Characterization of Parkinson's Disease using spectral features of kinetic tremor: correlation of on-line digitized handwriting and classical motor scales

Tremor is one of the motor impairments of Parkinson’s disease (PD). To date, kinetic tremor in PD is barely examined and there is lack of information on the relation between its severity and scales related to specific motor deficits [1]. In this study, we aimed at investigating the correlation between handwriting-related kinetic tremor and motor score measures by motor part of Unified Parkinson's Disease Rating Scale (UPDRS-III) [2], using a digitizing tablet. In this preliminary study, eight PD patients (7 M/1 F; age 74.5±8 years) draw an accurate Archimedes’ Spiral (AS) and fast, overlapped Circles (C) for a duration of 15 seconds. All patients underwent motor deficit assessment using UPDRS-III. Power Spectral Density of both velocity and acceleration profiles in their horizontal, vertical, and curvilinear components was estimated by using Welch’s method, with a Hamming window on intervals of 5 s and a 50% overlap. To analyze the power distribution related to different movement-associated phenomena, the ratio between two frequency bands (BME/BT) and the BT bandwidth (BW) were calculated for each subject. BME is the band of voluntary Movement Execution required by the task, ranging from 0.2 to 4 Hz, and BT is the band associated with involuntary Tremor, ranging from 4.0 to 12 Hz [3]. Normalized Jerk, a classic kinematic feature representing handwriting fluidity, was also estimated. The correlation between the parameters and UPDRS-III scores were assessed using Spearman’s rank correlation coefficients. All the evaluations were conducted in the pharmacological on state of PD patients. A positive correlation was found between Jerk and UPDRS-III scores (Table 1). On the contrary BME/BT correlates negatively with motor scale scores in horizontal and vertical velocities (Vx, Vy) for both tasks, vertical and curvilinear acceleration (Ay, Ac) for AS task, horizontal and vertical acceleration (Ax, Ay) for C task. Only Ay of C task shows correlation with motor scores. The results highlight that the severity of motor deficits in PD patients, as assessed by a widely employed motor scale, correlates with the outcomes of spectral and kinematic analysis of handwriting that indicate a loss of fluency, an increased power at BT level and a thinning of the spectral peak of BT. This suggests that handwriting assessment of parkinsonian dysgraphia can be used to implement clinical evaluation and represents a non-invasive, low-cost method for the identification of objective and reproducible biomarkers of kinetic tremor.