 The hallmark of downbeat nystagmus are down-beating jerk nystagmus in primary position of gaze. Typically, as in this patient, they're accentuated in down and especially down and lateral gaze. Note how the oscillations are more prominent when the patient gazes lateral and down. These are also a localizing nystagmus when seen, it should suggest a structural lesion at the cranioservical junction. Most commonly, this is an Arnold Kiari malformation. Other less common causes would include infarction or demyelination. Here's another example of down-beating nystagmus. See how in primary position of gaze, the nystagmus are jerk with the fast phase in the down direction. Once again, note how the oscillations increase in down and lateral gaze. And this is true to either side. This is one of the hallmarks of downbeat nystagmus. In this patient, the nystagmus are minimized in up gaze.