Do sensorimotor cortex activity, an individual's capacity for neuroplasticity, and psychological features during an episode of acute low back pain predict outcome at 6 months: a protocol for an Australian, multisite prospective, longitudinal cohort study

INTRODUCTION

Low back pain (LBP) is the leading cause of disability worldwide, with prevalence doubling in the past 14 years. To date, prognostic screening tools display poor discrimination and offer no net benefit of screening over and above a 'treat all' approach. Characteristics of the primary sensory (S1) and motor (M1) cortices may predict the development of chronic LBP, yet the prognostic potential of these variables remains unknown. The Understanding persistent Pain Where it ResiDes (UPWaRD) study aims to determine whether sensorimotor cortex activity, an individual's capacity for plasticity and psychosocial factors in the acute stage of pain, predict LBP outcome at 6 months. This paper describes the methods and analysis plan for the development of the prediction model.

METHODS AND ANALYSIS

The study uses a multicentre prospective longitudinal cohort design with 6-month follow-up. 120 participants, aged 18 years or older, experiencing an acute episode of LBP (less than 6 weeks duration) will be included. Primary outcomes are pain and disability.

ETHICS AND DISSEMINATION

Ethical approval has been obtained from Western Sydney University Human Research Ethics Committee (H10465) and from Neuroscience Research Australia (SSA: 16/002). Dissemination will occur through presentations at national and international conferences and publications in international peer-reviewed journals.

TRIAL REGISTRATION NUMBER

ACTRN12619000002189; Pre-results.

Connectivity as a Predictor of Responsiveness to Transcranial Direct Current Stimulation in People with Stroke: Protocol for a Double-Blind Randomized Controlled Trial

Background

Stroke can have devastating consequences for an individual’s quality of life. Interventions capable of enhancing response to therapy would be highly valuable to the field of neurological rehabilitation. One approach is to use noninvasive brain stimulation techniques, such as transcranial direct current stimulation, to induce a neuroplastic response. When delivered in combination with rehabilitation exercises, there is some evidence that transcranial direct current stimulation is beneficial. However, responses to stimulation are highly variable. Therefore biomarkers predictive of response to stimulation would be valuable to help select appropriate people for this potentially beneficial treatment.

Objective

The objective of this study is to investigate connectivity of the stimulation target, the ipsilesional motor cortex, as a biomarker predictive of response to anodal transcranial direct current stimulation in people with stroke.

Methods

This study is a double blind, randomized controlled trial (RCT), with two parallel groups. A total of 68 participants with first ever ischemic stroke with motor impairment will undertake a two week (14 session) treatment for upper limb function (Graded Repetitive Arm Supplementary Program; GRASP). Participants will be randomized 2:1 to active:sham treatment groups. Those in the active treatment group will receive anodal transcranial direct current stimulation to the ipsilesional motor cortex at the start of each GRASP session. Those allocated to the sham treatment group will receive sham transcranial direct current stimulation. Behavioural assessments of upper limb function will be performed at baseline, post treatment, 1 month follow-up and 3 months follow-up. Neurophysiological assessments will include magnetic resonance imaging (MRI), electroencephalography (EEG) and transcranial magnetic stimulation (TMS) and will be performed at baseline, post treatment, 1 month follow-up (EEG and TMS only) and 3 months follow-up (EEG and TMS only).

Results

Participants will be recruited between March 2018 and December 2018, with experimental testing concluding in March 2019.

Conclusions

Identifying a biomarker predictive of response to transcranial direct current stimulation would greatly assist clinical utility of this novel treatment approach.

Trial Registration

Australia New Zealand Clinical Trials Registry ACTRN12618000443291; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=12618000443291 (Archived by WebCite at http://www.webcitation.org/737QOXXxt)

Registered Report Identifier

RR1-10.2196/10848

Microcircuit mechanisms involved in paired associative stimulation-induced depression of corticospinal excitability

