The Hidden Metabolic Threat: A Case of Fatty Acid Oxidation Disorder Masquerading as Viral Myocarditis.

Fatty acid oxidation disorders (FAODs) are rare inherited metabolic diseases that can present with various features including cardiomyopathy, arrhythmias, hypoketotic hypoglycemia, and liver dysfunction. Their clinical and radiological manifestations may at times overlap with acquired conditions such as viral myocarditis and multisystem inflammatory syndrome in children (MIS-C), making timely diagnosis challenging. We describe a 10-month-old infant boy with undiagnosed long-chain FAOD (LC-FAOD) who presented with sudden cardiac arrest following a viral illness. The patient's family history was significant for cardiac disease and sudden death. The infant displayed early symptoms of cardiac dysfunction at three months and was misdiagnosed with COVID-19-related myocarditis at the age of six months. Despite initial cardiac recovery and metabolic stabilisation, the child remained comatose and succumbed to refractory respiratory distress syndrome. The initial workup identified metabolites suggestive of LC-FAOD and whole-exome sequencing (WES) identified a homozygous variant in HADHA gene, ultimately confirming the diagnosis of mitochondrial trifunctional protein deficiency (TFPD). Early recognition through newborn screening and genetic testing, coupled with timely initiation of targeted metabolic management, remains crucial to improving outcomes and preventing fatal consequences.