BACKGROUND: Human herpesvirus 6 (HHV-6) encephalitis is best recognized after allogeneic hematopoietic stem cell transplantation but can also occur in non-transplant immunocompromised patients. Hyponatremia and syndrome of inappropriate antidiuretic hormone secretion (SIADH) may precede or accompany HHV-6 encephalitis. Severe SIADH as an early dominant clue in non-transplant patients with smoldering adult T-cell leukemia/lymphoma (ATLL) is not fully described.

CASE REPORT: A 55-year-old woman with smoldering ATLL received systemic corticosteroid therapy for clinically diagnosed human T-cell leukemia virus type 1 (HTLV-1)-associated uveitis and was admitted for progressive pulmonary infection. Bronchoscopic specimens revealed polymicrobial infection, including Nocardia and Actinomyces; invasive pulmonary aspergillosis was also clinically suspected. Although pulmonary findings improved with antimicrobial and antifungal therapy, she developed nausea, acute impairment of consciousness, and severe SIADH-associated hyponatremia. Serum sodium decreased to 113 mmol/L, and consciousness did not adequately improve after sodium correction. Cerebrospinal fluid obtained by lumbar puncture on hospital day 13 showed HHV-6 DNA positivity (1.0×10³ copies/mL). Brain magnetic resonance imaging demonstrated bilateral cortical and limbic abnormalities, supporting a clinical diagnosis of HHV-6 encephalitis. Ganciclovir was initiated on hospital day 15 along with sodium correction, tolvaptan, and supportive care. The patient subsequently developed hypercapnic respiratory failure and CO₂ narcosis requiring mechanical ventilation. Her neurologic and respiratory status gradually improved, and she was discharged on hospital day 53.

CONCLUSIONS: In non-transplant patients with ATLL, corticosteroid exposure, and opportunistic infection, severe SIADH accompanied by new neurologic symptoms can be an early clue to HHV-6 encephalitis. Early recognition and prompt initiation of antiviral therapy may help to improve neurologic outcomes.

Keywords: Encephalitis, Hypercapnia, Roseolovirus Infections