Corin Bonnett

Assessment of Inter-Hemispheric Imbalance Using Imaging and Noninvasive Brain Stimulation in Patients With Chronic Stroke

OBJECTIVE To determine how interhemispheric balance in stroke, measured using transcranial magnetic stimulation (TMS), relates to balance defined using neuroimaging (functional magnetic resonance [fMRI], diffusion-tensor imaging [DTI]) and how these metrics of balance are associated with clinical measures of upper-limb function and disability. DESIGN Cross sectional. SETTING Laboratory. PARTICIPANTS Patients with chronic stroke (N = 10; age, 63 ± 9 y) in a population-based sample with unilateral upper-limb paresis. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Interhemispheric balance was measured with TMS, fMRI, and DTI. TMS defined interhemispheric differences in the recruitment of corticospinal output, size of the corticomotor output maps, and degree of mutual transcallosal inhibition that they exerted on one another. fMRI studied whether cortical activation during the movement of the paretic hand was lateralized to the ipsilesional or to the contralesional primary motor cortex (M1), premotor cortex (PMC), and supplementary motor cortex (SMA). DTI was used to define interhemispheric differences in the integrity of the corticospinal tracts projecting from the M1. Clinical outcomes tested function (upper extremity Fugl-Meyer [UEFM]) and perceived disability in the use of the paretic hand (Motor Activity Log [MAL] amount score). RESULTS Interhemispheric balance assessed with TMS relates differently to fMRI and DTI. Patients with high fMRI lateralization to the ipsilesional hemisphere possessed stronger ipsilesional corticomotor output maps (M1: r = .831, P = .006; PMC: r = .797, P = .01) and better balance of mutual transcallosal inhibition (r = .810, P = .015). Conversely, we found that patients with less integrity of the corticospinal tracts in the ipsilesional hemisphere show greater corticospinal output of homologous tracts in the contralesional hemisphere (r = .850, P = .004). However, an imbalance in integrity and output do not relate to transcallosal inhibition. Clinically, although patients with less integrity of corticospinal tracts from the ipsilesional hemisphere showed worse impairments (UEFM) (r = -.768, P = .016), those with low fMRI lateralization to the ipsilesional hemisphere had greater perception of disability (MAL amount score) (M1: r = .883, P = .006; PMC: r = .817, P = .007; SMA: r = .633, P = .062). CONCLUSIONS In patients with chronic motor deficits of the upper limb, fMRI may serve to mark perceived disability and transcallosal influence between hemispheres. DTI-based integrity of the corticospinal tracts, however, may be useful in categorizing the range of functional impairments of the upper limb. Further, in patients with extensive corticospinal damage, DTI may help infer the role of the contralesional hemisphere in recovery.

Stimulation targeting higher motor areas in stroke rehabilitation: A proof-of-concept, randomized, double-blinded placebo-controlled study of effectiveness and underlying mechanisms

PURPOSE To demonstrate, in a proof-of-concept study, whether potentiating ipsilesional higher motor areas (premotor cortex and supplementary motor area) augments and accelerates recovery associated with constraint induced movement. METHODS In a randomized, double-blinded pilot clinical study, 12 patients with chronic stroke were assigned to receive anodal transcranial direct current stimulation (tDCS) (n = 6) or sham (n = 6) to the ipsilesional higher motor areas during constraint-induced movement therapy. We assessed functional and neurophysiologic outcomes before and after 5 weeks of therapy. RESULTS Only patients receiving tDCS demonstrated gains in function and dexterity. Gains were accompanied by an increase in excitability of the contralesional rather than the ipsilesional hemisphere. CONCLUSIONS Our proof-of-concept study provides early evidence that stimulating higher motor areas can help recruit the contralesional hemisphere in an adaptive role in cases of greater ipsilesional injury. Whether this early evidence of promise translates to remarkable gains in functional recovery compared to existing approaches of stimulation remains to be confirmed in large-scale clinical studies that can reasonably dissociate stimulation of higher motor areas from that of the traditional primary motor cortices.

