It's not a disorder that's widely recognized because it's a symptom of other conditions. It can be devastating and life-altering.
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In Willis' movies, the quick-witted action star can fast talk his way out of any calamity. But now a serious language disorder is robbing the 67-year-old actor of the words he relies on.
On Instagram, his daughter Rumer Willis wrote "... our beloved Bruce has been experiencing some health issues and has recently been diagnosed with aphasia, which is impacting his cognitive abilities."
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Dr. May Kim-Tenser, a neurologist with Keck Medicine of USC, said how much the severity of the condition affecting Willis depends on the cause.
"Aphasia is basically the inability to communicate and speak and or understand language both verbal and written," she said. "If it happens suddenly, it's usually due to a stroke or it can be from head injury. But you can also see other types of aphasias that come on gradually from either a brain tumor that's slowly growing or from a degenerative progressive disease such as dementia."
Doctors remind us that aphasia is a symptom and the specifics of Willis' diagnosis have not been released. Over the past two years, Willis has released three films, such as "Gasoline Alley" and "American Siege."
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Kim-Tenser said living with aphasia can be quite frustrating.
"They're trying to communicate with someone and they're not able to get their words out or the words come out jumbled. Or they have something called word salad where they substitute other words," she said.
About 180,000 Americans get this brain disorder each year. Doctors say if these changes come on suddenly, it's important to call 911.
"It can be a stroke. And we have treatments in the ER that we can do to potentially reverse it," Kim-Tenser said.
On March 19, Willis celebrated his birthday with those he loved. Experts say strong family support can have a positive impact on living successfully with aphasia.
The disorder does not affect intelligence. Some people improve dramatically in a few months. Others may need to find other ways to communicate such as speech therapy and communication apps.