An Unusual Case of Hypersomnolence.

A young woman presented with acute-onset hypersomnia. Due to the acute onset, the possibility of secondary cause was considered. Magnetic resonance imaging of the brain was showing involvement of deep gray structures surrounding the third ventricle such as thalamus, hypothalamus, and caudate nucleus with true diffusion restriction. Differentials of vasculitic infarct or inflammatory cause were considered. Among inflammatory causes, neuromyelitis optica spectrum disorder (NMOSD) was considered due to predominant diencephalic involvement. Serum aquaporin-4 immunoglobulin G was positive. No other NMOSD core criteria fulfilling finding was there. Only hypersomnia presentation of NMOSD is uncommon. It should be considered in an acute hypersomnia patient as it is reversible with effective treatment.