He said: "Maybe 12 a week doesn't seem very many when you consider the thousands dying from heart attacks or heart failure in the UK every year.
"But what horrified me, was the age of these people - a lot of them were under eighteen."
CRY runs a national cardiac screening programme for anyone who may be at risk.
'Awareness must be raised'
Some people - like Tom - have no symptoms before they die. But many youngsters do get warning signs.
Chest pains or undue breathlessness when exercising, heart palpitations, bouts of dizziness or blackouts - all of these may indicate a heart defect.
Claire Prosser talks about Tom
And because these conditions are genetic, Dr Sharma believes there are other people who should consider heart screening too.
These would include anyone who has a close relative - a parent, sibling or child - with a potentially hereditary heart condition, and anyone who has a close relative who died suddenly under the age of forty.
It is quite likely sudden death may indicate an undiagnosed heart problem.
"We have to raise awareness about this in the general population", says Dr Sharma. "Most importantly we have to get the message out to GPs - they are the ones on the front line."
Dr Sharma sees over 30 patients a week in his clinics at both King's College Hospital and University Hospital in Lewisham. All of them are under 35. Treatment will vary according to how serious the condition is.
Dr Sanjay Sharma says he was 'horrified' by the age of those affected
He says: "We can now identify those at high risk, and we supply those patients with defibrillators.
"Some people will be treatable with drugs. For others it may just be a question of lifestyle modification.
"For instance, someone might get minor symptoms but only when they exercise. If we can persuade them not to exercise so vigorously, that may be all that's needed."
Since they lost Tom, his family have campaigned to raise awareness about heart defects in young people.
"The death of a child is just too huge for most people to comprehend", says Tom's mum, Claire Prosser.
"When Tom died, CRY understood what we were going through.
"If we can prevent even one family from going through what we have, it will be worthwhile."
'Life After Tom' will be broadcast on Monday 13th October on BBC Radio 4. It tells the story of Tom's mum, Claire Prosser, as she begins to rebuild her life without her son. You can listen again via the BBC iPlayer.
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