The 37-year-old, who played four Test matches for Australia in 2015, revealed he has been diagnosed with the same disease as retired South Sydney NRL star Jai Arrow.
During a Stan Sport interview with former Wallaby turned commentator Justin Harrison, Charles said he had only recently been diagnosed.
"For a long period there I just didn't know what it was," said Charles, who retired from professional rugby in 2018.
"I've had that many tests, seen that many doctors and it just wasn't getting any outcome, but my health just kept declining.
"And then recently, in the last few weeks, I was actually diagnosed with motor neurone disease, which has been extremely challenging and overwhelming. And, yeah, just at times quite cruel."
With no cure yet for the condition, Charles said his first thought was for his family, including his fiancee Steph.
"My first question was, 'How long have I got?' the former hooker said.
"I know that every person with MND has a different journey, a different path and it affects everyone differently.
"I know there's no cure and … your physicality, your body deteriorates and, yeah, your life cuts short."
Charles played Super Rugby for the Western Force and Melbourne Rebels, as well as stints in England and France.
Since retiring he has worked as chief executive of Rugby WA and helped run Sydney Uni rugby.
