Debated Issues
Which Target?
As detailed in the 'Other recognized indications of pallidal stimulation in dystonia' section, the largest available body of literature concerns the efficacy of the GPi ventrolateral target of stimulation, which is considered as the 'classical' target of stimulation for dystonia. However, other targets have also been tested and may be of interest for dystonia.
This is the case for the thalamus. Indeed, stimulation of the ventral oral nucleus of the thalamus has been shown to provide improvement of writer's cramp.[63] The ventral intermediate nucleus thalamic nucleus DBS has also been associated with a reduction of myoclonus in myoclonus dystonia, while dystonia was not improved.[64,65] In addition, the ventral intermediate nucleus thalamic stimulation failed to improve tardive dystonia.[66]
More recently, the subthalamic nucleus has been proposed as an interesting target of stimulation for dystonia, notably for more bradykinetic forms of dystonia, with the advantage of avoiding bradykinesia that might be induced by GPi DBS.[67] A recent, randomized, double-blind study has been performed to compare primary, generalized, focal or segmental dystonia subthalamic nucleus and GPi stimulation.[68] This study concerned 12 patients and suggests that subthalamic nucleus DBS was superior to GPi DBS in terms of motor aspects of the disease and as efficient as GPi DBS for quality of life improvement.
Recently, a single case report showed the improvement of axial dystonia or Pisa syndrome in Parkinson's disease after pedunculopontine stimulation.[69]
The combination of several targets of stimulation may also be considered as an option. This has been reported is some case reports.[70,71] The idea is to combine the advantages of each target of stimulation, for example, the thalamus for myoclonus and the GPi for dystonia in myoclonus dystonia.[71,72] On the other hand, using two areas of stimulation within the same target (i.e., the motor and limbic pats of the GPi) could be of interest in diseases associating severe motor, but also behavioral, disorders, such as in Lesch–Nyhan disease.[56] Finally, the progressive worsening of dystonia observed in some patients over time in primary dystonia, consistent with a progression of the pathological process, could beneficiate from a second implantation within the GPi in order to extend the area of stimulation.[8] It should, however, be noted that such multitarget procedures have only been reported in case reports, and their efficacy is not supported by large series.
All these issues will need further investigations and controlled studies before a conclusion can be made concerning alternatives to GPi target of stimulation and one versus several targets of stimulation.
Is There Still a Place for Pallidotomy?
Despite its side effects (especially for bilateral procedures), such as speech disturbances, pallidotomy may provide good improvement for tardive dystonia, status dystonicus and primary dystonia, although results are very heterogeneous among the published series.[73] This approach is also advantageous as it is less time-consuming, less expensive and will not have a risk of hardware complication. However no proper comparison exists between DBS and pallidotomy in dystonia. Interestingly, several case reports have suggested that, after a pallidotomy or thalamotomy, GPi DBS could lead to additional improvements in generalized dystonia.[74–78] On the other hand, it is of great interest to note that, when DBS has to be ceased for infectious reasons, for example, the quadripolar DBS lead can be used before its removal to perform radiofrequency lesioning of the GPi and maintain the clinical benefit of the DBS.[79]
Cortical Stimulation?
Although it is a different approach from DBS, cortical stimulation should also be discussed in the present review. Slow, repetitive, transcranial magnetic stimulation of the motor and premotor cortex has been demonstrated, by acting on cortex hyperexcitability, to improve writer's cramp.[80–82] Furthermore, transcranial alternating and current stimulation have been performed in a patient presenting severe cervical dystonia.[83] The authors noted a 54% improvement of the Toronto Western Spasmodic Torticollis Rating Scale total score, as well as a 75% improvement of the Toronto Western Spasmodic Torticollis Rating Scale pain score using TACS. This effect persisted after 30 days. However, this is, to our knowledge, the only reported example and has to be replicated before being considered as en efficient treatment.
On the other hand, epidural premotor cortical stimulation has been shown to provide some improvement in cervical dystonia and hand dystonia.[84] Similarly, another group reported an improvement of around 30% of fixed dystonia cases secondary to focal basal ganglia lesions after chronic extradural motor cortex stimulation.[85] However all of these series are open-label and the nature of fixed dystonia itself is still a matter of debate (psychogenic or not), making a conclusion difficult to draw on this technique of stimulation.[86,87] In addition, a recent series with double-blind assessment did not confirm these results and had to be stopped before its ending because of a lack of efficacy of motor cortex epidural stimulation in dystonia secondary to basal ganglia lesions.[88] Differences in terms of stimulation parameters (bipolar vs monopolar; shorter vs longer pulse width) could have participated in this discrepancy.[85,88] Taken together, these very heterogeneous results indicate that such techniques of stimulation need further controlled studies before they can be considered efficient in dystonia. Results are summarized in Table 1.
Future Neurology. 2014;9(1):77-87. © 2014 Future Medicine Ltd.
