Cause of Death Revealed for Girl, 9, Who Was Placed Under Anesthesia at Dentist

The cause of death has been revealed for a 9-year-old girl who underwent anesthesia for a dental procedure earlier this year.

Silvanna Moreno, a Southern California third-grader, died on March 18 hours after surgery at Dreamtime Dentistry in Vista, Calif., according to a new San Diego County Medical Examiner’s Office (SDCME) autopsy report obtained by PEOPLE.

Silvanna’s cause of death was listed as methemoglobinemia in the setting of recent nitrous oxide administration.

Methemoglobinemia occurs when excessive amounts of nitrous oxide get into someone’s hemoglobin (the protein contained in red blood cells that delivers oxygen to the tissues) and cause hypoxia, so tissues do not receive enough oxygen.

Nitrous oxide was among the anesthetics Silvanna was administered during her procedure, according to the autopsy report.

Silvanna’s autopsy states she had surgery after she complained of tooth pain in October 2024, She was referred to Dreamtime Dentistry, which booked her for a Tuesday, March 18, procedure. 

The day before, she had a fever but her symptoms resolved after being given children’s Tylenol, according to the autopsy report.

The day of her surgery, she complained of a headache “without appearing distressed,” according to the report. She was administered anesthesia at 10 a.m. for a three-hour procedure.

Once she woke up, she was moved to a recovery room and then discharged, the autopsy states. On the way home, she was described as being “out of it” and later was snoring heavily but was also initially able to open her eyes and stand.

Never miss a story — sign up for PEOPLE’s free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories.

Once Silvanna arrived home, she was carried inside and remained asleep for around 90 minutes before her snoring got noticeably quieter and slower, according to the autopsy.

Her grandmother then administered two separate readings of her heart rate. After the second reading, she remained unresponsive and 911 was called at 4:46 p.m. local time — about six hours after her getting anesthesia.

A dispatcher instructed the grandmother to administer chest compressions while they waited for emergency personnel to arrive.

She was taken to Rady Children’s Hospital Emergency Department, where she was in asystole — a condition in which the heart’s electrical system fails, causing the heart to stop pumping, according to the Cleveland Clinic.

Silvanna was administered several life-saving tactics but was pronounced dead at 5:44 p.m.

Dreamtime did not immediately respond to a call and email seeking comment on the autopsy.

In a previous statement, a representative for Dreamtime’s Dr. Ryan Watkins told PEOPLE, in part, “We are deeply saddened by the tragic passing of our young patient, which occurred several hours after her completed dental procedure.”

“Throughout the procedure, she was continuously monitored by our dentist anesthesiologist, who … has over 20 years of experience successfully administering general anesthesia on children, with no complications observed,” the statement continued.

“Following the procedure, [Silvanna] was discharged in stable condition—awake, with stable vital signs and protective reflexes intact—into her mother’s care, following our standard post-anesthesia protocols,” according to Dreamtime’s previous statement.

“The safety and well-being of our patients have always been and remain our highest priority,” Watkins said, adding, “Our hearts break for the family during this unimaginable time of grief, and we extend our deepest condolences as we maintain our focus on supporting the family and cooperating fully with the medical investigation.”