Verification of nerve decompression using mechanomyography.

BACKGROUND CONTEXT: 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 high 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. PURPOSE: The aim of this study was to assess the ability of MMG to quantitatively demonstrate successful nerve root decompression. STUDY DESIGN: Prospective cohort, Therapeutic Level III, Urban Level I Trauma Center. PATIENT SAMPLE: A total of 46 patients (72 affected nerve roots) undergoing decompression procedures for lower extremity radiculopathy caused by nerve root compression were enrolled in the study. The study population included 15 patients with herniated nucleus pulposus (HNP) and 31 with lateral recess stenosis (LRS). OUTCOME MEASURE: Visual analog scale (VAS) score. METHODS: A total of 72 nerves roots in 46 patients undergoing lumbar decompression procedures, for lower extremity radicular symptoms, were tested 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. VAS scores were collected pre- and postoperatively. RESULTS: Of the patients, 90% (65/72) had elevated stimulation thresholds (\u3e1 milliamp [mA]) before decompression. After decompression, 98% of patients (64/65) with elevated current thresholds exhibited a drop in threshold of ≥1 mA (p\u3c.001). A postdecompression increase in response amplitude was recorded in all patients. VAS scores improved postdecompression (6.8 vs. 1.1, p\u3c.001) with a positive correlation between decreased stimulation thresholds and degree of improvement in VAS scores (p\u3c.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