Duplex kidney

Most of us have two kidneys, one on either side of the lower back. Each is about the size of its owner’s clenched fist.

But sometimes a child is born with a duplex kidney. This simply means a ‘double’ kidney on one side. Very rarely, a child has a duplex kidney on both sides – known as bilateral duplex kidney.

In fact, a duplex kidney is one of the most common kidney problems people can be born with.

What causes a duplex kidney and who can get it?

It’s not clear exactly why this developmental problem happens. But it is thought that it is genetic – in other words, inherited from one or both parents.

What are the signs and symptoms of a duplex kidney?

The extent of the ‘doubling’ varies from child to child. In most cases it only affects a small part of one kidney and rarely causes problems.

But if the doubling affects a bigger part of the kidney and ureter (a thin tube leading from the kidney to the bladder), it can cause symptoms and treatment may be needed.

Usually, ‘doubling’ is limited to the kidney, including the ‘renal pelvis’. The renal pelvis is the area of the kidney where filtered urine (wee) collects before being passed down the ureter to the bladder, where it is stored.

When duplication only affects the kidney, it rarely causes problems. You won’t notice any symptoms and a child’s life won’t be affected in any way.

But sometimes, there is duplication of the ureter as well. This can be a partial duplication (so the ureter is a ‘Y’ shape) or there may be a completely separate extra ureter.

This more extensive doubling can mean a child is more prone to urine infections.

Also, the extra ureter can insert into the bladder in an abnormal place or even in the urethra (the tube leading from the bladder to the outside), causing constant urinary dribbling.

How is a duplex kidney normally diagnosed and treated?

The problem is often picked up before a child is born thanks to modern detailed ultrasound scanning techniques.

But sometimes it may not be detected until a baby is born – and only then if a child has investigations due to repeated urine infections, or if there are any other symptoms.

For most babies with a duplex kidney, no treatment is needed. But doctors may want to do repeated ultrasound examinations during pregnancy to monitor a baby's kidney development.

After birth a baby will be examined carefully by a paediatrician (doctor specialising in children's medical conditions). Tests may be suggested to find out more about the condition.

If the problem is thought to be big enough to cause any symptoms, a child is usually referred to a paediatric nephrologist – a doctor who specialises in children’s kidney problems.

More extensive testing can then be carried out. For instance, renal scans – which involve injecting radioactive dye into a vein and tracking its progress through the kidneys – will provide valuable information as to how efficiently the kidneys are working. This is usually carried out a few weeks after birth.

In the vast majority of cases no treatment is needed. But occasionally, if duplication is extensive and this is causing symptoms, an operation may be needed to correct the problem.

When to ask for medical help

If there is any concern that a child seems very prone to urine infections or is having trouble potty training beyond the age of around four, it’s a good idea for their parents to talk to their GP.

The child may need further investigations to rule out any underlying problem – such as a duplex kidney.