A mother may have to undergo a heart transplant to cure a one-in-500 defect which has killed her son and affected her grandson.

Margaret Mayhew, 63, of Ditchling Place, Brighton, found the body of her son, Steven, in his bed after he died suddenly from an enlarged heart in 1999.

A year later she was diagnosed with the same condition - known as dilated cardiomyopathy - and has been told her heart is a ticking timebomb.

Mrs Mayhew, who is awaiting assessment for a heart transplant at Harefield Hospital, said: "I know that in effect my heart could stop at any minute. I could go to bed and not wake up in the morning like Steven.

"We all keep smiling - it's what keeps us going. And who knows, one day - perhaps not in my time - a cure will be found."

Mrs Mayhew's grandson, 18-year-old Daniel Farrell, was diagnosed with cardiomyopathy caused by a viral condition when he was eight weeks old and had open heart surgery as a baby and two heart transplants.

Mrs Mayhew's daughter Kathy, 39, who lives in Wales with Daniel, also has the condition and is controlling it with medication.

Experts believe around 0.2 per cent of the population have cardiomyopathy but many do not know they have it.

Mrs Mayhew has had an internal defibrillator and pacemaker fitted which will automatically shock her heart if it develops the rhythm problems believed to have caused Steven's death.

Mrs Mayhew, who is married to Albert, 65, started to become ill in 2000.

She said: "I had gone away on holiday but while there I did not have any energy and found I was falling asleep all the time.

"I had also developed a hacking cough and just did not feel well at all.

"When I got home I still didn't improve and then my husband insisted I go and see a doctor. Doctors told me I had been walking around for weeks with a failing heart and I needed urgent treatment.

"Although I knew about the rest of the family, I did not think I had been affected. When I knew I had the same as the others, it was a real shock."

Mrs Mayhew was assessed for a heart transplant last year but medics felt she was healthy enough to not need one.

She said: "I am beginning to slow up a bit now.

"I can't get around the garden or sort out the house as much as I used to and I don't have as much energy as I did.

"We'll have to wait and see what happens."

Genetic tests on Mrs Mayhew and her daughter and grandson show that all three have a rogue gene that has caused the cardiomyopathy.

Medics believe Steven probably carried the same gene.

Mrs Mayhew said: "It was a tremendous shock when Steven died. He had not had any health problems in any way whatsoever. I went to wake him up one day and found him dead.

"After Steven's death my daughter started asking questions about my grandson's heart condition and whether they were linked in any way.

"They were not totally sure but when Daniel went for a routine check-up at Great Ormond Street Hospital in London, they checked Kathy as well as the condition can run in the family."

Medics found Kathy was borderline but three years later further tests showed she was developing the condition herself.

She has since responded to medication and her condition remains stable.

Mrs Mayhew is campaigning with the Cardiomyopathy Association for the need for more heart screening and genetic testing for affected families to help save lives.

Cardiomyopathy can be treated with surgery and drugs, with many sufferers going on to live long and full lives.

Mrs Mayhew said: "Before I heard about the association I thought we were the only family to be affected in this way but now I know there are others around us.

"It is vital that these conditions are picked up as soon as possible so people can start getting the treatment they need."

Call 0800 0181 024 or visit xww.cardiomyopathy.org