Age-Related Weakness of Proximal Muscle Studied with Motor Cortical Mapping: A TMS Study

Aging-related weakness is due in part to degeneration within the central nervous system. However, it is unknown how changes to the representation of corticospinal output in the primary motor cortex (M1) relate to such weakness. Transcranial magnetic stimulation (TMS) is a noninvasive method of cortical stimulation that can map representation of corticospinal output devoted to a muscle. Using TMS, we examined age-related alterations in maps devoted to biceps brachii muscle to determine whether they predicted its age-induced weakness. Forty-seven right-handed subjects participated: 20 young (22.6±0.90 years) and 27 old (74.96±1.35 years). We measured strength as force of elbow flexion and electromyographic activation of biceps brachii during maximum voluntary contraction. Mapping variables included: 1) center of gravity or weighted mean location of corticospinal output, 2) size of map, 3) volume or excitation of corticospinal output, and 4) response density or corticospinal excitation per unit area. Center of gravity was more anterior in old than in young (p<0.001), though there was no significant difference in strength between the age groups. Map size, volume, and response density showed no significant difference between groups. Regardless of age, center of gravity significantly predicted strength (β = −0.34, p = 0.005), while volume adjacent to the core of map predicted voluntary activation of biceps (β = 0.32, p = 0.008). Overall, the anterior shift of the map in older adults may reflect an adaptive change that allowed for the maintenance of strength. Laterally located center of gravity and higher excitation in the region adjacent to the core in weaker individuals could reflect compensatory recruitment of synergistic muscles. Thus, our study substantiates the role of M1 in adapting to aging-related weakness and subtending strength and muscle activation across age groups. Mapping from M1 may offer foundation for an examination of mechanisms that preserve strength in elderly.

Neurophysiological correlates of aging-related muscle weakness

Muscle weakness associated with aging implicates central neural degeneration. However, role of the primary motor cortex (M1) is poorly understood, despite evidence that gains in strength in younger adults are associated with its adaptations. We investigated whether weakness of biceps brachii in aging analogously relates to processes in M1. We enrolled 20 young (22.6 ± 0.87 yr) and 28 old (74.79 ± 1.37 yr) right-handed participants. Using transcranial magnetic stimulation, representation of biceps in M1 was identified. We examined the effect of age and sex on strength of left elbow flexion, voluntary activation of biceps, corticospinal excitability and output, and short-interval intracortical and interhemispheric inhibition. Interhemispheric inhibition was significantly exaggerated in the old (P = 0.047), while strength tended to be lower (P = 0.075). Overall, women were weaker (P < 0.001). Processes of M1 related to strength or voluntary activation of biceps, but only in older adults. Corticospinal excitability was lower in weaker individuals (r = 0.38), and corticospinal output, intracortical inhibition and interhemispheric inhibition were reduced too in individuals who poorly activated biceps (r = 0.43, 0.54 and 0.38). Lower intracortical inhibition may reflect compensation for reduced corticospinal excitability, allowing weaker older adults to spread activity in M1 to recruit synergists and attempt to sustain motor output. Exaggerated interhemispheric inhibition, however, conflicts with previous evidence, potentially related to greater callosal damage in our older sample, our choice of proximal vs. distal muscle and differing influence of measurement of inhibition in rest vs. active states of muscle. Overall, age-specific relation of M1 to strength and muscle activation emphasizes that its adaptations only emerge when necessitated, as in a weakening neuromuscular system in aging.

Post-exercise depression following submaximal and maximal isometric voluntary contraction.

It is well known that corticomotor excitability is altered during the post-exercise depression following fatigue within the primary motor cortex (M1). However, it is currently unknown whether corticomotor reorganization following muscle fatigue differs between magnitudes of force and whether corticomotor reorganization occurs measured with transcranial magnetic stimulation (TMS). Fifteen young healthy adults (age 23.8±1.4, 8 females) participated in a within-subjects, repeated measures design study, where they underwent three testing sessions separated by one-week each. Subjects performed separate sessions of each: low-force isometric contraction (30% maximal voluntary contraction [MVC]), high-force isometric contraction (95% MVC) of the first dorsal interosseous (FDI) muscle until self-perceived exhaustion, as well as one session of a 30-min rest as a control. We examined changes in corticomotor map area, excitability and location of the FDI representation in and around M1 using TMS. The main finding was that following low-force, but not high-force fatigue (HFF) corticomotor map area and excitability reduced [by 3cm(2) (t(14)=-2.94, p=0.01) and 56% respectively t(14)=-4.01, p<0.001)]. Additionally, the region of corticomotor excitability shifted posteriorly (6.4±2.5mm) (t(14)=-6.33, p=.019). Corticomotor output became less excitable particularly in regions adjoining M1. Overall, post-exercise depression is present in low-force, but not for HFF. Further, low-force fatigue (LFF) results in a posterior shift in corticomotor output. These changes may be indicative of increased sensory feedback from the somatosensory cortex during the recovery phase of fatigue.

