Introduction The effect of the variability of electrode placement on outcomes after bilateral deep brain stimulation of subthalamic nucleus has not been sufficiently studied, especially with respect to the sequence of hemisphere implantation.

Methodology We retrospectively analysed the clinical and radiographic data of all the consecutive patients with Parkinson's disease who underwent surgery at our centre and completed at least 1 year follow-up. The dispersion in electrode location was calculated by the square of deviation from population mean, and the direction of deviation was analysed by comparing the intended and final implantation coordinates. Linear regression analysis was performed to analyse the predictors of postoperative improvement of the motor condition, also controlling for the sequence of implanted hemisphere.

Results 76 patients (mean age 58±7.2 years) were studied. Compared with the first side, the second side electrode tip had significantly higher dispersion as an overall effect (5.6±21.6 vs 2.2±4.9 mm², p=0.04), or along the X-axis (4.1±15.6 vs 1.4±2.4 mm², p=0.03) and Z-axis (4.9±11.5 vs 2.9±3.6 mm², p=0.02); the second side stimulation was also associated with a lower threshold for side effects (contact 0, p<0.001 and contact 3, p=0.004). In the linear regression analysis, the significant predictors of outcome were baseline activities of daily living (p=0.010) and dispersion of electrode on the second side (p=0.005).

Conclusions We observed a higher dispersion for the electrode on the second implanted side, which also resulted to be a significant predictor of motor outcome at 1 year.