After further detail emerged about the cause of death following an autopsy on Norwegian world swim champion Alex Dale Oen, a a second medical officer has recommended that the athlete's family have heart checks because of a condition that is often hidden in under-50s and may have been hereditary.

Dale Oen, among favourites for Olympic gold over 100m breaststroke this summer until his death on the last day of April during a training camp in Arizona, had an enlarged heart and at least one previous 'heart attack' that went undetected, a medical examiner states in the investigation report that followed an autopsy.

All three major arteries in Dale Oen's heart were extremely narrowed, up to 90 per cent, because of the buildup of plaque, said Shannon Mackey-Bojack, medical director of the Jesse Edwards Registry of Cardio Vascular Disease in St. Paul, Minnesota.

Further, she reported, the heart had prior damage from smaller heart attacks that had apparently gone undiagnosed despite Dale One having had heart checks during standard medical tests. "If you're having chest pain and you're 26, you probably are not thinking you're having a heart attack," Mackey-Bojack noted. "You're just thinking that you worked out too hard."

Plaque buildup is detected through an angiogram but few under-50s are checked, the examiner noted. "This isn't something you routinely look for who is 26 years old," she said.

The common risk factors for heart disease are obesity (poor diet, lack of exercise), smoking and family history. The latter is the only one that might apply to Dale Oen, Kimbal Babcock, clinical services manager for the Coconino County Public Health Services District, suggested.

The swimmer's grandfather died suddenly at 42, the cause unknown, according to Mackey-Bojack. Her work recommends that the One family, including the swimmer's older brother Robin Dale Oen, have heart checks targeted at the problem detected in the autopsy of the late athlete.

The results of a second set of tests on Dale Oen was released on Tuesday this week by the Coconino unit. The verdict: the swimmer died of atherosclerotic coronary artery disease (hardening of the arteries caused by plaque buildup).

"Based on the autopsy findings and the investigative history that is available to me, it is my opinion that Alexander Dale-Oen died of atherosclerotic coronary artery disease," Coconino County medical examiner Lawrence Czarnecki wrote. "Given the decedent's young age and significant atherosclerotic disease, a follow-up and evaluation of family members is recommended," he added.

In his sibling's memory, Robin Dale Oen is continuing the work he started with his brother to launch a foundation, The Dale Oen Experience, to help children achieve their dreams.

The Coconino County Medical Examiner found no anatomic cause of death in an initial examination on May 1. An autopsy at the Jesse Edwards Registry of Cardiovascular Disease, which specializes in diagnosing heart disease, followed, as did additional toxicology inquiries. No illegal substances were detected.