Last updated at 09:34 21 February 2007

With her sparkling green eyes and mischievous smile, the oxygen and feeding tube Erin Cottington wears constantly is the only outward clue of her uphill struggle for survival.

But the key to the 20-month-old's battle to beat the health problems she has suffered since she was born is rather unusual - regular doses of the sex drug Viagra.

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The toddler was first given the treatment when she was just a few days old to help her overcome life-threatening circulatory problems, caused by her underdeveloped heart and lungs.

Now she has one of the small blue pills each day to help keep her blood pressure down and stay healthy.

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Although this may surprise those who know it as a cure for impotence, Viagra was originally developed to improve blood flow to the heart in angina sufferers.

Last night Erin's mother, Alison, 33, said: "We can laugh about it now and find it amusing that Erin is being treated with Viagra, but it has helped save her life.

"At the time she was so terribly poorly, the doctors thought we could lose her at any time, but she has fought it all.

"The drug has been vital for helping swell her arteries and keep her blood pressure down. She has a bit more than one pill a day, crushed up in water, but the doctors think that as she grows the dosage will get smaller and she could eventually grow out of it.

"We are just thankful to have her with us, her will to live has stunned everyone."

Doctors at St Mary's Hospital, Manchester, first realised Erin was not developing properly in the womb when married Mrs Cottington went for her 20 week scan.

But it wasn't until she was born, one week premature, weighing just 5lb 13oz, in June 2005, that they were able to assess exactly what was wrong.

They diagnosed a diaphragmatic hernia - a rare condition which affects around 4,000 babies each year that is caused by a hole in the diaphragm.

In Erin's case, she had virtually no diaphragm on the right-handside of her body which meant that her intestines and bowels had pushed into her chest cavity as she was growing in the womb, crushing her right lung and heart and stopping them from developing properly.

Immediately after she was born doctors rushed Erin to intensive care and put her on a life-support machine. But they knew that without specialist treatment that she wouldn't make it through the night, so they decided to fly Erin by air ambulance to Yorkhill Hospital, Glasgow.

There, medics placed Erin on a heart and lung bypass machine to pump oxygen into her blood and help her breathe. They also pumped artificial blood into her crushed right lung to help it increase its capacity.

Erin was given Viagra to open up her blood vessels and improve the blood flow around her body. She also had eight operations, including three to repair her diaphragm and open heart surgery to stem bleeding between her heart and lungs, before she was three months old.

"The doctors basically told us that if Erin stayed in Manchester she would not make it," Mrs Cottington, a full-time mother, from Blackley, Manchester, said.

"It was awful, they prepared us for the worst because they didn't think she would make the transfer in the helicopter, let alone cope with the treatment. But she kept battling and we were thrilled when they said she could come home."

After more than 10 months in hospital, Erin was finally allowed out of hospital in April last year and is now on the road to recovery.

She is still fed through a tube and requires oxygen at night, but doctors are pleased with her progress and say she is developing like any other normal child.

Dr Carl Davies, a consultant paediatric surgeon at the Royal Hospital for Sick Children in Glasgow, who treats around six children a year with similar conditions, said: "Erin was very sick when she arrived with us - she was the most intensively-treated child in the world at that time.

"We have been using Viagra for a very small number of children for a few years and believe it can be very useful for a few patients. She will be tested to see its impact and will grow out of it. Most children stop taking it by the time they are two or three."

Last week, doctors in Newcastle revealed they had also used Viagra to help a premature baby boy to breathe. Lewis Goodfellow, who was born 16 weeks premature, was given the drug to help open tiny blood vessels in his seriously underdeveloped lungs.