Every year there are potentially more than 500 sudden unexplained deaths in England, reveals a nationwide study published ahead of print in the journal Heart.
This figure is around eight times higher than previously thought, the data suggest.
The research team drew on cause of death judgments relating to sudden unexplained deaths in people from 117 coroners (out of 122) across England between October 1997 and May 1999.
The judgments were all based on post mortem examination reports. None of those who died had a history of heart disease, and they had all last been seen alive within 12 hours of death. All were aged between 4 and 64. Each case was assessed by an expert to eliminate other identifiable causes of death.
National figures for SADS (sudden adult death syndrome) were calculated and compared with official death statistics for SADS and unknown causes of death.
The results showed that only around a third of the 56 cases of SADS were correctly identified. Causes of death had been attributed to heart attack or other causes, such as epilepsy and drowning.
SADS cases tended to be young, with an average age of 32. But their ages ranged from 7 to 64. Almost two thirds (63%) were male.
Four had had some heart symptoms in the 48 hours before death, and two thirds had experienced cardiac symptoms at some point in the past.
Futhermore, almost one in five (18%) had a family history of sudden unexplained deaths before the age of 45.
Initially, the authors calculated that the total annual numbers of SADS cases per 100,000 of the population was 0.16. But official statistics for unknown causes of death were 1.34 per 100,000, up to eight times higher than initially estimated.
The authors say that this equates to more than 500 such deaths every year in England.
Families affected by SADS have an inherited genetic heart defect, which makes them prone to severe abnormalities in heart rhythm, and sudden death, suggest the authors.
SADS should be a certifiable cause of death, which should prompt automatic screening of other family members, they contend.