. Introduction The effect of subthalamic nucleus (STN) deep brain stimulation (DBS) on postural balance is controversial. Variability of results may be due, in part, to discrepancies in DBS contact location within the STN. The ability of PD patients to accurately perceive their risk of falling and whether STN DBS localization has any effect on this ability is also unclear. Therefore, we sought to investigate the effect of STN DBS localization on postural balance (sway) in dynamic and static test conditions designed to challenge the afferents responsible for postural balance maintenance to varying degrees. In addition, we evaluate the effect of STN DBS localization and test condition on the perceived sense of balance of patients with advanced PD. Introduction The effect of subthalamic nucleus (STN) deep brain stimulation (DBS) on postural balance is controversial. Variability of results may be due, in part, to discrepancies in DBS contact location within the STN. The ability of PD patients to accurately perceive their risk of falling and whether STN DBS localization has any effect on this ability is also unclear. Therefore, we sought to investigate the effect of STN DBS localization on postural balance (sway) in dynamic and static test conditions designed to challenge the afferents responsible for postural balance maintenance to varying degrees. In addition, we evaluate the effect of STN DBS localization and test condition on the perceived sense of balance of patients with advanced PD. Methods Six subjects (Table 1) with advanced idiopathic PD and bilateral STN DBS, with at least one dorsal and one ventral STN DBS contact bilaterally, were tested for postural balance (using a strain-gauge force platform) in a double-blind fashion during three DBS conditions (after overnight withdrawal of medication): bilateral DBS of either the dorsal or ventral STN (2.5V/60μs/185Hz) and both stimulators off. Sway length (SL) and sway area (SA) were measured in static test conditions (Eyes Open (EO), Eyes Closed (EC), Eyes Open on foam (FO), and Eyes Closed on foam (FC) ) and dynamic test conditions (Sudden Loading Good Light/Poor Light (SLT-GL, SLT-PL) and Repetitive Bend and Reach Good Light/Poor Light (RBRT-GL, RBRT-PL)) designed to simulate daily living tasks. The Perceived Sense of Postural Sway and Instability (PSPSI) scale (Table 2) was administered immediately after each postural balance test to evaluate the PSPSI of each patient. Methods Six subjects (Table 1) with advanced idiopathic PD and bilateral STN DBS, with at least one dorsal and one ventral STN DBS contact bilaterally, were tested for postural balance (using a strain-gauge force platform) in a double-blind fashion during three DBS conditions (after overnight withdrawal of medication): bilateral DBS of either the dorsal or ventral STN (2.5V/60μs/185Hz) and both stimulators off. Sway length (SL) and sway area (SA) were measured in static test conditions (Eyes Open (EO), Eyes Closed (EC), Eyes Open on foam (FO), and Eyes Closed on foam (FC) ) and dynamic test conditions (Sudden Loading Good Light/Poor Light (SLT-GL, SLT-PL) and Repetitive Bend and Reach Good Light/Poor Light (RBRT-GL, RBRT-PL)) designed to simulate daily living tasks. The Perceived Sense of Postural Sway and Instability (PSPSI) scale (Table 2) was administered immediately after each postural balance test to evaluate the PSPSI of each patient. Results Effect of DBS Localization on Sway Length (Fig 1A). DBS localization did not affect SL significantly during static conditions. However, SL was significantly greater with ventral in comparison to dorsal stimulation during SLT-GL. Increases in SL with ventral stimulation in comparison to off DBS trended towards significance during SLT-PL and RBRT-GL. Effect of DBS Localization on Sway Area (Fig 1B). During EC, FO, and SLT-GL, SA was significantly greater with ventral DBS in comparison to dorsal and “off” DBS. Effect of DBS Localization on PSPSI Rating (Fig 1C). PSPSI rating was significantly lower with ventral DBS in comparison to “off” DBS during FO and FC. In addition, PSPSI rating was lowest in every test condition with the exception of RBRT-GL. Results Effect of DBS Localization on Sway Length (Fig 1A). DBS localization did not affect SL significantly during static conditions. However, SL was significantly greater with ventral in comparison to dorsal stimulation during SLT-GL. Increases in SL with ventral stimulation in comparison to off DBS trended towards significance during SLT-PL and RBRT-GL. Effect of DBS Localization on Sway Area (Fig 1B). During EC, FO, and SLT-GL, SA was significantly greater with ventral DBS in comparison to dorsal and “off” DBS. Effect of DBS Localization on PSPSI Rating (Fig 1C). PSPSI rating was significantly lower with ventral DBS in comparison to “off” DBS during FO and FC. In addition, PSPSI rating was lowest in every test condition with the exception of RBRT-GL. Conclusions The current study demonstrated that a mismatch between actual and perceived balance exists in PD patients. Stimulation of the ventral STN further increases this mismatch and may increase fall risk. In addition, postural sway increases with ventral STN stimulation when either visual or proprioceptive input is disturbed and also during dynamic conditions. Patients seemed to adopt a “posture second” strategy during dynamic conditions that may increase risk of falling. In regards to minimizing fall risk, stimulation of the dorsal STN appears to be a better option than ventral stimulation. However, in order to optimize patient outcomes, further study is needed to determine the differential effects of electrode localization and stimulation parameters. Conclusions The current study demonstrated that a mismatch between actual and perceived balance exists in PD patients. Stimulation of the ventral STN further increases this mismatch and may increase fall risk. In addition, postural sway increases with ventral STN stimulation when either visual or proprioceptive input is disturbed and also during dynamic conditions. Patients seemed to adopt a “posture second” strategy during dynamic conditions that may increase risk of falling. In regards to minimizing fall risk, stimulation of the dorsal STN appears to be a better option than ventral stimulation. However, in order to optimize patient outcomes, further study is needed to determine the differential effects of electrode localization and stimulation parameters. This study was supported by the Gardner Family Center Research Grant, Davis Phinney Foundation, and Cincinnati VAMC. Effect of Dorsal & Ventral STN DBS on Actual and Perceived Sense of Postural Balance Travis R. Larsh 1 ; Amit Bhattacharya, PhD 2 ; Andrew P. Duker, MD 1,3 ; Ashutosh Mani 2 ; Cyndy Cox 2 ; Rmya Raghavan 1 ; Maureen Gartner, MSN, NP-C 1,3 ; and Fredy J. Revilla, MD 1,3,4 University of Cincinnati (UC) College of Medicine: Department of Neurology and Rehabilitation Medicine 1 ; Biomechanics-Ergonomics Research Laboratory, Department of Environmental Health 2 ; UC Neuroscience Institute 3 ; Veterans Affairs Medical Center 4, Cincinnati, Ohio, USA Table 1: Patient Demographics Sex 3 male, 3 female Age (years+/-se)63.0 +/-1.5 Disease Duration (years+/-se)17.0+/-2.5 Time since DBS surgery (years+/-se)3.5+/-0.6 OffDorsalVentral UPDRS-III (rating+/-se)36.8+/ / /-3.7 Axial UPDRS (rating+/-se)8.8+/ / /-0.9 Postural Stability (rating+/-se) 0.7+/ /-0.20 Table 2: PSPSI Scale 1. How much did you feel your body sway (i.e., rotate, pivot)? a little some a lot Did you have any difficulty in maintaining balance (how much did you or your muscles compensate for your movement)? a little some a lot Did you feel at any time that you would fall? a little some a lot What would you say was the overall difficulty of the task? a little some a lot A. Sway Length B. Sway Area C. PSPSI Rating (#p<0.1; *p<0.05; **p<0.01)