Researchers assessed the long term effect of Subthalamic nucleus Deep Brain Stimulation (DBS) on Parkinson's Disease. People with Parkinson's Disease were assessed before DBS and 1, 3, 5 years after DBS had begun.
DBS involves the use of electrodes that are implanted into the brain and connected to a small electrical device called a pulse generator that can be externally programmed. DBS can reduce the need for L-dopa and related drugs, and reduce dyskinesia. For more information go to : http://www.ninds.nih.gov/disorders/deep_brain_stimulation/deep_brain_stimulation.htm
As a result of DBS the quality of life improved by 58% after 3 years but gradually declined afterwards. Sleep, cognition, and emotion were mostly unchanged. After 5 years, when assessed without medication DBS improved Parkinson's Disease motor symptoms by 35%. However, after 5 years, when assessed with the simultaneous use of medication, motor symptoms were similar to those at the outset. L-dopa intake was reduced from 660mg to 310mg after 5 years. STN DBS can therefore improve Parkinson's Disease and reduce the need for L-dopa but there is a gradual decline and diminished efficacy after five years of use.
Reference : Chinese Medical Journal  128 (18) : 2433-2438 (L.L.Jiang, J.L.Liu, X.L. Fu, W.B.Xian, J.Gu, Y.M.Liu, J.Ye, J.Chen, H.Qian, S.H.Xu, Z.Pei, L.Chen)
Complete abstract : http://www.ncbi.nlm.nih.gov/pubmed/26365958