There are only a few studies that have directly compared patients who have undergone GPi or STN DBS and most of these studies are not randomized and the number of patients is small. Krack et al. (42) retrospectively compared eight patients who had STN DBS with five who had GPi DBS six months after surgery. In the medication off state, UPDRS motor scores improved by 71% with STN stimulation and by 39% with GPi stimulation and UPDRS ADL scores were also more improved in the STN group. Rigidity and tremor showed comparable improvement in both groups but bradykinesia was more improved in the STN group. There was a reduction in levodopa equivalence dose only in the STN group. Scotto di Luzio et al. (43) retrospectively compared nine patients who had undergone STN stimulation to five who had undergone GPi stimulation 12 months after surgery. UPDRS motor scores in the medication off/stimulation on condition improved by 56.6% with STN stimulation and 41.7% with GPi stimulation. Both groups had a significant reduction in dyski-nesia but there was a reduction in levodopa dose only in the STN group. Volkmann et al. (44) retrospectively compared 16 patients who had undergone STN stimulation with 11 patients who had undergone GPi stimulation. There was a 67% improvement in UPDRS motor scores with STN stimulation compared with a 54% improvement with GPi stimulation. However, there were no significant differences between the two groups in terms of off medication motor function, dyskinesia, or motor fluctuations. Medication was reduced only in the STN group and the GPi group required significantly more electrical power compared to the STN group. In another study, Krause et al. (45) compared six GPi patients with 12 STN patients 12 months after surgery. GPi stimulation directly reduced dyskinesia, whereas STN stimulation reduced antiparkinsonian medications resulting in a reduction of dyskinesia. STN stimulation also improved UPDRS motor scores but GPi stimulation did not have a similar effect.
There has been only one blinded, randomized trial of GPi DBS versus STN DBS published to date. Anderson et al. (21) compared 10 patients who received bilateral STN DBS with 10 patients who received bilateral GPi DBS 12 months after surgery. UPDRS scores were significantly improved for both groups and there were no significant differences between the STN and GPi DBS groups. More specifically, UPDRS motor scores were improved 48% with STN DBS and 39% with GPi DBS; UPDRS ADL scores were improved by 28% with STN DBS and 18% with GPi DBS; bradyki-nesia was improved by 44% with STN DBS and 33% with GPi DBS; tremor was improved 89% with STN DBS and 79% with GPi DBS; rigidity was improved 48% with STN DBS and 47% with GPi DBS; and axial symptoms were improved 44% with
STN DBS and 40% with GPi DBS. In addition, dyskinesia was reduced by 62% with STN DBS and 89% with GPi DBS; however, there was a levodopa reduction of 38% with STN DBS and only 3% with GPi DBS. Although the sample size was small, this study suggests that cognitive and behavioral problems may be more common after STN DBS compared to GPi DBS. It was also suggested that follow-up care may be more difficult after STN DBS due to the medication adjustments during stimulation parameter optimization, which are generally not required after GPi DBS.
In summary, the majority of the small retrospective reports have suggested that STN DBS is more effective than GPi DBS. In fact, the American Academy of Neurology Practice Parameter: Treatment of Parkinson disease with motor fluctuations and dyskinesia (an evidence-based review) (41) concluded that STN DBS is possibly effective in improving motor function and reducing motor fluctuations, dyskinesia, and antiparkinsonian medication usage and may be considered as a treatment option in PD patients. In contrast, it was concluded that there were insufficient data to determine whether GPi DBS is an effective treatment option for PD. However, given the results of Anderson et al.'s blinded, randomized study, which found the outcomes from the two targets to be very similar, the results of larger, randomized, blinded studies are necessary before the two targets can be adequately compared.
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