Synaptic weight changes induced by temporal correlations between the spikes of pre- and postsynaptic neurons are referred to as spike-timing-dependent plasticity (STDP). Transcranial magnetic stimulation (TMS) induces long-lasting effects on corticospinal excitability, if it is repetitively paired with stimulation of afferents from a corresponding contralateral hand region at short intervals (paired associative stimulation, PAS). PAS-induced plasticity has been linked with synaptic STDP. We aimed to investigate which elements of the cortical microcircuitry sustain and govern PAS-induced depression of corticospinal excitability in the target muscle representation (and enhancement of excitability in its functional surround). We show that the time window during which the interaction between both stimulus-induced cortical events leads to immediate post-interventional depression is short (<4.5 ms). The depressant PAS effects at the target representation were completely blocked by applying a subthreshold magnetic pulse 3 ms before the principal TMS pulse, even when the strength of the latter was adjusted to generate a motor-evoked potential of similar amplitude to that with the unconditioned magnetic pulse. Epidural recordings from the cervical cord of a patient showed that under this condition late TMS-evoked I-waves remain suppressed. When the intensity of the TMS component during PAS was lowered - sufficient to allow activation of inhibitory neurons, but insufficient to activate corticospinal neurons - excitability of short-latency intracortical inhibition remained unchanged. PAS-induced facilitation in the functional surround followed the same pattern as the centre-depressant effects. These findings may suggest that excitability-depressant PAS-induced effects are due to weakening of excitatory synapses between upper cortical layer principal neurons, but not those located on the corticospinal neuron, or inhibitory synapses. Inhibitory interneurons involved in short-latency intracortical inhibition are gate-keepers to producing centre-depressant/surround-facilitatory PAS effects. Based on these and earlier findings we propose a model specifying the composition and laminar location of the involved microcircuit of PAS-induced plasticity that may enhance its utility as a model of STDP in humans.

Transcranial electric and magnetic stimulation: technique and paradigms

Transcranial electrical and magnetic stimulation techniques encompass a broad physical variety of stimuli, ranging from static magnetic fields or direct current stimulation to pulsed magnetic or alternating current stimulation with an almost infinite number of possible stimulus parameters. These techniques are continuously refined by new device developments, including coil or electrode design and flexible control of the stimulus waveforms. They allow us to influence brain function acutely and/or by inducing transient plastic after-effects in a range from minutes to days. Manipulation of stimulus parameters such as pulse shape, intensity, duration, and frequency, and location, size, and orientation of the electrodes or coils enables control of the immediate effects and after-effects. Physiological aspects such as stimulation at rest or during attention or activation may alter effects dramatically, as does neuropharmacological drug co-application. Non-linear relationships between stimulus parameters and physiological effects have to be taken into account.

Differential modulation of motor cortex excitability in BDNF Met allele carriers following experimentally induced and use-dependent plasticity

The purpose of this study was to investigate how healthy young subjects with one of three variants of the brain-derived neurotrophic factor (BDNF) gene modulate motor cortex excitability following experimentally induced and use-dependent plasticity interventions. Electromyographic recordings were obtained from the right first dorsal interosseous (FDI) muscle of 12 Val/Val, ten Val/Met and seven Met/Met genotypes (aged 18-39 years). Transcranial magnetic stimulation of the left hemisphere was used to assess changes in FDI motor-evoked potentials (MEPs) following three separate interventions involving paired associative stimulation, a simple ballistic task and complex visuomotor tracking task using the index finger. Val/Val subjects increased FDI MEPs following all interventions (≥ 25%, P < 0.01), whereas the Met allele carriers only showed increased MEPs after the simple motor task (≥ 26%, P < 0.01). In contrast to the simple motor task, there was no significant change in MEPs for the Val/Met subjects (7%, P = 0.50) and a reduction in MEPs for the Met/Met group (-38%, P < 0.01) following the complex motor task. Despite these differences in use-dependent plasticity, the performance of both motor tasks was not different between BDNF genotypes. We conclude that modulation of motor cortex excitability is strongly influenced by the BDNF polymorphism, with the greatest differences observed for the complex motor task. We also found unique motor cortex plasticity in the rarest form of the BDNF polymorphism (Met/Met subjects), which may have implications for functional recovery after disease or injury to the nervous system in these individuals.

A checklist for assessing the methodological quality of studies using transcranial magnetic stimulation to study the motor system: an international consensus study

In the last decade transcranial magnetic stimulation (TMS) has been the subject of more than 20,000 original research articles. Despite this popularity, TMS responses are known to be highly variable and this variability can impact on interpretation of research findings. There are no guidelines regarding the factors that should be reported and/or controlled in TMS studies. This study aimed to develop a checklist to be recommended to evaluate the methodology and reporting of studies that use single or paired pulse TMS to study the motor system. A two round international web-based Delphi study was conducted. Panellists rated the importance of a number of subject, methodological and analytical factors to be reported and/or controlled in studies that use single or paired pulse TMS to study the motor system. Twenty-seven items for single pulse studies and 30 items for paired pulse studies were included in the final checklist. Eight items related to subjects (e.g. age, gender), 21 to methodology (e.g. coil type, stimulus intensity) and two to analysis (e.g. size of the unconditioned motor evoked potential). The checklist is recommended for inclusion when submitting manuscripts for publication to ensure transparency of reporting and could also be used to critically appraise previously published work. It is envisaged that factors could be added and deleted from the checklist on the basis of future research. Use of the TMS methodological checklist should improve the quality of data collection and reporting in TMS studies of the motor system.

