The effects of changing head position and posture on head tremor in individuals with essential tremor involving the head
Badke, Nicole Jacqueline
essential tremor , head kinematics , spectral analysis , tilt-twist , disability , electromyography , postural control , neck muscles , vision , movement disorder
Objective: To determine the effects of head position and of different postural control demands on head tremor measures in participants with essential tremor. Methods: Seventeen participants with essential tremor (ET) of the head and 17 control participants took part. Individuals held their heads in varying degrees of rotation, flexion, and extension. Subsequently, individuals sat and stood in different postures, incorporating different foot placements (feet apart and together), surfaces (solid and foam), and vision conditions (eyes open and closed). Neck muscle activity was recorded from three muscles bilaterally (trapezius, sternocleidomastoid, splenius capitis). Three-dimensional head and thorax positions were recorded using an Optotrak system, and head angular velocity with respect to thorax was calculated by differentiating tilt-twist angles. Fourier analysis was used to determine tremor power. Results: ET participants showed sharp peaks at their tremor frequency in spectral plots of kinematic data, whereas CN participants did not. Electromyography data was too noisy for frequency analysis. ET participants displayed increased tremor power in head positions 25° from neutral compared to neutral and positions 50° from neutral. Tremor power increased with increasing difficulty of posture for both participant groups. Removal of vision resulted in decreased tremor power in ET participants; power was significantly decreased in the easier postures, and progressively less so with increasing difficulty of posture. Interestingly, tremor direction was inconsistent in both groups, and two ET participants displayed multiple tremor peaks. Conclusions: The tilt-twist method is a feasible way of measuring head kinematics. Changing stiffness of the neck likely mediates the effect of head position on head tremor power, with the resultant interplay of the central driver and the mechanical resonance driving the amplitude changes. Decreasing stability of posture increases head tremor, likely due to the associated increase in postural sway and stress on the postural control system. Vision appears to exacerbate head tremor through the addition of tremor-related visual noise and an implicit task to stabilize vision; this is possible evidence of a visuomotor deficit. However, stabilizing vision becomes less important with increasing difficulty of posture, resulting in a narrowing gap in tremor power between vision and no vision conditions.