The retrograde (i.e. backward) flow of urine from the urinary bladder into the ureters is medically referred to as vesicoureteral reflux (VUR).
Normally, urine flows in a one-way direction from the kidneys into the ureters, which are the muscular tubes that connect the kidneys to the bladder. There is a valve within each ureter at its point of entry into the bladder. This location is known as the ureterovesical junction (UVJ). From the bladder, urine then passes through the urethra, which is another tube, to exit the body.
VUR is generally a condition that manifests in the pediatric population and it may be primary or secondary. In the first form, which is more common, the patient presents with VUR despite having a lower urinary tract that is otherwise functional. On the other hand, patients with secondary VUR typically have a dysfunctional lower urinary tract that may be due to obstruction or neuropathology. In both cases, there is failure of the UVJ valve to prevent the backward flow of urine. This may lead to serious consequences due to the migration of lower urinary tract bacteria to an upper urinary tract that should be sterile.
The distal part of the ureter at the UVJ passes through a muscular opening in the detrusor muscle of the bladder and a submucosal tunnel before finally reaching the lumen of the bladder. This passage through the detrusor muscle hiatus and submucosal tunnel functions as a competent valve when the length of the ureter is sufficient. Insufficient ureteric length results in an incompetent UVJ valve.
Causes of VUR
Primary VUR is typically a congenital condition and is believed to have a genetic etiology, but the exact cause has not been elucidated to date. It is obvious that there is a defect in UVJ valve formation during the development of the child in utero. The end result of this malformation and/or abnormal development is a UVJ that is too short. This allows urine to flow backwards in response to the normal increase in bladder pressure that occurs during the process of micturition. Primary VUR may improve or even disappear during childhood, because the ureters lengthen with the growth of the child.
In contrast to the primary form, secondary VUR arises due to a malfunction within the urinary system. Most commonly, recurrent urinary tract infections (UTIs) are implicated as the etiological agents of secondary VUR. Several studies suggest that UTIs may cause the ureters to swell leading to the occurrence of obstruction within the urinary system. Many investigators, however, believe that reflux may develop independently from UTIs and the correlation noted with recurrent UTIs is simply because these children are routinely screened for reflux. Nonetheless, rates of VUR are increased in those with obstruction of the bladder outlet and neuropathology of the bladder (e.g. neurogenic bladder).