How the dementia Bruce Willis has progresses, from trouble speaking to physical problems

• Bruce Willis' family announced that he has rare type of dementia.
• He retired from acting last year because of challenges with communication called aphasia.

Bruce Willis has been diagnosed with a rare condition called frontotemporal dementia, according to his family.

His family announced last year that the 67-year-old "Die Hard" star was retiring from acting after being diagnosed with aphasia, a language and communication disorder.

In a statement on Thursday, the Willis family said his condition had progressed and he had a more specific diagnosis of frontotemporal dementia — the most common form of dementia for people under 60 — of which aphasia is an early symptom.

The statement, signed by his daughters, wife, and and ex-wife said: "Unfortunately, challenges with communication are just one symptom of the disease Bruce faces.

"While this is painful, it is a relief to finally have a clear diagnosis."

They said that if he could today, Willis would want to bring global attention to the disease.

FTD can damage the front or sides of the brain

FTD is a group of dementias that mainly affect personality and behavior or language and speech because of damage to the front or sides of the brain, Sebastian Crutch, professor of neuropsychology at UCL's Dementia Research Centre, UK, told Insider.

The condition is caused by an abnormal build-up of certain proteins in brain cells, but it is not known why this happens, and there is no known cure, according to the charity Rare Dementia Support.

There is often a genetic link, with around one in eight people who develop FTD having relatives also affected by the condition, the UK's National Health Services says.

Symptoms of FTD include loss of vocabulary and difficulty swallowing

There are a variety of FTD symptoms including behavioral changes, language problems, issues with mental abilities, and physical problems, the NHS says.

Behavioral changes usually come first, with the sufferer often unaware. These can include being insensitive or rude, compulsively eating or drinking alcohol, neglecting hygiene, and changing food preferences. These changes come on gradually and increase as the condition worsens.

Some people suffer from language changes, including using words in the wrong order or incorrectly — calling a sheep a dog, for example, the NHS says. Some people with FTD become completely mute over time.

Willis' family hasn't confirmed what type of FTD he has, but because of his difficulty speaking he likely has primary progressive aphasia, Dr. Henry Paulson, director of the Michigan Alzheimer's Disease Center at the University of Michigan, told CNN.

"PPA may start with difficulty simply finding words, so people begin to use simpler words or more generic words for things they can't quite recall," Paulson said.

He said that if language is more effortful on a daily basis for a person, they should see a doctor.

As the condition progresses, people with FTD might struggle with planning or working things out, among other mental ability issues.

In later stages, patients may experience physical changes including a loss of bladder and bowel control, slow stiff movements, and difficulty swallowing.

The NHS says how quickly FTD worsens varies by person, but the average survival time after symptoms start is around 8 to 10 years.