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:

Patty Duke through the years
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Sean Astin says mother Patty Duke was 'suffering terribly' before her death
Child actress Patty Duke roller skates in Central Park in New York City, April 6, 1961. (AP Photo)
Gregory Peck, holding pocket watch, jokes with, from left, Patty Duke, Joan Crawford and Ed Begley backstage at the Academy Awards at Santa Monica Civic Auditorium in Los Angeles, Ca., April 8, 1963. Peck won best actor for "To Kill a Mockingbird"; Duke won best supporting actress for "The Miracle Worker"; Crawford accepted the Oscar for Anne Bancroft, who won best actress for "The Miracle Worker"; and Begley won best supporting actor for "Sweet Bird of Youth." (AP Photo)
Child actress Patty Duke is filling out the letters of her name as it appears on the marquee at the Playhouse Theatre in New York City, advertising the play "The Miracle Worker," on March 25, 1960. 13-year-old Patty, who stars opposite Anne Bancroft in the hit show, is held and supported by Mannie Kay, president of the sign company. (AP Photo)
Actress Patty Duke, star of ABC-TV's "The Patty Duke Show," and Colette Daiute, right, who recently won the Miss Teenage America contest, meet at a Hollywood beauty salon as they prepare for their roles on Patty's show, Dec. 3, 1965. Fixing Patty's hair is hair stylist Jeanne Bodel. (AP Photo)
Winners of the best supporting performers Oscars pose with the presenters at the annual Academy Awards at Santa Monica, Ca., on April 8, 1963. Left to right; presenter George Chakiris, Patty Duke who won the best supporting actress for her role in The Miracle Worker, Ed Begley, names best supporting actor for the Sweet Bird of Youth, and Rita Moreno who presented Begley with his Oscar. (AP Photo)
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)
Actress Patty Duke appears as a witness before a House Appropriations Subcomittee holding hearings on funding for mental health programs on Capitol Hill in Washington, D.C., April 27, 1989. At left is chairman of the panel Rep. William Natcher of Kentucky. (AP Photo/Dennis Cook)
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
FILE - In this Aug. 17, 2004 file photo, actress Patty Duke is honored with a star on the Hollywood Walk of Fame in Los Angeles.Duke, who won an Oscar as a child at the start of an acting career that continued through her adulthood, died Tuesday, March 29, 2016, of sepsis from a ruptured intestine. She was 69. (AP Photo/Damian Dovarganes, File)
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)
Actress Patty Duke is seen during an interview in New York, Nov. 18, 2002. Duke, Helen Keller in "The Miracle Worker," 1960s TV icon and pill-popping heroine of "Valley of the Dolls," has joined the cast of the Broadway revival of "Oklahoma!" (AP Photo/Louis Lanzano)
Academy and Emmy award-winning actress, Patty Duke is shown during a news conference at the Paley Center for Media in Beverly Hills, Calif., on Tuesday, March 23, 2010. The cast of the 1960s TV comedy "The Patty Duke Show," are promoting the Social Security Administration's service allowing baby boomers to apply online for retirement and Medicare benefits. (AP Photo/Damian Dovarganes)
The cast of the television hit 1960s sitcom: âThe Patty Duke Show," Paul O'Keefe, left, and Patty Duke, smile during a Social Security news conference at the Paley Center for Media in Beverly Hills, Calif., on Tuesday, March 23, 2010. The cast of the 1960s TV comedy now as seasoned citizens, are promoting the Social Security Administration's service allowing baby boomers to apply online for retirement and Medicare benefits. (AP Photo/Damian Dovarganes)
Patty Duke speaks at the national premiere of her new Social Security Public Service Snnouncement at the Paley Center for Media in New York City. January 6, 2009 Credit: Dennis Van Tine/MediaPunch/IPX
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)

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:

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

January 1 -- Former U.S. Sen. Dale Bumpers, D-Ark., seen here in 2006, passed away at the age of 90. Bumpers' signature moment on the national stage came in 1999, just weeks after leaving the Senate, when he defended Pres. Bill Clinton — who had worked for a challenger's 1974 campaign — before the U.S. Senate during his impeachment trial.

(AP Photo/Danny Johnston)

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 11 -- Singer David Bowie died after battling cancer for 18 months. He was 69.

(AP Photo/Marty Lederhandler, FIle)

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)

January 29 -- Paul Kantner, guitarist, vocalist and founding member of the band Jefferson Airplane, passed away at age 74 of multiple organ failure following a heart attack.

(AP Photo/Shawn Baldwin, File)

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)

February 13 -- United States Supreme Court justice, Antonin Scalia.

