Digitizing Tablet and Fahn–Tolosa–Marín Ratings of Archimedes Spirals have Comparable Minimum Detectable Change in Essential Tremor

Background: Drawing Archimedes spirals is a popular and valid method of assessing action tremor in the upper limbs. We performed the first blinded comparison of Fahn–Tolosa–Marý´n (FTM) ratings and tablet measures of essential tremor to determine if a digitizing tablet is better than 0–4 ratings in detecting changes in essential tremor that exceed random variability in tremor amplitude.

Methods: The large and small spirals of FTM were drawn with each hand on two consecutive days by 14 men and four women (age 60¡8.7 years [mean¡SD]) with mild to severe essential tremor. The drawings were simultaneously digitized with a digitizing tablet. Tremor in each digitized drawing was computed with spectral analysis in an independent laboratory, blinded to the clinical ratings. The mean peak-to-peak tremor displacement (cm) in the four spirals and mean FTM ratings were compared statistically.

Results: Test–retest intraclass correlations (ICCs) (two-way random single measures, absolute agreement) were excellent for the FTM ratings (ICC 0.90, 95% CI 0.76–0.96) and tablet (ICC 0.97, 95% CI 0.91–0.99). Log10 tremor amplitude (T) and FTM were strongly correlated (logT=αFTM + β, α≈0.6, β≈–1.27, r=0.94). The minimum detectable change for the tablet and FTM were 51% and 67% of the initial assessment.

Discussion: Digitizing tablets are much more precise than clinical ratings, but this advantage is mitigated by the natural variability in tremor. Nevertheless, the digitizing tablet is a robust method of quantifying tremor that can be used in lieu of or in combination with clinical ratings.