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To explore the patterns and mechanisms of jerky seesaw nystagmus in internuclear ophthalmoplegia (INO), we analyzed the nystagmus patterns in 33 patients with dissociated torsional-vertical nystagmus and INO. In 11 (33%) patients, the nystagmus was ipsiversive torsional in both eyes with vertical components in the opposite directions. In contrast, 18 (55%) patients showed ipsiversive torsional nystagmus with a larger upbeat component in the contralesional eye. Four (12%) patients exhibited ipsiversive torsional nystagmus with a greater downbeat component in the ipsilesional eye. At least one component of contraversive ocular tilt reaction was associated in most patients (30/33, 91%). The patterns of jerky seesaw nystagmus in INO suggest a disruption of neural pathways from the contralateral vertical semicircular canals with or without concomitant damage to the fibers from the contralateral utricle in or near the medial longitudinal fasciculus. [ABSTRACT FROM AUTHOR]