Time course of induction of increased human motor cortex excitability by nerve stimulation

Manipulation of afferent input induces changes in the excitability and organisation of human corticomotor representations. These changes are generally short lived, although can be prolonged by repetition. Here, we charted the time-course of the change of motor cortex excitability induced by electrical stimulation of radial and ulnar nerves. Corticomotor excitability was evaluated by measuring the amplitude of the motor evoked potentials in the first dorsal interosseous muscle by transcranial magnetic stimulation of the optimal cortical area. Measurements were carried out before the start of peripheral nerve stimulation, and then during the peripheral nerve stimulation at 15 min intervals over a period of 2 h. The amplitudes of the motor evoked potentials significantly increased during the 2 h period of peripheral nerve stimulation. Cortical excitability peaked after about 45-60 min stimulation. These clear-cut changes in cortical excitability following peripheral nerve stimulation may reveal some of the mechanisms underlying motor learning and cortical plasticity.

Effect of transcranial magnetic stimulation over the cerebellum on the excitability of human motor cortex

There have been conflicting reports over whether it is possible to stimulate the human cerebellum through the intact scalp using transcranial magnetic stimulation. Here we attempt to clarify the situation in normal subjects by comparing the various methods which have been used. EMG responses evoked by magnetic stimulation over the motor cortex could be suppressed by a prior magnetic stimulus over the cerebellum but the onset latency of the effect varied according to the type of magnetic coil used. Inhibition began at a latency which ranged from 5 to 9 msec in different subjects if conditioning stimuli were given through a flat figure-of-eight coil held horizontally over the basal occiput. The effect lasted a further 6-10 msec. With a larger double cone coil, held vertically over the basal occiput, inhibition began earlier and at a more constant latency of 5 msec. It lasted only 3 msec. Stimulation of the C6/7 nerve roots in the brachial plexus with either an electrical or magnetic stimulus also could suppress EMG responses evoked by cortical stimulation. This began at a conditioning-test interval of 7 or 8 msec and lasted for some 5 msec. We suggest that two types of motor cortical suppression may be elicited from stimulation over the posterior neck/skull: a cerebellar effect starting at 5 msec, and a peripheral nerve effect starting later at 7/8 msec. Stimulation with a horizontal large figure-of-eight coil may produce a mixture of effects because the lower wing of the coil overlaps the posterior neck and can activate peripheral nerve fibres in the brachial plexus.

Comparison of cervical magnetic stimulation and bilateral percutaneous electrical stimulation of the phrenic nerves in normal subjects

Cervical magnetic stimulation is a new technique for stimulating the phrenic nerves, and may offer an alternative to percutaneous electrical stimulation for assessing diaphragmatic strength in normal subjects and patients in whom electrical stimulation is technically difficult or poorly tolerated. We compared cervical magnetic stimulation with conventional supramaximal bilateral percutaneous electrical stimulation in nine normal subjects. We measured oesophageal pressure (Poes), gastric pressure (Pgas) and transdiaphragmatic pressure (Pdi). The maximal relaxation rate (MRR) was also measured. The mean magnetic twitch Pdi was 36.5 cmH2O (range 27-48 cmH2O), significantly larger than electrical twitch Pdi, mean 29.7 cmH2O (range 22-40 cmH2O). The difference in twitch Pdi was explained entirely by twitch Poes, and it is possible that the magnetic technique stimulates some of the nerves to the upper chest wall muscles as well as the phrenic nerves. We compared bilateral, rectified, integrated, diaphragm surface electromyographic (EMG) responses in three subjects and found results within 10% in each subject, indicating similar diaphragmatic activation. The within occasion coefficient of variation, i.e. same subject/same session, was 6.7% both for magnetic and electrical twitch Pdi. The between occasion coefficient of variation, i.e. same subject/different days, was 6.6% for magnetic stimulation and 8.8% for electrical. There was no difference between relaxation rates measured with either technique. We conclude that magnetic stimulation is a reproducible and acceptable technique for stimulating the phrenic nerves, and that it provides a potentially useful alternative to conventional electrical stimulation as a nonvolitional test of diaphragm strength.