Abnormal, retrograde flow of urine from the urinary bladder into the proximal urinary tract (ureters, renal pelvis, collecting system). Affects 1–2% of otherwise normal children.
- Most common type, caused by an incompetent or inadequate ureterovesical tunnel. With the normal ureterovesical junction, the ureter tunnels through the detrusor muscle of the bladder before terminating at the ureteral hiatus (intramural ureter). The intramural ureter is compressed passively as the bladder fills, preventing retrograde flow of urine. If the intramural tunnel is too short, this flap-valve mechanism fails and VUR ensues.
- Caused by abnormally high pressure within the bladder. This pressure is transmitted to the ureter and causes a failure of the flap-valve mechanism. Clinical conditions include neurogenic bladder, posterior urethral valves and other causes of bladder obstruction.
- Urine backs up into the ureter only.
- Urine backs up into the ureter, renal pelvis and calyces; the renal pelvis appears healthy and has sharp calyces.
- Urine backs up into the ureter and collecting system; the ureter and pelvis appear mildly dilated, and the calyces are mildly blunted.
- Urine backs up into the ureter and collecting system; the ureter and pelvis appear moderately dilated, and the calyces are moderately blunted.
- Urine backs up into the ureter and collecting system; the pelvis is severely dilated, the ureter appears tortuous, and the calyces are severely blunted.