High width variability during spiral drawing: further evidence of cerebellar dysfunction in essential tremor

Essential tremor (ET) is among the most prevalent neurological diseases, yet the location of the primary disease substrate continues to be a matter of debate. The presence of intention tremor and mild gait ataxia suggests an underlying abnormality of the cerebellum and/or cerebellar pathways. Uncovering additional signs of cerebellar dysfunction would further substantiate the proposition that ET is a disease of the cerebellar system. We evaluated 145 ET cases and 34 normal controls clinically and by computerized spiral analysis. Spiral analysis is a program that objectively characterizes kinematic and physiologic features of hand-drawn spirals using specific calculated spiral indices that correlate with spiral shape and motor execution. We used the spiral width variability index (SWVI), a measure of loop-to-loop spiral width variation with the influence of tremor removed, as a metric of drawing ataxia. The SWVI was higher in cases than controls (0.91 ± 1.94, median=0.46 vs. 0.40 ± 0.29, median=0.30, p<0.001). Cases with higher SWVI also had greater intention tremor during the finger-nose-finger maneuver, r=0.27, p=0.001), and cases with intention tremor of the head had the highest SWVI (1.57 ± 3.44, median=0.51, p<0.001). There was a modest association between SWVI and number of missteps during tandem gait (r=0.16, p=0.06). The primary anatomical substrate for ET continues to be a matter of speculation, yet these and other clinical data lend support to the notion that there is an underlying abnormality of the cerebellum and/or its pathways.