Challenges in Recruitment for the Study of Noninvasive Brain Stimulation in Stroke: Lessons from Deep Brain Stimulation

OBJECTIVE Noninvasive brain stimulation (NIBS) can augment functional recovery following stroke; however, the technique lacks regulatory approval. Low enrollment in NIBS clinical trials is a key roadblock. Here, we pursued evidence to support the prevailing opinion that enrollment in trials of NIBS is even lower than enrollment in trials of invasive, deep brain stimulation (DBS). METHODS We compared 2 clinical trials in stroke conducted within a single urban hospital system, one employing NIBS and the other using DBS, (1) to identify specific criteria that generate low enrollment rates for NIBS and (2) to devise strategies to increase recruitment with guidance from DBS. RESULTS Notably, we found that enrollment in the NIBS case study was 5 times lower (2.8%) than the DBS trial (14.5%) (χ(2) = 20.815, P < .0001). Although the number of candidates who met the inclusion criteria was not different (χ(2) = .04, P < .841), exclusion rates differed significantly between the 2 studies (χ(2) = 21.354, P < .0001). Beyond lack of interest, higher exclusion rates in the NIBS study were largely due to exclusion criteria that were not present in the DBS study, including restrictions for recurrent strokes, seizures, and medications. CONCLUSIONS Based on our findings, we conclude and suggest that by (1) establishing criteria specific to each NIBS modality, (2) adjusting exclusion criteria based on guidance from DBS, and (3) including patients with common contraindications based on a probability of risk, we may increase enrollment and hence significantly impact the feasibility and generalizability of NIBS paradigms, particularly in stroke.

Influence of Corticospinal Tracts from Higher Order Motor Cortices on Recruitment Curve Properties in Stroke

Background: Recruitment curves (RCs) acquired using transcranial magnetic stimulation are commonly used in stroke to study physiologic functioning of corticospinal tracts (CST) from M1. However, it is unclear whether CSTs from higher motor cortices contribute as well. Objective: To explore whether integrity of CST from higher motor areas, besides M1, relates to CST functioning captured using RCs. Methods: RCs were acquired for a paretic hand muscle in patients with chronic stroke. Metrics describing gain and overall output of CST were collected. CST integrity was defined by diffusion tensor imaging. For CST emerging from M1 and higher motor areas, integrity (fractional anisotropy) was evaluated in the region of the posterior limb of the internal capsule, the length of CST and in the region of the stroke lesion. Results: We found that output and gain of RC was related to integrity along the length of CST emerging from higher motor cortices but not the M1. Conclusions: Our results suggest that RC parameters in chronic stroke infer function primarily of CST descending from the higher motor areas but not M1. RCs may thus serve as a simple, in-expensive means to assess re-mapping of alternate areas that is generally studied with resource-intensive neuroimaging in stroke.

Reproducibility of transcranial magnetic stimulation metrics in the study of proximal upper limb muscles

OBJECTIVE Reproducibility of transcranial magnetic stimulation (TMS) metrics is essential in accurately tracking recovery and disease. However, majority of evidence pertains to reproducibility of metrics for distal upper limb muscles. We investigate for the first time, reliability of corticospinal physiology for a large proximal muscle - the biceps brachii and relate how varying statistical analyses can influence interpretations. METHODS 14 young right-handed healthy participants completed two sessions assessing resting motor threshold (RMT), motor evoked potentials (MEPs), motor map and intra-cortical inhibition (ICI) from the left biceps brachii. Analyses included paired t-tests, Pearsons, intra-class (ICC) and concordance correlation coefficients (CCC) and Bland-Altman plots. RESULTS Unlike paired t-tests, ICC, CCC and Pearsons were >0.6 indicating good reliability for RMTs, MEP intensities and locations of map; however values were <0.3 for MEP responses and ICI. CONCLUSIONS Corticospinal physiology, defining excitability and output in terms of intensity of the TMS device, and spatial loci are the most reliable metrics for the biceps. MEPs and variables based on MEPs are less reliable since biceps receives fewer cortico-motor-neuronal projections. Statistical tests of agreement and associations are more powerful reliability indices than inferential tests. SIGNIFICANCE Reliable metrics of proximal muscles when translated to a larger number of participants would serve to sensitively track and prognosticate function in neurological disorders such as stroke where proximal recovery precedes distal.