What is the pathophysiology of downbeat nystagmus?

Updated: Oct 17, 2018
  • Author: Christopher M Bardorf, MD, MS; Chief Editor: Edsel Ing, MD, MPH, FRCSC  more...
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Answer

Downbeat nystagmus is defined as nystagmus with the fast phase beating in a downward direction. [4] The nystagmus usually is of maximal intensity when the eyes are deviated temporally and slightly inferiorly. With the eyes in this position, the nystagmus is directed obliquely downward. In most patients, removal of fixation (eg, by Frenzel goggles) does not influence slow phase velocity to a considerable extent; however, the frequency of saccades may diminish. [5]

The presence of downbeat nystagmus is highly suggestive of disorders of the craniocervical junction (eg, Arnold-Chiari malformation). This condition also may occur with bilateral lesions of the cerebellar flocculus and bilateral lesions of the medial longitudinal fasciculus, which carries optokinetic input from the posterior semicircular canals to the third nerve nuclei. It may also occur when the tone within pathways from the anterior semicircular canals is relatively higher than the tone within the posterior semicircular canals. Under such circumstances, the relatively unopposed neural activity from the anterior semicircular canals causes a slow upward pursuit movement of the eyes with a fast, corrective downward saccade. [6, 7]


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