Neurophysiology and Nerve Transfers
Ross Mandeville, M.D., UCSD; Justin M. Brown, MD; Geoffrey L Sheean, MD, MBBS, UCSD
Department of Neurosurgery, University of California, San Diego, San Diego, CA
Introduction: Our ability to detect subclinical damage to nerves is important for optimizing nerve transfer outcomes. Clinical assessment alone is often poor at determining the source of weakness. Nerve stimulation and electromyography (EMG) provide valuable subclinical information to better describe the relevant weakness. However, quantification of any deficit found has historically been difficult, typically categorized into “normal/near normal” and “clearly abnormal”. Our experience is that options for nerve transfer often are limited to clearly abnormal nerves. Therefore, the question becomes, “How abnormal is too abnormal?”. We describe our approach to neurophysiological assessment for nerve transfer as well as potential methods to enhance quantification.
Current Neurophysiological Approach: Acknowledging concerns regarding the relationship between motor units potentials (MUP) and state of innervation, when evaluating Donor nerves, we categorize the interference pattern as either “Good”, “Fair”, or “Poor”; predominantly based on total number of MUPs within the majority of 6-12 sampled areas. Characteristics of MUPs ( rate and size) allow for some degree of subjective modification. Additional semi-quantitative methods include Motor Unit Number Index (MUNIX) and Turns-amplitude analysis (TAA; utilizing cut-offs). Again, acknowledging the imperfect relationship between compound muscle action potential (CMAP) and state of innervation, we evaluate Recipientmuscles mainly through surface or monopolar needle stimulation with surface or, less preferably, intramuscular needle electrode recording. Tetanic stimulation provides some reassurance as to potential force (beyond that achieved with a single twitch). Motor Unit Number Estimation (MUNE), when possible, provides a direct estimate of the total number of motor units within the muscle, hopefully detecting cases where very few motor units make up the CMAP.
Potential Further Quantification: A number of other quantitative methods could potentially be useful. One approach involves extracting size index (SI) and firing rate (FR) from an interference pattern and estimating total innervation through the sum of the product of each SI and FR, divided by the number of MUPs in the interference pattern. This innervation quotient would be high in denervated muscles. Intraoperative neurophysiology provides an excellent opportunity for more detailed analysis, especially of deep muscles not amenable to evaluation in the clinic setting. Possible methods include recording direct nerve action potentials, as well as deep muscle CMAPs and MUNE.
Conclusion: We describe the semi-objective evaluation practiced in our program and note that opportunities for increased quantification in neurophysiologic assessment for nerve transfer are abundant.
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