Famous US actor Bruce Willis has been diagnosed with frontotemporal dementia, a "cruel disease," his family said in a statement posted to the Association for Frontotemporal Degeneration's website on Thursday. 

The actor best known for his part in the "Die Hard" franchise left Hollywood in March and has been out of the spotlight since.

"Since we announced Bruce's diagnosis of aphasia in spring 2022, Bruce's condition has progressed and we now have a more specific diagnosis: frontotemporal dementia," a statement said.

It is worth noting that Frontotemporal dementia (FTD) is an umbrella term for illnesses that impair the parts of the brain responsible for personality, conduct, and language.

"Unfortunately, challenges with communication are just one symptom of the disease Bruce faces," the family statement said. "While this is painful, it is a relief to finally have a clear diagnosis. "

"Today there are no treatments for the disease, a reality that we hope can change in the years ahead."

The frontal and temporal lobes of the brain decrease in FTD patients, as per doctors.

What causes this is unknown, however, it might result in personality changes or behavioral changes that make someone appear socially incorrect, impetuous, or uncaring towards those around them.

Other patients lose their ability to communicate.

FTD can develop between the ages of 40 and 65 and accounts for up to one-fifth of all dementia cases, as per the Mayo Clinic.

Willis' current wife, Emma Heming Willis, as well as his former wife, actress Demi Moore, and his daughters Rumer, Scout, Tallulah, Mabel, and Evelyn, issued a family statement saying the actor had always fought to raise awareness about vital causes.

"We know in our hearts that -– if he could today -- he would want to respond by bringing global attention and a connectedness with those who are also dealing with this debilitating disease and how it impacts so many individuals and their families," it said.

A die-hard career

Willis, 67, rose to prominence on the TV sitcom Moonlighting before becoming a prolific action star famed for his sarcastic delivery. Aside from the Die Hard films, he received praise for his performances in Pulp Fiction, The Fifth Element, The Last Boy Scout, Twelve Monkeys, The Sixth Sense, Unbreakable, Looper, and Moonrise Kingdom.

Willis, one of the most bankable performers of the 1980s and 1990s, carved out a significant niche in the family comedy, most notably as the voice of a baby in Look Who's Talking and its sequel.

He has also released three albums as a singer (his cover of Under the Boardwalk peaked at No. 2 on the UK singles chart) and appeared in numerous theatrical plays, including a revival of Misery on Broadway in 2015.

The action icon was married to Hollywood star Demi Moore for 23 years and together have three daughters. Following his divorce from Moore, he married Emma Heming Willis, with whom he has two daughters.

Willis has recently continued his acting career, while also playfully satirizing it in films such as Red and The Expendables. He appeared in seven films that were released in 2021, while three films featuring Willis were released this year.

A further eight completed Willis films are currently set for release in 2023.