Fig. 2. Risk factors for axial impairment progression in people with Parkinson’s disease and subthalamic deep brain stimulation.

Kaplan–Meier curves show axial impairment progression in the total sample (a) and subgroups of people with Parkinson’s disease (PD) and subthalamic deep brain stimulation based on preoperative axial scores (b), frontal scores (c), and age at disease onset (d). The “axial worsening-free survival” reflects axial impairment progression intended as the achievement of an axial score ≥7. Squares and circles in the Kaplan–Meier curves indicate censored data.