Functional magnetic resonance imaging and control over the biceps muscle after intercostal—musculocutaneous nerve transfer

Abstract

Object. Recent progress in the understanding of cerebral plastic changes that occur after an intercostal nerve (ICN)—musculocutaneous nerve (MCN) transfer motivated a study with functional magnetic resonance (fMR) imaging to map reorganization in the primary motor cortex. Methods. Eleven patients with traumatic root avulsions of the brachial plexus were studied. Nine patients underwent ICN—MCN transfer to restore biceps function and two patients were studied prior to surgery. The biceps muscle recovered well in seven patients who had undergone surgery and remained paralytic in the other two patients. Maps of neural activity within the motor cortex were generated for both arms in each patient by using fMR imaging, and the active pixels were counted. The motor task consisted of biceps muscle contraction. Patients with a paralytic biceps were asked to contract this muscle virtually. The location and intensity of motor activation of the seven surgically treated arms that required good biceps muscle function were compared with those of the four arms with a paralytic biceps and with activity obtained in the contralateral hemisphere regulating the control arms. Activity could be induced in the seven surgically treated patients whose biceps muscles had regained function and was localized within the primary motor area. In contrast, activity could not be induced in the four patients whose biceps muscles were paralytic. Neither the number of active pixels nor the mean value of their activations differed between the seven arms with good biceps function and control arms. The weighted center of gravity of the distribution of activity also did not appear to differ. Conclusions. Reactivation of the neural input activity for volitional biceps control after ICN—MCN transfer, as reflected on fMR images, is induced by successful biceps muscle reinnervation. In addition, the restored input activity does not differ from the normal activity regulating biceps contraction and, therefore, has MCN acceptor qualities. After ICN—MCN transfer, cerebral activity cannot reach the biceps muscle following the normal nervous system pathway. The presence of a common input response between corticospinal neurons of the ICN donor and the MCN acceptor seems crucial to obtain a functional result after transfer. It may even be the case that a common input response between donor and acceptor needs to be present in all types of nerve transfer to become functionally effective.