Elevated cerebrospinal fluid IgG index in herpes simplex encephalitis post‐HSV‐1 clearance: A preliminary study

Abstract Herpes simplex encephalitis (HSE) is an acute form of encephalitis that can lead to poor neurological outcomes. Although the exact pathogenesis of HSE remains elusive, recent reports suggest a significant role for postinfectious immune‐inflammatory processes in the central nervous system (CNS). This study aimed to clarify the association between CNS autoimmune responses and clinical presentation in patients with HSE, focusing on cerebrospinal fluid (CSF) characteristics, particularly the IgG index. We retrospectively analyzed 176 consecutive patients suspected of having aseptic meningitis /encephalitis for chronological changes in CSF findings and clinical presentations. These patients underwent PCR screening for herpesviruses (HV) in their CSF. We identified seven patients positive for herpes simplex virus type 1 (HSV‐1), 20 patients positive for varicella‐zoster virus, and 17 patients who met the criteria for aseptic meningitis but were PCR‐negative for HV. Patients in the HSV‐1‐positive group exhibited a significant increase in the IgG index at the time of PCR‐negative conversion compared with on admission ( p = 0.0156), while such a change was not observed in the other two groups. Additionally, all patients in the HSV‐1‐positive group tested negative for anti‐neural autoantibodies in CSF and serum samples collected approximately 3 weeks after onset. This study, therefore, highlights that CSF IgG index elevation occurs even after PCR‐confirmed HSV‐1 clearance, which might indicate immunopathogenesis that is independent of antibody‐mediated mechanisms.