Tremor is defined as a rhythmic, involuntary oscillation that can affect the upper and lower extremities, head, jaw, face, tongue, voice and trunk.¹ Irrespective of the aetiology, tremor has a detrimental effect on the patient’s quality of life. Surgery is offered when tremor is functionally disabling and medically refractory, or when patients develop complications from drug therapy that limit effective tremor control. A number of subcortical nuclei and white matter tracts have been defined that, when lesioned or stimulated, control different aspects of tremor. This article discusses the merits and drawbacks of stimulating each of these surgical targets for different forms of tremor and why deep brain stimulation (DBS) of the caudal or motor part of the zona incerta nucleus (cZI) is effective in alleviating all forms of tremor.

Ventrolateral Nucleus of the Thalamus
Since the 1950s, stereotactic surgery for tremor has targeted the ventrolateral (VL) nucleus of the thalamus (Hass ers ventralis intermedius) and it has been the target of choice to effectively suppress distal limb tremor having a resting or postural component, including tremor of Parkinsons disease (PD) and essential tremor (ET). Long-term studies (up to seven years) have demonstrated a 50–90% improvement in symptoms for this condition.^2–6 However, proximal tremor and the action component of distal tremor respond poorly to DBS,⁷ with only one-third of the patients showing any significant improvement.^8,9 Similarly, inconsistent or transient results are seen following DBS for proximal action tremor, as in multiple sclerosis (MS), with an initial 40–50% improvement in tremor but with little improvement in patient activities of daily living.^10,11 Nguyen et al. have previously suggested that stimulating the dorsal part of the VL nucleus could suppress proximal action tremor, as single unit recordings¹² have revealed somatotopic organisation in the VL thalamus, with the he d, neck and proximal part of the limb represented in the dorsal part of the nucleus and the hands in the ventral part.¹³ Improvement in head and neck and voice tremor is variable, with 15–51% improvement following unilateral DBS and 39–100% with bilateral procedures.^{3–5,14} The key limiting factor in effectively suppressing tremor by DBS of this nucleus is the high incidence (30–50%) of bilateral-stimulation-related dysarthria and disequilibrium.^{8,9,15–17} These side effects are generally reversible by adjusting the stimulation parameters, although this may be at the expense of satisfactory tremor control. Therefore, some centres only implant unilateral DBS leads, contralateral to the tremor dominant side. Another side effect compromising effective tremor control is that patients develop tolerance (tissue habituation) to continuous stimulation, even when the amplitude is increased.^8,9,18 Patients are advised to turn their stimulators off at night, take stimulation hol days for weeks8 or use the DBS only ‘on demand’¹⁹ to prevent tissue habituation. Tissue habituation also probably contributes to decline in tremor control in the long term. There are a few case reports suggesting that unilateral DBS of the VL nucleus can improve distal primary writing tremor,^20,21 post-traumatic tremor^22–25 and orthostatic tremor.²⁶

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