18-year-old Rina Matsuno's sudden death was caused by a lethal arrhythmia, a medical certificate from her doctor confirmed.
SME Records, the label of Shiritsu Ebisu Chuugaku, released a statement on February 10 that announced Matsuno's cause of death.
An arrhythmia is a heart condition where the heartbeat becomes irregular. Left untreated, an arrhythmia can cause loss of consciousness and sudden death as the heart becomes unable to properly pump blood through the body.
On February 7, Matsuno was supposed to participate in a concert with her group Shiritsu Ebisu Chuugaku. However, due to poor health, she stayed at home instead to rest. Throughout the day, the idol is said to have appeared fine, and there was no cause for alarm.
Matsuno's parents were with her while she was at home resting. At approximately 5am on February 8, her parents noticed that something unusual was happening with her daughter and called emergency services.
An ambulance rushed Matsuno to the hospital for emergency treatment. However, by the time she arrived at the hospital, the idol had already lost consciousness. She was found to be dead upon arrival.