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Horizontal nystagmus is also classified into three degrees as follows:
First degree nystagmus is present only on lateral gaze, and has the fast phase
in the direction of gaze;
Second degree nystagmus is present in the primary (neutral) position of gaze;
Third degree nystagmus is present in the same direction in all gaze positions
Such distinctions help to identify the anatomical source of the nystagmus. First
degree nystagmus usually originates in the brainstem or cerebellum, while second
and third degree nystagmus are usually vestibular in origin.
Other (extremely) rare pathologic nystagmuses are gaze paretic, rebound,
fixation, congenital and dissociated nystagmus.
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