What Causes VUR? Primary and Secondary Reflux

The ureter does not enter the bladder at a right angle; it travels an oblique course for a few centimeters between the muscle layer and the inner lining of the bladder wall before opening into the bladder. The point where the ureter joins the bladder is called the ureterovesical junction (UVJ).

The great majority of reflux seen in children is primary; that is, the bladder is healthy and the problem lies only at the junction. The ureter's tunnel in the bladder wall is short from birth, or the ureteral opening sits further to the side and higher up than it should. This is the result of a small variation, occurring before birth, in how the ureteric bud meets the bladder.

In some children, the junction is normal at birth, but the pressure inside the bladder is continuously, or during voiding, very high; the valve cannot withstand this pressure, and urine flows backward. This is called secondary reflux; the underlying problem is in the bladder.

In some children, reflux does not occur alone but together with a structural variation of the ureter and bladder. The most common is a duplex collecting system (duplication, colloquially a 'double kidney'): two separate ureters arise from the kidney, and because the ureter belonging to the lower half opens further to the side, reflux is seen in that ureter.

Sterile urine — urine free of germs — flowing backward under low pressure generally does not harm the kidney; in most children, reflux exists silently and then disappears. Harm results when two conditions come together: bacteria present in the urine, and that urine reaching the kidney tissue.