Sean Astin says mother Patty Duke was 'suffering terribly' before her death

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EXCLUSIVE: Sean Astin Says Mother Patty Duke Was 'Suffering Terribly'

Sean Astin was with his mother, Anna "Patty Duke" Pearce, when she died on Tuesday morning and he shared her final moments with ET in his only on-camera interview.

"She was surrounded by her family," Astin told ET. "It was in the middle of the night and it was a race to get there. It was a very powerful experience. And then, in her last moments, she was at peace."

WATCH: Sean Astin Remembers Late Mother Patty Duke

Her peace came after a painful battle with sepsis due to a ruptured intestine.

"She had been suffering terribly," Astin said.

Patty Duke through the years:

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Patty Duke through the years
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Sean Astin says mother Patty Duke was 'suffering terribly' before her death
UNITED STATES - NOVEMBER 12: JOURNEY TO THE UNKNOWN - 'Last Visitor' 6/16/69 Patty Duke (Photo by ABC Photo Archives/ABC via Getty Images)
UNITED STATES - NOVEMBER 12: JOURNEY TO THE UNKNOWN - 'Last Visitor' 6/16/69 Patty Duke (Photo by ABC Photo Archives/ABC via Getty Images)
LOS ANGELES, CA - FEBRUARY 2: Patty Duke attends 27th Annual Golden Globe Awards on February 2, 1970 at the Ambassador Hotel in Los Angeles, California. (Photo by Ron Galella, Ltd./WireImage)
LOS ANGELES, CA - FEBRUARY 2: Patty Duke attends 27th Annual Golden Globe Awards on February 2, 1970 at the Ambassador Hotel in Los Angeles, California. (Photo by Ron Galella, Ltd./WireImage)
Patty Duke during 27th Annual Golden Globe Awards at Ambassador Hotel in Los Angeles, California, United States. (Photo by Ron Galella/WireImage)
CANADA - JULY 25: Patty Duke's childhood of sorrows. 'Perfect teen' image hid tale of exploitation and abouse/H1 (Photo by Andrew Stawicki/Toronto Star via Getty Images)
John Astin and Patty Duke during 'Hedda Gabler' Los Angeles Premiere at Huntington Hartford Theater in Hollywood, California, United States. (Photo by Ron Galella/WireImage)
MARCUS WELBY, M.D. - 'Unindicted Wife' - Aired on February 25, 1975. (Photo by ABC Photo Archives/ABC via Getty Images) JOHN ASTIN;PATTY DUKE
Award-winning actress Patty Duke poses for photographers with her granddaughters, 2-year-old Elizabeth Astin (L) and 7-year-old Alexandra Astin following an unveiling ceremony, honoring Duke with the 2,260th star on the Hollywood Walk of Fame in Los Angeles, California August, 17, 2004. Duke won the Oscar for best supporting actress for "The Miracle Worker" at 16, the youngest at the time to be so named. REUTERS/Jim Ruymen JR/GN
HOLLYWOOD - AUGUST 17: Actor David Ogden Stiers kisses actress Patty Duke at the ceremony honoring Duke with a star on the Hollywood Walk of Fame August 17, 2004 in Hollywood, California. (Photo by Vince Bucci/Getty Images)
Award-winning actress Patty Duke (C) poses with sons, MacKenzie (L) and Sean Astin following an unveiling ceremony honoring Duke with the 2,260th star on the Hollywood Walk of Fame in Los Angeles, California August, 17, 2004. Duke won the Oscar for best supporting actress for "The Miracle Worker," at 16, the youngest at the time to be so named. REUTERS/Jim Ruymen JR
Patty Duke during Patty Duke Honored with a Star on the Hollywood Walk of Fame for Her Achievements in Film at 7000 Hollywood Blvd. in Hollywood, California, United States. (Photo by Jesse Grant/WireImage)
HOLLYWOOD, CA - MARCH 7: Actress Patty Duke Astin receives her award for Favorite Dual-Role on stage at the 2nd Annual TV Land Awards held on March 7, 2004 at The Hollywood Palladium, in Hollywood, California. (Photo by Kevin Winter/Getty Images)
Actress Patty Duke talks about working on a student production of 'The Four Children of Tander Welch,' in Columbia, South Carolina, on November 20, 2006. (Photo by Gerry Melendez/The State/MCT via Getty Images)
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Duke died at the age of 69 at a hospital in Coeur D'Alene, Idaho. She was perhaps best known for portraying Helen Keller in 1962's The Miracle Worker, a role that earned her an Academy Award for Best Supporting Actress. At the time, she was the youngest Oscar winner ever at 16.

