Mumps Vaccine-Associated Aseptic Meningitis Complicated by Syndrome of Inappropriate Antidiuretic Hormone Secretion.

Mumps vaccine is generally safe; however, aseptic meningitis remains a rare but recognized adverse event. We report the case of a 15-year-old male who developed a progressive headache and hyponatremia, leading to a diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADH). As his symptoms worsened, CSF analysis revealed pleocytosis with monocyte predominance and elevated protein levels, resulting in a diagnosis of aseptic meningitis. A detailed history showed that he had received a mumps vaccine 27 days prior to admission. PCR testing of the CSF detected mumps virus, and direct sequencing confirmed 100% identity with the Torii vaccine strain. Although the incidence of vaccine-associated aseptic meningitis is significantly lower than that following natural mumps infection, clinicians should remain vigilant, particularly in patients presenting with neurological symptoms and SIADH after mumps vaccination. Early recognition of CNS involvement and careful review of vaccination history are essential for accurate diagnosis and appropriate management. This case underscores the importance of considering vaccine-associated aseptic meningitis in the differential diagnosis of patients with hyponatremia and progressive headache following mumps vaccination, even when vaccination occurs beyond the routine immunization age.