Visual and Positional Modulation of Pendular Seesaw Nystagmus: Implications for the Mechanism.

Background: The mechanisms of pendular seesaw nystagmus (SSN) remain unknown.

Methods: We evaluated modulation of pendular SSN by removal of visual fixation, convergence, and positional changes in 2 patients, one with bitemporal hemianopia due to a traumatic damage of the optic chiasm and the other with platybasia compressing the medulla and lower cerebellum.

Results: In both patients, the pendular SSN markedly decreased or disappeared with convergence, without visual fixation in darkness, during static head tilt toward each shoulder while sitting and while supine.

Positional suppression of periodic alternating nystagmus.

A 43-year-old man with a high-grade glioma involving the cerebellar nodulus showed a near-complete suppression of periodic alternating nystagmus (PAN) in the lateral decubitus position to either side. This positional modulation of PAN is consistent with suppression of the velocity storage mechanism by head position changes (tilt dumping) and is supportive of the role of the velocity storage mechanism in generating PAN.

Pursuit-paretic and epileptic nystagmus in MELAS.

A 27-year-old man with MELAS 3243 mutation developed a right homonymous hemianopia, left beating nystagmus, and impaired smooth pursuit (SP) to the left. Intermittently, the left beating nystagmus changed to right beating with rightward eye and head deviation and associated altered mental status. Each episode lasted several minutes.