Objective: To investigate how ST modulate the postural sway in patients with idiopathic cervical dystonia (iCD) when proprioceptive and vestibular stimulation are provided simultaneously.

Background: We previously showed that the postural sway induced by proprioceptive stimulation was attenuated in iCD without sensory tricks (ST), whereas it was preserved in those with effective ST. However there was no modulation of the sway in either group, when ST were applied, thus pointing to a group specific trait in gating afferents rather than providing a holistic explanation for ST. We hypothesized that ST-specific effects on gating afferents were more relevant in the context of multimodal afferents.

Method: Postural sway was investigated in 15 iCD with ST and 11 CD without during I) simple standing II) while standing and performing ST (or ST-like movement in the control group) and III) while elevating the arm without touching the face. Neck muscle vibration (i.e. proprioceptive stimulation), galvanic vestibular stimulation (GVS) and combined stimulation were applied for 5 sec each. The sway was measured by capturing the trajectory of a marker fixed on the sternum. The absolute sway amplitude and sway amplitude normalized to the intra-individual average sway for each stimulation across conditions I-III were analysed.

Results: During neck muscle vibration and combined stimulation iCD with ST showed a larger postural sway along the sagittal axis, i.e. the expected sway direction during proprioceptive stimulation, than iCD without ST. During GVS the sway along the lateral axis, the expected direction of GVS-induced sway, was comparable in both groups. There was no ST-specific effect in either group. Normalizing the responses revealed a relative increase of the sway along the sagittal axis in iCD without ST during simultaneous GVS and proprioceptive stimulation. This effect was not seen in iCD with ST.

Conclusion: This study confirms our previous findings of larger postural sways upon proprioceptive stimulation in iCD with ST. Vestibular responses, however, were comparable in both groups. During multimodal simultaneous stimulation the gain for proprioceptive afferents was upregulated in iCD without ST, but surprisingly not in those with effective ST. One explanation could be that in iCD with ST the gain for proprioceptive afferents is already maximized thus leading to a ceiling effect that does not allow any further increase of the sway.

References: Sensory Trick Efficacy in Cervical Dystonia Is Linked to Processing of Neck Proprioception. F Brugger, A Peters, D Georgiev, G Kägi, B Balint, KP Bhatia, BL Day. 61, 50-56 Apr 2019. Parkinsonism Relat Disord

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-impact-of-the-sensory-trick-on-gating-multimodal-afferents-in-cervical-dystonia/