·         PUJ- Hydronephrosis is the most common cause of a neonatal abdominal mass and often results from PUJ obstruction. Approximately 10—30% of PUJ obstructions are bilateral, but involvement is usually asymmetric and severe bilateral obstruction is rare. In rare cases the renal pelvis may dilate to create a giant abdominal cyst with little or no recognizable renal parenchyma. OH may result with severe bilateral obstruction and, paradoxically, PH may also occur in 25% of cases. A urinoma or urinary ascites may result from urinary tract rupture in utero. A PUJ obstruction may be suspected when there is marked hydronephrosis with a prominent renal pelvis without hydroureter. Identical appearances may occur with severe vesicoureteric reflux if the ureter is not distended with urine at the time of scanning.

 

·         Vesicourteric reflux-Vesicoureteric reflux is suspected by the presence of dilatation of the renal tract that varies with time. Intermittent dilatation of the ureter is a strong indicator of reflux.

 

 

·         Vesicoureteric junction obstruction- VUJ obstruction may occur in isolation or in association with a duplex kidney and double ureters. In duplex kidneys, the ureter from the upper renal moiety inserts into the bladder or distal urinary tract at a low ectopic site. This ectopic ureter is prone to obstruct and may dilate and herniate into the bladder or urethra as a ureterocele. The upper moiety may thus become hydronephrotic: a dilated ureter from obstruction is seen as a cystic serpiginous tubular structure in the lower abdomen running behind the bladder. Dilatation of the lower moiety is likely to be due to reflux.

 

·         Multucystic dysplastic kidney type IIb-In type IIb disease there are multiple cysts of varying sizes accompanied by dilatation of the renal pelvis. The multicystic dvsplastic kidney has no significazt function.

 

·         Urethral obstruction- Urethral obstruction occurs more frequently in males than females and is most commonly due to posterior urethral valves. Obstruction to bladder emptying results in persistent dilatation of the bladder, thickened bladder wall and dilated collecting systems. Urinary ascites, urinomas and dystrophic bladder wall calcification are other associated abnormalities which may occur. Oligohydramnios indicates a poor prognosis. It may be associated with cystic dysplastic changes in the renal parenchyma. Dilated bladder is seen in the female in cloacal abnormalities.

 

 

·         Miscellaneous non-obstructive dilatations- Generalized or focal non-obstructive dilatation may be seen in infants in congenital megacalyces, exomphalos, macroglossia, Beckwith—Wiedemann syndrome and the Laurence_Moon_Biedl syndrome. Rarely, dilated calyces may be seen in ectopic, malrotated or fused kidneys. In primary megaureter, dilatation is usually confined to the ureters only. Chromosomal abnormalities have been reported in association with PUJ obstruction, bladder outlet obstruction and mild pelvic dilatation, but this is uncommon.

 

·         Prune belly syndrome- The syndrome is characterized by severe deficiency of abdominal wall musculature associated with gross dilatation of the bladder and ureters. The collecting system dilatation is non-obstructive. Despite the presence of gross ureteric dilatation the kidneys are small and the calyces may not be particularly dilated. It may result from resolution of urinary ascites in utero.

 

 

·         Pressure from pelvic masses- Ovarian masses, anterior meningocele, pelvic sacrococcygeal teratoma and hydrocolpos may compress the urinary bladder and/or the lower ureters, causing upper renal tract dilatation.

 

NB- After 19 weeks gestation measurement of the AP diameter of the fetal renal pelvis (PD) and kidney (KD) provides useful information on the degree of hydronephrosis. A PD of 5—9 mm and a PD:KD ratio of less than 0.5 in the absence of rounded calyces is usually physiological. A PD of 10 mm or more and a PD:KD ratio of greater than 0.5 is usually pathological, particularly in the presence of rounded calyces. Hydronephrosis should only be diagnosed in the presence of calyceal dilatation.