Flipping vertical nystagmus with vergence

It can be helpful to evaluate the effects of convergence on vertical nystagmus, both upbeat nystagmus (UBN) and downbeat nystagmus (DBN). For example, patients presenting urgently with spontaneous UBN due to thiamine deficiency (Wernicke's) will often have DBN with convergence (see patient #1 in the video). Over time, the spontaneous UBN transitions to DBN, at which time the nystagmus may transition back to UBN with convergence (see patient #2 in the video). Although neither of these patients have Wernicke's, the theorized mechanism of "flipped" vertical nystagmus is presumably the same - i.e., medullary involvement of medial and lateral vestibular subnuclei (which process otolithic inputs) disrupts the translational vestibulo-ocular reflex (t-VOR) pathways, which are dependent on orbital position and vergence angle. Therefore, faulty t-VOR signals result in an abnormal response (flipped vertical nystagmus) with convergence.