Actress Carrie Fisher was unabashedly vocal about her lifelong battles with mental illness and drug abuse. She once defiantly told ABC News, "I am mentally ill. I can say that. I am not ashamed of that. I survived that, I'm still surviving it. But bring it on."

Her candor inspired a generation of women. If a cool and funny Hollywood icon could be so open about getting help for her struggles, then so could they.

But a disorder that ultimately contributed to Fisher's death was something she hadn't publicly said much about: sleep apnea.

Fisher died Dec. 27, four days after she had a heart attack on a flight from London to Los Angeles.

In a news release Friday, the Los Angeles County Medical Examiner's Office said sleep apnea - where a person repeatedly stops breathing during sleep, sometimes more than 100 times a night - was a contributing factor in her death, according to The Associated Press.

The medical examiner still listed the cause of Fisher's death as "undetermined," according to the AP. Other contributing factors were her ingestion of multiple unspecified drugs and a buildup of fatty tissue in her arteries.

But that the 60-year-old dealt with sleep apnea wasn't widely known. It's unclear if Fisher even knew she had it.

Grace Pien, a sleep specialist with Johns Hopkins School of Medicine, said that silence is something Fisher has in common with the legion of female sufferers of sleep apnea.

More than 18 million Americans have been diagnosed with sleep apnea, according to the National Sleep Foundation. Doctors cannot detect the disorder during a routine examination and it is the leading cause of daytime fatigue, according to the National Institutes of Health.

Symptoms include snoring and gasping for breath in one's sleep. Untreated, sleep apnea could increase the risk of high blood pressure, heart attack and heart failure, according to the National Institutes of Health. And one study found that properly treating sleep apnea with a CPAP machine can alleviate issues with the heart and blood sugar.

"What you die of is not the sleep apnea," John Bouzis, a dentist who works with sleep specialists to help apneic patients, told The Washington Post. "You die of the cardiovascular disease. You die of the stroke. You die of the pulmonary problems . . . Sleep apnea is a time bomb."

Women, particularly older ones, have a greater chance of never being diagnosed with sleep apnea - and never being treated for it. Female patients are also more likely to play down the symptoms, Pien said.

Pien told the National Sleep Foundation that women with sleep apnea are diagnosed less frequently than men. By some counts, as many as nine men are diagnosed with sleep apnea for every woman, although studies have shown that the ratios are much closer.

Part of the problem, Pien told The Washington Post, is that physicians have a stereotype of sleep apnea sufferers - namely middle-aged, overweight or obese men.

In women, the incidence of sleep apnea increases after menopause, which means people with the ailment may believe their sudden lethargy is due to other changes.

"Especially if women don't have a regular bed partner," she said. "They may not know some of the overnight symptoms of apnea. They may feel just more tired during the day."

"Women tend to be less forthcoming about having some of the symptoms like snoring just because they are more embarrassed about that," she said.

Bouzis said doctors can miss out on the chance to have a conversation with female patients if they are too polite.

"It's not a comfortable conversation for a physician to have with a woman," he told The Post. "We're likely to say to a man you need to lose some weight. It's not a conversation we have with a lot of women. I catch myself wanting to bring it up, hoping that they bring it up so that we can easily have that conversation."

What remained unclear on Saturday was what role the unspecified drug use played in Fisher's death.

In a statement to People Magazine, Fisher's daughter, Billie Lourd, alluded to her mother's struggles with addiction:

"My mom battled drug addiction and mental illness her entire life. She ultimately died of it. She was purposefully open in all of her work about the social stigmas surrounding these diseases.

"She talked about the shame that torments people and their families confronted by these diseases. I know my Mom, she'd want her death to encourage people to be open about their struggles. Seek help, fight for government funding for mental health programs. Shame and those social stigmas are the enemies of progress to solutions and ultimately a cure. Love you Momby."

At the time of her death, Fisher had wrapped up her role in the latest installment of the "Star Wars" movie series, but she also had recurring roles on the online comedy "Catastrophe" and on Fox's "Family Guy," according to Variety.