Disruption of cortical synaptic homeostasis in individuals with chronic low back pain.

OBJECTIVE: Homeostatic plasticity mechanisms regulate synaptic plasticity in the human brain. Impaired homeostatic plasticity may contribute to maladaptive synaptic plasticity and symptom persistence in chronic musculoskeletal pain. METHODS: We examined homeostatic plasticity in fifty individuals with chronic low back pain (cLBP) and twenty-five pain-free controls. A single block (7-min) of anodal transcranial direct current stimulation ('single tDCS'), or two subsequent blocks (7-min and 5-min separated by 3-min rest; 'double tDCS'), were randomised across two experimental sessions to confirm an excitatory response to tDCS applied alone, and evaluate homeostatic plasticity, respectively. Corticomotor excitability was assessed in the corticomotor representation of the first dorsal interosseous muscle by transcranial magnetic stimulation-induced motor evoked potentials (MEPs) recorded before and 0, 10, 20, and 30-min following each tDCS protocol. RESULTS: Compared with baseline, MEP amplitudes increased at all time points in both groups following the single tDCS protocol (P < 0.003). Following the double tDCS protocol, MEP amplitudes decreased in pain-free controls at all time points compared with baseline (P < 0.01), and were unchanged in the cLBP group. CONCLUSION: These data indicate impaired homeostatic plasticity in the primary motor cortex of individuals with cLBP. SIGNIFICANCE: Impaired homeostatic plasticity could explain maladaptive synaptic plasticity and symptom persistence in cLBP.

Novel adaptations in motor cortical maps: the relation to persistent elbow pain.

INTRODUCTION: Unilateral elbow pain results in sensorimotor dysfunction that is frequently bilateral, affects local and remote upper limb muscles, and persists beyond resolution of local tendon symptoms. These characteristics suggest supraspinal involvement. Here, we investigated 1) the excitability and organization of the M1 representation of the wrist extensor muscles and 2) the relation between M1 changes and clinical outcomes in lateral epicondylalgia (LE) (n = 11) and healthy control subjects (n = 11). METHODS: Transcranial magnetic stimulation was used to map the M1 representation of the extensor carpi radialis brevis (ECRB) and extensor digitorum (ED). RESULTS: The cortical representations of ECRB and ED were more excitable, and the centers of gravity for the two muscles were located closer together in LE than that in healthy controls. Increased ECRB excitability and closer location of the center of gravity were associated with higher pain severity at rest and/or in the preceding 6 months. A novel finding was a reduced number of discrete peaks in the representations of ECRB and ED in participants with LE compared with that in healthy controls. CONCLUSIONS: This finding may have broad implications for the control of the wrist extensor muscles in LE. These data provide evidence that cortical organization may be maladaptive in LE and suggest that reorganization may be associated with persistence/recurrence of pain.

The effect of electrical stimulation on corticospinal excitability is dependent on application duration: a same subject pre-post test design.

BACKGROUND: In humans, corticospinal excitability is known to increase following motor electrical stimulation (ES) designed to mimic a voluntary contraction. However, whether the effect is equivalent with different application durations and whether similar effects are apparent for short and long applications is unknown. The aim of this study was to investigate whether the duration of peripheral motor ES influenced its effect on corticospinal excitability. METHODS: The excitability of the corticomotor pathway to abductor pollicis brevis (APB) was measured in fourteen health subjects using transcranial magnetic stimulation before, immediately after and 10 minutes after three different durations (20-, 40-, 60-min) of motor ES (30Hz, ramped). This intervention was designed to mimic a voluntary contraction in APB. To control for effects of motor ES on the peripheral elements (muscle fibre, membrane, neuromuscular junction), maximum compound muscle actions potentials (M-waves) were also recorded at each time point. Results were analysed using a repeated measures analysis of variance. RESULTS: Peripheral excitability was reduced following all three motor ES interventions. Conversely, corticospinal excitability was increased immediately following 20- and 40-min applications of motor ES and this increase was maintained at least 20-min following the intervention. A 60-min application of motor ES did not alter corticospinal excitability. CONCLUSIONS: A 20-min application of motor ES that is designed to mimic voluntary muscle contraction is as effective as that applied for 40-min when the aim of the intervention is to increase corticospinal excitability. Longer motor ES durations of 60-min do not influence corticospinal excitability, possibly as a result of homeostatic plasticity mechanisms.

Interaction between simultaneously applied neuromodulatory interventions in humans.

