Objective: To examine the association between clinical features of cervical dystonia (CD) and cerebellar inhibition of the motor cortex (M1).

Background: The involvement of cerebellar outflow pathways in cervical and other idiopathic dystonias is suggested by neuroimaging and animal models. Cerebellar brain inhibition (CBI) is a paired pulse TMS paradigm that assesses the inhibitory effect of cerebellar output on motor cortex excitability, and is likely to be supported by these pathways. Previous evidence showed reduced CBI efficiency in idiopathic dystonia, but sample size was too small to assess its association with clinical features.

Method: CD subjects (ages 41-72, median 63.5) and age-matched controls were recruited. Testing involved delivering a cerebellar conditioning stimulus (intensity = resting motor threshold of contralateral abductor pollicis brevis muscle) followed by a test stimulus over contralateral M1 using 3, 5, 7 and 15ms interstimulus intervals (ISIs). We calculated average ratios of conditioned:unconditioned MEP amplitudes as measure of CBI (higher CBI ratio, lower CBI efficiency).

Results: 20 CD subjects and 10 healthy age-matched controls were recruited. CD subjects were subsequently dichotomized by median Toronto Western Spasmodic Torticollis (TWSTRS) severity subscore. One-way ANOVA, conducted independently for each ISI, showed a ‘Clinical Group’ effect only for the 5ms ISI (F(2,24)=9.13, p=0.001). Bonferroni post hoc testing revealed that CBI ratio when using an ISI of 5ms were higher in subjects with more severe dystonia vs. less severe dystonia (p=0.002) or controls (p=0.006). CBI at 5ms was significantly correlated with TWSTRS dystonia severity subscore (partial correlation coefficient, adjusted for age, disease duration and sex= 0.48, p=0.04) and TWSTRS pain subscore (PCC = 0.49, p=0.04). CBI efficiency did not differ between CD patients with and without tremor. We ruled out direct cortico-spinal tract activation by comparing conditioned and unconditioned MEP onset latencies at 5ms ISI (F(5,39)=0.467, p=0.799).

Conclusion: CBI efficiency at 5ms ISI diminishes with increasing dystonia severity and dystonia-related pain in subjects with CD. Our results provide initial evidence of a relationship between cerebellar influences on M1 excitability and the burden of motor symptoms and pain in CD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cerebellar-brain-inhibition-and-cervical-dystonia-exploring-clinico-electrophysiological-associations/