Effectiveness of deep brain stimulation in subthalamic nucleus in Parkinson’s disease — a somatotopic organisation

Abstract

Deep Brain Stimulation (DBS) in subthalamic nucleus (STN) has substantially improved the suppression of symptoms in Parkinson’s disease (PD). However, a quantitative understanding of the influence of high-frequency stimulation on the neuronal elements surrounding the electrode and the resulting effects for the different symptoms for upper and lower extremities is missing. The aim of the study is to investigate if there is a somatotopic organisation of the STN. With the use of inertial sensors motor performance was observed in a number of PD patients who performed certain movement tests. Four different stimulator settings were tested. From these movement registrations the effect of DBS on bradykinesia and tremor on the left and right side of the body, and upper and lower extremities was investigated. With the use of a volume conduction model in combination with an axon model the activation area resulting from stimulation was estimated for different, clinically used, electrode positions and stimulation settings.

Patient experiments showed that differences could be found in the occurrence and severity of symptoms for the left and right side of the body and for upper and lower extremities for different DBS settings. Tremor and bradykinesia were found to respond differently to DBS. The model results showed that stimulation of large fibers (>3 μm), large stimulus amplitudes (>3.3 V), large pulse widths (>90 μs), and electrode contacts positioned near the border of the STN may easily cause stimulation outside the STN.

In conclusion, a somatotopic organisation may be expected to be present in STN as well as a functional organisation for tremor and bradykinesia. Furthermore, stimulation outside the STN may also form part of the effectiveness of the therapy.