Comparison of the efficacy of unipolar and bipolar electrode configuration during subthalamic deep brain stimulation

Deep brain stimulation of the subthalamic nuclei (STN) is a well established treatment in advanced Parkinson's disease (PD). Based on the clinical efficacy and elicited side-effects, both unipolar and bipolar stimulation modes may be applied. Bipolar stimulation usually produces a more focused and therefore thinner area of tissue activated during stimulation than unipolar stimulation does. The primary aim of our clinical study was to quantify the different clinical efficacy between these two stimulation modes. Twenty-one patients with PD previously underwent bilateral STN DBS implantation were involved in the study. Approximately three years after the implantation, we evaluated rigidity, tremor and bradykinesia according to the Unified Parkinson's disease Rating Scale in a practically off condition. Keeping the cathode of the chronic stimulation setting constant, the amplitude of stimulation was changed between 0 and 3.6 V by 0.2 V steps. Subsequently, the improvements in rigidity, tremor and bradykinesia were compared between unipolar and bipolar modes using 60 μs pulse-width and 130 Hz frequency. Within the examined amplitude range, unipolar stimulation usually had a significantly higher efficacy than bipolar stimulation; however, also with a higher rate of side-effects (19% vs. 0%). Depending on the evaluated parkinsonian symptoms, the efficacy of uni- and bipolar stimulation was different. To achieve the same level of improvement during bipolar stimulation, approximately 0.4-0.5 V higher amplitude was required than in unipolar mode. However in some cases, the efficacy of bipolar stimulation was unable the reach that of unipolar stimulation within the examined amplitude range.