BACKGROUND: Transcranial direct current stimulation (tDCS) is a neuromodulatory technique with the potential to enhance the efficacy of traditional therapies such as neuromuscular electrical stimulation (NMES). Yet, concurrent application of tDCS/NMES may also activate homeostatic mechanisms that block or reverse effects on corticomotor excitability. It is unknown how tDCS and NMES interact in the human primary motor cortex (M1) and whether effects are summative (increase corticomotor excitability beyond that of tDCS or NMES applied alone) or competitive (block or reduce corticomotor excitability effects of tDCS or NMES applied alone). OBJECTIVE: To investigate corticomotor excitability in response to NMES after concurrent application of tDCS protocols that enhance (anodal tDCS) or suppress (cathodal tDCS) excitability of M1. METHODS: We used transcranial magnetic stimulation (TMS) to examine corticomotor excitability before and after the concurrent application of: i) NMES with anodal tDCS; and ii) NMES with cathodal tDCS. Effects were contrasted to four control conditions: i) NMES alone, ii) anodal tDCS alone, iii) cathodal tDCS alone, and iv) sham stimulation. RESULTS: Concurrent application of two protocols that enhance excitability when applied alone (NMES and anodal tDCS) failed to induce summative effects on corticomotor excitability, as predicted by homeostatic plasticity mechanisms. Combined cathodal tDCS and NMES suppressed the enhanced excitation induced by NMES, an effect that might be explained by calcium dependent anti-gating models. CONCLUSIONS: These novel findings highlight the complex mechanisms involved when two neuromodulatory techniques are combined and suggest that careful testing of combined interventions is necessary before application in clinical contexts.

Primary sensory and motor cortex excitability are co-modulated in response to peripheral electrical nerve stimulation.

Peripheral electrical stimulation (PES) is a common clinical technique known to induce changes in corticomotor excitability; PES applied to induce a tetanic motor contraction increases, and PES at sub-motor threshold (sensory) intensities decreases, corticomotor excitability. Understanding of the mechanisms underlying these opposite changes in corticomotor excitability remains elusive. Modulation of primary sensory cortex (S1) excitability could underlie altered corticomotor excitability with PES. Here we examined whether changes in primary sensory (S1) and motor (M1) cortex excitability follow the same time-course when PES is applied using identical stimulus parameters. Corticomotor excitability was measured using transcranial magnetic stimulation (TMS) and sensory cortex excitability using somatosensory evoked potentials (SEPs) before and after 30 min of PES to right abductor pollicis brevis (APB). Two PES paradigms were tested in separate sessions; PES sufficient to induce a tetanic motor contraction (30-50 Hz; strong motor intensity) and PES at sub motor-threshold intensity (100 Hz). PES applied to induce strong activation of APB increased the size of the N(20)-P(25) component, thought to reflect sensory processing at cortical level, and increased corticomotor excitability. PES at sensory intensity decreased the size of the P25-N33 component and reduced corticomotor excitability. A positive correlation was observed between the changes in amplitude of the cortical SEP components and corticomotor excitability following sensory and motor PES. Sensory PES also increased the sub-cortical P(14)-N(20) SEP component. These findings provide evidence that PES results in co-modulation of S1 and M1 excitability, possibly due to cortico-cortical projections between S1 and M1. This mechanism may underpin changes in corticomotor excitability in response to afferent input generated by PES.

Characteristics of student preparedness for clinical learning: clinical educator perspectives using the Delphi approach.

BACKGROUND: During clinical placements, clinical educators facilitate student learning. Previous research has defined the skills, attitudes and practices that pertain to an ideal clinical educator. However, less attention has been paid to the role of student readiness in terms of foundational knowledge and attitudes at the commencement of practice education. Therefore, the aim of this study was to ascertain clinical educators' views on the characteristics that they perceive demonstrate that a student is well prepared for clinical learning. METHODS: A two round on-line Delphi study was conducted. The first questionnaire was emailed to a total of 636 expert clinical educators from the disciplines of occupational therapy, physiotherapy and speech pathology. Expert clinical educators were asked to describe the key characteristics that indicate a student is prepared for a clinical placement and ready to learn. Open-ended responses received from the first round were subject to a thematic analysis and resulted in six themes with 62 characteristics. In the second round, participants were asked to rate each characteristic on a 7 point Likert Scale. RESULTS: A total of 258 (40.56%) responded to the first round of the Delphi survey while 161 clinical educators completed the second (62.40% retention rate). Consensus was reached on 57 characteristics (six themes) using a cut off of greater than 70% positive respondents and an interquartile deviation IQD of equal or less than 1. CONCLUSIONS: This study identified 57 characteristics (six themes) perceived by clinical educators as indicators of a student who is prepared and ready for clinical learning. A list of characteristics relating to behaviours has been compiled and could be provided to students to aid their preparation for clinical learning and to universities to incorporate within curricula. In addition, the list provides a platform for discussions by professional bodies about the role of placement education.

Priming the brain to learn: the future of therapy?

Neuromodulatory techniques with the ability to alter cortical excitability are gaining interest for their potential to enhance the brain's sensitivity to traditional therapies. Neuromodulatory techniques that prime the brain prior to manual or exercise therapy hold therapeutic promise for enhancing clinical outcomes in musculoskeletal and neurological conditions. The integration of these techniques into physiotherapy practice represents an exciting opportunity for the therapists of the future. Here, an overview is provided of three neuromodulatory techniques (peripheral electrical stimulation, transcranial direct current stimulation and repetitive transcranial magnetic stimulation) and the potential implications of these techniques for therapists discussed. Understanding these techniques and their therapeutic implications will ensure that therapists are well positioned to contribute to their clinical translation and adoption into clinical practice in an appropriate time frame. A therapeutic landscape defined by neuromodulatory techniques and improved clinical outcomes across a range of conditions is no longer far-fetched.