A comparison of the directionality of head tremor between essential and dystonic tremor

An essential tremor (ET) is the most common movement disorder in adults. In clinical practice, it is often difficult to distinguish between an essential tremor with head tremor, or dystonic head tremor (DT) in cervical dystonia [1,2]. Therefore, we developed an instrumental method using an accelerometer to precisely measure and analyze head tremor to find a parameter evaluating the directionality of the tremor. Is head tremor in patients with ET one directional (typically “yes yes” or “no no” tremor) while DT is more complex? The head was measured with a Wireless XSens accelerometer (MTw Awinda). Probands were measured for at least 20 seconds. The accelerometer was placed on the subject’s forehead and the head tremor was measured in sitting position. We included 25 patients fulfilling the criteria for ET (10 M, 15 F, mean age 65.9, SD 11.5) and 23 patients with cervical dystonia (5 M, 18 F, mean age 64.4, SD 10.8). After preprocessing steps, involving removing the gravitational component of the acceleration data in all three axes (x,y,z), we normalized the acceleration data by subtracting the mean of each axis and dividing by the mean value of the Euclidean acceleration distance. Subsequently, we calculated the principal components of the tremor using principal component analysis (PCA) on preprocessed acceleration data. To compare groups of patients with ET and DT we evaluated the ratio of the mean of the first component to the second component of PCA. We also calculated the dominant frequency and power of the tremor using Welch’s power spectral density estimation at frequency range 2-12 Hz. Data was compared using the Wilcoxon rank-sum test. The group of DT patients had a higher ratio of the first component to the second component PCA in comparison to patients with ET (p=0.02). There were no significant differences in frequency and power of tremor. Figure: Data analysis pipeline. Subject A had essential head tremor and subject B had dystonic head tremor.Download : Download high-res image (291KB)Download : Download full-size image We created a method to assess the directionality of head tremor based on principal component analysis. Our results show that one direction tremor is more dominant in patients with dystonic tremor compared to essential tremor. This result challenges the standard thinking that ET is primarily in one direction only.