Serotonin Syndrome Induced by Fentanyl Alone in an Adult Patient After Cardiac Surgery: A Case Report

Serotonin syndrome is a rare but potentially fatal condition characterized by altered mental status, autonomic hyperactivity, and neuromuscular abnormalities. Although fentanyl is known to be a causative agent of serotonin syndrome, most reports have shown that fentanyl-related serotonin syndrome is caused by multiple drug interactions, and only one case of serotonin syndrome caused by fentanyl alone has been reported in a pediatric patient. In this report, we describe a case of postoperative serotonin syndrome caused by fentanyl alone in an adult patient after cardiac surgery. A 66-year-old male was diagnosed with unstable angina pectoris and underwent off-pump coronary artery bypass grafting. Two hours after the intensive care unit (ICU) admission, he exhibited symptoms of sweating, tremors, and muscle rigidity. Four hours later, the body temperature rose to 40.0 °C, suggesting malignant hyperthermia or a similar condition. Dantrolene was administered to the patient, and all symptoms improved within several minutes. However, the patient experienced a relapse of symptoms every four to six hours, requiring additional dantrolene treatment each time. Although no other serotonergic agents were used, we suspected serotonin syndrome induced by fentanyl alone and discontinued its use on postoperative day three. Following the discontinuation of fentanyl, no further episodes were observed. The patient was discharged from the hospital without any complications on postoperative day 29. During a subsequent check-up, the patient was found to have a sternal dehiscence and underwent one-stage sternal reconstruction. General anesthesia was induced and maintained without the use of fentanyl. The patient was discharged 10 days after surgery without symptoms of serotonin syndrome. In a patient with postoperative hyperthermia and neuromuscular abnormalities, serotonin syndrome should be considered when fentanyl is administered. Dantrolene may be beneficial in managing serotonin syndrome caused by fentanyl alone and/or benzodiazepine resistance.