How we pass urine

Urine is made in each kidney and then passes through its ureter to the bladder. The bladder fills up with urine, like a balloon. When it is full, it sends signals to the brain that ‘tell’ the bladder muscles to push urine out through the urethra. This is how we pass urine (wee).

As most people urinate (wee), the end of each ureter that goes into the bladder squeezes tight. It acts like a one-way valve – urine can go into the bladder but cannot leave the bladder to go back up the ureter.

How VUR happens

In VUR there is a problem with the way one or both ureters enter the bladder. The ‘valve’ does not work properly. This problem is usually present at birth.

As children with VUR urinate, some urine refluxes (goes back up) one or both ureters, and may reach the kidney.

VUR may affect one ureter and kidney (unilateral VUR) or both ureters and kidneys (bilateral VUR).

After urinating, this urine settles back into the bladder. In severe VUR, this means the bladder is never completely empty.

What causes VUR?

It is not always known what causes VUR.

It is not caused by anything that the mother does during her pregnancy.

It can run in families. If either parent or one of your children has VUR, your doctor may recommend that your other children are also tested, especially if they have any complications such as frequent urinary tract infections (UTIs).

VUR that occurs by itself is called primary VUR.

Sometimes VUR is caused by, or happens with, another problem in the urinary system that makes it difficult to pass urine. This is called secondary VUR. Most of these problems are present at birth.

These include:

• Posterior urethral valves (PUV) – a blockage in part of the urethra (nearest the bladder) that happens in some boys.
• Neuropathic bladder (or neurogenic bladder) – a problem with the nervous system (including the brain), which affects how the bladder works. Affected children are unable to control how their bladder works and may dribble urine or have bladder spasms.
• Ureterocele – the ureter dilates (swells) where it enters the bladder because the opening is very small. This is often found where a kidney and ureter develop in two parts (Duplex kidneys) – in these cases, sometimes one of the ureters has a ureterocele and one has reflux.

Other problems

VUR may also happen with other kidney problems that develop while the baby is growing in the womb. These include renal dysplasia or renal hypoplasia, when one or both kidneys do not fully develop and are often smaller than usual.

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