The Notebook star diagnosed with Alzheimer's 20 years after playing character with same illness

The Notebook's Gena Rowlands has Alzheimer's disease, her son has revealed.

Rowlands played the older version of Rachel McAdams' Allie in the 2004 film.

The film, which also stars Barbie's Ryan Gosling, follows the love story between Allie and Noah (Gosling) as they face pressure from their families due to their different socioeconomic backgrounds.

The 94-year-old's son, Nick Cassavetes, has now revealed his mom's diagnosis with Entertainment Weekly as The Notebook celebrated its 20th anniversary on Tuesday (25 June).

Gena Rowlands has Alzheimer's, her son says (Jeffrey Mayer/WireImage)

"I got my mom to play older Allie, and we spent a lot of time talking about Alzheimer's and wanting to be authentic with it, and now, for the last five years, she's had Alzheimer's." Cassavetes said.

"She's in full dementia. And it's so crazy — we lived it, she acted it, and now it's on us."

According to the Alzheimer's Association, Alzheimer's is a common cause of dementia, which is often used as an umbrella term to describe those who are living with a variety of symptoms.

Symptoms of dementia are usually a decline in thinking skills, which can impact a person's daily life and relationships.

While an exact cause is not known, it is believed that Alzheimer's can be triggered by age or family history.

Rowlands' mother and Cassavetes' grandmother also had the condition, and the actor had previously spoken about how it affected her decision to take the role.

Rowlands played the older version of McAdams' character (New Line Cinema)

In a 2004 interview with O magazine, she said: "This last one — The Notebook, based on the novel by Nicholas Sparks — was particularly hard because I play a character who has Alzheimer's.

"I went through that with my mother, and if Nick hadn't directed the film, I don't think I would have gone for it — it's just too hard. It was a tough but wonderful movie."

While the film is one of Rowlands' better known roles in recent years, the star has had a massive career, having earned two Oscar nominations for her work in 1974's Woman Under The Influence and 1980's Gloria.

Given The Notebook's tremendous legacy, Cassavetes revealed that he is proud of the film as it 'holds up pretty good'.

"It's always a shock to hear that as much time has gone by as it has, but it makes sense. I'm just happy that it exists," the director said.

"It seems to have worked and I'm very proud of it."