Disturbance of verticality perception and postural dysfunction in Parkinson's disease

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Impaired postural control is a central feature of Parkinson's disease (PD), predisposing to falls and interfering with social participation.1 Functionally, postural control can be divided into orientation and stabilization components, reflecting the processing of one's own position with respect to gravity and compensatory body reactions, respectively.2 In this concept, stooped posture and camptocormia in PD patients indicate orientation‐related deficits, whereas impaired postural reflexes and consecutive balance problems reflect stabilization deficits.3
Postural control involves the integration of input from different sensory channels, among others allowing for relatively precise verticality perception.4 The latter can be most easily determined by the subjective visual vertical (SVV), a measure of discrepancy between assumed and realistic perpendicular orientation of a straight line. Commonly, the SVV is described in terms of deviation, defined as its counter/clockwise tilt in relation to objective verticality, and variability, being the variance of deviation over subsequent measurements and regarded as an index of uncertainty about verticality.8 Distinct underpinnings of these parameters were proposed, based on differential associations with balance abnormalities and brain lesions in stroke patients. Whereas high deviation is thought to point to impairments in otholithic or vestibular pathways,8 increased variability is conceived as a higher‐order dysfunction of polymodal sensory integration, in particular related to neglect syndromes and right cerebral lesions.8
In PD patients, the degree of SVV deviation appears to correlate with the extent of postural instability,13 but clinical associations with SVV variability are unknown so far. We therefore investigated whether changes in the different dimensions of verticality perception were associated with orientation vs stabilization‐related deficits of postural control in PD. Further, as the development of either SVV measure throughout the disease has not been studied, but may provide hints to the basis of the complex evolution of postural PD sequels, we were interested in the specific relations of deviation and variability to the disease stage.
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