Objective: To evaluate whether tactile and proprioceptive dysfunction in cervical dystonia is associated with co-existing tremor.

Background: Cervical dystonia (CD) is characterized by involuntary neck muscle contractions that cause abnormal head postures. Tremor can be an additional phenotypic feature of CD indicating that CD is also a network disorder involving cerebro-cerebellar circuitry. CD is associated with a generalized somatosensory deficit that also affects non-dystonic motor systems. It is unknown, if the different phenotypes of CD exhibit different levels of somatosensory impairment.

Method: We enrolled 24 treatment-free patients with focal CD, (12 with tremor and 12 without) and 22 age-matched healthy subjects (HS). As markers of somatosensory function we obtained 1) tactile temporal discrimination thresholds of the non-dystonic forearm, and 2) proprioceptive acuity measures in both the affected (head/neck) and unaffected body segments (forearm/hand) using a joint position-matching task. The head or the wrist was passively displaced by the experimenter or through a robotic exoskeleton to a distinct joint position (head lateral rotation and head lateral bending, wrist flexion-extension and abduction-adduction). Then participants actively reproduced the experienced joint position, which was tracked by a motion capture system for the head or by encoders of the robotic exoskeleton for the wrist. The absolute joint position matching error between target and the reproduced positions served as a marker of proprioceptive acuity.

Results: Mean tactile temporal discrimination thresholds were significantly elevated in both CD subgroups when compared to controls. Proprioceptive acuity of both the affected and unaffected body segments was abnormal only in patients with cervical dystonia and tremor.

Conclusion: Tactile abnormalities were a shared dysfunction of both CD phenotypes, while proprioceptive dysfunction was primarily observed in CD patients with tremor. These findings suggest that the pathophysiology in CD patients with tremor is characterized by two abnormal neural processes: First, a dysfunctional somatosensory gating mechanism related to basal ganglia defective inhibitory activity and Second, abnormal processing of proprioceptive information within a defective cortico-cerebellar loop.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/tactile-and-proprioceptive-dysfunction-differentiates-between-cervical-dystonia-with-and-without-tremor/