Cortical Somatosensory Excitability Is Modulated in Response to Several Days of Muscle Soreness

Facilitation of Early and Middle Latency SEP after tDCS of M1: No Evidence of Primary Somatosensory Homeostatic Plasticity

Homeostatic plasticity is a mechanism that stabilizes cortical excitability within a physiological range. Most homeostatic plasticity protocols have primed and tested the homeostatic response of the primary motor cortex (M1). This study investigated if a homeostatic response could be recorded from the primary sensory cortex (S1) after inducing homeostatic plasticity in M1. In 31 healthy participants, homeostatic plasticity was induced over M1 with a priming and testing block of transcranial direct current stimulation (tDCS) in two different sessions (anodal and cathodal). S1 excitability was assessed by early (N20, P25) and middle-latency (N33-P45) somatosensory evoked potentials (SEP) extracted from 4 electrodes (CP5, CP3, P5, P3). Baseline and post-measures (post-priming, 0-min, 10-min, and 20-min after homeostatic induction) were taken. Anodal M1 homeostatic plasticity induction significantly facilitated the N20-P25, P45 peak, and N33-P45 early SEP components up to 20-min post-induction, without any indication of a homeostatic response (i.e., reduced SEP). Cathodal homeostatic induction did not induce any significant effect on early or middle latency SEPs. M1 homeostatic plasticity induction by anodal stimulation protocol to the primary motor cortex did not induce a homeostatic response in SEPs.

Modulation of central pain mechanisms using high-definition transcranial direct current stimulation: A double-blind, sham-controlled study

BACKGROUND

The use of high-definition transcranial direct current stimulation (HD-tDCS) has shown analgesic effects in some chronic pain patients, but limited anti-nociceptive effects in healthy asymptomatic subjects.

METHODS

This double-blinded sham-controlled study assessed the effects of HD-tDCS applied on three consecutive days on central pain mechanisms in healthy participants with (N = 40) and without (N = 40) prolonged experimental pain induced by intramuscular injection of nerve growth factor into the right hand on Day 1. Participants were randomly assigned to Sham-tDCS (N = 20 with pain, N = 20 without) or Active-tDCS (N = 20 with pain, N = 20 without) targeting simultaneously the primary motor cortex and dorsolateral prefrontal cortex for 20 min with 2 mA stimulation intensity. Central pain mechanisms were assessed by cuff algometry on the legs measuring pressure pain sensitivity, temporal summation of pain (TSP) and conditioned pain modulation (CPM), at baseline and after HD-tDCS on Day 2 and Day 3. Based on subject's assessment of received HD-tDCS (sham or active), they were effectively blinded.

RESULTS

Compared with Sham-tDCS, Active-tDCS did not significantly reduce the average NGF-induced pain intensity. Tonic pain-induced temporal summation at Day 2 and Day 3 was significantly lower in the NGF-pain group under Active-tDCS compared to the pain group with Sham-tDCS (p ≤ 0.05). No significant differences were found in the cuff pressure pain detection/tolerance thresholds or CPM effect across the 3 days of HD-tDCS in any of the four groups.

CONCLUSION

HD-tDCS reduced the facilitation of TSP caused by tonic pain suggesting that efficacy of HD-tDCS might depend on the presence of sensitized central pain mechanisms.

The effect of theta burst stimulation over the primary motor cortex on experimental hamstring pain: A randomised, controlled study

