Cerebellar Brain Inhibition Is Associated With the Severity of Cervical Dystonia.

Purpose: Cerebellar connectivity is thought to be abnormal in cervical dystonia (CD) and other dystonia subtypes, based on evidence from imaging studies and animal work. The authors investigated whether transcranial magnetic stimulation-induced cerebellar brain inhibition (CBI), a measure of cerebellar efficiency at inhibiting motor outflow, is abnormal in patients with CD and/or is associated with clinical features of CD. Because of methodological heterogeneity in CBI reporting, the authors deployed additional controls to reduce potential sources of variability in this study.
Methods: Cerebellar brain inhibition was applied in 20 CD patients and 14 healthy control subjects. Cerebellar brain inhibition consisted of a cerebellar conditioning stimulus delivered at four different interstimulus intervals (ISIs) before a test stimulus delivered to hand muscle representation in the motor cortex. The average ratio of conditioned to unconditioned motor evoked potential was computed for each ISI. Cervical dystonia clinical severity was measured using the Toronto Western Spasmodic Torticollis Rating Scale. Control experiments involved neuronavigated transcranial magnetic stimulation, neck postural control in patients, and careful screening for noncerebellar pathway inhibition via cervicomedullary evoked potentials.
Results: There was no difference between CBI measured in healthy control subjects and CD patients at any of the four ISIs; however, CBI efficiency was significantly correlated with worsening CD clinical severity at the 5 ms ISI.
Conclusions: Cerebellar brain inhibition is a variable measure in both healthy control subjects and CD patients; much of this variability may be attributed to experimental methodology. Yet, CD severity is significantly associated with reduced CBI at the 5 ms ISI, suggestive of cerebello-thalamo-cortical tract dysfunction in this disorder.
Competing Interests: R. E. Sondergaard holds a Natural Sciences and Engineering Research Council of Canada (NSERC) studentship and N. D. J. Strzalkowski held University of Calgary Eyes High, Parkinson Alberta and Parkinson Foundation postdoctoral fellowships. Funding was also provided by NSERC (RGPIN/04126-2017) to Z. H. T. Kiss and Dystonia Medical Research Foundation of Canada grant (Z.H.T.K. and D.M.). The remaining authors have no conflicts of interest to disclose.
(Copyright © 2021 by the American Clinical Neurophysiology Society.)