title: Long-term measurement of therapeutic electrode impedance in deep brain stimulation.
abstract: OBJECTIVE
  Deep brain stimulation technology now allows a choice between constant current and constant voltage stimulation, yet clinical trials comparing the two are lacking. Impedance instability would theoretically favor constant current stimulation; however, few publications address this with long-term follow-up. In this report, we review our series for impedance change and discuss our findings and their implications for future study design.
  
  
  METHODS
  A retrospective chart review was performed of all consecutive patients seen in the outpatient clinic for deep brain stimulation adjustments at the University of Wisconsin-Madison from February 2006 to May 2007. The following data were extracted: Quadrapolar contact selection, frequency, voltage, pulse width, and measured impedance at the therapeutic parameters. Patients were selected if consecutive measurements of therapeutic impedances for the same patient were performed with the same frequency, pulse width, voltage, and configuration of active contacts.
  
  
  RESULTS
  A total of 63 patients with 110 electrodes had 301 documented programming visits. From these, 16 patients had 20 consecutive measurements with unchanged parameters in 19 electrodes at a median interval of 68 days and median follow-up of 549 days after implantation. No significant intra-patient intra-electrode therapeutic impedance variability was observed in this study (SD = 105.3 Ω, paired t-test, p= 0.312). In contrast, marked inter-patient variability in impedance was noted. This variability could not be explained by stimulation target, measurement interval, time since implantation, monopolar vs. bipolar stimulation, stimulation voltage, or stimulation frequency.
  
  
  CONCLUSIONS
  No significant change in the same electrode therapeutic impedance was identified. Given the assumption that stimulation current is the critical parameter influencing clinical outcomes, these findings would not disadvantage constant voltage stimulation. However, inter-patient variability suggests a possible advantage for constant current stimulation when generalizing experience and comparisons over multiple patients. Further study of the relationship of stimulation efficacy to stimulation mode and impedance change is warranted.
authors: Department of Neurological Surgery, University of Wisconsin Hospitals and Clinics, Madison, WI, USA;             Department of Neurology, University of Wisconsin Hospitals and Clinics, Madison, WI, USA;             National Primate Research Center, Madison, WI, USA;             Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, WI, USA; and             Department of Communicative Disorders, University of Wisconsin Hospitals and Clinics, Madison, WI, USA.
authors: Karl A
authors: KA
authors: Sillay
authors: Jason C
authors: JC
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authors: EB
authors: Montgomery