Objective: To determine if a digitizing tablet is better than Fahn-Tolosa-Marín part B (FTM-B) ratings in detecting changes in essential tremor (ET) that exceed random variability in tremor amplitude.

Background: Tremor rating scales provide crude, nonlinear, subjective assessments of tremor severity. By contrast, digitizing tablets are capable of providing precise, linear, objective measures of tremor in writing and drawings. However, the advantages of tablets over rating scales are diminished when random variability in tremor is large. Tablets will measure random variability precisely, but changes in tremor must exceed random variability to be recognizable as true change due to a treatment or disease progression.

Methods: The large and small spirals of FTM-B were drawn by 18 patients with mild to severe ET in an unpublished open-label pharmacokinetic/pharmacodynamic study of sodium oxybate. Baseline assessments on consecutive days were used to compute test-retest reliability (intraclass correlation) and minimum detectable change (MDC) for the FTM ratings and digitizing tablet measurements. Regression between FTM ratings and log (base 10) digitizing tablet measurements was computed to obtain the Weber-Fechner relationship. Tremor amplitude in each digitized drawing was computed using spectral analysis (Elble et al. Mov Disord 1996;11:70-78) by a technician who was blinded to the tremor ratings. The grand average of peak-to-peak tremor displacement (cm) in the four spirals (2 spirals, right and left hands) was compared with the grand average of the four FTM ratings.

Results: Regression analysis revealed a very strong linear Weber-Fechner relationship between FTM and log(tablet) for baseline 1 (log(tablet) = 0.59 FTM – 1.24, r = 0.95) and baseline 2 (log(tablet) = 0.61 FTM – 1.29, r = 0.94). The MDC for the digitizing tablet was 50% of the baseline geometric mean tremor amplitude. The MDC for FTM was 90% of mean baseline rating. Using the Weber-Fechner equation (slope = 0.6), the FTM score was converted to actual tremor amplitude (Deuschl et al. Lancet Neurol 2011;10:148-161), and the MDC for the converted FTM ratings was 66%.

Conclusions: The variability in ET amplitude is so great that MDC of the digitizing tablet (i.e., the smallest change exceeding random variability) is comparable to MDC of FTM-B tremor ratings. Detectable change in ET is limited by its considerable natural variability over time.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/digitizing-tablet-and-fahn-tolosa-marn-tremor-ratings-have-comparable-minimum-detectable-change/