The comedian explained on Twitter that watching her mother's mental state deteriorate at a rapid rate over the past few months has been "devastating".
In 2005, Griffin launched a reality series on Bravo called Kathy Griffin: My Life on the D List, which featured her parents Maggie Griffin and John Griffin.
Maggie has since become a firm favourite among Griffin's fans due to her sharp wit and sense of humour.
Griffin regularly shares videos on social media of her mother with her dog, Twinkle, much to the delight of her two million followers.
A) As heartbreaking as this is, I feel the need to share some important info about my mom Maggie. I've always been honest with you all, but this one is really hard. The pic below, taken in September, was the last time I was able to have a proper/coherent conversation with her. pic.twitter.com/oqBcQTEDAB— Kathy Griffin (@kathygriffin)January 3, 2019
Some people had noticed that it had been a while since the comedian had mentioned her mother on Twitter, which is why Griffin decided to discuss her mother's dementia diagnosis openly for the first time.
"As heartbreaking as this is, I feel the need to share some important info about my mom Maggie," she tweeted.
"I've always been honest with you all, but this one is really hard.
"The pic below, taken in September, was the last time I was able to have a proper/coherent conversation with her."
She continued, explaining that the disease had taken a huge toll on her mother's mind over the past few months.
C) My mom is 98 and up until this past couple years she was so sharp she always kept me on my toes. Her mind was so naturally quick, funny, and smart. No one could get anything past her. Watching that slip away so fast has been devastating.— Kathy Griffin (@kathygriffin)January 3, 2019
"Since that photo was taken, she has rapidly fallen into the throes of dementia," she said.
"This is never easy for any child, I know this is a reality that millions of people deal with every year."
Griffin explained that her mother's sharp-witted mind has always been one of her most admirable assets, and that watching it "slip away so fast has been devastating".
The comedian has assured her fans that her mother isn't suffering from any pain and that she's being taken care of 24 hours a day.
She also revealed that her mother is currently only able to recall Griffin's name and to say "I love you".
Many people have been sending well-wishes to Griffin and some have been sharing their personal experiences of coping with loved ones being diagnosed with dementia.
— The Memories Project (@JoyMemories)January 3, 2019
I’m so sorry Kathy. Maggie is the BEST. I can’t imagine how hard it must be to watch a loved one struggle with dementia. 💔 https://t.co/0maZdQdMlc— kristen johnston (@thekjohnston)January 3, 2019
"We all felt we knew Maggie through your Twitter posts and now we all share your pain as you lose a part of her. Sending her love and warm wishes," wrote Star Wars actor Mark Hamill.
"My mum is very sick too," one person tweeted. "I'd trade anything to be able to take a walk with her one more time."
Another person explained how they'd taken care of their 94-year-old grandmother when she was diagnosed with dementia, describing it as "one of the hardest things to go through".
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There are approximately 850,000 people with dementia in the UK, according to the Alzheimer's Society.
This number is expecting to surpass one million by 2025.