Article
Motor threshold: the key to optimizing motor cortex stimulation parameters?
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Authors
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Philipp J. Slotty
- Division of Neurosurgery, University of British Columbia, Vancouver, Canada; Abteilung für Stereotaxie und Funktionelle Neurochirurgie, Heinrich-Heine Universität, Düsseldorf, Deutschland
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Stephano Chang
- Division of Neurosurgery, University of British Columbia, Vancouver, Canada
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Christopher R. Honey
- Division of Neurosurgery, University of British Columbia, Vancouver, Canada
| Published: | June 2, 2015 |
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Outline
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Objective: Motor cortex stimulation can be effective in the treatment of neuropathic pain disorders. Unfortunately, no widely accepted programming guidelines exist. Large differences in response to MCS can be seen between patients and within individual patients at different times. These variations can be reduced by studying each individual's stimulation intensity as a percentage of their motor threshold (intensity required to cause a motor contraction) rather than as a raw voltage or current.
We propose that effective stimulation voltage can be determined as a percentage of motor threshold (PMT).
Method: Seven patients being treated with motor cortex stimulation for chronic neuropathic pain were studied. Patients had their stimulation voltage randomized to their baseline PMT; 10% higher; 10% lower. Patients were blinded to their settings and settings were applied in a randomized order. Treatment outcome was assessed with the visual analogue scale for pain and the SF-36 quality of life questionnaire.
Results: The best treatment response was seen with medium settings which were at a mean of 62% of MT (VAS 3.4). High and low settings both resulted in a significant increase in pain compared to medium settings (VAS 6.0 and 6.3 respectively) and a significant decrease in SF-36 scores. No significant difference in pain control was observed between high and low settings. The mean time from changes in treatment settings to changes in pain levels was 2.9 days (± 1.0 day).
Conclusions: When considering MCS parameters, the absolute value of the voltage (or current) is probably far less important than the degree to which the stimulation is influencing the motor cortex. We propose that the percentage of motor threshold (PMT) represents an important parameter that measures the degree to which MCS may be affecting the motor cortex. In our prospective randomized blinded assessment, it appeared that a mean PMT of 62% was required for effective pain relief. Higher settings did not results in increased therapeutic efficacy but in a significant increase in pain, probably induced by current spread to the sensory cortex.
Targeting therapy to a PMT level may speed initial programming, allow more consistent longitudinal follow-up and be a basis for a standardized programming paradigm.
