Neural Adaptation After Tendon Transfer and Training in Tetraplegia

Overview

The number of people in the United States who have survived SCI is estimated to be approximately 273,000 persons. Around 50% of the injuries are to the cervical spine resulting in tetraplegia. An important rehabilitation goal in this population is recovery of upper limb function, which could decrease medical costs and improve their quality of life. Re-establishing active grasp and pinch strength to the hand can be accomplished by surgeries that transfer the tendon of a strong muscle to restore strength to a paralyzed muscle, but the outcomes of the surgeries are variable. The investigators have demonstrated in an ongoing study, the functional gains after surgery can be improved with a focused therapy program to retrain the transferred muscle. The propose of this study is to examine the cortical mechanisms that drive successful muscle re-education after surgery. Understanding the neural (brain) activity associated with functional performance can help to predict who will respond to therapy and will guide evidence-based rehabilitation programs to improve upper limb function in tetraplegia.

Restoring upper limb function is rated among the highest priorities for individuals with tetraplegia. Re-establishing active grasp and pinch strength to the hand can be accomplished by tendon transfer procedures in which the tendon of a strong proximal muscle is surgically re-attached to the tendon of a paralyzed muscle. A common procedure to restore lateral (key) pinch is to transfer the distal tendon of one of the three elbow flexor muscles, the brachioradialis (Br) to the tendon of the paralyzed thumb flexor, the flexor pollicis longus (FPL). Recovery of functional pinch depends on how well the patient learns to activate the Br to flex the thumb through its new distal attachment, and also to control flexion at the elbow through its proximal attachment. The investigators' previous work shows that Br to FPL recipients do not activate the transferred Br fully and may not reach optimal functional status on their own or with traditional therapies. The investigators propose that participation in a postoperative task-based training program will drive cortical changes that impact functional (pinch) ability. Recent studies of individuals with cervical SCI show substantial cortical reorganization can occur after the injury, but neural substrates of motor learning after tendon transfer have not been studied. For these patients, very little is known about what functional brain changes accompany improved performance in response to additional intervention. The Br to FPL transfer alters the central feedback from the periphery and may permit new or adaptive neural pathways that can achieve greater functional use of the tendon transfer. Neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) have become important tools for understanding plasticity in the neuromuscular system and for assessing the neural underpinnings of successful novel interventions. The objective of the proposed study is to identify the neural pattern that is associated with the best functional outcomes (highest pinch force) after Br to FPL transfer. The investigators will use fMRI and functional performance measures to find neural predictors and correlates of muscle re-education. That is, the investigators expect that successful postoperative muscle re-education will depend on increased cortical drive to the transferred Br in combination with new synergists, and this will be reflected in the neural imaging results. The purpose of the study is to evaluate neural activity from Br to FPL transfer recipients after conventional therapy and in response to an additional task-based training home program that aims to improve voluntary activation of the transferred Br in functional pinch tasks. The 10-week training program is under evaluation in RRD Pilot (B0583P) study and includes producing pinch force in different upper limb postures, biofeedback from a pinch dynamometer, and practicing selected pinch tasks. The investigators anticipate that increases in the amplitude and distribution of fMRI blood oxygen level dependent (BOLD) responses in sensorimotor cortices will underlie improved motor control post-surgically and following successful intervention to promote motor relearning. Specific Aims 1. Define the cortical representation of pre-training pinch function in SCI patients. The investigators will quantify the pattern (location, volume and intensity) of cortical activation associated with voluntary pinch in individuals who are one-year post Br to FPL tendon transfer surgery and a conventional therapy program. The investigators hypothesize (H1) greater volume and intensity of brain activation will correlate to better pinch function measured by pinch strength and the magnitude Br activation in pinch. Secondary analyses will determine if the location of the brain activation varies with pinch function (force) and specificity relative to voluntary elbow flexion. 2. Evaluate the cortical response to the task-based home therapy program. Correlates of task-based adaptation from pre- to post-training will be assessed using fMRI. H2: Greater activation (intensity and volume) in the primary motor cortex (M1) and sensory cortex will translate to increased voluntary activation of the transferred Br in pinch compared to elbow flexion. Task-based training outcome measures will include isometric pinch force magnitude, EMG quantification of Br activation in pinch and elbow flexion. 3. Determine neural signatures of surgical and training induced motor improvements. H3: After directed task-based training, brain activity during voluntary pinch will extend to adjacent areas (larger representation, greater activity), in sensorimotor brain areas, facilitating the ability to voluntarily increase the transferred Br activation in pinch. Secondary analyses will contrast size and activation level of brain changes with pinch activation in patients who receive training relative to non-impaired and non-surgical participants. The postoperative therapy protocols after tendon transfer procedures are not well defined, inconsistently applied, and lack evidence for their effectiveness. The study proposed here will investigate cortical change to assess outcome dependent plasticity. Thus, it may be possible to predict why some individuals do not re-train the transferred muscle as well as others. Establishing this relationship can lead to understanding the mechanisms of successful interventions and may identify brain based dynamics that could become the focus of future treatments (e.g. biofeedback, brain stimulation, etc.).