Objective: We used a paradigm of neck vibration to test the hypothesis that the sensory trick (ST), which is used by some patients with idiopathic cervical dystonia (CD) to alleviate their dystonic posture, modulates the processing of proprioceptive information.

Background: Muscle vibration activates muscle spindles and can thus be used in the experimental setting as a proprioceptive stimulus simulating muscle lengthening. Applied over neck muscles during stance, vibration modulates global body orientation, which usually leads to a tonic forward sway in healthy persons. This postural response has been reported to be diminished or absent in CD patients.

Methods: 21 CD patients with a ST, 15 CD patients without ST and 16 healthy controls (HC) were recruited. Neck muscle vibration (100 Hz for 10 sec.) was applied bilaterally over the upper trapezius under three different conditions: 1) Quiet standing; 2) standing while performing the ST (or ST-like movement in the control groups); 3) standing while elevating the flexed arm without touching any part of the body (i.e. sham ST). Centre of pressure position and ankle, hip and neck angles in the sagittal plane were analysed.

Results: For HC in all three conditions, neck muscle vibration led to an initial forward sway of the body that slowly increased during the prolonged vibration. CD patients with a ST showed a similar pattern of sagittal sway. However, in patients without a ST, the initial sagittal sway was significantly reduced in all three conditions (p<0.001) and the later slow increase was absent (p=0.024). In general, the response was mediated by an ankle flexion in combination with a simultaneous hip extension and a tendency to extend the neck. Ankle flexion and neck extension were reduced in patients without a ST, whereas hip extension was significantly reduced in those with a ST. However, contrary to our hypothesis, the ST did not have an effect that was specific to either or both CD groups for any aspect of the postural response.

Conclusions: Our results suggest that processing of proprioceptive input from the neck muscles in CD patients with an effective ST is similar to HC, while patients without any ST are far less sensitive to this input. This difference in sensitivity to proprioceptive input for postural control could be an important trait that determines if CD patients benefit from ST or not.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/differences-in-processing-proprioceptive-input-from-the-neck-muscles-may-explain-the-benefit-from-sensory-tricks-in-patients-with-idiopathic-cervical-dystonia/