Wales rugby union international Jonathan Davies has been forced to quit the sport after doctors diagnosed him with epilepsy.
Doctors believe that repeated head injuries to the Worcester Warriors forward, who played 67 times for his country, have brought on the condition.
ITV News Sports Editor Steve Scott reports.
Concussion has become a major issue in the sport this year after two high profile injuries to one of Thomas' compatriots George North and England's Mike Brown who were both sidelined for months after suffering serious head injuries.
Support staff at the World Cup, which begins this week, have been given access to all camera footage so injuries won't be missed and slow motion shots can help any diagnoses.
Thomas admits that during his career he never really thought about the consequences of head injuries.
"To be a rugby player at the elite level of the game you have to almost have that gladiatorial mindset but there's a difference between digging in with a sore knee or sore shoulder to digging in when you've had a knock to the head."