Abstract
The distribution of neuromyelitis optica (NMO)-characteristic brain lesions corresponds to sites of high aquaporin-4 (AQP4) expression, and the brainstem and hypothalamus lesions that express high levels of AQP4 protein are relatively characteristic of NMO. The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is one of the important causes of hyponatremia and results from an abnormal production or sustained secretion of antidiuretic hormone (ADH). SIADH has been associated with many clinical states or syndromes, and the hypothalamic-neurohypophyseal system regulates the feedback control system for ADH secretion. We report the case of a 63-year-old man with NMO, whose initial manifestation was hyponatremia caused by SIADH. Retrospective analysis revealed that the serum anti-AQP4 antibody was positive, and an MRI scan showed a unilateral lesion in the hypothalamus. SIADH recovered completely with regression of the hypothalamic lesion. As such, NMO should even be considered in patients who develop SIADH and have no optic nerve or spinal cord lesions but have MRI-documented hypothalamic lesions.