Fisher passed away in December last year at the age of 60 after she suffered a mid-air heart attack.
Coroner's officials released the autopsy results for Fisher's death late Friday, according to The Hollywood Reporter.
They, however, said investigators were not able to determine an exact cause.
The release stated that Fisher showed signs of having taken multiple drugs, but the investigators could not find out whether they were something to do with her death.
Sleep apnoea is a condition in which a person's breathing pauses during sleep.