Bruce Willis' daughter thought he'd lost interest in her. Then she realized it was an early dementia symptom.

• Bruce Willis developed a "vague unresponsiveness," his daughter Tallulah Willis wrote in Vogue.
• The family put it down to "Hollywood hearing loss" from being in films like "Die Hard."

Bruce Willis' daughter thought her father had lost interest in her, but he was actually suffering from a type of dementia, she wrote in Vogue.

In the past, Bruce had a "vague unresponsiveness" about him, Tallulah Willis wrote, which the family put down to "Hollywood hearing loss" from filming movies like "Die Hard."

She said his unresponsiveness "broadened," and she took it personally at times, thinking she wasn't interesting enough for her father now he had two new children with her step-mom, Emma Heming Willis.

"Though this couldn't have been further from the truth," she wrote.

In Spring of last year, his family revealed that he had been diagnosed with aphasia, which in February this year they said had progressed to frontotemporal dementia, a group of dementias that can affect personality, behavior, language, and speech.

There is no known cure for FTD, which has a variety of symptoms that usually start with behavioral changes, Insider previously reported, which can include being insensitive or rude.

The patient can struggle with using the wrong words, and some may become mute over time.

Tallulah also talked about her struggles with an eating disorder and her mental health. She said she had an ache in her heart wondering what her dad would have done if he was his "full self" when he'd seen her at her lowest weight.

As FTD progresses, patients may struggle with planning, working things out, and other mental ability issues.

"I keep flipping between the present and the past when I talk about Bruce: he is, he was, he is, he was. That's because I have hopes for my father that I'm so reluctant to let go of," Tallulah said.