Her death certificate said in January that the 60-year-old, best known for her role as Princess Leia, had suffered a cardiac arrest.
But in a statement the LA coroner said the exact cause was unknown.
Sleep apnoea is a common condition in which a person stops breathing during sleep, either for a few seconds or minutes.
As well as listing sleep apnoea as a cause of death, the coroner's statement cited other factors, including heart disease and drug use.
In a statement released to People magazine after the coroner's ruling, Fisher's daughter Lourd said:
"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."
Her manner of death would be listed as undetermined, the coroner's statement said.
Fisher had been on tour promoting her book The Princess Diarist when she was taken ill on a flight from London to Los Angeles on 23 December.
She never regained consciousness and died on 27 December at the Ronald Reagan UCLA Medical Centre.
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