Theta burst stimulation (TBS) over the primary motor cortex (M1) is an emerging technique that may have utility in the treatment of musculoskeletal pain. However, previous work exploring the analgesic effects of noninvasive brain stimulation has been limited largely to the arm or hand, despite 80% of acute musculoskeletal injuries occurring in the lower limb. This is a pertinent point, given the functional and neurophysiological differences between upper and lower limb musculature, as well as evidence suggesting that reorganization of corticomotor pathways is region-specific. This study investigated the effect of excitatory TBS on pain, function, and corticomotor organization during experimentally induced lower limb pain. Twenty-eight healthy participants attended 2 experimental sessions. On Day 0, participants completed 10 sets of 10 maximal eccentric contractions of the right hamstring muscles to induce delayed onset muscle soreness. Four consecutive blocks of either active or sham TBS were delivered on Day 2. Measures of mechanical sensitivity, pain (muscle soreness, pain intensity, pain area) function (single-leg hop distance, maximum voluntary isometric contraction, lower extremity functional scale), and corticomotor organization were recorded before and after TBS on Day 2. Pain and function were also assessed daily from Days 2 to 10. Active TBS reduced mechanical sensitivity compared to sham stimulation (P = .01). Corticomotor organization did not differ between groups, suggesting that improvements in mechanical sensitivity were not mediated by changes in M1. Subjective reports of pain intensity and function did not change following active TBS, contrasting previous reports in studies of the upper limb. PERSPECTIVE: M1 TBS reduces mechanical sensitivity associated with experimentally induced hamstring pain. Though further work is needed, these findings may hold important implications for those seeking to expedite recovery or reduce muscle sensitivity following hamstring injury.

The Effect of Acute and Sustained Pain on Corticomotor Excitability: A Systematic Review and Meta-Analysis of Group and Individual Level Data

Pain alters motor function. This is supported by studies showing reduced corticomotor excitability in response to experimental pain lasting <90 minutes. Whether similar reductions in corticomotor excitability are present with pain of longer durations or whether alterations in corticomotor excitability are associated with pain severity is unknown. Here we evaluated the evidence for altered corticomotor excitability in response to experimental pain of differing durations in healthy individuals. Databases were systematically searched for eligible studies. Measures of corticomotor excitability and pain were extracted. Meta-analyses were performed to examine: (1) group-level effect of pain on corticomotor excitability, and (2) individual-level associations between corticomotor excitability and pain severity. 49 studies were included. Corticomotor excitability was reduced when pain lasted milliseconds-seconds (hedges g's = -1.26 to -1.55) and minutes-hours (g's = -0.55 to -0.9). When pain lasted minutes-hours, a greater reduction in corticomotor excitability was associated with lower pain severity (g = -0.24). For pain lasting days-weeks, there were no group level effects (g = -0.18 to 0.27). However, a greater reduction in corticomotor excitability was associated with higher pain severity (g = 0.229). In otherwise healthy individuals, suppression of corticomotor excitability may be a beneficial short-term strategy with long-term consequences. PERSPECTIVE: This systematic review synthesised the evidence for altered corticomotor excitability in response to experimentally induced pain. Reduced corticomotor excitability was associated with lower acute pain severity but higher sustained pain severity, suggesting suppression of corticomotor excitability may be a beneficial short-term adaptation with long-term consequences.

Modulation Of Experimental Prolonged Pain and Sensitization Using High-Definition Transcranial Direct Current Stimulation: A Double-Blind, Sham-Controlled Study

High definition transcranial direct current stimulation (HD-tDCS) targeting brain areas involved in pain processing has shown analgesic effects in some chronic pain conditions, but less modulatory effect on mechanical and thermal pain thresholds in asymptomatic subjects. This double-blinded study assessed the HD-tDCS effects on experimental pain and hyperalgesia maintained for several days in healthy participants. Hyperalgesia and pain were assessed during three consecutive days following provocation of experimental pain (nerve growth factor injected into the right-hand muscle) and daily HD-tDCS sessions (20-minutes). Forty subjects were randomly assigned to Active-tDCS targeting primary motor cortex and dorsolateral prefrontal cortex simultaneously or Sham-tDCS. Tactile and pressure pain sensitivity were assessed before and after each HD-tDCS session, as well as the experimentally-induced pain intensity scored on a numerical rating scale (NRS). Subjects were effectively blinded to the type of HD-tDCS protocol. The Active-tDCS did not significantly reduce the NGF-induced NRS pain score (3.5±2.4) compared to Sham-tDCS (3.9±2.0, P > .05) on day 3 and both groups showed similarly NGF-decreased pressure pain threshold in the right hand (P < .001). Comparing Active-tDCS with Sham-tDCS, the manifestation of pressure hyperalgesia was delayed on day 1, and an immediate (pre-HD-tDCS to post-HD-tDCS) reduction in pressure hyperalgesia was found across all days (P < .05). PERSPECTIVE: The non-significant differences between Active-tDCS and Sham-tDCS on experimental prolonged pain and hyperalgesia suggest that HD-tDCS has no effect on moderate persistent experimental pain. The intervention may still have a positive effect in more severe pain conditions, with increased intensity, more widespread distribution, or increased duration and/or involving stronger affective components.