WATCH: Melissa Gilbert Remembers Patty Duke After Her Death: 'She Was My Second Mom'

Along with her Oscar win, she will also be remembered for helping destigmatize mental illness. After bravely going public about her struggle with manic depression, Duke traveled the nation lobbying as a mental health advocate.

"The more she opened up, the more she shared of her pain. It is very instructive for people," Astin said. "This [1982] bipolar diagnosis was a watershed. Now she had a new identity and a whole new mission -- a whole new sense of purpose -- and that was to share what she had gone through with other people."

The actress, along with other advocates like former First Lady Rosalynn Carter and Mariel Hemingway, spoke out publicly about her bipolar disorder and other psychiatric health issues at a time when mental illnesses carried an even heavier stigma. Duke even opened up to ET about her diagnosis in a 1992 interview.

"I happen to prefer calling my disorder manic depression," Duke told ET at the time. "It is known as bipolar now. And if that works for other people, fine, but I prefer to call it manic depression because it sounds like it felt."

WATCH: Patty Duke Dies at 69 -- Hollywood Pays Tribute

To honor his mother's legacy, Astin has set up a fundraiser for The Patty Duke Mental Health Project, which will continue Duke's campaign of awareness.

"Above all, the way that I think of my mother, the thing that gives me such joy and reverence for her, is that above all else, she was a warrior," Astin said.

Notable people we lost in 2016:

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Notable people we lost in 2016
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Sean Astin says mother Patty Duke was 'suffering terribly' before her death

January 4 -- Robert Stigwood, manager of the Bee Gees and Cream.

(Photo by Michael Putland/Getty Images)

January 4 -- Country singer Craig Strickland was found dead at age 29 from hypothermia.

(Photo via Instagram)

January 6 -- Pat Harrington, Jr., actor in 'One Day at a Time,' died at 86 from complications from Alzheimer's disease.

(Photo by CBS via Getty Images)

January 14 -- Actor Alan Rickman, popular for playing Professor Snape in the 'Harry Potter' films, died at 69 after battling cancer.

(Photo by Mike Pont/WireImage)

January 18 -- Glenn Frey, founding member of The Eagles, died at age 67 due to complications from multiple ailments.

(Photo by Tommaso Boddi/WireImage)

February 2 -- Jibri Bryan, #34, of the Mercer Bears was killed in a shooting at age 23.

(Photo by Streeter Lecka/Getty Images)

February 4 -- Maurice White, founder of Earth, Wind & Fire, died from the effects of Parkinson's disease at age 74.

(Photo by Richard E. Aaron/Redferns)

February 4 -- Astronaut Edgar D. Mitchell, the sixth person to walk on the moon, passed away at 85 under hospice care.

(Photo via NASA)

March 9 -- Beatles producer Sir George Martin passed away at age 90, as announced by Ringo Starr on Twitter.

(Photo by Vince Bucci/Getty Images)

March 16 -- Singer Frank Sinatra Jr.

(Photo by Express/Express/Getty Images)

March 29 -- Oscar winning actress Patty Duke, at the age of 69.

(Photo by ABC Photo Archives/ABC via Getty Images) 
Actor Tommy Ford passed on October 12, he was 52 years old. (Photo by Moses Robinson/Getty Images)

Agnes Nixon the creator of 'All my children' left us on September 28th at the age of 93.

(Photo by ABC Photo Archives/ABC via Getty Images)

Darrell Ward the star of 'Ice Road Truckers' life was taken in a plane crash, he was 52 years old. 
Popular voice actor Jack Riley passed on August 19th. (Photo by Michael Buckner/Getty Images)
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