Patient-Specific Analysis of Neural Activation During Spinal Cord Stimulation for Pain.

OBJECTIVE: Despite the widespread use of spinal cord stimulation (SCS) for chronic pain management, its neuromodulatory effects remain poorly understood. Computational models provide a valuable tool to study SCS and its effects on axonal pathways within the spinal cord. However, these models must include sufficient detail to correlate model predictions with clinical effects, including patient-specific data. Therefore, the goal of this study was to investigate axonal activation at clinically relevant SCS parameters using a computer model that incorporated patient-specific anatomy and electrode locations. METHODS: We developed a patient-specific computer model for a patient undergoing SCS to treat chronic pain. This computer model consisted of two main components: 1) finite element model of the extracellular voltages generated by SCS and 2) multicompartment cable models of axons in the spinal cord. To determine the potential significance of a patient-specific approach, we also performed simulations with standard canonical models of SCS. We used the computer models to estimate axonal activation at clinically measured sensory, comfort, and discomfort thresholds. RESULTS: The patient-specific and canonical models predicted significantly different axonal activation. Relative to the canonical models, the patient-specific model predicted sensory threshold estimates that were more consistent with the corresponding clinical measurements. These results suggest that it is important to account for sources of interpatient variability (e.g., anatomy, electrode locations) in model-based analysis of SCS. CONCLUSIONS: This study demonstrates the potential for patient-specific computer models to quantitatively describe the axonal response to SCS and to address scientific questions related to clinical SCS.

Randomized clinical trial of deep brain stimulation for poststroke pain.

OBJECTIVE: The experience with deep brain stimulation (DBS) for pain is largely based on uncontrolled studies targeting the somatosensory pathways, with mixed results. We hypothesized that targeting limbic neural pathways would modulate the affective sphere of pain and alleviate suffering. METHODS: We conducted a prospective, double-blinded, randomized, placebo-controlled, crossover study of DBS targeting the ventral striatum/anterior limb of the internal capsule (VS/ALIC) in 10 patients with poststroke pain syndrome. One month after bilateral DBS, patients were randomized to active DBS or sham for 3 months, followed by crossover for another 3-month period. The primary endpoint was a ≥50% improvement on the Pain Disability Index in 50% of patients with active DBS compared to sham. This 6-month blinded phase was followed by an 18-month open stimulation phase. RESULTS: Nine participants completed randomization. Although this trial was negative for its primary and secondary endpoints, we did observe significant differences in multiple outcome measures related to the affective sphere of pain (eg, Montgomery-Åsberg Depression Rating Scale, Beck Depression Inventory, Affective Pain Rating Index of the Short-Form McGill Pain Questionnaire). Fourteen serious adverse events were recorded and resolved. INTERPRETATION: VS/ALIC DBS to modulate the affective sphere of pain represents a paradigm shift in chronic pain management. Although this exploratory study was negative for its primary endpoint, VS/ALIC DBS demonstrated an acceptable safety profile and statistically significant improvements on multiple outcome measures related to the affective sphere of pain. Therefore, we believe these results justify further work on neuromodulation therapies targeting the affective sphere of pain. Ann Neurol 2017;81:653-663.

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.

Cerebellar deep brain stimulation for chronic post-stroke motor rehabilitation: a phase I trial.

Upper-extremity impairment after stroke remains a major therapeutic challenge and a target of neuromodulation treatment efforts. In this open-label, non-randomized phase I trial, we applied deep brain stimulation to the cerebellar dentate nucleus combined with renewed physical rehabilitation to promote functional reorganization of ipsilesional cortex in 12 individuals with persistent (1-3 years), moderate-to-severe upper-extremity impairment. No serious perioperative or stimulation-related adverse events were encountered, with participants demonstrating a seven-point median improvement on the Upper-Extremity Fugl-Meyer Assessment. All individuals who enrolled with partial preservation of distal motor function exceeded minimal clinically important difference regardless of time since stroke, with a median improvement of 15 Upper-Extremity Fugl-Meyer Assessment points. These robust functional gains were directly correlated with cortical reorganization evidenced by increased ipsilesional metabolism. Our findings support the safety and feasibility of deep brain stimulation to the cerebellar dentate nucleus as a promising tool for modulation of late-stage neuroplasticity for functional recovery and the need for larger clinical trials. ClinicalTrials.gov registration: NCT02835443 .

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.

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 (ß = -0.34, p = 0.005), while volume adjacent to the core of map predicted voluntary activation of biceps (ß = 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.

Effectiveness and neural mechanisms associated with tDCS delivered to premotor cortex in stroke rehabilitation: study protocol for a randomized controlled trial.

