This has to do with a backflow of urine into the kidneys, before and during miction (urination) due to a missing anti-reflux mechanism at the junction where the ureter empties into the bladder. This disorder is normally congenital, can be associated with other abnormalities (doubling, megaloureter) and is seen relatively frequently (8 time more frequent in girls).
Normally, the uretero-vesical junction functions as a sphincter and urine can only flow out through the urethra. However, the urine can also flow back to the kidneys via the ureters as a result of a change of the vesico-ureteral junction, when the intravesical pressure is elevated. This leads to frequent urinary tract infections that, by the way, should be a diagnostic hint that a vesico-ureteral reflux may be present.
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