May 18, 2010 -- Angel Burton had repeated and painful kidney infections from the time she was born in Louth in Lincolnshire, England.

But when she was 5, doctors attempted surgery and discovered the little girl had two complete drainage systems -- one piggy-backed on top of the other in each kidney.

When one set of ureters failed, the second set kicked in, restoring her health and perhaps saving her life.

Angel has duplex or double kidneys, two functioning units within the same kidney.

"I was surprised we had one (a duplex) in Angel because none of the previous x-rays had shown that," said Dr. Prasad Godbole, consultant pediatric urologist at Sheffield Children's Hospital, who operated on the little girl.

"Sometimes you could miss it but I was surprised because looking at all the previous x-rays there was no suggestion of this," he said. "Her kidneys are working fine."

Today, Angel is 8 and in excellent health.

"It's a real miracle," her mother, Claire Burton, told the Cavendish Press in Manchester. "It's absolutely amazing that none of her earlier scans picked [it] up. We're just so grateful to have Angel back to her happy, healthy self."

This genetic abnormality occurs in about 1 in 100 children, according to a statement from Sheffield Children's NHS Foundation, the hospital where she had her surgery in 2007.

"In the vast majority of cases, duplex kidneys never cause problems and can go undetected but, occasionally, can cause urine infections and need investigations and treatment, which may include surgery," said the hospital spokesman.

The kidneys are two bean-shaped organs located near the middle of the back, just below the rib cage. Waste and extra water are removed from the blood in the kidneys and sent to the bladder as urine via the ureters.

After Angel's birth in 2001, her health began to decline. Doctors diagnosed her with bilateral reflux as urine backed up into her kidneys, causing infections. Over time, she was regularly screened and monitored, but none of the doctors noticed the two additional tubes.

By the time she was 5, her kidneys had been scarred from the repeated infections, and in 2007, doctors attempted to create an artificial valve.

It wasn't until Angel underwent surgery that doctors discovered she had duplex kidneys.

"They'd been in there an hour and a quarter when the surgeon came out," said her mother. "He said: 'We've found something very, very strange considering the amount of scans Angel's had this year."

"He went on to explain they'd put a camera in Angel's bladder so they could repair the valves," said Burton. "They'd found the two openings that were supposed to be there and they'd gone ahead and repaired the two valves."

"But then they'd found two more openings and when they'd put the camera up, these openings led up to two more perfectly healthy kidneys, just sitting on top of the other ones," said Burton.

'He said: 'They are totally independent. We are just stunned that these have never been picked up on a scan. She's been having intensive scanning for five years and they've never shown up. It looks like the healthy ones have taken over.''

Angel doesn't exactly have four kidneys, she has a "duplication of the collecting system," according to Dr. Bryan Becker, president of the National Kidney Foundation and professor of medicine at the University of Wisconsin School of Medicine and Public Health.

Duplex Kidney Is Rare Congenital Condition

"In this congenital condition, more than one ureter, the tube from the kidney to the bladder, drains a different pole of the kidney," he said. In Angel's case, one of the ureters in each kidney remained free of damage.

Angel's deformity likely occurred during her development in utero when tissue masses split during the evolution of the kidneys, said Becker, who was not involved in her treatment. The condition can be related to a number of different syndromes.

Becker speculated that when doctors discovered the functional ureters during surgery, they "left everything in there and let the kidneys keep working and do their thing."

Kidney disease is relative rare in children; adults are about 20 times more likely to have problems and the risk increases with age.

In the general population, slightly more than 30 people in every 100,000 develop kidney failure each year. But in children, aged 19 and under, the annual rate is only one or two new cases per 100,000 children, according to the National Kidney and Urologic Disease Information Clearing House (NKUDIC), which is part of the National Institutes of Health.

Boys are more susceptible than girls to have kidney failure due to birth defects and other hereditary diseases. African Americans in their late teens are three times more likely than Caucasians in the same age group to develop kidney failure.

"When children develop kidney failure, not only do they have the same set of co-morbidity conditions that adults get, they are more at risk on how it takes a toll in their lifetime," said Becker.

The kidneys also regulate blood pressure, balance chemicals like sodium and potassium, and make hormones to help bones grow and keep the blood healthy by making new red blood cells.

Children with kidney disease are especially at risk for growth disturbances. Becker cited the "classic example" of such an illness in Tiny Tim, the sickly character from Charles Dickens novel, "A Christmas Carol."

"The kidneys don't work well and they don't develop muscular-skeletally and it affects their ability to function," he said.

When kidney failure occurs, doctors are more apt to recommend an organ transplant, rather than dialysis, according to Becker.

In children aged 1 to 5, the transplant success rate is nearly 80 percent for five years and 63 percent for 10 years. In children 6 to 11 years, that rate is about the same for five years and drops closer to adult rates at 50 percent over 10 years.

"Dialysis for a child is very complex," he said. "Even with the ability to give growth hormone, it's a very time-consuming and personally intensive treatment that requires a huge commitment of the family and child to be successful."