As doctors and families gather in London for an international conference looking at genetic causes for Sads, BBC News Online hears Wendy's story.

The day before she died, 19-year-old Lucy Allum went to work as normal, and spent the evening watching videos with her friend.

The following morning, when her mother went to wake her up, Lucy was dead.

Since that day in January 2001, Wendy, her husband Clive and her son Chris, 24, have been searching for answers about exactly why their beloved daughter died so suddenly.

Wendy, from Chalfont St Peters in Buckinghamshire, said: "The only thing wrong was that in the months before she died, she started complaining of headaches.

It's very hard to live with not knowing

Wendy Allum

"But she had them while she was working with a computer, and I thought that her eyes were affected.

"The day she died, she was going to go for an eye test."

When she went in to wake her daughter up, Lucy, who had been healthy and sporty, was lying face down, with her arm dragging on the floor.

"I knew as soon as I saw the way her arm was, there was something wrong. She was very stiff, she had been dead for some hours."

'Need to know'

The ambulance service and the police were called. Her family thought Lucy may have taken her own life, but the police surgeon said that was not the case.

But because there was no clear cause of death, Lucy's body, a post-mortem had to be carried out.

The post-mortem failed to establish a cause of death, so the pathologist asked for permission to remove Lucy's heart and brain for further investigation.

"I just said yes because we needed to know," said her mother.

But still nothing was found, and Lucy's cause of death was recorded as Sads.

Heart problems

Sads can occur during sport, non-competitive physical activity, emotional upsets, or even during sleep.

It normally affects young adults aged between 18 and 25 years old.

It is often linked to a disturbance of the heart's electrical system, causing an abnormality of the heart beating or rhythm, called an arrhythmia.

A condition called Long QT Syndrome is a common cause.

But in Lucy's case, no underlying cause was found.

Her family even had tests after her death to ensure there was no underlying genetic cause for her death which could have also affected them.

Wendy said: "The worry gets worse because we have another child."

Since his sister's death, Chris has been diagnosed with insulin dependent diabetes, and has suffered from anxiety and depression.

Because of the inquiries into how Lucy died, her family could not hold her funeral until five months later.

But Wendy said the occasion was a real tribute to her daughter, with around 500 of her friends and family filling the local church.

Research

Wendy, who is involved with the Sads UK charity, said the worst thing for all the family was the lack of answers about Lucy's death.

"The coroner's office we dealt with were very good, they recognise Sads. But in a lot of areas, they don't.

"Parents can be given very strange reasons for why their children have died."

She added more research needed to be done into the causes of Sads.

"It's very hard to live with not knowing."