A gaze-triggered downbeat nystagmus persisting in primary position in a patient with hypomagnesemia combined with anti-SOX1

•There is a gaze-triggered downbeat nystagmus persisting for many seconds in the primary eye position..•Two unrelated simultaneous pathophysiologies are probably underlying: hypomagnesemia and anti-SOX1 paraneoplasia.•The gaze-triggered downbeat nystagmus disappeared due to magnesium supplementation.•The cerebellar oculomotor syndrome with dizziness was only successfully treated by additional administration of steroids.•An intraductal papillary mucinous neoplasm was found in CT and MRI scan.