On the effect of vibrotactile stimulation in Essential Tremor

Abstract

Motivation: Essential Tremor (ET) is a neurological disease characterized by tremors of hands that causes motor dysfunction, interfering with activities of daily living and compromising quality of life. It is most common in individuals over the age of 65, with a prevalence of around 4.5%. However, ET can affect anyone at any age. Despite available treatments to manage tremor, ET still has no cure. This is because there is no understanding of its origin. In addition, the mechanisms behind the generation and manifestation of tremor, as well as the effect of external inputs on these mechanisms, are unclear. Studies have been conducted on the effect of external stimuli, such as inertial loads and electrical stimuli, on the motor behaviour of individuals affected by tremor. However, there is few evidence on vibrotactile stimulation. Objective: In this context, this study aimed to evaluate the effect of peripheral vibrotactile stimulation on the dynamics of postural tremor in individuals with ET. Methods: For this purpose, the involuntary movements of 18 individuals with ET were analysed during the maintenance of posture in the absence of vibrotactile stimulation and presence of four different patterns of vibratory stimulus. The simulus were applied to the fingertips, palm of the hand and forearm, using piezoelectric actuators. The involuntary movements were recorded by two inertial measurement units positioned in the hand and forearm of the limb most affected by the condition. Two types of analysis were designed, one in the time domain (focused on changes in amplitude and regularity of tremor before, during and after stimulation) and the other in the frequency domain (focused on changes in power spectral density of the tremor with and without stimulation). Thus, different signal processing methods were proposed to describe the tremor in terms of amplitude, frequency and regularity (approximate entropy). Results: As a result, it was observed that the response to vibrotactile stimulation was different among individuals. When comparing the involuntary activity before and after stimulation, the stimulus at 250 Hz was the only one that caused a reduction in the amplitude of tremor, mainly in the hand (η² > 0.1, p-value < 0.05). However, in terms of regularity, the tremor became more unpredictable after stimulation of the limb at 250 Hz (η² > 0.1, p-value < 0.05), showing a small effect size only for the Y axis (η² = 0.012). For many individuals, stimulation at 250 Hz and random frequency reduced the peak power of tremor compared to the peak power when the limb was not stimulated (η² > 0.1, p-value < 0.05). In terms of frequency, it was also found that vibrotactile stimulation shifted the peak frequency, regardless of the stimulus pattern (η² > 0.12, p-value < 0.05). Conclusion: Therefore, vibrotactile stimulation is a potential way to change the dynamics of postural tremor and could be a useful clinical tool for more than only dampening tremor.