When Neurology Overlaps: Herpes Simplex Virus Encephalitis in a Patient with Progressive Multiple Sclerosis

Herpes simplex virus encephalitis (HSVE) is the most common cause of sporadic viral encephalitis in adults and carries significant morbidity and mortality if untreated. We report the case of a middle-aged female patient with progressive multiple sclerosis (MS) who developed acute gastrointestinal symptoms followed by rapid neuropsychiatric deterioration, expressive aphasia, and focal seizures. Initial evaluation suggested infectious gastroenteritis, hyponatraemia, and a possible MS exacerbation. Despite partial biochemical correction, her neurological status worsened. Later on, she developed focal seizures, which warranted urgent stroke and neurological review. Neuroimaging demonstrated temporal lobe and insular involvement (edema and hyperintensity in T2/Fluid Attenuated Inversion Recovery (FLAIR)), and cerebrospinal fluid (CSF) polymerase chain reaction (PCR) confirmed Herpes simplex virus (HSV)-1 infection. She was treated with intravenous acyclovir for 21 days. Although virological clearance and radiological resolution were achieved, she developed persistent neurocognitive sequelae, including aphasia, seizures, and personality changes. This case highlights the diagnostic complexity of HSVE in the context of MS, where overlapping features can obscure early recognition. Prompt initiation of acyclovir and multidisciplinary management are crucial to optimizing outcomes and minimizing long-term disability.