Vestibular Functions and Parkinson's Disease

Abstract

For decades it has been speculated that Parkinson's Disease (PD) is associated with dysfunction of the vestibular system, especially given that postural instability is one of the major symptoms of the disorder. Nonetheless, clear evidence of such a connection has been slow to emerge. There are still relatively few studies of the vestibulo-ocular reflexes (VORs) in PD. However, substantial evidence of vestibulo-spinal reflex deficits, in the form of abnormal vestibular-evoked myogenic potentials (VEMPs), now exists. The evidence for abnormalities in the subjective visual vertical is less consistent. However, some studies suggest that the integration of visual and vestibular information may be abnormal in PD. In the last few years, a number of studies have been published which demonstrate that the neuropathology associated with PD, such as Lewy bodies, is present in the central vestibular system. Increasingly, stochastic or noisy galvanic vestibular stimulation (nGVS) is being investigated as a potential treatment for PD, and a number of studies have presented evidence in support of this idea. The aim of this review is to summarize and critically evaluate the human and animal evidence relating to the connection between the vestibular system and PD.

Keywords: Parkinson's disease; VEMPs; dopamine; striatum; vestibular system; vestibulo-ocular reflexes; vestibulo-spinal reflexes.

Figures

Figure 1

Possible neuronal pathways connecting the vestibular nucleus complex to the striatum. PFN, Parafascicular nucleus; PPT, pedunculopontine tegmental nucleus; SNc, Substantia nigra pars compacta; VNC, vestibular nucleus complex. Reproduced from Stiles et al. (59) with permission from the publisher.

Figure 2

Estimated number of c-Fos positive cells in the striatum following vestibular stimulation. ***P ≤ 0.0001 for the comparison of the higher current with both the sham groups and the lower current group. From Stiles et al. (59) with permission.

Figure 3

Examples of the firing patterns of the 6 single striatal neurons responding to electrical stimulation of the vestibular labyrinth in a phase-locked manner, (A) with examples of their action potential waveforms (averages of 200 action potentials (B); mean ± SD in red). From Stiles et al. (59) with permission.

Figure 4

Peri-stimulus histograms of neuronal responses to electrical vestibular stimulation. (A) Combined histogram of firing of all non-responsive neurons at 1 × (top) and 2 × (bottom) the threshold of nystagmus. (B) Combined firing of all 6 responsive neurons, at 3 × the threshold of nystagmus, phase-locked to the stimulus. Red bar represents the stimulation period. Spikes from the stimulus artifact have been removed for clarity. Data are presented as mean (black bars) and standard deviation (gray bars). From Stiles et al. (59) with permission.