Bruce Willis' functions will get "worse," Dr. Agus says

Bruce Willis' functions will get "worse and worse" as his frontotemporal dementia continues to advance, CBS News medical contributor Dr. David Agus told "CBS Mornings."

"This is a progressive disorder ... He won't be able to do many activities that we all do in life," Agus said.

His family announced his diagnosis on Thursday in a statement, writing that while it is a challenging diagnosis, the family was relieved to have "clear answers."

Agus said that on average it usually takes several years to make the diagnosis of frontotemporal dementia from when symptoms first present. His family announced last year the Hollywood star was stepping away from acting because of aphasia, in which patients struggle to find words and communicate verbally. In their statement this week, the family said his condition has progressed.

FTD is a degenerative brain disorder that happens when one of the proteins in the neurons becomes dysfunctional, causing it to accumulate and deteriorate the frontal and/or temporal lobes of the brain, Agus said.

What is frontotemporal dementia? Bruce Willis' diagnosis, explained

It makes up about 20% of dementia cases, second only to Alzheimer's, making FTD not "uncommon" and, Agus said, often underdiagnosed.

Approximately 1 of every 8 cases of FTD is genetic, and most happen to people in their 40s, 50s and 60s. Agus said that people can slow or delay the progression of dementia by "keeping that brain active."

"Every year you delay retirement, you reduce dementia by 3%. So, do things that make you uncomfortable — and it doesn't have to be your primary job, but don't just sit back and relax," Agus said.

Agus said that Willis will probably go through some of the symptoms of FTD and may require in-home care as a result.

"They can get nursing to help the family. But it is a tremendous stress to the family and obviously the patient and one of the underappreciated heroes are the people who take care of people with dementia," Agus said.

There is currently neither a cure nor treatments available for the condition but the Willis family — which includes his five daughters, his wife, Emma Heming Willis, and his ex-wife, Demi Moore — said in their statement that they hope that Willis' diagnosis helps shine a light on the "disease that needs far more awareness and research."

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