(NEXSTAR) – Bruce Willis’ family released a statement Thursday revealing that he has been diagnosed with frontotemporal dementia (FTD), a progressive disease that has no cure.

FTD symptoms often arise in younger patients, between the ages of 40 and 65, according to the Johns Hopkins School of Medicine.

“It’s not unusual that we see people in the prime of their life who are working, raising kids … and they come down with this condition that changes their personality, changes their ability to communicate and worsens relentlessly to where they become completely disabled,” Dr. Constantine Lyketsos, an Alzheimer’s researcher at Johns Hopkins, told Nexstar.

After Alzaheimer’s, FTD is the second most common form of dementia in middle age patients with most patients falling into one of two groups, according to Lyketsos.

“One group is what’s referred to as aphasia, and that essentially means difficulty with using language,” Lyketsos said. “It could be trouble finding words; it could be trouble with halting speech, expressing oneself fluently; it could also be trouble understanding things, although with FTD it’s more likely the expressive side.

The other common type of FTD symptoms involve behavior and personality.

“These have to do with changes in behavior and personality in social context, primarily,” Lyketsos said. “So people are inappropriate in social settings, they might say the wrong thing. They might get stuck on things around them in conversations that is uncharacteristic of them. They occasionally could become inappropriate or violent, although that’s very uncommon.”

Family says Willis’ condition has ‘progressed’

Nearly a year after the “Die Hard” actor’s family announced that he would step away from acting after being diagnosed with aphasia, family says Willis’ “condition has progressed.”

In Thursday’s statement, his family said communication challenges were just one symptom of his frontotemporal dementia.

“Today there are no treatments for the disease, a reality that we hope can change in the years ahead,” the statement read. “As Bruce’s condition advances, we hope that any media attention can be focused on shining a light on this disease that needs far more awareness and research.”

The statement was posted on the website for The Association for Frontotemporal Degeneration and signed by Willis’ wife, Emma Heming Willis, his ex-wife Demi Moore, and his five children, Rumer, Scout, Tallulah, Mabel and Evelyn.

“Our hearts are with Bruce Willis and his family, as is our gratitude for his family’s decision to bravely share Bruce’s diagnosis of frontotemporal dementia,” the Alzheimer’s Association said in a statement.

According to Johns Hopkins Medicine, FTD affects women and men in equal numbers. While there is no cure, treatments include antidepressants, antipsychotic medicine, behavior modification and speech therapy.

Over a four-decade career, Willis’ movies earned more than $5 billion at the worldwide box office. While beloved for hits like “Die Hard” and “The Sixth Sense,” the prolific actor had in recent years primarily featured in direct-to-video thrillers.

The Associated Press contributed to this report.