Pain's Adverse Impact on Training-Induced Performance and Neuroplasticity: A Systematic Review

Motor training is a widely used therapy in many pain conditions. The brain’s capacity to undergo functional and structural changes i.e., neuroplasticity is fundamental to training-induced motor improvement and can be assessed by transcranial magnetic stimulation (TMS). The aim was to investigate the impact of pain on training-induced motor performance and neuroplasticity assessed by TMS. The review was carried out in accordance with the PRISMA-guidelines and a Prospero protocol (CRD42020168487). An electronic search in PubMed, Web of Science and Cochrane until December 13, 2019, identified studies focused on training-induced neuroplasticity in the presence of experimentally-induced pain, 'acute pain' or in a chronic pain condition, 'chronic pain'. Included studies were assessed by two authors for methodological quality using the TMS Quality checklist, and for risk of bias using the Newcastle–Ottawa Scale. The literature search identified 231 studies. After removal of 71 duplicates, 160 abstracts were screened, and 24 articles were reviewed in full text. Of these, 17 studies on acute pain (n = 7) or chronic pain (n = 10), including a total of 258 patients with different pain conditions and 248 healthy participants met the inclusion criteria. The most common types of motor training were different finger tasks (n = 6). Motor training was associated with motor cortex functional neuroplasticity and six of seven acute pain studies and five of ten chronic pain studies showed that, compared to controls, pain can impede such trainings-induced neuroplasticity. These findings may have implications for motor learning and performance and with putative impact on rehabilitative procedures such as physiotherapy.

Ischemic preconditioning of the muscle reduces the metaboreflex response of the knee extensors

Purpose

This study investigated the effect of ischemic preconditioning (IP) on metaboreflex activation following dynamic leg extension exercise in a group of healthy participants.

Method

Seventeen healthy participants were recruited. IP and SHAM treatments (3 × 5 min cuff occlusion at 220 mmHg or 20 mmHg, respectively) were administered in a randomized order to the upper part of exercising leg’s thigh only. Muscle pain intensity (MP) and pain pressure threshold (PPT) were monitored while administrating IP and SHAM treatments. After 3 min of leg extension exercise at 70% of the maximal workload, a post-exercise muscle ischemia (PEMI) was performed to monitor the discharge group III/IV muscle afferents via metaboreflex activation. Hemodynamics were continuously recorded. MP was monitored during exercise and PEMI.

Results

IP significantly reduced mean arterial pressure compared to SHAM during metaboreflex activation (mean ± SD, 109.52 ± 7.25 vs. 102.36 ± 7.89 mmHg) which was probably the consequence of a reduced end diastolic volume (mean ± SD, 113.09 ± 14.25 vs. 102.42 ± 9.38 ml). MP was significantly higher during the IP compared to SHAM treatment, while no significant differences in PPT were found. MP did not change during exercise, but it was significantly lower during the PEMI following IP (5.10 ± 1.29 vs. 4.00 ± 1.54).

Conclusion

Our study demonstrated that IP reduces hemodynamic response during metaboreflex activation, while no effect on MP and PPT were found. The reduction in hemodynamic response was likely the consequence of a blunted venous return.

