What is hydronephrosis?

Hydronephrosis is a condition that occurs when a kidney swells because urine cannot drain normally from the kidney to the bladder, according to Healthline. It is a structural condition, not a disease, since it results from a blockage at some location in the urinary tract.

A sudden obstruction in the ureter due to a kidney stone is the most common cause of hydronephrosis. Other causes include blood clots, tumors, scar tissue or birth defects, according to Healthline. The condition is most commonly diagnosed via ultrasound, reports the National Kidney Foundation, although an X-ray, CT scan, MRI or cystoscopy may also be used for diagnosis.