Stimulation-Free Concentric-Needle Motor Unit Number Estimate – A Potential New Biomarker For Nerve Health
Merle Göben, cand. med.1, Reiner B. See, M.D.2, Carl Froilan D Leochico, MD3, Austin Birmingham, MD4, Art Eleanore, MD4, Stanley Bazarek, MD, PhD5, Benjamin R Johnston, MD, PhD6, Jasmine A. Thum, MD7, Justin M. Brown, MD4, Daniel Stashuk, PhD8 and Ross Mandeville, MD1, 1Beth Israel Deaconess Medical Center, Boston, MA, 2Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, 3St. Luke's Medical Center, Taguig City, Philippines, Philippines, 4Massachusetts General Hospital, Boston, MA, 5Brigham & Women's Hospital, Boston, MA, 6Alpert Medical School, Brigham & Women's Hospital, Harvard Medical School, Boston, MA, 7University of California, Los Angeles, Los Angeles, MA, 8University of Waterloo, Waterloo, ON, Canada
Introduction
Accurate, rapid, and reliable pre-operative quantification of nerve health is crucial for good outcomes of peripheral nerve surgeries, including nerve transfers. Of particular importance is the proportion of remaining functional motor units within a muscle. In this study, we introduce Stimulation-Free Concentric-Needle Motor Unit Number Estimate (SFCN-MUNE) as a new quantitative measure of nerve health and correlate it with strength outcomes after nerve transfer surgery.
Methods
We conducted a retrospective study of patients with chronic cervical spinal cord injury, who underwent nerve transfer surgery to restore finger extension by coapting the radial nerve branches supplying the supinator muscle to the posterior interosseous nerve (PIN). Patients were included in the study if both their pre-operative EMG recordings and post-operative medical research council (MRC) muscle grading scores after at least 12 months were recorded and stored in the clinical database. SFCN-MUNE is conceptually similar to a recently published stimulation-free MUNE method. It is calculated using the mean absolute amplitude over a 1s epoch of the EMG signal at maximal voluntary contraction as a surrogate for the Compound Muscle Action Potential size, with the mean area of a sample of Motor Unit Potentials multiplied by an estimate of their mean occurrence rate as a surrogate for mean Single Motor Unit Potential size.
The SFCN-MUNE measurements were correlated with post-operative strength outcomes graded using the MRC Score as well as average decomposition-based motor unit recruitment counts (DB-AMURC), considered the most comprehensive, albeit time-consuming, characterization of the functional MU status of a nerve.
Results
16 supinator radial nerve branch to PIN transfers in 12 patients were identified. SFCN-MUNE correlated moderately well with MRC strength outcomes (r=0.63) and well with DB-AMURC (r=0.85). In this cohort, a cut-off of 10 Motor Unit Potential Trains (MUPTs) reliably predicted those who would achieve an MRC outcome of 4 from those who did not.
Conclusion
SFCN-MUNE shows great potential as a new automatic, rapid, accurate, and reliable measure of nerve health that can easily be implemented within a standard EMG protocol using standard equipment and without additional training. Its application to those with nerve injury promises to aid surgical decision-making and provides understandable, quantitative information most relevant to the surgeon.
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