How does having a personality disorder affect Frontal Temporal Dementia?

I can certainly speak to this issue a little. My mom had some kind of personality disorder probably all of her life, at least as long as I've been around (45 years). She was never diagnosed because she refused to believe anything was wrong with her, but in hindsight, we're pretty certain she had borderline personality disorder and/or narcissistic personality disorder and perhaps a little paranoid/delusional disorder thrown in the mix. She developed symptoms of early onset Alzheimers in her mid 50s and was dead by age 69 from very advanced Alzheimers symptoms. In the end, all of the same personality disorder signs were just as strong as they had ever been. She had ZERO short term memory but would still react to situations emotionally the same way she always had--with distrust, paranoia, self-pity, and self absorption. Her diagnosis at the end was "Alzheimers with severe psychiatric disturbance." Think personality disorder on steroids. Or Alzheimers on steroids. Either way, it was miserable to watch and miserable to live with.
One thing I've thought about that's hard to articulate, but it sounds like you may understand what I mean: because we watched her all of our lives be "not quite right" emotionally and mental health-wise, once the Alzheimers symptoms started, for years we just felt like it was more of the same ol' same ol' from her. I think her prior lifetime of showing a personality disorder stripped us of the ability to feel much of a sense of urgency when the Alzheimers symptoms started. It didn't feel like much out of the ordinary to see her "losing her mind." That's sad to say now, but I look back and know that we just never felt things like "oh it's so sad and unusual for mom to act this way." No, instead, we watched her decline into Alzheimers and thought "yeah, that's how she's always been." So sad. I hope no one judges me for admitting that. Until you've lived with a very mentally ill parent, you just can't imagine what it's like.
And, by the way, at the end, there was one neurologist who believed she did not have Alzheimers but instead had FrontoTemporal Dementia, just as you have mentioned.
Best wishes to you. It can be a long hard road to navigate. Use this site for support. It's what got me through!

I recommend you research on the site for the AFTD, Association for FrontoTemporal Dementia. There are specific support groups through them from people who understand FTD.