Abstract
Injury to the cervical segments of the spinal cord causes significant functional deficits to the upper extremities, affecting the performance of daily life activities and quality of life. Functional impairments following a cervical spinal cord injury (SCI) are primarily attributed to the damage to ascending and descending tracts that hinders the flow of information to and from supraspinal centers and the hand muscles, but there is also evidence that post-injury patterns of cortical reorganization can also play an important role. This thesis explored the influence of different approaches used to activate the motor cortex combined with repetitive task practice, and assessed their effects on hand function and corticomotor excitability. In chapter 1, we explored the literature regarding the primary processes (the lesion itself) and secondary processes (cortical reorganization and anatomical changes) that may further contribute to deficits in hand function after tetraplegia. In addition, we explored the evidence regarding the use of stimulation approaches that either target the motor cortex directly (repetitive transcranial magnetic stimulation [rTMS] and transcranial direct current stimulation [tDCS]), or indirectly (transcutaneous electrical nerve stimulation [TENS] and peripherally applied vibration [VIB]) for their effects on hand function and corticomotor excitability and thus, their potential as adjunct tools in neurorehabilitation. In the study described in chapter 2, we assessed the safety and late effects of a multi-day intervention consisting of a novel electrode montage using tDCS aimed at increasing bi-hemispheric cortical excitability (and thus, targeting the bimanual motor deficits observed after SCI and the cortical activation that exists in bimanual movements) interleaved with bimanual typing task performance in neurologically healthy participants. We found that a multi-day intervention consisting of 5 days of BAC-tDCS was associated with significantly greater gains in bimanual typing performance when compared with sham-tDCS, which was not retained after one week. In chapter 3, we describe a study where we assessed the late effects of a 3-day intervention of 10 Hz rTMS (a frequency that is associated with increased corticomotor excitability) interleaved with performance of a fine motor task on hand function and cortical excitability in individuals with tetraplegia and neurologically healthy controls. We found that participants who received rTMS made greater improvements in skilled hand function and assessed by the Jebsen-Taylor Hand Function Test and grasp force than participants who received sham-rTMS. The improvement in skilled hand function was not accompanied by changes in pinch grip force or corticomotor excitability. In Chapter 4, we undertook a study to assess whether there are differences in outcomes of repetitive task practice when combined with each of the clinically accessible approaches to increase corticomotor excitability either directly (tDCS) or indirectly (TENS, VIB) . Individuals with tetraplegia participated in this crossover study consisting of one session of each stimulation approach, simultaneously delivered with repetitive task practice. We found transient increases in pinch force in the VIB condition, and increases in skilled hand function measured by the Nine-hole Peg Test with the TENS and tDCS conditions. In addition, TENS and tDCS seemed to have shared similarities in terms of early increases in corticomotor excitability, whereas VIB seemed to follow a different pattern characterized by late increases in corticomotor excitability. Finally, in chapter 5 we explore discuss the results of these studies and their applicability to upper extremity rehabilitation of individuals with tetraplegia.