A case report: an 89-year-old man taking prednisolone for chronic inflammatory demyelinating polyradiculoneuropathy diagnosed with human herpesvirus 6 variant A encephalitis

Human herpesvirus 6 (HHV-6) is a neurotropic virus comprising two variants, HHV-6A and HHV-6B. Although HHV-6B is known to cause exanthem subitum in children, the clinical significance of HHV-6A remains poorly understood, particularly in adult and elderly populations. Here, we report a rare case of HHV-6A encephalitis in an immunosuppressed elderly patient. An 89-year-old male patient with chronic inflammatory demyelinating polyradiculoneuropathy was administered immunosuppressive therapy with prednisolone. His consciousness progressively worsened over 4 days. Brain magnetic resonance imaging with fluid-attenuated inversion recovery demonstrated a hyperintense lesion in the left medial temporal lobe. The FilmArray meningitis/encephalitis panel tested positive for HHV-6. Further testing revealed elevated HHV-6A DNA levels in both the cerebrospinal fluid and plasma, confirming the diagnosis of HHV-6A encephalitis. A moderate increase in HHV-6A DNA and a slow clinical course suggest that chromosomally integrated HHV-6 (ciHHV-6) was unlikely. The treatment consisted of tapering prednisolone and initiating foscarnet, which gradually improved the patient's level of consciousness. This case highlights the importance of HHV-6A in the differential diagnosis of encephalitis, particularly in elderly patients receiving immunosuppressive therapy. Early diagnosis using a multiplex PCR panel, combined with prompt initiation of antiviral treatment, may lead to more favorable outcomes. Further studies are required to clarify the pathogenesis, clinical spectrum, and optimal management of HHV-6A-associated central nervous system diseases.