Quantitative Separation of Tremor and Ataxia in Essential Tremor

Abstract

Objective This study addresses an important problem in neurology, distinguishing tremor and ataxia using quantitative methods. Specifically, we aimed to quantitatively separate dysmetria, a cardinal sign of ataxia, from tremor in essential tremor (ET). Methods In Experiment 1, we compared 19 participants diagnosed with ET undergoing thalamic deep brain stimulation (DBS; ETDBS) to 19 healthy controls (HC). We quantified tremor during postural tasks using accelerometry and dysmetria with fast, reverse‐at‐target goal‐directed movements. To ensure that endpoint accuracy was unaffected by tremor, we quantified dysmetria in selected trials manifesting a smooth trajectory to the endpoint. Finally, we manipulated tremor amplitude by switching DBS ON and OFF to examine its effect on dysmetria. In Experiment 2, we compared 10 ET participants with 10 HC to determine whether we could identify and distinguish dysmetria from tremor in non‐DBS ET. Results Three findings suggest that we can quantify dysmetria independently of tremor in ET. First, ETDBS and ET exhibited greater dysmetria than HC and dysmetria did not correlate with tremor (R2 < 0.01). Second, even for trials with tremor‐free trajectories to the target, ET exhibited greater dysmetria than HC (p < 0.01). Third, activating DBS reduced tremor (p < 0.01) but had no effect on dysmetria (p > 0.2). Interpretation We demonstrate that dysmetria can be quantified independently of tremor using fast, reverse‐at‐target goal‐directed movements. These results have important implications for the understanding of ET and other cerebellar and tremor disorders. Future research should examine the neurophysiological mechanisms underlying each symptom and characterize their independent contribution to disability. ANN NEUROL 2020;88:375–387.

Suppression of Axial Tremor by Deep Brain Stimulation in Patients with Essential Tremor: Effects on Gait and Balance Measures

“…Interestingly, stride length variability reduced to the level of HC, suggesting that VIM DBS may possibly have reduced the risk of falls for these subjects although our dataset did not have a validated measure of falls to compare. Consistent with previous studies [ 1 16 27 ], DBS was effective in reducing upper limb tremor. Here, we found reduction in upper limb tremor during an overground walking and postural balance task.…”

“…We quantified tremor during the postural balance task as the sum of power from 4–8 Hz in the middle 20 s of the raw acceleration signal during quiet standing for each wearable sensor (lumbar, sternum, left ankle, right ankle, left wrist, right wrist) [ 26 27 ]. To achieve this, we performed a power spectral analysis of the acceleration signal of the transverse, sagittal, and frontal planes for each body location ( Figure 1B ) using a customized MATLAB program (Math Works TM Inc., Natick, Massachusetts, USA).…”

Dysmetria and Errors in Predictions: The Role of Internal Forward Model

“…Indeed, in a wave-variable processor model where the cerebellum is assumed to contribute to the servo-motor mechanism, a reduction in cerebellar output results in large regular oscillations [86]. Furthermore, dysmetria and tremor are independent of each other in essential tremor [87].…”

Decomposition of Reaching Movements Enables Detection and Measurement of Ataxia

“…Furthermore, the scoring criteria of BARS and the International Cooperative Ataxia Rating Scale (ICARS) [17] suggest that segmentation of movement increases with disease severity on the finger-to-nose test (FNT). Prior work has also demonstrated that information extracted from the FNT is related to the presence of ataxia and its severity [10,18,19].…”