Jan. 16, 2014 -- Marianna Palka, who left Scotland to pursue a career in filmmaking in the United States, knew she had a "50-50" chance of getting Huntington's disease – a rare, but devastating genetic disease that has been described as Parkinson's, Alzheimer's and ALS, all rolled into one.
It began to destroy her father his late 30s, so Palka, now 32 and symptom-free, decides to find out if she, too, has the genetic mutation that will rob eventually her of her mobility and her mind.
Filmmaker Lucy Walker, who received Oscar nominations for "Waste Land" (2010) and "The Tsunami and the Cherry Blossom," (2011) follows Palka as she prepares to go to the doctor and open the envelope that will seal her fate. Her short documentary, "The Lion's Mouth Opens," premieres at the Sundance Film Festival Jan. 18.
Surrounded in the film by family and close friends, Palka was struck by fear, determination and an overwhelming sense that she will be able to control her destiny, no matter what the test reveals.
"I have always felt like I don't have it. I always know I don't have it. I don't feel like any good will come from me having it," she says in the opening of the 16-minute film, as she rolls the dice in a game with her closest friends on the eve of her doctor's appointment. "It's strange to have that news and to know it's sitting there."
"Getting a positive result is like a death sentence," she says. "But if I get the opposite result, I get to be like everyone else."
The documentary takes its name from the Bob Dylan poem, "Last Thoughts," after he visited folk singer Woody Guthrie, who was dying of Huntington's disease at a New York City psychiatric hospital: "And the lion's mouth opens and y'er staring at his teeth, and his jaws start closin' with you underneath."
Such is the fear that Palka said she had to overcome after watching the deterioration of her own father, one that her mother describes in the film as "each brick from the castle … falling out and the castle … caving in."
Walker and Palka had met through a mutual friend, the musician Moby, at Sundance. The director was moved by Palka's idea to film her journey.
"My heart went out to her, and I thought this is such a daunting prognosis, that I was immediately able to say I would drop everything," Walker told ABCNews.com.
"This film is so much about the moment where life really throws you an unimaginable challenge and the profoundest of difficulties," she said. "Marianna had incredible courage sharing this moment with the world, and it is a service to others. … Rare talents have the gift to open themselves to the camera. It's amazing when they let you in."
Palka's father, 63, is in a nursing home in Glasgow. "He's not a living person," she told ABCNews.com. "He died a long time ago."
"Because I have a family, we all know how hard and horrifying this disease is," Palka said. "It's not abstract, not like I might get a future disease I have never heard of. People at risk know what it entails."
But Palka also said that a close relationship with her mother, whom she called a "shining star," and sharing her fear with others helped her cope.
Initially, Palka said she had wanted to learn if she had the disease so she could make plans to have a child, perhaps through IVF, where the embryo can be tested. But, there was more to her journey.
"Having the sense of the unknown my whole life and going back and forth psychologically to plan a life you don't know exists, when you don't know what your road is, always vacillating. If I have it what does [life] it look like and if I don't, what does it look like."
An estimated 30,000 Americans live with Huntington's disease, one that affects their ability to walk, to think and to regulate their mood. But an even larger group – about 250,000 – are, like Palka, at risk, but symptom-free.
Huntington's is an autosomal dominant disease caused by the presence of a mutation in the length of the Huntington protein. The child of a parent with the mutation has a 50-50 chance of inheriting the mutation and the disease.
"It actively affects the biological processes of your body throughout your development," said Louise Vetter, CEO of the Huntington's Disease Society of America (HDSA). "Science has come to understand that the body compensates for a certain amount of time for the neurons that are dying."
"It will hit you at some point," she said. "The difference depends on the length of the mutation. Through genetic testing you are actually being diagnosed. Unlike the vast majority of test, this is not a risk factor. You have this in your body and it's just a matter of time."
Depending on the length of the repeat on the gene, the onset of the disease could be anywhere from the 70s down to the 30s, or earlier, according to Vetter.
"It was absolutely an incredibly brave and courageous act … to be prepared to deal with the deck that's dealt," said Vetter of Palka's decision to get the genetic test. "Typically, the decision comes when young couples think about planning a family. … It takes a proactive and special kind of person to say they are prepared for whatever they learn and know they can handle this."
Palka wrote, directed and starred in the award-winning 2009 film, "Good Dick," with Charles Durning and Tom Arnold. Her other co-producer and co-star in that film, Jason Ritter, is one of the close friends who supports her through Walker's latest documentary.
Ritter's own father, actor John Ritter, died of an aortic aneurysm at the age of 54, and so he has shared his fundraising experience with Palka to help find a cure for Huntington's. She calls him, "my family."
They have also supported the legislative efforts of HDSA to fight discrimination and to secure equal rights to medical and disability benefits.
At the heart of Palka's story is the support of a close circle of friends who help prepare her for the doctor's visit and accompany her to learn if she has Huntington's, test results that will not be revealed here.
"Lucy's film is built around a decisive moment of genetic truth that everyone in Marianna's family and close circle of friends fear the most," said Judith Helfand, creative director and co-founder of Chicken & Egg Pictures, which provided the grant for the film.
"The incredible trust she has for them, can be felt, in every corner of the frame, and by extension, in the trust she has for director Lucy Walker," she said in an email. "Soon the audience is a part of this extended family privy, in real time, to the most intimate and terrifying line in the sand, life before-life after moments."