It's unclear from the report what role drugs played in her death.
Her manner of death would be listed as undetermined, the agency said.
A watch commander for the coroner's office declined to provide additional details on the findings, referring questions to a representative who was not immediately available.
Sleep apnoea causes the walls of the throat to relax during sleep, interrupting normal breathing, according to the NHS.
Her brother Todd Fisher said: "There's nothing about this that is enlightening".
Fisher was 60 when she went into cardiac arrest on December 23 aboard a flight from London to Los Angeles.
The actress, best known as Star Wars' Princess Leia Organa, suffered a heart attack at the end of a year ago.
Before her death, the daughter of actor Debbie Reynolds and the late singer Eddie Fisher had been on an airplane returning from England where she was shooting the third season of the British sitcom "Catastrophe".
Mr Fisher said his sister had written a lot about her drug use and that numerous things she took had been prescribed by doctors for her mental health conditions.
"I would tell you, from my perspective that there's certainly no news that Carrie did drugs". Paramedics removed her from the flight and rushed her to a nearby hospital, where she was treated for a heart attack.
She said she smoked marijuana at 13, used LSD by 21 and was diagnosed as having bipolar disorder at 24.
He said his sister's heart condition was probably worsened by her smoking habit, as well as the medications she took.
"They're working on her right now", the pilot said in a public recording of the conversation on liveatc.net. Let's celebrate not doing them by doing them.' I got into trouble each time.
Fisher made her feature film debut opposite Warren Beatty in the 1975 hit Shampoo.