Human herpesvirus-6 (HHV-6) encephalitis and myelitis following allogeneic hematopoietic stem cell transplantation (HSCT) is frequently life-threatening. We retrospectively evaluated the clinical significance of hyponatremia in cases of HHV-6 encephalitis/myelitis. Using an institutional database and medical records, we identified and retrospectively analyzed 16 cases of HHV-6 encephalitis and/or myelitis after allogeneic HSCT. HHV-6 encephalitis and myelitis were defined as the symptoms/signs with HHV-6-DNA in the cerebrospinal fluid. Seizure and memory disorder were defined as symptoms/signs of encephalitis, and dysesthesia and vesicorectal disorder as those of myelitis. Five patients developed encephalitis with or without myelitis, and 11 patients developed myelitis alone. Hyponatremia (median 129.1 mEq/L; range 125.9–130.1) was observed in all five patients with HHV-6 encephalitis at diagnosis, and values were significantly lower than those in patients with HHV-6 myelitis alone (median 137.6; range 134.0–142.2; P < 0.01). In three of the five patients with encephalitis, the decrease in sodium level preceded the clinical onset of encephalitis by one or two days. These results suggest that hyponatremia may be an important manifestation of HHV-6 encephalitis, but not of myelitis, and could be a useful tool for the early prediction or diagnosis of HHV-6 encephalitis.
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