Hydronephrosis is dilation of the renal pelvis and calyces due to obstruction of urinary flow. This obstruction may occur anywhere along the urinary tract. Increased ureteral pressure leads to changes in glomerular filtration rate, tubular function, and renal blood flow. The obstruction may be acute or chronic, and unilateral or bilateral.

In children, hydronephrosis is most often secondary to an anatomic abnormality such as posterior urethral valves, vesicoureteral reflux, or ureteropelvic junction (UPJ) obstruction. Congenital anomalies account for a higher incidence of hydronephrosis in children compared to adults.

The Society for Fetal Urology (SFU) has developed a grading system for the severity of hydronephrosis. Severity ranges from grade 0 (mild; no renal pelvis dilation) to grade 4 (severe; dilation of the renal pelvis and calyces and thinning of the renal parenchyma).

Patients with isolated hydronephrosis are typically pain free; the presence of pain is usually in the setting of stones, acute bladder distension, or infection. Upper ureteral or renal pelvic lesions typically cause flank pain, whereas lower ureteral obstruction causes pain that radiates to the ipsilateral testicle or labia. Other presenting symptoms include hypertension, change in urine output, hematuria, or new creatinine elevation on routine laboratory studies.