The cause of death has been released for a 9-year-old Southern California girl who died earlier this year following a dental procedure.

Silvanna Moreno, a third-grade student, passed away on March 18 just hours after undergoing surgery at Dreamtime Dentistry in Vista, California. According to an autopsy report from the San Diego County Medical Examiner’s Office, her death was caused by methemoglobinemia, a rare condition linked to the use of nitrous oxide, commonly known as laughing gas.

Methemoglobinemia occurs when too much nitrous oxide interferes with hemoglobin—the protein in red blood cells that carries oxygen—leading to dangerously low oxygen levels in the body (a condition known as hypoxia). The report confirms that nitrous oxide was one of the anesthetics administered during Silvanna’s procedure.

Silvanna had originally reported tooth pain in October, and after being referred for treatment, Dreamtime Dentistry scheduled surgery for March. The night before her appointment, she had a mild fever that responded to children’s Tylenol. On the morning of her surgery, she complained of a headache but otherwise appeared fine. She was placed under anesthesia at 10 a.m. for a procedure that lasted about three hours.

Following the surgery, Silvanna was moved to a recovery room and then discharged. On the ride home, her family noticed she seemed disoriented and began snoring heavily, although she was still able to open her eyes and stand.

Back at home, she napped for roughly 90 minutes, but her breathing became slower and quieter. Her grandmother checked her pulse twice and, noticing something was wrong, called 911 at 4:46 p.m.—about six hours after the procedure.

While waiting for paramedics to arrive, the 911 dispatcher guided Silvanna’s grandmother through CPR. When she arrived at Rady Children’s Hospital, doctors found she was in asystole—a state where the heart has stopped beating. Despite emergency efforts, Silvanna was pronounced dead at 5:44 p.m.

Dr. Ryan Watkins, a representative from Dreamtime Dentistry, told PEOPLE that methemoglobinemia is “an extremely rare condition” that sometimes occurs with nitrous oxide use. He added that patients typically show low oxygen readings in the 80s on pulse oximeters when the condition occurs. However, Watkins said Silvanna’s oxygen levels were normal throughout the procedure and during recovery.

He emphasized that the dental team had carefully reviewed Silvanna’s medical and dental history before the procedure and that she was closely monitored the entire time.

Although her death has been ruled accidental, Dreamtime Dentistry is conducting a full internal review. “We are committed to learning from this tragic event and will implement any additional safety measures that may help prevent such occurrences in the future,” said Watkins. “We extend our deepest condolences to Silvanna’s family and ask for continued respect for their privacy during this time of grief.”