CNN —

Emma Heming Willis is opening up about the challenges facing her husband, Bruce Willis, as he lives with frontotemporal dementia.

Three years after Willis stepped away from his career after having been diagnosed , his wife spoke with Diane Sawyer for a special titled, "Emma & Bruce Willis: The Unexpected Journey."

According to the Mayo Clinic , frontotemporal dementia (FTD) is an "umbrella term for a group of brain disorders that primarily affect the frontal and temporal lobes of the brain. These areas of the brain are generally associated with personality, behavior and language."

Heming Willis told Sawyer that while Willis is mobile and generally in good health, it is "his brain that is failing him."

"The language is going, and, you know, we've learned to adapt," she said.

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