Motor Threshold: A Possible Guide to Optimizing Stimulation Parameters for Motor Cortex Stimulation

No widely accepted programming guidelines for motor cortex stimulation (MCS) exist. We propose that an individual's effective stimulation voltage can be predicted as their percentage of motor threshold (PMT).

Seven patients already successfully treated with MCS for neuropathic pain were included. Patients received stimulation that was the same as their baseline PMT ("medium"), 10% higher ("high") or 10% lower ("low") in a blinded, randomized study. Outcome was assessed after 14 days with the visual analogue scale for pain, the McGill pain questionnaire, and the SF-36 questionnaire.

The best treatment response (mean VAS 3.4) was seen with the medium setting which was at a mean of 62% PMT. High and low settings both resulted in a significant increase in pain compared with the medium setting (mean VAS 6.0 and 6.3, respectively) and a significant decrease in SF-36 scores. No significant difference in pain control was observed between the high and low settings. The mean time from changes in treatment settings to reported change in pain level was 2.9 days (±1.0 day).

We propose that the PMT represents an important parameter that measures the degree to which MCS may be affecting the motor cortex. A mean PMT of 62% was required for effective pain relief. Higher settings did not result in increased therapeutic efficacy but rather in a significant increase in pain. 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.