Almost six months after the 60-year-old’s death, the coroner said in a statement that Fisher also had atherosclerotic heart disease and had used drugs.
It is not clear how much influence these had in her death.
Sleep apnoea is a common condition in which a person stops breathing during sleep, either for a few seconds or minutes.
Her brother Todd Fisher said: “There’s nothing about this that is enlightening.”
He told Associated Press: “I would tell you, from my perspective, that there’s certainly no news that Carrie did drugs.”
Fisher had previously said she had smoked pot at 13, used LSD at 21 and had been diagnosed as bipolar at 24, for which she was given electroshock therapy and medication.
Mr Fisher said his sister had written a lot about her drug use and that many of the things she took had been prescribed by doctors for her mental health conditions.
He said: “They were doing their best to cure a mental disorder. Can you really blame them?
“Without her drugs, maybe she would have left long ago.”
Fisher, daughter of actor Debbie Reynolds and late singer Eddie Fisher, was best known for her role as Princess Leia in the Star Wars franchise.
Just before her death she had been returning from England after filming the third season of sitcom Catastrophe.
She became unresponsive and died in hospital four days later on 27 December.
The day after her death, Reynolds suffered a stroke and died, aged 84.