Objective: To characterize the local field potential (LFP) properties within the globus pallidus (GP) during active deep brain stimulation (DBS) in a patient with Holmes tremor.

Background: Recent studies have demonstrated the potential of using pallidal DBS as a treatment for Holmes tremor [1]. However, the pathophysiology of this tremor and the mechanisms by which DBS improves it remain poorly understood. The recent development of DBS systems capable of sensing neural signals during active stimulation now allows for novel exploration of the neurophysiology underlying Holmes tremor and its treatment.

Method: A 46-year-old man with a left posterior cerebral artery infarct involving the posterior thalamus developed a debilitating right upper extremity Holmes tremor three months afterwards. Two years later, a DBS system with sensing capabilities was implanted into the left GP. DBS stimulation parameters were optimized for tremor control per standard of care. At the initial programming session, LFPs from the left GP were collected when the patient was at rest, both with and without active stimulation. Power spectral density (PSD) estimates were calculated for each stimulation condition.

Results: A monopolar review identified that stimulation at the second most dorsal of a four-contact DBS lead provided partial improvement of the patient’s tremor and allowed for concurrent recording of LFPs during active stimulation. In both stimulation conditions, the dominant spectral content was in the delta-theta range. The OFF-stimulation condition demonstrated a spectral peak at 9.8 Hz.

Conclusion: This case report presents one of the first described recordings of pallidal LFP signals in a patient with Holmes tremor outside of the operative setting. The data reproduce a previously reported low-frequency concentration of pallidal LFP spectral power in Holmes tremor, in contrast to beta-band peaks frequently seen in Parkinson’s disease [2]. The data also demonstrate the presence of an alpha-band peak during the OFF-stimulation condition. These results provide important first steps towards further investigation into the neurophysiology of Holmes tremor.

References: [1] C. Kilbane et al., “Pallidal stimulation for Holmes tremor: Clinical outcomes and single-unit recordings in 4 cases,” J. Neurosurg., vol. 122, no. 6, pp. 1306–1314, 2015. [2] A. Ramirez-Zamora et al., “Clinical outcome and characterization of local field potentials in holmes tremor treated with pallidal deep brain stimulation,” Tremor and Other Hyperkinetic Movements, vol. 6, no. 388, pp. 1–8, 2016.

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