Objective: To investigate whether primary writing tremor (PWT) and dystonia share a common pathophysiology and clarify if they are different phenomenological expressions of the same trait.

Background: PWT is probably the most common form of task-specific tremor and it occurs during handwriting, considerably interfering with it (1). PWT pathophysiology is still debated and there is controversy about it being a type of focal dystonia, a variant of essential tremor or a separate entity (2). Some clinical features (i.e. task/position-specificity) and observations (family history for dystonia) might support the notion that PWT is dystonic in nature.

Methods: We recruited 5 patients with PWT. All of them were clinically evaluated through the Fahn-Tolosa-Marin (FTM) tremor scale and underwent the following neurophysiological tests: blink recovery cycle (BRC), eye-blink classical conditioning (EBCC), short and long-intracortical inhibition (SICI-LICI). Moreover, motor-evoked potentials (MEP), SICI and LICI were measured pre and post paired associative stimulation (PAS) plasticity paradigm. Data were compared with those obtained in 5 dystonic patients (DYT), all of them presenting with cervical and/or hand dystonic tremor and writing dystonia. EBCC and BRC were also studied in 12 healthy subjects (HS).

Results: FTM tremor scale score was 6.43.0 and 24.013.5, respectively in PWT and DYT group. Data analysis for BRC showed a smaller suppression of the R2 component in both patients’ groups compared to HS; a similar inhibition between DYT and PWT was observed. There was no difference between DYT and PWT in the number of conditioned response (CR) during EBCC and, compared with HS, patients had a smaller number of CR. Baseline SICI and LICI were similar in the two patients’ group, but whereas they were unchanged in PWT after PAS, they were reduced in DYT.

Conclusions: The present results suggest that PWT and DYT have similar brainstem excitability and associative learning tested by means of the EBCC. Interestingly, cortical plasticity induced by PAS had a different effect on SICI and LICI in the two groups. Overall this data suggests that PWT and DYT share some pathophysiological mechanisms. The dissimilarity might be explained by the different clinical presentation. Hence, we may conclude that PWT is a form fruste of dystonia.

References: 1. Bhatia KP, Bain P, Bajaj N, Elble RJ, Hallett M, Louis ED, Raethjen J, Stamelou M, Testa CM, Deuschl G; Tremor Task Force of the International Parkinson and Movement Disorder Society. Consensus Statement on the classification of tremors. from the task force on tremor of the International Parkinson and Movement Disorder Society. Mov Disord. 2018 Jan;33(1):75-87. 2. Rana AQ1, Vaid HM. A review of primary writing tremor. Int J Neurosci. 2012 Mar;122(3):114-8.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/is-primary-writing-tremor-a-form-fruste-of-dystonia/