Editorial: Dystonia and tremor

Pattamon Panyakaew^1,2*Aparna Wagle Shukla³

• ¹Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand
• ²Chulalongkorn Centre of Excellence for Parkinson's Disease & Related Disorders, Bangkok, Thailand
• ³Norman Fixel Institute for Neurological Diseases, Department of Neurosurgery, College of Medicine, University of Florida, Gainesville, Florida, United States

Beylergil et al. determined the prevalence of tremors in CD enrolled in the Dystonia Coali4on cohort. 4 The study found that approximately 45% of pa4ents, par4cularly women, experienced head tremors, with nearly 75% exhibi4ng irregular head tremors (beTer classified as jerky dystonia) and 25% presen4ng with regular head tremors. The predictors of head tremors included increased disease-severity, increased disease dura4on, and increased age, in that order, whereas the presence of regular head tremors was associated with decreased disease severity and older age. This underpins that jerky dystonia and regular head tremors seen in CD should be regarded as dis4nct en44es.Jabarkheel et al. prospec4vely compared the electrophysiological characteris4cs of head and arm tremors seen in pa4ents with focal CD vs. segmental dystonia. 5 While the mean frequency of the head tremor was observed to be low (4.3 ± 0.9; range 3.5 -6 Hz), the arm tremor exhibited a slightly higher frequency (5.5 ± 0.6; range 3.5 -7 Hz). The frequency of head tremors in younger par4cipants was higher than in older par4cipants, as previously described in pa4ents with essen4al tremor (ET). When comparing focal vs segmental dystonia, the head tremor in CD had lower peak frequency and amplitude with longer EMG burst dura4on. Arm tremor in pa4ents grouped as focal dystonia (CD plus arm tremor without dystonic features) exhibited lower amplitude compared to segmental dystonia (CD plus arm tremor with dystonic features). The head and arm tremors tended to be less severe in pa4ents repor4ng alcohol responsiveness. The study concluded that the physiological characteris4cs of tremors in focal and segmental dystonia differ to some extent, indica4ng that the progression of dystonia symptoms across body regions may influence the underlying physiology of co-occurring tremors.The effects of botulinum toxin (BoNT) injec4on were inves4gated regarding the regularity of head oscilla4on in CD in a small sample size (N=8 with documented head movements by the magne4c search coil). 7 The regularity of head tremors was quan4fied by calcula4ng the dispersion of head movements in 4me series values. BoNT injec4on could change the regularity of head tremor to a certain "set-point" in the oscillatory network by possibly modula4ng propriocep4ve feedback to the head neural integrator. 8 In addi4on, the randomness of head movements was not changed with BoNT injec4on, suppor4ng that the head movements in this study were consistent with jerky dystonia rather than tremor based on the current viewpoint. Overall, the amplitude and frequency of head movement decreased with BoNT injec4on, with a pronounced reduc4on in head orienta4on in pa4ents with high-intensity head oscilla4on prior to the injec4on.Whether task-specific tremor (TST) should be classified as a form of task-specific dystonia (TSD) or as a variant of ET remains unclear. The electrophysiology of TST is poorly characterized. Kuo and Chen reviewed the current evidence of the underlying physiology of TST. 9 Most of the studies were conducted in pa4ents diagnosed with primary wri4ng tremor and TST presen4ng in musicians. The electromyographic results showed co-ac4va4on between the antagonist pairs and overflow muscle ac4vi4es to the adjacent muscles, similar to dystonia. However, the loss of inhibi4on in spinal, brainstem, and cor4cal levels was not iden4cal to dystonia. The reciprocal inhibi4on, the physiological technique to assess spinal inhibi4on, was normal in TST. GABAergic cor4cal inhibi4on was slightly impaired, while the cor4cal silent period was within the normal range. Func4onal imaging revealed decreased func4onal connec4vity between the cerebellum and other parts of the brain, but less widespread compared to dystonia. Taken together, TST may be under the subtype of dystonia and tremor rather than ET. Nevertheless, it could be a separate en4ty since it was not en4rely congruent with the physiology of dystonia and tremor.Finally, gait and balance problems have been iden4fied in pa4ents with CD. However, these aspects have never been addressed in CD with head tremor. Wagle Shukla et al. inves4gated the clinical and spa4otemporal parameters of gait in this specific group. 6 They demonstrated that nearly half of the pa4ents with CD and tremors experienced clinical gait and balance difficul4es, including slower walking speed and impaired performance on the Berg Balance Scale. In their assessments, more than 20% of pa4ents exhibited a shorter step length, wider stride width, and increased double support 4me when walking on a gait mat compared to healthy individuals, sugges4ng that an abnormal cerebellar network contributed to these findings. However, when comparing with ET and pa4ents with orthosta4c tremor, the dystonia and tremor group exhibited less pronounced abnormali4es in objec4ve gait and balance variables, sugges4ng that a rela4vely lesser degree of dysfunc4on within the cerebellar network was present. The study also highlighted that gait and balance dysfunc4on in CD with head tremors could also stem from factors beyond cerebellar dysfunc4on, including impairments in ves4bular and propriocep4ve pathways due to abnormal head posi4oning and constant head shaking. Reduced control over voluntary neck movements may further hinder naviga4on in complex environments. These findings emphasize the importance of incorpora4ng rehabilita4on strategies into outpa4ent management plans for dystonia and tremors.To summarize, this special issue of Dystonia and Tremor emphasizes the clinical characteris4cs, physiological aspects, and pathophysiological understanding of dystonia and tremor. The repe44ve jerky movements of a par4cular body part in dystonia should no longer be classified as a tremor. Physiological findings can offer clinicians valuable insights for improving diagnosis and pa4ent management while also guiding researchers in designing more robust studies. The