Heming opened up in an interview with Town & Country on Tuesday, October 29, revealing that the family initially overlooked some of the actor's symptoms due to his history of having a stutter in childhood.
"Bruce has always had a stutter, but he has been good at covering it up," Heming, 46, told the outlet.
Heming added that Willis's stutter ultimately led to him pursuing acting as a career after his theater teacher in college made him realize he could memorize a script and say the words without stuttering.
"As his language started changing, it [seemed like it] was just a part of a stutter, it was just Bruce," Heming explained. "Never in a million years would I think it would be a form of dementia for someone so young."
She continued: "For Bruce, it started in his temporal lobes and then has spread to the frontal part of his brain. It attacks and destroys a person's ability to walk, think, make decisions. I say that FTD whispers, it doesn't shout. It's hard for me to say, 'This is where Bruce ended, and this is where his disease started to take over.' He was diagnosed two years ago, but a year prior, we had a loose diagnosis of aphasia, which is a symptom of a disease but is not the disease."
Willis's family shared in 2022 that the Sixth Sense actor, 69, had been diagnosed with aphasia, a disorder that affects how a person is able to communicate.
According to Mayo Clinic, frontotemporal dementia (FTD) is an "umbrella term for a group of brain diseases that mainly affect the frontal and temporal lobes of the brain" - areas that are "associated with personality, behavior and language."
Heming shares two children Mabel, 12, and Evelyn, 10 with Willis. The actor also is father to Rumer, 36, Scout, 33, and Tallulah, 30, with ex-wife Demi Moore.
In the interview, Heming also shared an update on how she was dealing with her husband's disease.
"Today I'm much better than I was when we first received the FTD diagnosis," Heming said. "I'm not saying it's any easier, but I've had to get used to what's happening so that I can be grounded in what is, so that I can support our children. I'm trying to find that balance between the grief and the sadness that I feel, which can just crack open at any given moment, and finding joy."