(AP Photo/Haraz N. Ghanbari)

February 19 -- Author of the beloved novel To Kill A Mockingbird, Harper Lee.

(AP Photo, File)

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 24 -- Comedian Garry Shandling at age 66. 

(AP Photo/Chris Carlson, File)

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

(Photo by ABC Photo Archives/ABC via Getty Images) 
FILE - In this Dec. 6, 1973 file photo, political activist Tom Hayden, husband of Jane Fonda, tells newsmen in Los Angeles that he believes public support was partially responsible for the decision not to send him and others of the Chicago 7 to jail for contempt. Hayden, the famed 1960s anti-war activist who moved beyond his notoriety as a Chicago 8 defendant to become a California legislator, author and lecturer, has died at age 76. His wife, Barbara Williams, says Hayden died on Sunday, Oct. 23, 2016, in Santa Monica of a long illness. (AP Photo/George Brich, File)
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)

 In this July 22, 2007 photo, Alexis Arquette arrives at the "Comedy Central Roast of Flavor Flav" in Burbank, Calif. Arquette died from a heart attack and battled HIV for 29 years, according to her death certificate. The transgender character actress and sibling of actors David, Rosanna, Richmond and Patricia Arquette died Sept. 11, 2016, in Los Angeles. She was 47. (AP Photo/Matt Sayles, File)
In this April 9, 2008 photo, actor Gene Wilder listens as he is introduced to receive the Governor's Awards for Excellence in Culture and Tourism at the Legislative Office Building in Hartford, Conn. Wilder, who starred in such film classics as "Willy Wonka and the Chocolate Factory" and "Young Frankenstein" has died. He was 83. (AP Photo/Jessica Hill, File)
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)
In this April 28, 2012, photo, John McLaughlin arrives at the White House Correspondents' Association Dinner on in Washington. McLaughlin, the conservative political commentator and host of the namesake long-running television show that pioneered hollering-heads discussions of Washington politics, died Tuesday, Aug. 16, 2016. He was 89. (AP Photo/Kevin Wolf)
In this April 10, 2016 file photo, Garry Marshall arrives at the 2016 TV Land Icon Awards at Barker Hangar in Santa Monica, Calif. Writer-director Marshall, whose TV hits included "Happy Daysâ "Laverne & Shirley" and box-office successes included "Pretty Woman" and "Runaway Bride," has died at age 81. Publicist Michelle Bega says Marshall died Tuesday, July 19, 2016, in at a hospital in Burbank, Calif., of complications from pneumonia after having a stroke. (Photo by Rich Fury/Invision/AP, File)
This image provided by Sports Illustrated on Saturday, June 4, 2016 shows the newest Sports Illustrated Cover Tribute to Muhammad Ali. The portrait was shot by Neil Leifer while Ali trained at 5th Street Gym in Miami Beach on Oct. 9, 1970. This will be the June 13 issue cover on stands June 8. Ali, the magnificent heavyweight champion whose fast fists and irrepressible personality transcended sports and captivated the world, has died according to a statement released by his family Friday, June 3. He was 74. (Sports Illustrated via AP)
FILE - In this Wednesday, Sept. 3, 2014, file photo, Actor Anton Yelchin poses for portraits during the 71st edition of the Venice Film Festival in Venice, Italy. Yelchin, a charismatic and rising actor best known for playing Chekov in the new "Star Trek" films, has died at the age of 27. He was killed in a fatal traffic collision early Sunday morning, June 19, 2016, his publicist confirmed. (AP Photo/Domenico Stinellis, File)
FILE - In this June 29, 2014, file photo, "The Voice" Season 6 contestant Christina Grimmie performs as part of "The Voice Tour" at Cobb Energy Centre, in Atlanta. Family and friends are gathering in New Jersey on Friday, June 17, 2016, for a viewing and memorial service to remember Grimmie, who was fatally shot a week earlier, as she signed autographs after a show in Orlando, Fla. (Photo by Robb D. Cohen/Invision/AP, File)
FILE - In this July 31, 1997 file photo, actor Alan Young of the "Mister Ed" television series, poses with Mister Ed-For-A-Day, "Champagne," a 13-year-old Palomino mare, in San Francisco. Young, who played straight man to the talking horse in the 1960s sitcom, has died. Jaime Larkin, a spokeswoman for the Motion Picture and Television Home in Los Angeles, says that Young died Thursday, May 19, 2016. He was 96. (AP Photo/Susan Ragan, File)
