Abstract
Background/Objectives: Deep Brain Stimulation (DBS) of the subthalamic nucleus (STN) is a widely used intervention for Parkinson's disease (PD) and obsessive-compulsive disorder (OCD). While motor and OCD symptom benefits are established, increasing evidence highlights psychiatric side effects. The underlying mechanisms involve stimulation parameters, electrode positioning, and medication adjustments. This systematic review aims to evaluate the short-term and long-term psychiatric effects of STN-DBS and identify influencing factors. Methods: A systematic literature search (PubMed, Scopus, Web of Science, Embase; 2015-2024) was conducted following PRISMA guidelines. Studies examining psychiatric effects of STN-DBS in PD or OCD, reporting quantitative/qualitative psychiatric measures, and specifying stimulation parameters were included. Risk of bias was assessed using the Newcastle-Ottawa Scale (NOS) for observational studies and the Cochrane Risk of Bias Tool for randomized controlled trials (RCTs). Results: A total of 16 studies met the inclusion criteria, with sample sizes from 10 to 149 patients and short- to long-term follow-ups (up to 17 years). Short-term effects included transient hypomania, euphoria, increased impulsivity (especially with medial STN stimulation), and sometimes anxiety reduction. Long-term effects showed a tendency towards apathy and depression (apathy increased significantly in one large cohort), particularly linked to ventromedial STN stimulation or dopaminergic medication reduction. Impulse control disorders (ICDs) improved long-term in one study following medication reduction, while impulsivity slightly worsened in another. Verbal fluency decline was commonly reported, though global cognition often remained stable. Psychiatric outcomes (mood/apathy, attention/memory) depended on stimulation location within STN subregions. Higher total electrical energy delivered (TEED) correlated with depressive trait shifts in one study. Conclusions: STN-DBS has complex psychiatric consequences. Electrode positioning, stimulation parameters (including location within STN subregions and possibly TEED), and medication adjustments significantly influence outcomes. Careful patient selection, preoperative psychiatric screening, optimized programming targeting specific STN subregions, and cautious medication management are essential to minimize psychiatric risks while maximizing therapeutic benefits for motor and OCD symptoms.