Effects of thalamic stimulation frequency on intention and postural tremor

Abstract

Deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM) of the thalamus improves essential tremor. Suppression of the amplitude of the postural tremor component with VIM DBS depends on stimulation frequency. The purpose of this study was to determine the effect of DBS frequency on the intention tremor component, that is, tremor that is enhanced by target-directed movement, and to compare it to the effect of DBS frequency on postural tremor in people with essential tremor. We measured tremor frequency and amplitude during trials of postural holding and voluntary reaching between two targets at 10 different stimulation frequency settings between 0 and 185 Hz. Tremor frequency did not change with changes in stimulation frequency. Amplitude suppression of both intention and postural tremor depended on stimulation frequency. Maximal tremor reduction occurred at approximately 130 Hz for both forms of tremor. However, at optimal frequencies, the percent reduction in tremor amplitude relative to the DBS OFF condition was greater for postural than for intention tremor. These results suggest that VIM DBS stimulation frequencies near 130 Hz may provide maximal control of intention and postural tremor. Identification of optimal stimulation settings should consider assessment of intention tremor, not just postural tremor, as intention tremor may not be as well controlled as postural tremor but may be a better gauge for functional benefit.

Figure 1

Illustration of intention tremor in an individual subject across six different stimulation frequencies. Tremor magnitude decreases as stimulation frequency increases up to 130 Hz, after which little additional reduction in tremor occurred in this individual. Note that tremor is most apparent at the two extremes of the reach as the subject approaches the chin or the target, but is not obvious during the movements between targets.

Figure 2

Illustration of postural tremor in an individual subject across six different stimulation frequencies. Tremor magnitude decreases as stimulation frequency increases up to 130 Hz, after which little additional reduction in tremor occurred in this individual.

Figure 3

FFT plots for the 0 Hz and 185 Hz postural tremor trials shown in Fig. 2. There is no substantial change in peak spectral frequency, which is in the range of 3–4 Hz for both trials, despite the substantial reduction in tremor amplitude at 185 Hz.

Figure 4

Illustration of individual tremor amplitudes for intention (4A) and postural (4B) tremor across stimulation frequencies. Values are normalized to the DBS OFF condition which was set at 100%.

Figure 5

Illustration of average absolute tremor amplitudes for intention (black circles) and postural (gray triangles) tremor across stimulation frequencies (5A). Values represent the average length of path of the index finger per second. Figure 5B illustrates average normalized tremor amplitudes (DBS OFF = 100%). The intention tremor data points were best fit by the curve Y = 35.5 + 172.1/(1+exp(− (X+48.6)/− 85.8)). The postural tremor data points were best fit by the curve Y = 29.2 + 76.8/(1+exp(− (X−52.1)/− 17.8)). Graphed values are means +/− SE.