A Rare Case of Isolated One and Half Syndrome Associated with Choroidal Plexus Papilloma

One-and-a-half syndrome (OAHS) is characterized by ipsilateral conjugate horizontal gaze palsy and internuclear ophthalmoplegia. We report a case of a 24-year-old gentleman who presented with a complaint of binocular diplopia, with limitation of extraocular movements. There was gaze-evoked nystagmus on attempted abduction in the right eye and down-beating nystagmus on downgaze. MRI brain showed well-defined homogeneously enhancing mass lesion in the fourth ventricle, which was surgically managed, and histopathology showed choroid plexus papilloma. This is a rare case report of Choroidal plexus papilloma causing OAHS.