladder dysfunction, ultimately leading to a secondary functional obstruction that may require

PUV, also known as *congenital obstructing posterior urethral membrane*, is found in 1 out of 5000-8000 live births, and occurs only in males (King, 1985). Oligohydramnios is a common consequence, and renal dysplasia may also be present. Using postnatal fluoroscopic VCUG, the gold standard for diagnosis of PUV and other lower urinary tract obstructions (Riedmiller et al., 2001), the classic finding for PUV is a linear filling defect in the column of radiocontrast filling a markedly dilated posterior urethra. However, this distinct linear radiolucent band corresponding to the "valve" is not always seen, because the obstructing membrane can become distended and take on a more sail-like or windsock appearance, as shown in Figure 4. Secondary VUR is found in 25-50% of PUV cases (Agarwal, 1999). In a subset of patients, unilateral VUR may provide a "pop-off" effect, whereby renal tissue and function on the non-refluxing side is preserved at the expense of severe dysplasia and

Although PUV is the most common cause of congenital lower urinary tract obstruction postnatally, detailed postmortem analysis of fetuses with megacystis and hyperechogenic kidneys showed that isolated severe lower urinary tract obstruction before 25 weeks' gestation was as likely to be due to urethral atresia or stenosis as PUV (Robyr et al., 2005). Urethral atresia may arise in association with complex collections of other genitourinary and/or gastrointestinal anomalies. Moreover, urethral atresia may be a cause of bladder outlet obstruction in females whereas PUV is not. Urethral atresia is incompatible with life unless an alternative connection between the bladder and the amniotic sac is present. Prenatal surgical decompression has been performed to relieve this obstruction, although a spontaneous fistula or patent urachus may also provide the necessary bladder drainage

**3.1.4 Other upper urinary tract obstructions** 

**3.2 Lower urinary tract obstructions** 

careful management to optimize renal outcomes.

dysfunction in the refluxing kidney (Greenfield et al., 1983).

**3.2.2 Urethral atresia, stenosis, or hypoplasia** 

**3.2.1 Posterior urethral valves** 

retroiliac or retrocaval course; and mid-ureteral stricture.

Fig. 4. Radiological findings associated with PUV in a 2 day old male. **A - C.** Ultrasound of urinary bladder (A), right (B) and left (C) kidneys. The bladder (BL) is rounded with a thickened and trabeculated wall. This patient had severe hydroureter bilaterally, although only the left ureter (LU) is clearly observed in image A. Moderate to severe hydronephrosis is present bilaterally, with thinning of the cortex, increased echogenicity relative to the liver (LIV), and poor corticomedullary differentiation. One fluid-filled area (CY) in the right kidney did not clearly communicate with the collecting system, and likely represents a large cyst. **D and E.** VCUG. The lobulated and undulating contours of the urinary bladder (BL) reflect thickening and trabeculation of the bladder wall. The posterior urethra (PU) is dilated. Rather than the classic abrupt transition to a normal caliber anterior urethra, this patient has the "wind-sock" appearance generated by distal prolapse or distention of the valve membrane (arrow). VUR into a tortuous and dilated left ureter (LU) is obvious.

(Gonzalez et al., 2001; Herndon & Casale, 2002). An association between urethral atresia and prune belly syndrome has been recognized (Reinberg et al., 1993). Progression to ESRD is

Congenital Obstructive Nephropathy: Clinical Perspectives and Animal Models 11

Fig. 5. Radiological findings associated with prune belly syndrome in a 2 day old male. **A - E.** Ultrasound of urinary bladder (A), right (B) and left (C) ureters, right (D) and left (E) kidneys. The bladder (BL) is decompressed but bladder wall thickness is normal. Tortuous dilated ureters (U, RU, LU) are observed bilaterally in the lower abdomen. The kidneys are

dysplastic and amorphous in appearance, with cysts of varying sizes. There is no corticomedullary differentiation in either kidney. **F.** VCUG. The protuberant abdomen resulting from lack of abdominal wall musculature is evident from the position of bowel loops (arrows) on this lateral view. This patient has an unusual configuration of the bladder, bladder base and posterior urethra. There is absence of the normal prostate and a very distended posterior urethra (PU) connecting to a dysplastic-appearing bladder (BL). No evidence of true PUV was found on this or any subsequent investigations. Reflux is observed into the distended right ureter (RU) but not the left. **G.** VCUG from another patient 10 days postnatally shows a more typical trabeculated bladder with the characteristic tubular shape of the bladder base in prune belly syndrome.

common, although not universal, in surviving patients with urethral atresia (Gonzalez, et al., 2001).

#### **3.2.3 Prolapsing ureterocele**

Large ureteroceles can prolapse through the urethra, which may result in bladder outlet obstruction. This is most frequently an acquired condition, although rarely prolapse may occur *in utero*, leading to features of congenital obstructive nephropathy (Sozubir et al., 2003).

### **3.2.4 Other urethral obstructions**

Urethral diverticula and anterior urethral valves are rare causes of infravesicular obstruction. Interestingly, although bladder pathology and variable degrees of hydroureteronephrosis result, renal function is usually not impaired after surgical correction of the obstruction (Arena et al., 2009; Gupta & Srinivas, 2000; Rawat et al., 2009).