FILE - In this Feb. 4, 2007 file photo, Prince performs during the halftime show at the Super Bowl XLI football game at Dolphin Stadium in Miami. The disclosure that some pills found at Prince's Paisley Park home and studio were counterfeit and contained the powerful synthetic opioid fentanyl strongly suggests they came to the superstar illegally. Prince died April 21, 2016, of an accidental fentanyl overdose. (AP Photo/Chris O'Meara, File)
FILE - In this Nov. 16, 2003, file photo, Joanie Laurer, former pro wrestler known as Chyna, flexes her bicep as she arrives at the 31st annual American Music Awards, in Los Angeles. Chyna's manager, Anthony Anzaldo, says he believes the former wrestling star died of an accidental overdose of a prescription sleeping pill and a tranquilizer. Anzaldo found the body of the 46-year-old in bed at her Redondo Beach home on April 20, 2016. (AP Photo/Kevork Djansezian, File)
FILE - In this Nov. 1, 2015 file photo, Doris Roberts arrives at the Hollywood Film Awards in Beverly Hills, Calif. Family spokeswoman said Monday, April 18, 2016, that Roberts died overnight Sunday in her sleep in Los Angeles. She was 90. (Photo by Jordan Strauss/Invision/AP, File)
FILE - In this Jan. 30, 2016 file photo, Ken Howard, president of SAG-AFTRA, speaks at the 22nd annual Screen Actors Guild Awards in Los Angeles. Howard, who starred in 1970s series âThe White Shadowâ and has led the Screen Actors Guild for years, died, Wednesday, March 23, 2016, at age 71. No cause of death was given. (Photo by Vince Bucci/Invision/AP, File)
FILE - In this Sunday, Aug. 28, 1988 file photo, Larry Drake holds the Emmy he won for best supporting actor in a drama series for his role as a mentally challenged character in "L.A. Law," at the 40th annual Emmy Awards in Pasadena, Calif. Drake, who earned back-to-back Emmys for his portrayal of Benny Stulwicz, was found dead in his Los Angeles-area home on Thursday, March 17, 2016. He was 66. (AP Photo/Lennox McLendon)
FILE - In this April 3, 2011 file photo, Joey Martin Feek, left, and Rory Lee Feek of "Joey + Rory" arrive at the 46th Annual Academy of Country Music Awards in Las Vegas, Nev. Joey Feek is recovering after undergoing surgery to treat a reoccurrence of cervical cancer. The singer underwent surgery on Thursday, July 9, 2015, at the Cancer Treatment Centers of America in Chicago and said she was doing âgreat.â(AP Photo/Chris Pizzello, File)
FILE - In this Dec. 18, 2011, file photo, Angela Raiola, better known as Big Ang, arrives at "Vh1 Divas Celebrates Soul" in New York. Raiola from the reality TV series "Mob Wives" has died following a nearly yearlong battle with cancer, series producer Jennifer Graziano said Thursday, Feb. 18, 2016. (AP Photo/Charles Sykes, File)
FILE - In this March 22, 1985 file photo, actor Taimak, left, and Vanity arrive at the Century Plitt Theater for the premiere of their film "The Last Dragon," in Los Angeles. Vanity, a Prince protege who renounced her sexy stage persona to become a Christian minister, has died at age 57. The singer and actress, born Denise Matthews, died Monday, Feb. 15, 2016, at a hospital in Fremont, Calif, said Gisela Hernandez, a spokeswoman for Washington Hospital Health Care System. (AP Photo/Liu Heung Shing, File)
FILE - In this June 30, 2009, file photo, Glenn Frey, of the the Eagles, performs at Belfast's Odyssey Arena in Northern Ireland. Frey, who co-founded the Eagles and with Don Henley became one of history's most successful songwriting teams with such hits as "Hotel California" and "Life in the Fast Lane," has died at age 67. He died Monday, Jan. 18, 2016, in New York. (AP Photo/Peter Morrison, File)

Celine Dion's husband Rene Angelil Passes Away at 73.

 Â©RTNRD / MediaPunch/IPX

FILE - In a Monday, March 2, 2015 file photo, singer Natalie Cole performs at "An Evening of SeriousFun Celebrating the Legacy of Paul Newman," at Avery Fisher Hall, in New York. Cole, the daughter of jazz legend Nat "King" Cole who carried on his musical legacy, died Thursday night, Dec. 31, 2015, according to publicist Maureen O'Connor. She was 65.(Photo by Evan Agostini/Invision/AP, File)
FILE - This June 26, 2015 file photo shows Motorhead bassist Lemmy Kilmister performing on the Pyramid stage during Glastonbury Music Festival at Worthy Farm, Glastonbury, England. Ian "Lemmy" Kilmister, the Motorhead frontman whose outsized persona made him a hero for generations of hard-rockers and metal-heads, has died on Monday, Dec. 28, 2015. (Photo by Joel Ryan/Invision/AP, file)

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