Dr. Greene’s Answer:
Normally the kidneys manufacture urine. This urine is transported through two tubes called ureters, toward the bladder, a muscular sac a bit like a thick-walled water balloon. When you urinate, the urine is excreted from the bladder through an exit tube called the urethra. When the bladder muscle contracts, the openings from the ureters into the bladder are normally squeezed shut, forcing the urine to go one-way, out the urethra. In some children, however, the connection where the ureters enter the bladder muscle allows urine to go back up toward the kidneys as well as down through the urethra when the bladder muscle is contracted. This is called vesicoureteral reflux.
There are three problems associated with vesicoureteral reflux. First, when the bladder contracts, causing urine to be forced upwards, the urine puts pressure on the kidneys that they are not built to handle. This can produce scarring of the kidneys. The kidneys and ureters can become dilated from the back-up, leading to poor function. The second problem is that the urine that travels backwards quickly returns to the bladder. We depend on the bladder to completely expel the urine in order to get rid of the small amounts of bacteria that can enter the bladder (especially in girls, who have short, straight urethras connecting the bladder to the outside world). Effective bladder emptying is a major defense against infection. Kids with reflux get bladder infections more easily because the urine is not fully expelled. The third problem is that the urine that goes backward can carry the infection found in the bladder back up to the kidney, resulting in serious infections and possible kidney damage.
Thankfully, only 1% of newborns have some form of reflux and the great majority of children with reflux outgrow this over the course of several years ( J Am Soc Nephrol 1998; 9:2377). Reflux is divided into grades I, II, III, IV, and V. In children with grades I and II, 80% will resolve spontaneously by school age. Spontaneous resolution is progressively less common in grades III and IV (only about 10% of grade IV involving both sides), and rare in grade V. Progress can be followed by using ultrasounds and x-rays (particularly one called a voiding cystourethrogram, or VCUG, which takes pictures of the bladder contracting and the urine exiting). Surgical interventions are available for cases in which reflux does not spontaneously resolve.