As many as five people under 35 die in Australia each week from sudden cardiac arrest and in half the cases the first sign of something wrong “is when they die suddenly”, a leading cardiologist says.

On Sunday 27-year-old Chris Head died during the City2Surf fun run in Sydney. He went into cardiac arrest as he approached the finishing line just before 11am and died shortly after arriving at St Vincent’s hospital.

His family were now likely to be screened for genetic heart disease, said Professor Chris Semsarian, a cardiologist at Royal Prince Alfred Hospital in Sydney and a world expert on sudden cardiac death in young people.

Head was a fitness enthusiast who appeared to be in great physical shape. Semsarian said young people who suffered sudden cardiac death often seemed perfectly healthy.

“For half of these types of deaths, the first sign of something wrong with their heart is when they die suddenly,” Semsarian said.

“It’s really, really sad and it’s a big challenge as a clinician to diagnose in advance because there are so often no symptoms, and that’s the really tricky part.”

Head’s younger brother, Jonathan Head, wrote on Facebook that Chris’s death was “an indescribable shock to us all and we loved him more than we can say”.

Semsarian said when people over 35 went into cardiac arrest, doctors could be fairly confident coronary artery disease was involved – where major blood vessels supplying the heart with blood, oxygen and nutrients become damaged and narrowed.

This was not the case in younger people, he said. “In this age group, it’s an entirely different game,” he said.

“We start thinking more about whether genetics may be involved, and in about one-third of these cases genetics will be at play. In the other two-thirds, we just don’t know. And while that’s not optimal, five years ago we knew of no genetic causes at all.

“Even if we can’t find a cause, we still test the immediate family members, who undergo clinical and genetic screening.”

Genetic screening was generally carried out when no cause of death was found from a postmortem examination, Semsarian said.

His team at the University of Sydney and Royal Prince Alfred hospital’s Centenary Institute are world leaders in such research.

Breakthroughs by his team in genetic technologies mean whole exome and whole genome sequencing of all 22,000 genes in the human body have been carried out to better understand genetic causes of heart disease.

“It means we’re no longer looking at one or five genes in postmortem, but hundreds and thousands in one test, and were getting loads of information back about genetic changes beyond what we have ever evaluated before,” Semsarian said.

“The challenge is we now have to work out, one by one, what each of these thousands of genetic changes mean. But it is a major step forward in using sequencing technology.”

Deaths such as Head’s should prompt people with a family history of sudden deaths to at least tell their doctor and have a basic checkup, he said.

“There are warning signs in some of these cases, and the biggest one is unexplained blackouts,” he said.

“If you’re looking at blood and feel like passing out, that’s normal, but if you’re walking down the street or driving and black out, that’s a major problem. A racing heartbeat when doing no exercise may also warrant a chec-up.”

Not always being able to tell people why their relative had died was one of the hardest parts of his job, Semsarian said. “This is the worst time of their life, they’ve suddenly lost a young sister, brother, son or daughter,” he said.

“When I started my training as a cardiologist, this type of death was an area for which virtually no research existed, and that cardiologists turned away from because it is so difficult.

“These are not cases of blocked arteries requiring a stent, or high cholesterol needing a tablet.

“I realised there was an urgent need to look after these families who are dealing with many unknowns about why their loved ones died and what it may mean for them, and as part of this work you develop lifelong relationships with them.

“Despite their pain they walk out at least knowing someone in the world cares about their family and what happened, and that there’s a plan ahead for them.”

It is a pain that Bronwen Fallens knows well, after her brother Justin Middling died while sitting in a university lecture aged just 33. A postmortem identified no cause for his sudden cardiac arrest.

“I actually get anxiety now thinking my own heart is going to stop,” Fallens said. “I had all of the genetic testing and the stress test and heart ultrasound carried out after Justin died, and even though they found no genetic cause I am extremely aware of my heart now and sometimes panic.

“My doctor said that’s normal for family members to feel that way.”

While it was a relief knowing she had no genetic heart condition, Fallens said it would always be difficult not knowing why her seemingly healthy brother died.

“I told the doctors that people have heart attacks all the time and survive, why couldn’t my brother?” she said.

“What I didn’t know at the time was cardiac arrests are completely different from heart attacks, because heart attacks have warning signs and a cardiac arrest means your heart literally just stops suddenly with no warning.

“When someone you love dies, it’s normal to just want answers.”