Prospective Analysis Utilizing Intraoperative Neuromonitoring for the Evaluation of Inter-Burst Frequencies

Abstract

Background: Intraoperative neuromonitoring (IONM) for spinal cord stimulation (SCS) uses electromyography (EMG) responses to determine myotomal coverage as a marker for dermatomal coverage.

Objective: These responses can be utilized to evaluate the effects of stimulation platforms on the nervous system.

Methods: Eight patients were tested at inter-burst frequencies of 10 Hz, 20 Hz, 30 Hz, and 40 Hz using DeRidder Burst stimulation to determine the amplitude of onset of post-synaptic signal generation. Three patients had additional data recording amplitude of onset of tonic stimulation prior to and post DeRidder Burst stimulation at each inter-burst frequency. This represented post-synaptic excitability.

Results: In all patients, the DeRidder Burst waveform generated EMG responses under all inter-burst frequencies including temporal summation, deeper fiber recruitment, and compounded action potentials. There was a non-significant decrease of 7.6-7.8% in amplitudes to generate response under 40 Hz, compared to the other frequencies. However, there was a 73.1% reduction in energy requirements at 10 Hz. The enhanced post-synaptic excitability effect was demonstrated at all frequencies.

Conclusion: DeRidder Burst has similar effects of temporal summation, deeper fiber recruitment, and compounded action potentials under IONM at 40 Hz, 30 Hz, 20 Hz, and 10 Hz. In addition, the hyperexcitability phenomenon was also observed regardless of the frequency. This demonstrates that postsynaptic responses captured via IONM may be a sensitive biomarker to SCS mechanism of action. In addition, lower inter-burst frequencies may have a similar clinical effect on pain relief thus reducing power consumption even further than current dosing paradigms.

Keywords: Burst; DeRidder Burst; EMG; SCS; SSEPS; electromyogram; somatosensory evoked potentials; spinal cord stimulation.

Figure 1

EMG myotomal coverage as a marker for dermatomal coverage. Coverage demonstrating bilateral EMG activity slightly favoring the left side that start in the abdominal muscles and progressively work to more distal muscle groups. There are a total of five muscle groups seen in this figure for both the left and right sides. The muscles groups, in order, are abdominal, iliopsoas, quadriceps, tibialis anterior, and abductor hallicus. Time-locked spikes can be seen on the left side with higher amplitudes compared to the right side.

Figure 2

DeRidder Burst (BurstDR) waveform with closely packed five pulse-train pattern, with a quiescent phase, in the abdominal muscles which is the first line on lower portion of image for both left and right sides of the figure. EMG responses were propagated into one large EMG spike as you follow the time locked responses through the subsequent muscle groups. The right side is larger than the left. In the final muscle group of abductor hallicus you can see a single very large spike. The top half of the image demonstrates stimulation artifact within the same muscle groups which is earlier onset than time locked complete EMG responses.

Figure 3

Post-synaptic activation thresholds across the different frequencies of DeRidder Burst. There was a slight reduction in the activation amplitude necessary to elicit a response after burst stimulation from baseline at 40 Hz compared to 30 Hz-10 Hz, but this did not reach statistical significance. There was also no difference when comparing 30 Hz, 20 Hz, and 10 Hz.

Figure 4

(A) Post-synaptic excitability effect in each of the final three patients. For each of the three patients a baseline tonic stimulation threshold was obtained. These were 5.0 mA, 7.0 mA, and 10 mA. The threshold of activation after each DeRidder Burst frequency is recorded. There was a trend for higher required amplitudes with decreasing frequency in each individual. (B) Average post-synaptic excitability effect when compared to baseline tonic stimulation. Baseline tonic is statistically significantly different from tonic post burst 40–10 Hz. There is also statistically significant difference between tonic post burst 40 Hz and 10 Hz, but not comparing between the others.

Figure 5

Average amplitudes for tonic, post DeRidder Burst tonic, and DeRidder Burst. The DeRidder Burst averages include all 8 patients while the post DeRidder Burst tonic include the three patients tested for hyperexcitability. There is a statistically significant decrease in thresholds for activation with traditional tonic stimulation with the post-synaptic excitability effect. There is also the expected significant decrease in thresholds with DeRidder Burst, which is statistically significant from both baseline tonic and tonic post DeRidder Burst.