A potential role for early hyponatremia in the diagnosis of Borna-virus encephalitis?

Background The first case of fatal Borna Virus-1 encephalitis (BoDV-1) was reported in 2018. Here, we report another fatal case of BoDV-1 encephalitis with early severe hyponatremia, indicative of neurohypophysial dysfunction. Case presentation: A 77-year-old female living in southern Germany was admitted to hospital in 2020 due to rapidly progressing word-finding difficulties, personality changes, global disorientation, diffuse cognitive slowness, and gait ataxia, initially without fever. After a rapid deterioration with fever, gait instability and ataxia, rapid cognitive decline, meningism, epileptic seizures, aphasia, and signs of latent right hemiparesis, the suspicion of a (meningo-)encephalitis was set. Furthermore, an unexplained, severe hyponatremia had been present since admission. Laboratory workup in cerebrovascular fluid (CSF) and serum as well as brain imaging was negative. Despite extensive empirical antiviral, antimicrobial, and immunosuppressive treatment efforts, the patient fell into coma (day 5), lost all brainstem functions (day 18), and remained fully dependent on invasive mechanical ventilation. Finally, she clinically developed a status of brain death and died 42 days after initial admission. Brain autopsy confirmed an extensive, diffuse, and severe affection of neocortical, subcortical and cerebellar structures as well as the neurohypophysis due to infection with BoDV-1. In light of the autopsy results, the hyponatremia could imply an early basal brain involvement, which could narrow down the initial differential diagnosis. Conclusion The diagnosis of BoDV-1 encephalitis remains clinically challenging. The disease progresses quickly to irreversible brain damage. An early, unexplained, hyponatremia in the presence of severe and rapidly evolving encephalitis may narrow down the diagnosis.