**Conflict of interest**

*Neurostimulation and Neuromodulation in Contemporary Therapeutic Practice*

evidence in this field is severely limited and still inconclusive.

sleep maintenance.

**4.4 Sensory functions**

promise for the future of DBS.

understanding of brain physiology improves.

**5. Conclusion**

neurodegenerative diseases and has been a subject of study after initiating DBS. Polysomnographic studies show that DBS in STN leads to an objective improvement sleep quality [72], which is seen in subjective-based studies as well [73, 74]. Improvement in sleep and pain was also observed in dystonia patients who were treated with STN DBS [75]. Interestingly, daytime sleepiness does not seem to be affected by DBS [76]. A possible reason for this could be a lacking effect on REM sleep, as DBS patients have an increased risk of developing REM sleep behavioral disorder in the case of STN stimulations [77]. The effect on restless legs syndrome appears to be positive in moderate to severe cases, but new cases of the syndrome can appear after initiating DBS [78, 79]. Overall, it seems that constant stimulations improve sleep quality over the long term by influencing nighttime mobility and

Somatosensory dysfunction is often reported in neurodegenerative diseases and presents a significant burden in everyday life of the patients. Problems with the sense of smell and taste arise in the early phases of neurodegeneration. Subjective improvement in both smell and taste was reported in a recent prospective study [64]. It appears that the improvement in smell stems from improved odor information processing, as only odor discrimination and identification were improved, while the detection was not affected by DBS [80, 81]. There are improvements in visual function as well, mostly due to the effect on ocular smooth motor function and improving problems with saccade movement [82]; however, the amount of

Fortunately, the DBS effects on pain are more apparent. Most studies suggest a beneficial effect of DBS on pain [83], especially in patients who suffer from pain in off periods [84]. There are varied types of pain that can be present in patients, but it appears that STN DBS has a substantial effect in curbing dystonic and musculoskeletal pain, while central and neuropathic pain are less affected [85]. However, severe neuropathic pain can be treated with DBS if the target for stimulations is the periaqueductal gray matter, possibility due to an increased release of endogenous opioids [86, 87]. There are promising results in chronic pain as well, with the anterior cingulate cortex showing potential, though it is too early to tell due to a lack of studies in this field [88]. This finding underlines the importance of selecting the right target for neuromodulation depending on the wanted results, which holds

Nonmotor symptoms represent a challenge in the treatment of neurodegenera-

tive diseases and have a significant influence on the quality of life. DBS shows promise in alleviating these symptoms, depending significantly on the target of stimulation. The main problem is the lack of studies in this field, as most have methodological issues or small sample sizes, which limit the strength of the evidence. Likewise, only a handful of studies have nonmotor symptoms and primary end points. The number of approved indications for DBS is still small and mostly focused on extrapyramidal symptoms, and therefore, most studies are focused on the effects of DBS in STN or GPi. However, it is clear that DBS has a promising future in the treatment of neurodegenerative diseases in general and will have an important role in personalized medicine as functional neuroimaging and our

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The authors declare no conflict of interest.
