Analysis of 3D spatial trajectories in Parkinsonian, essential and physiological tremors.

The clinical differentiation of the tremor in Parkinson's disease (PD) and essential tremor (ET) can sometimes be challenging, especially in the early stage of the disease. As different neural oscillators are involved in the generation of these two types of tremor, their trajectorial analysis could also be different. The goal of this study was to investigate whether some disease-specific patterns related to their tremor trajectories in fact exist. The three-axial accelerometer signals of the hand tremor obtained from a total of 369 participants [49 with PD, 25 with isolated resting tremor (iRT), 133 with ET, and finally 162 normal subjects with physiological tremor (Ph)] were subjected to vector analysis using a custom-made mathematical program. Subsequently, detailed trajectorial analysis was performed. The key discrimination ability between the PD and ET groups was represented by the ratio of the vector in the y-z plane and the spatial vector. The great majority of the patients with PD and iRT showed significantly higher values as compared to those with ET. The differences between the PD and iRT groups and between ET and Ph were not statistically significant. We suggest that the newly introduced three-axial accelerometry with analysis of tremor trajectories could be beneficial in differentiating between tremors in PD and ET.

Direction-dependent excitatory and inhibitory ocular vestibular-evoked myogenic potentials (oVEMP) produced by oppositely directed accelerations along the midsagittal axis of the head [corrected].

Oppositely directed displacements of the head need oppositely directed vestibulo-ocular reflexes (VOR), i.e. compensatory responses. Ocular vestibular-evoked myogenic potentials (oVEMPs) mainly reflect the synchronous extraocular muscle activity involved in the process of generating the VOR. The oVEMPs recorded beneath the eyes when looking up represent electro-myographic responses mainly of the inferior oblique muscle. We aimed: (1) to study the properties of these responses as they were produced by head acceleration impulses to the forehead and to the back of the head; (2) to investigate the relationships between these responses and the 3-D linear head accelerations that might reflect the true stimulus that acts on the vestibular hair cells. We produced backward- and forward-directed acceleration stimuli in four conditions (positive and negative head acceleration impulses to the hairline and to the inion) in 16 normal subjects. The oVEMPs produced by backward- and forward-directed accelerations of the head showed consistent differences. They were opposite in the phase. The responses produced by backward accelerations of the head began with an initial negativity, n11; conversely, those produced by accelerations directed forward showed initially a positive response, p11. There was a high inter-subject correlation of head accelerations along the head anteroposterior and transverse axes, but almost no correlation of accelerations along the vertical axis of the head. We concluded that backward-directed head accelerations produced an initial excitatory response, and forward-directed accelerations of the head were accompanied by an initial inhibitory response. These responses showed dependence on acceleration direction in the horizontal plane of the head. This could be consistent with activation of the utricle.

Vestibulo-ocular (oVEMP) responses produced by bone-conducted sound stimuli applied to the mid-sagittal plane of the head.

Recently several studies have yielded evidence that impulses of bone-conducted (BC) sound can produce short-latency myogenic responses in the extraocular muscles, which are probably mediated by otolithic afferents. These responses, although miniscule, can be recorded with surface electrodes and are termed ocular vestibular evoked myogenic potentials (oVEMP). It is assumed that in response to low-frequency BC-sound stimuli the head moves predominantly along the axis from the site of the applied stimulus to the opposite side. Thus, oppositely-directed accelerations along a particular axis would produce oppositely-directed compensatory vestibulo-ocular responses (VOR) and oVEMPs. The aim of this study was to investigate whether the oVEMPs would reflect these direction-dependent VOR responses. Single cycles of 125 and 250 Hz BC tones were applied to opposite sides of two approximately orthogonal, naso-occipital (x) and vertical (z) axes of the head. oVEMP responses were recorded with standard bilateral vertical EOG montages. The responses in all twelve healthy subjects showed consistent differences with regard to the latency and/or shape of the response to stimuli applied to opposite sides of the head. These differences likely reflect different patterns of electro-myographic activity of the extraocular muscles, which may be mediated by groups of vestibular (probably otolithic) afferents with differently-orientated spatial polarization vectors.