Actor Bruce Willis has been diagnosed with dementia, evolving from a previous diagnosis of aphasia last spring, his family announced Thursday.

More specifically, Willis has frontotemporal dementia, which can include aphasia, which brings challenges with speaking and writing.

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

Willis's family said last year the actor would be stepping away from his decades-long career due to his impaired cognition.

What is frontotemporal dementia?

Frontotemporal dementia, also known as FTD, is one of several types of dementia and causes nerve damage in the frontal and temporal lobes, which leads to a loss of function in those areas, according to the Alzheimer's Association.

There are different types of frontotemporal dementia. Behavioral variant frontotemporal dementia causes nerve loss in the areas of the brain that control empathy, judgment and conduct.

Primary progressive aphasia deteriorates parts of the brain that control speaking, writing and comprehension. The onset of symptoms typically begins before age 65, but can occur later.

FTD can also disrupt motor function and movement, which could be classified as Lou Gehrig's disease, also known as ALS.

How is FTD different from Alzheimer's?

Diagnosis of FTD tends to happen between a person in their 40s and 60s, while Alzheimer's happens at a later age. Alzheimer's is also more closely tied to hallucinations, memory loss and issues with spatial orientation, such as getting lost.

Treatment and diagnosis

Doctors use brain imaging technology, such as MRIs, to diagnose FTD. The results are analyzed in tandem with a patient's medical history and symptoms. About 30% of people with frontotemporal degeneration inherit the disease; there are no known risk factors.

There are medications that can help relieve symptoms, but the disease eventually gets worse with time.