Study of Sensory Attenuation in Functional Movement Disorders

Functional movement disorders (FMD) are conditions where people experience unusual movements or difficulties with walking, not caused by a specific brain or nerve injury but related to how the brain controls movements. Functional movement disorders are common in clinical practice and can lead to significant disability and healthcare costs.

A key feature of FMD is a problem with self-agency-the feeling that we are in control of our own movements. Many patients with FMD feel that their abnormal movements happen without their control. Sensory attenuation is closely linked to self-agency. It's the brain's way of reducing the intensity of sensations caused by our own movements. For example, you can't tickle yourself because your brain knows it's your own action. In people with FMD, this process doesn't work properly. As a result, they might feel their movements are involuntary.

Previous research shows that sensory attenuation is reduced in FMD, but the studies so far have been small. This study will investigate sensory attenuation in a larger group of FMD patients and compare it with healthy individuals. The goal is to see if reduced sensory attenuation could be used as a marker to measure the severity of FMD.

Objective: The primary objective of the study is to investigate sensory attenuation in patients with functional movement disorders using the force-matching paradigm. The secondary objective is to determine whether sensory attenuation metrics are correlated with disability and functional movement disorder scores.

Materials: The data will be collected using a stand-alone, mechatronic system, capable of applying precise forces to a patient's finger and capturing a patient's force inputs into the system. This device has an integrated data collection system that ensures all results are captured and stored on an SD card.

Experimental Design: Thirty-five patients diagnosed with FMD and 35 healthy controls will be included in the study. Patients will be enrolled from the integrated functional movement disorders clinic and the movement disorders clinic at the London Health Sciences Center.

Each participant will be tested on two conditions: (1) Self condition: matching a target force by pressing on a movement arm with an integrated pressure transducer over the non-dominant index finger. (2) External condition: Matching a target force by moving a slider that controls the pressure generated by the movement arm placed over the non-dominant index finger. Five different target forces, with 0.5 Newtons increments, from 1 Newton to 3 Newtons, will be randomly presented in both conditions. All subjects will complete a total of 30 randomly generated forces for each condition. The applied forces are small and not painful.

Procedure: Participants will sit in front of a table and place the tip of their non-dominant index finger under the force transducer. In the self-condition (condition 1), the motor will exert one of the five target forces in each trial. The participants will then start matching the target force by directly pressing onto the movement arm, which rests on the non-dominant index finger, using the contralateral index finger. In the external condition (condition 2), the motor will exert one of the five target forces. The participant will then match the target force by moving a potentiometer slider, which controls the force generated by the movement arm placed over the non-dominant index finger. The study visit will be completed in one hour.

The ratio between the matched force and the target force will be calculated and serve as a sensory attenuation measure. This measure will be averaged across trials to give the mean attenuation for each force level and condition.

Statistical analysis: A Mixed Design ANOVA will be used to measure the differences between the within-subjects factors (conditions and force levels) and the between-subjects factor (healthy control vs. FMD groups). Post-hoc analysis will be conducted using Bonferroni correction for multiple comparisons to determine which specific means are significantly different. Pearson's correlation will be used to determine how scores on the World Health Organization Disability Assessment Schedule (WHODAS2) and the Psychogenic Movement Disorders Scale (PMDS) are related to the degree of sensory attenuation.

Eligibility Criteria:
Inclusion Criteria:

(1) Diagnosed with a functional movement disorder (2) Age \>18 years

Exclusion Criteria:

(1) Moderate to severe action tremor in the dominant hand that interferes with the tasks. (2) Upper limb peripheral neuropathy. (3) Inability to read or understand English.

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