The 20-year-old's sudden death was probed during an inquest that began on Monday, more than three years after the emerging talent fell ill at pre-season training.
Noting body cramps while stretching after a cardio workout, Titmuss suffered a seizure at the club's base at Narrabeen, on Sydney's northern beaches, on November 23, 2020.
He later died at Royal North Shore Hospital after being taken there in an ambulance.
Keith Titmuss's family have attended the inquest into his death. (Flavio Brancaleone/AAP PHOTOS)
While an autopsy was unable to ascertain the cause of death, counsel assisting the coroner Adam Casselden SC told the court that Titmuss most likely suffered exertional heat stroke.
Paramedics who arrived at the scene noted Titmuss's temperature was 41.9C, while his heart rate was over 140 beats per minute (bpm).
Manly coaching staff noted Titmuss had struggled throughout the training session, when his weight increased from 112kg to 116.9kg.
But staff and medical practitioners did not identify that Titmuss was suffering heat stroke.
The inquest was told no ice packs were used, with one physician giving the young athlete a paper bag to treat hyperventilation.
Former Manly prop Lloyd Perrett collapsed in similar circumstances in 2017 but survived when then-chief medical officer Luke Inman treated the player for heat stroke.
Like Titmuss, Perrett exhibited a body temperature of over 40C, while he had a heart rate above 160bpm.
After the defibrillator showed no problems with the player's heart, Dr Inman told the inquest he placed ice packs on areas with large arteries to cool Perrett.
"Without the education, it's extremely difficult (to diagnose) even for a doctor," he said.
"But if the defibrillator shows there's no need for resuscitation, there's a high chance it's heat stroke.
"If it's not detected or treated, they're dead after 20 or 30 minutes.
"They'll cook internally. It's nearly impossible to get them back after that point."
Dr Inman had recommended Perrett ease into training following a health screening to determine his condition before preseason.
"They recommend a period of acclimatisation in the NRL, but there's no exact way of guiding teams on what that entails," he said.
"What it should mean is a graded increase in exposure to the heat so the body can adjust gradually and build a tolerance to it.
"But it wouldn't be done at any NRL club - no one will comply to that in a performance setting.
"They would be feeling that they would be two weeks behind any other team if they put the brakes on."
Titmuss's death came three days into his time as a member of the club's top-30 NRL squad.
He had been a rising star at the club, having delivered the Sea Eagles their maiden Holden Cup title in 2017 with a try on the siren to beat Parramatta.
He scored the match-winning try for Wayne Lambkin's side in the last cup grand final before being named players' player in the 2019 Jersey Flegg Cup team.
The inquest, attended by Titmuss's family, continues.
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