Objective: To investigate cortical integration of proprioceptive information in patients with orthostatic tremor (OT), measured as event-related synchronisation (ERS) response to proprioceptive stimulation using magnetencephalography (MEG) [1-3].

Background: OT is a rare neurological disorder where a fast synchronised tremor occurs in leg and trunk muscles when activated in standing position [4-6]. Besides of the tremor, patients are strongly affected by an intense feeling of unsteadiness while standing [6-8]. This may be explained by a pathological integration of proprioceptive information either by disturbance of the fast tremor activity affecting afferent proprioceptive signals or by pathological changes impeding the integration of proprioceptive information [9-12].

Method: Fifteen OT patients and fifteen age and gender matched healthy controls (HC) were examined in a whole-head MEG system containing 102 triplets of magnetometer and 2 planar gradiometers. Proprioceptive stimulation by passive movements of the right index finger and the right forefoot was evoked by custom-made pneumatic MEG-compatible actuators [figure 1 and 2] [13,14]. Sensor-level time-frequency responses were extracted in a frequency range of 8 to 30 Hz, including both alpha (8-13 Hz) and beta (14-30 Hz) rhythms. The gradiometer pair with the highest amplitude change time-locked to the proprioceptive stimulus was used to calculate the ERS. Electromyography (EMG) tracked muscle activity on the right arm and leg to enable detection of possible voluntary movements during the passive movements.

Results: Finger stimulation did induce ERS followed by a rebound as expected in both groups. The stimulation of the foot, however, only induced a slight ERS. Cluster-based permutation tests comparing the HC and OT groups did not result in any significant differences in the time-frequency responses to the proprioceptive stimulation of the right index finger, nor to the right foot [figure 3].

Conclusion: Proprioceptive stimulation induced a stable ERS in OT without evidence for a difference compared with healthy individuals. Thus, the characteristic intense feeling of instability in OT could not be referred to an altered cortical integration of proprioceptive information.

Fig1. Passive movement of the right foot.

Fig2. Passive movement of the right index finger.

Fig3. Time-frequency response.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cortical-response-to-proprioceptive-stimulation-in-orthostatic-tremor-a-magnetoencephalography-study/