Unstable impedance of a single electrode contact resulting in loss of DBS therapy-a case report.

Open and short circuits of electrode contacts are important technical dysfunctions of DBS. Here, we report on another type of dysfunction restricted to a single electrode contact: impedance instability within regular absolute values. After 9-year subthalamic DBS, a Parkinson patient developed unilateral motor symptoms and intermittent dysaesthesia due to impedance instability of the active contact. DBS efficacy could be restored without surgical revision by activation of the neighboring contact. During 3-year-follow-up, impedances of the dysfunctional contact varied between 1 and 3 kΩ whereas the other three contacts remained stable. Impedance documentation is crucial to identify such dysfunctions.