137 Electrophysiological effects of deep brain stimulation in anorexia nervosa Limitations Limitations of this study are the very small sample size due to the highly specific setting and population of the study (an experimental high risk intervention study in a physically severely compromised patient group). When combined, average effects did not always reach significance, however, nominal significance was observed that might result in significant overall effects when more patients are investigated for the effect of DBS on brain function in the future. Possible suboptimal band power selection and the large number of channels could have contributed to the fact that we did not find significance in summarized power. The current results suggest that some level of idiosyncratic response to DBS in AN is to be expected. The parameters that affect this differential response (electrode placement, stimulation duration and response, other confounding variables) are beyond the scope of the current investigation. We did find changes in functional connectivity. However, the direction of these changes and whether this indeed reflects a form of disorganisation of pathological connectivity which is later restored to normalized connectivity is of course highly speculative. A larger sample size, EEG-measurements during symptom-provoking tests and the establishment of a correlation with clinical symptom improvement in future studies could ameliorate these limitations. Conclusion To our knowledge this is the first study on DBS in AN examining the effect that DBS has on brain function as revealed in EEG signals. Within-subject effects of stimulation were large, widespread over frequencies, and visible across wide brain areas and networks, even with DBS turned off. Prolonged stimulation (>one year) seemed to reinstate organization in the functional brain networks. Our results support the observations that effects of DBS are not merely local, but influence widespread pathological network activity and that, after an initial period of disorganisation, the brain seems to adapt to the stimulation with reorganization in a potentially more healthy state. The functional relevance of these findings must be established in future studies. Future studies may extend our findings by increasing sample size, evaluation of the parameters that best predict treatment outcome, and the influence of DBS on source localized EEG activity.
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