Three Hertz postural leg tremor impairs posture maintenance in multiple system atrophy-cerebellar type

Three-Hz postural leg tremor has recently been identified as highly prevalent in patients with the cerebellar type of multiple system atrophy, but its impact on posture maintenance remains poorly understood.

Thirty-seven patients with spinocerebellar ataxia and 58 others with cerebellar type of multiple system atrophy were given Synapsys posturography examinations. Fifty-three healthy controls were also tested. Low, medium, and high-frequency sway were recorded to compute energy values. Frequency shift and postural strategy predominance were evaluated from the postural sway distributions, mainly from the proportions of higher frequency values among the total values. The trajectories of postural sway components were evaluated with the generalized additive mixed model. Distributions of the components and their relationships with falls and tremors were assessed through repeated measures correlation analysis.

As the test difficulty increased, the standard controls showed slight increases in the energy values at every frequency. Distributions of the higher frequency (>0.5 Hz) values increased escalatingly with test difficulty, illustrating frequency shifts and hip strategy predominance. Medium and high-frequency values were strongly and positively inter-correlated in normal stances, but this was not observed among the spinocerebellar ataxia or multiple system atrophy patients. Unlike normal stances, the proportion of medium frequency values was negatively related to the total value among the spinocerebellar ataxia and multiple system atrophy patients, implying a failure of frequency shift in response to perturbation. Medium frequency proportions were also inversely correlated with tremors among the multiple system atrophy patients.

The observed synchronized changes in medium and high-frequency postural sway indicate that they constitute a complete hip strategy for posture control. The strategy was rigid in those with spinocerebellar ataxia but completely disrupted in those with multiple system atrophy. Three Hertz postural leg tremors destabilize the ankle joints and interfere with postural adjustment among those with multiple system atrophy.