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CNS Adaptation and Cortical Plasticity After Nerve Transfer: How Complete Is It?
Douglas C. Ross, MD, MEd; Thomas Allen Miller, MD
University of Western Ontario, London, ON, Canada
Background: Cortical plasticity has been defined as the capacity of the central nervous system to adapt or regenerate after trauma. Peripheral nerve transfer induces a “traumatic” change in peripheral nerve functioning which requires adaptive, central changes to enhance and restore function. The prototype for “modern” nerve transfers is the Oberlin or “double transfer” (of Mackinnon) which restores elbow flexion utilizing fascicles originally subserving finger and/or wrist flexors and it has been observed that these transfers are easily relearned and function extremely well. It is our hypothesis that this particular transfer is atypical in that the donor nerve/muscle unit normally fires during elbow flexion and thereby enhances the results without any significant change in cortical organization. Furthermore, the results of other nerve transfers are attenuated by the lack of complete, "functional" cortical reorganization.
Purpose: The purpose of this study is to evaluate the function of selected peripheral nerve transfers based upon whether the donor nerve/muscle unit is recruited or not during volitional movements.
Methods: Four peripheral nerve transfers were studied: 1) double nerve (FCU fascicle of ulnar nerve and FDS fascicle of median nerve) transfer for elbow flexion, 2) radial nerve branch (long head triceps) to axillary nerve, 3) anterior interosseous nerve (AIN) (pronator quadratus) to ulnar motor and 4) tibial nerve (lateral gastrocnemius) to peroneal. Target muscles were assessed clinically (MRC scale), objectively (peak power measurements), and electrophysiologically (motor unit recruitment) in two anatomic positions: one which would normally “fire” the donor nerve/mucle unit (eg forearm pronation for AIN to ulnar transfer) and the other which would not normally recruit the donor unit (eg forearm supination for AIN to ulnar transfer). Each of the measurements was compared for statistical significant differences using non-parametric tests.
Results: A representative group of 12 patients were assessed. Mean time from time of surgery to testing was greater than 24 months. On average, subjects were a full MRC grade stronger when the limb was positioned to fire the donor nerve/muscle unit. Peak power measurements were also significantly higher in those positions. Finally, motor unit recruitment on EMG was significantly different in the two anatomic positions for all transfers but different among different transfers (greatest for tibial to peroneal).
Conclusions: While cortical reorganization does occur, the ease and magnitude may have been overestimated based upon the success iof transfers for elbow flexion. These findings may have implications for rehabilitation both centrally and peripherally after nerve transfers.
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