Abstract
Downbeat nystagmus in primary position usually reflects lesions in the posterior fossa near the cranio-cervical junction such as Arnold-Chiari malformation (type I). It is said that about one third of the patients with this sign have a Chiari malformation. However, the pathophysiological mechanism of downbeat nystagmus remains controversial. Several lesions responsible for downbeat nystagmus have been proposed, such as that in the medulla or cerebellum (flocculus, nodulus).
In this study, the electrically elicited blink reflex was investigated in 6 patients (one male, five female, average age 53.3 yr.) with Arnold-Chiari malformation (Type I) presenting with downbeat nystagmus to evaluate brainstem function. Latencies of each components were in the normal ranges in all 6 patients; R1 (11.13+0.66 ms), ipsi-R2 (32.45+4.47 ms), and contra-R2 (33.98+5.09 ms).
In addition, there were no abnormalities in any component of ABR in any patients. Eye tracking test (ETT) showed a saccadic pursuit pattern, especially downward. The frequencies of horizontal optokinetic nystagmus were preserved, whereas slow phase velocities were relatively decreased. Caloric nystagmus could not be suppressed during gaze in the visual suppression test.
From these findings we concluded that downbeat nystagmus in Arnold-Chiari malformation may be due to cerebellar (flocculo-nodular) lesions, rather than medulla lesions.
