Hydronephrosis is usually diagnosed in one of two ways: A prenatal ultrasound reveals that the fetus has dilated kidneys, or an ultrasound that's performed to evaluate another medical problem, such as a urinary tract infection or incontinence, shows hydronephrosis. Prenatal ultrasounds detect hydronephrosis in about one out of every 100 pregnancies.

Once hydronephrosis is noted, the baby will often need additional tests to find out the severity of the condition. These tests are important because diagnosing and treating a potential abnormality early can prevent urinary tract infections and permanent kidney damage or scarring.

Tests include the following:

  • Voiding Cystourethrogram (VCUG) — A VCUG provides important information about the shape and size of the child's bladder, the bladder neck or opening, and the tubes that drain the urine from the kidneys into the bladder, called ureters. It is used to diagnose reflux, the abnormal backflow of urine from the bladder into the ureter and up to the kidney. It also provides anatomic information about the urethra, the tube that takes urine from the bladder outside the body, to make sure there's no blockage, a condition called posterior urethral valves.
  • Kidney (Renal) Scan — A kidney scan may be performed depending on the child's history of urinary tract infections, the result of the VCUG and the severity of the hydronephrosiss. Kidney scans show the function and drainage of the kidneys and can reveal if there's kidney damage and scarring from a previous urinary tract infection or long-standing hydronephrosis. Two types of kidney scans are typically performed, depending on the diagnosis:
    • Lasix Renogram or MAG-III diuretic renogram: Tests for significant blockage in the urinary tract.
    • DMSA (dimercaptosuccinic acid) scan: Tests for scarring or damage to the kidney tissue, which is more common in patients with vesicoureteral reflux.

If your newborn child had hydronephrosis noted on a prenatal ultrasound, another ultrasound should be performed one to three days after birth. Certain conditions seen on the ultrasound, such as severe hydronephrosis in both kidneys or a dilated bladder, may warrant more tests. A VCUG will be performed within the next several weeks of life.

Hydronephrosis is graded on a scale from zero to four, with one being the mildest form and four the most severe. The degree of hydronephrosis is used to help decide how to treat the condition that's causing it. More severe grades of hydronephrosis require more extensive tests. For example, grade III and IV hydronephrosis (that are not due to vesicoureteral reflux) typically require a renal scan.

Reviewed by health care specialists at UCSF Benioff Children's Hospital.

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