Postural instability is a sign of progression of Parkinson's disease (PD) and often resistant to levodopa treatment. To explore the effect of bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) on postural stability and gait, full body gait analyses were performed without medication, OFF and ON DBS in eight PD patients and 12 healthy age-matched controls. DBS setting was changed at least 3 hours before gait analysis. To describe asymmetry most and least affected sides (MAS and LAS) were rated with the Unified Parkinson's Disease Rating Scale, motor part and quantitative gait analysis with the Vicon 612 gait analysis system. Stride length and gait velocity but not cadence improved ON DBS. The distances between the heel markers and center of mass (COM) were asymmetric and reduced OFF DBS. STN DBS increased the distances significantly and reduced asymmetry. The improvement in heel to COM distance was larger on the MAS compared with the LAS. OFF DBS knee momentum asymmetry was inversed so that LAS was more impaired than MAS. ON DBS asymmetry improved. PD patients OFF DBS place the heel too close to COM. The most affected body side has the most impaired swing and the result is a smaller knee moment on the opposite and least affected body side and an asymmetric gait pattern with disturbed balance OFF STN DBS. The asymmetry OFF DBS improved ON DBS. We suggest that DBS facilitates symmetric gait and thereby improves balance during gait.

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Survival of 70% after a mean of 25 years of PD including 10 years with DBS illustrates that this is a selected group of PD patients. The prevalence of dementia steadily increased after surgery as expected from disease progression and can be an early event. Compared with the few similar long-term studies, the present study presents a larger cohort followed at the same DBS center for a longer period of time and none was lost to follow-up, making conclusions more valid. The present findings are of significant prognostic help for the patient, caregiver and physician when treatment with DBS has to be decided.

Deep brain stimulation of the dorsal STN is superior to stimulation of the ventral STN. Possible different effects of stimulation inside the nucleus underline the need for exact knowledge of the active stimulation site position to target the most effective area.

Our study suggests that simple "bedside" PIGD tests - particularly the combination of tandem gait performance, TUG and retropulsion test - can discriminate APD from PD.