Hydronephrosis describes hydrostatic dilation of the renal pelvis and calyces as a result of obstruction to urine flow downstream.

The signs and symptoms of hydronephrosis depend upon whether the obstruction is acute or chronic, partial or complete, unilateral or bilateral.

[2] Blood tests may show impaired kidney function (elevated urea or creatinine) or electrolyte imbalances such as hyponatremia or hyperchloremic metabolic acidosis.

Urinalysis may indicate an elevated pH due to the secondary destruction of nephrons within the affected kidney, which impairs acid excretion.

Physical examination in a thin patient may detect a palpable abdominal or flank mass caused by the enlarged kidney.

Some of these congenital defects have been identified as inherited conditions, however, the benefits of linking genetic testing to early diagnosis have not been determined.

[4] Compression of one or both ureters can also be caused by other developmental defects not completely occurring during the fetal stage such as an abnormally placed vein, artery, or tumor.

[citation needed] Sources of obstruction that can arise from other various causes include kidney stones, blood clots or retroperitoneal fibrosis.

[5] A 2019 review found three cases of hydronephrosis with renal colic were caused by malpositioned menstrual cups pressing on a ureter.

[8] Prenatal diagnosis is possible,[9] and in fact, most cases in pediatric patients are incidentally detected by routine screening ultrasounds obtained during pregnancy.

[citation needed] Urinalysis is usually performed to determine the presence of blood (which is typical for kidney stones) or signs of infection (such as a positive leukocyte esterase or nitrite).

[citation needed] Imaging studies, such as an intravenous urogram (IVU), renal ultrasonography, CT, or MRI, are also important investigations in determining the presence and/ or cause of hydronephrosis.

Whilst ultrasound allows for visualisation of the ureters and kidneys (and determine the presence of hydronephrosis and / or hydroureter), an IVU is useful for assessing the anatomical location of the obstruction.

)[12] Kay recommends that a neonate born with untreated in utero hydronephrosis receive a renal ultrasound within two days of birth.

In the case of renal colic (one sided loin pain usually accompanied by a trace of blood in the urine) the initial investigation is usually a spiral or helical CT scan.

This is generally done within the first few days after birth, although there is some risk that obtaining an imaging study this early may miss some cases of mild hydronephrosis due to the relative oliguria of a newborn.

[15] A voiding cystourethrogram (VCUG) is also typically obtained to exclude the possibility of vesicoureteral reflux or anatomical abnormalities such as posterior urethral valves.