Abstract
Introduction
Assessment of nerve root decompression in surgery is largely based on visualization and tactile feedback. Often times, visualization can be limited, such as in minimally invasive surgery, and tactile feedback is a subjective assessment that makes the evaluation of successful nerve decompression difficult. Electromyography (EMG) has been proposed as an assessment tool but EMG responses are often difficult to quantify. Alternatively, mechanomyography (MMG) provides a quantifiable response with minimal signal to noise ratio compared with EMG. MMG provides a sensitive tool to accurately quantify mechanical responses to motor action potentials generated by electrical stimulus, allowing more reliable assessment of nerve decompression. The objective of this study was to assess the ability of mechanomyography to quantitatively demonstrate successful nerve root decompression.
Materials and Methods Seventy-two (72) nerves roots in 46 patients undergoing lumbar decompression procedures were examined using MMG. Nerves were stimulated upstream from the compression site and the lowest threshold current needed to generate a muscle response was determined. Signal response sizes were recorded before and after decompression. Visual-analog scale (VAS) scores were collected pre and post-operatively.
Results
Ninety percent (90%) of patients (65/72) had elevated stimulation thresholds (>1mA) prior to decompression. After decompression, 98% of patients (64/65) with elevated current thresholds exhibited a drop in threshold of ≥ 1mA (p < 0.001). A post-decompression increase in response amplitude was recorded in all patients. VAS scores improved post-decompression (6.8 versus 1.1, p < 0.001) with a positive correlation between decreased stimulation thresholds and improved VAS scores (p < 0.001).
Conclusion
MMG is an effective tool that can be used to differentiate normal and compressed nerves by quantifying the mechanomyographic response to a stimulating current. MMG allows one to measure the effect of decompression, judge its effectiveness in real time, and eliminate the subjectivity seen in tactile feedback methods. When the adequacy of decompression is uncertain, MMG can guide the surgeon toward additional or alternative procedures to ensure complete nerve root decompression.