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Mechanisms of deep brain stimulation for essential tremor

Rodger J. Elble
DOI: http://dx.doi.org/10.1093/brain/awt347 4-6 First published online: 13 January 2014

Although essential tremor is often thought of as benign, it can be disabling to the point of justifying invasive deep brain stimulation (DBS). However, the mechanisms by which thalamic and subthalamic stimulation suppress tremor are poorly understood.

In this issue of Brain, Groppa et al. (2014) quantified the performance of seven patients with essential tremor in a reaching task to determine the DBS target structures associated with therapeutic reduction in tremor and with the side effect of stimulation-induced ataxia. The optimum site of stimulation was usually in the posterior subthalamic area, beneath the inferior border of the thalamic nucleus ventralis intermedius. Probabilistic diffusion tractography identified the dentatothalamic tract as the key therapeutic DBS target structure. Supratherapeutic stimulation of this site produced upper limb ataxia, despite continued suppression of tremor. The authors systematically varied stimulation pulse strength and duration to find that stimulation chronaxies of 27 and 52 µs were associated with tremor suppression and the induction of ataxia, respectively. Chronaxie is loosely defined as the minimum duration at which a current twice threshold (minimum current with indefinitely long pulse duration) elicits a response and provides a clue as to the properties of the axons recruited. They reasoned that two different populations of large myelinated nerve fibres were affected: (i) dentatothalamic axons for the therapeutic effect; and (ii) afferent or efferent axons of the red nucleus for the stimulation-induced ataxia. However, the involvement of rubral pathways could not be confirmed with diffusion tractography. This work is an important contribution to a growing literature addressing the mechanisms of DBS and the preferred site of DBS stimulation for essential tremor.

Ventralis intermedius has historically been the preferred stereotactic target for thalamotomy and DBS in the treatment of essential tremor and other tremor disorders (Miocinovic et al., 2013), but recent postoperative correlations …

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