Symptomatic hydronephrosis is a totally different entity from congenital asymptomatic hydronephrosis. It generally occurs in a school-age child or adolescent, which presents with intermittent abdominal pain, sometimes together with vomiting. Urinary tract infection may or may not be present. Perinatal US may be negative, or a previous hydronephrosis may have solved spontaneously. Children as young as 3 years may present with colicky pain due to hydronephrosis. In the vast majority of symptomatic cases, aberrant vessels may be found going to the lower pole of the kidney and causing intermittent compression of the UPJ. In situation of hyperdiuresis, the UPJ is compressed and the pelvis gets distended; this in turn determines reduction of diuresis and reduction of the hydronephrosis. The pain is therefore intermittent, as is the US finding of hydronephrosis, Nonetheless, this mechanism can lead to significant loss of kidney function. Grade of hydronephrosis can be extremely variable at US. MAG3 scan can be nonobstructive. The treatment is straightforward surgical.

Three peculiar cases are possible: hydronephrosis in duplex system, hydronephrosis in ectopic kidney, and in horseshoe kidney. In most cases of renal anomalies, an anatomical defect of UPJ is present and spontaneous resolution of obstruction is less plausible.

Hydronephrosis in horseshoe kidney occurs more commonly than in normal kidneys. Vascular anomalies with aberrant vessels may be present as well as the anomalies of the UPJ with very high insertion of the ureter on the pelvis. The surgical approach must be decided, keeping in mind the abnormal position of the renal pelvis (anterior and medial to the kidney), and a laparoscopic transperitoneal approach should be considered nowadays the surgical approach of choice. Section of the hystums is no longer an option.

Hydronephrosis in the ectopic kidney can be associated with reduced function due to renal hypoplasia. At US, the pelvis is generally malrotated. Evaluation of UPJO and indication to surgery may not be so evident.

Hydronephrosis in the duplex kidney is the less common. It generally occurs in the lower pole. Compression by aberrant vessels to the lower pole can be a possible cause. Hypoplasia of UPJ in case of incomplete duplex system (two pelvis draining in a single ureter) is also a possible occurrence.

The patient, adequately hydrated, receives 99mTc-MAG3 (mercaptoacetyltriglycine) as a bolus injection contextually with the starting of 30-min dynamic acquisition The administered activity of radiotracer is adjusted to the patient’s weight, according to EANM dosage card and to the national regulations.

Region-of-interest (ROI) around the heart, the kidneys, and the area around the kidneys (“background”) are drawn on a summed image of dynamic frames using a dedicated software; thus, time/activity curves are obtained, of expression of perfusion (flow T/A) and of renal function and drainage (renograms), respectively. In renograms, the ascending part represents the parenchymal phase (single kidney function) and the descending part reflects renal transit and emptying; with the same method, differential renal function (DRF or split function) is also determined.

If renal drainage is impaired at the end of the dynamic study, an additional image after 5 min standing upright is acquired (“gravity-assisted drainage,” GAD1). In case of still poor drainage (cutoff = 30 %), a 1 mg/kg dose of furosemide is given intravenously and an additional 20-min dynamic acquisition follows. If pyelic drainage is still incomplete (half-time >20 min, drainage <30 %), a second “gravity-assisted drainage” test (GAD-2) is also performed. The drainage is classified as impaired on the basis of all the previous mentioned three tests.