Influenza B-Associated Acute Necrotizing Encephalopathy in a Previously Healthy Child: A Case Report of Fulminant Neurological Deterioration.

Acute necrotizing encephalopathy (ANE) is a rare, rapidly progressive, immune-mediated encephalopathy most commonly triggered by viral infections such as influenza, characterized by acute encephalopathy, seizures, and a high risk of severe neurological sequelae or death. We report a six-year-old previously healthy female who presented with several days of fever, reduced oral intake, progressive altered mental status, and new-onset seizures, requiring intensive care admission and mechanical ventilation due to worsening encephalopathy and respiratory compromise. Laboratory evaluation demonstrated systemic inflammation, and respiratory viral testing confirmed Influenza B infection. Neuroimaging revealed characteristic bilateral, symmetric lesions involving the thalami, external capsules, brainstem, and periventricular white matter with diffusion restriction, consistent with ANE. After exclusion of alternative infectious, metabolic, toxic, and autoimmune etiologies, a diagnosis of Influenza B-associated ANE was established. The patient was managed with high-dose corticosteroids, intravenous immunoglobulin, empirical antimicrobials, and comprehensive pediatric intensive care support, including antiseizure therapy and mechanical ventilation. Her course was complicated by ventilator-associated pneumonia, transient arrhythmias, and electrolyte disturbances, but she gradually improved with multidisciplinary management, achieving successful extubation and partial neurological recovery. Follow-up imaging demonstrated regression of acute lesions with residual changes. This case highlights the importance of early recognition, prompt neuroimaging, and aggressive immunomodulatory and supportive therapy in ANE, while underscoring its potential for significant morbidity despite intensive treatment.