The influence of positional effects on the spontaneous decrease of downbeat nystagmus in the course of the day

Introduction:

Downbeat nystagmus (DBN), often being caused by cerebellar degeneration (1), is the most frequent central spontaneous nystagmus (1, 2). Prior research has indicated that slow-phase velocity (SPV) of DBN is dependent on head position relative to gravity (3, 4) and that it decreases during the daytime when the head is in upright position (5). Based on these results, we examined patients in different resting positions (sitting upright, lying supine/prone) to determine whether the position has an effect on the DBN-decrease in the course of the day.

Methods:

Nine patients (five females, 44 to 72 years old, mean 63. 7 years, SD±7.9 years, mean duration of DBN 7.8 years, SD±4.5 years) had DBN due to either cerebellar (N=2) or unknown etiology (N=7). Eye movements were recorded with video-oculography. Mean slow-phase velocities (SPV) of DBN (with gaze straight ahead and attempted fixation onto a laser-projected dot) were determined before and after resting. A baseline measurement (9 am) took place before resting; the other two measurements (11 am, 1 pm) were preceded by a resting interval of approximately 2 hours each. Whole-body positions during resting were upright, supine, or prone, all in complete darkness. Repeated measurement ANOVAs were carried out for upright, supine, and prone measurements. The data of upright measurements have the highest clinical relevance, as this is the usual position of people during daytime.

Results:

Before resting (9 am), the average SPV ranged from 3.05 deg/s to 3.6 deg/s on the separate days of measurement (no significant difference, p>0.98). When being assessed in the typical daytime position (=upright), a significantly lower average SPV value (0.65 deg/s, SE=0.2) was associated with previous rest in the upright position than previous rest in the prone position (2.22 deg/s, SE=0.73, p<0.05) or supine position (2.1 deg/s; SE=0.77, p<0.05). SPV values in the prone position and supine position did not differ from each other (p>0.97). The respective ANOVA revealed a significant effect of the resting interval, F(2, 16)=5.54, p=0.01, no main effect for daytime, F(1, 8)=0.06, p=0.805, and no resting interval * daytime interaction, F(2, 16)=1.5, p=0.25). Equivalent ANOVAS with respect to supine/prone measurements did not show significant effects.

Conclusion:

DBN in an upright position becomes minimal after resting upright. The spontaneous decrease of DBN is less pronounced when patients lie down to rest. The clinical relevance of this finding is to recommend patients with DBN to rest in an upright position in the course of the day.

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