BACKGROUND: More than 60% of stroke survivors experience residual deficits of the paretic upper limb/hand. Standard rehabilitation generates modest gains. Stimulation delivered to the surviving Primary Motor Cortex in the stroke-affected hemisphere has been considered a promising adjunct. However, recent trials challenge its advantage. We discuss our pilot clinical trial that aims to address factors implicated in divergent success of the approach. We assess safety, feasibility and efficacy of targeting an alternate locus during rehabilitation- the premotor cortex. In anticipating variance across patients, we measure neural markers differentiating response from non-response. METHODS/DESIGN: In a randomized, sham-controlled, double-blinded pilot clinical study, patients with chronic stroke (n = 20) are assigned to receive transcranial direct current stimulation delivered to the premotor cortex or sham during rehabilitation of the paretic arm/hand. Patients receive the designated intervention for 30 min, twice a day for 3 days a week for 5 weeks. We assess hand function and patients' reports of use of paretic hand. A general linear mixed methods model will analyze changes from pre- to post-intervention. Responders and non-responders will be compared upon baseline level of function, and neural substrates, including function and integrity of output tracts, bi-hemispheric balance, and lesion profile. Incidence of adverse events will be compared using Fisher's Exact test, while rigor of blinding will be assessed with Chi-square analysis to ascertain feasibility. DISCUSSION: Variable success of cortical stimulation in rehabilitation can be related to gaps in theoretical basis and clinical investigation. Given that most patients with severe deficits have damage to the primary motor cortex or its output pathways, it would be futile to target stimulation to this site. We suggest targeting premotor cortex because it contributes substantially to descending output, a role that is amplified with greater damage to the motor cortex. With regards to clinical investigation, paired cortical stimulation in rehabilitation has been compared to rehabilitation alone in unblinded trials or to unconvincing sham conditions. Transcranial direct current stimulation, a noninvasive technique of brain stimulation, which offers a more effective placebo and has a favorable safety-feasibility profile, may improve scientific rigor. Neural markers of response would help inform patient selection for future clinical trials so we can address limitations of recent negative studies. TRIAL REGISTRATION: NCT01539096.

Myoelectrical manifestation of fatigue less prominent in patients with cancer related fatigue.

PURPOSE: A lack of fatigue-related muscle contractile property changes at time of perceived physical exhaustion and greater central than peripheral fatigue detected by twitch interpolation technique have recently been reported in cancer survivors with fatigue symptoms. Based on these observations, it was hypothesized that compared to healthy people, myoelectrical manifestation of fatigue in the performing muscles would be less significant in these individuals while sustaining a prolonged motor task to self-perceived exhaustion (SPE) since their central fatigue was more prominent. The purpose of this study was to test this hypothesis by examining electromyographic (EMG) signal changes during fatiguing muscle performance. METHODS: Twelve individuals who had advanced solid cancer and cancer-related fatigue (CRF), and 12 age- and gender-matched healthy controls performed a sustained elbow flexion at 30% maximal voluntary contraction till SPE. Amplitude and mean power frequency (MPF) of EMG signals of the biceps brachii, brachioradialis, and triceps brachii muscles were evaluated when the individuals experienced minimal, moderate, and severe fatigue. RESULTS: CRF patients perceived physical "exhaustion" significantly sooner than the controls. The myoelectrical manifestation of muscular fatigue assessed by EMG amplitude and MPF was less significant in CRF than controls. The lower MPF even at minimal fatigue stage in CRF may indicate pathophysiologic condition of the muscle. CONCLUSIONS: CRF patients experience less myoelectrical manifestation of muscle fatigue than healthy individuals near the time of SPE. The data suggest that central nervous system fatigue plays a more important role in limiting endurance-type of motor performance in patients with CRF.

Lack of muscle contractile property changes at the time of perceived physical exhaustion suggests central mechanisms contributing to early motor task failure in patients with cancer-related fatigue.

CONTEXT: Fatigue is one of the most common symptoms reported by cancer survivors, and fatigue worsens when patients are engaged in muscle exertion, which results in early motor task failure. Central fatigue plays a significant role, more than muscle (peripheral) fatigue, in contributing to early task failure in cancer-related fatigue (CRF). OBJECTIVES: The purpose of this study was to determine if muscle contractile property alterations (reflecting muscle fatigue) occurred at the end of a low-intensity muscle contraction to exhaustion and if these properties differed between those with CRF and healthy controls. METHODS: Ten patients (aged 59.9±10.6 years, seven women) with advanced solid cancer and CRF and 12 age- and gender-matched healthy controls (aged 46.6±12.8 years, nine women) performed a sustained contraction of the right arm elbow flexion at 30% maximal level until exhaustion. Peak twitch force, time to peak twitch force, rate of peak twitch force development, and half relaxation time derived from electrical stimulation-evoked twitches were analyzed pre- and post-sustained contraction. RESULTS: CRF patients reported significantly greater fatigue as measured by the Brief Fatigue Inventory and failed the motor task earlier, 340±140 vs. 503±155 seconds in controls. All contractile property parameters did not change significantly in CRF but did change significantly in controls. CONCLUSION: CRF patients perceive physical exhaustion sooner during a motor fatigue task with minimal muscular fatigue. The observation supports that central fatigue is a more significant factor than peripheral fatigue in causing fatigue feelings and limits motor function in cancer survivors with fatigue symptoms.