Slowing in Peak-Alpha Frequency Recorded After Experimentally-Induced Muscle Pain is not Significantly Different Between High and Low Pain-Sensitive Subjects

Peak alpha frequency (PAF) reduces during cutaneous pain, but no studies have investigated PAF during movement-related muscle pain. Whether high-pain sensitive (HPS) individuals exhibit a more pronounced PAF response to pain than low-pain sensitive (LPS) individuals is unclear. As a pain model, twenty-four participants received nerve growth factor injections into a wrist extensor muscle at Day 0, Day 2, and Day 4. At Day 4, a subgroup of twelve participants also undertook eccentric wrist exercise to induce additional pain. Pain numerical rating scale (NRS) scores and electroencephalography were recorded at Day 0 (before injection), Day 4, and Day 6 for 3 minutes (eyes closed) with wrist at rest (Resting-state) and extension (Contraction-state). The average pain NRS scores in contraction-state across Days were used to divide participants into HPS (NRS-scores≥2) and LPS groups. PAF was calculated by frequency decomposition of electroencephalographic recordings. Compared with Day 0, contraction NRS-scores only increased in HPS-group at Day 4 and Day 6 (P < .001). PAF in Contraction-state decreased in both groups at Day 6 compared with Day 0 (P = .011). Across days, HPS-group showed faster PAF than LPS-group during Resting-state and Contraction-state (P < .04). Average pain NRS-scores across days during Contraction-states correlated with PAF at Day 0 (P = .012). Pain NRS-scores were associated with PAF during Contraction-state at Day 4 and Day 6 (P < .05). PERSPECTIVE: PAF was slowed during long-lasting movement-related pain in both groups, suggesting a widespread change in cortical excitability independent of the pain sensitivity. Moreover, HPS individuals showed faster PAF than LPS individuals during muscle pain, which may reflect a different cognitive, emotional, or attentional response to muscle pain among individuals.

The Relationship Between Corticomotor Reorganization and Acute Pain Severity: A Randomized, Controlled Study Using Rapid Transcranial Magnetic Stimulation Mapping

OBJECTIVE

Although acute pain has been shown to reduce corticomotor excitability, it remains unknown whether this response resolves over time or is related to symptom severity. Furthermore, acute pain research has relied upon data acquired from the cranial "hotspot," which do not provide valuable information regarding reorganization, such as changes to the distribution of a painful muscle's representation within M1. Using a novel, rapid transcranial magnetic stimulation (TMS) mapping method, this study aimed to 1) explore the temporal profile and variability of corticomotor reorganization in response to acute pain and 2) determine whether individual patterns of corticomotor reorganization are associated with differences in pain, sensitivity, and somatosensory organization.

METHODS

Corticomotor (TMS maps), pain processing (pain intensity, pressure pain thresholds), and somatosensory (two-point discrimination, two-point estimation) outcomes were taken at baseline, immediately after injection (hypertonic [n = 20] or isotonic saline [n = 20]), and at pain resolution. Follow-up measures were recorded every 15 minutes until 90 minutes after injection.

RESULTS

Corticomotor reorganization persisted at least 90 minutes after pain resolution. Corticomotor depression was associated with lower pain intensity than was corticomotor facilitation (r = 0.47 [P = 0.04]). These effects were not related to somatosensory reorganization or peripheral sensitization mechanisms.

CONCLUSIONS

Individual patterns of corticomotor reorganization during acute pain appear to be related to symptom severity, with early corticomotor depression possibly reflecting a protective response. These findings hold important implications for the management and potential prevention of pain chronicity. However, further research is required to determine whether these adaptations relate to long-term outcomes in clinical populations.

Effect of anodal high-definition transcranial direct current stimulation on the pain sensitivity in a healthy population: a double-blind, sham-controlled study

ABSTRACT

High-definition transcranial direct current stimulation (HD-tDCS) of brain areas related to pain processing may provide analgesic effects evident in the sensory detection and pain thresholds. The somatosensory sensitivity was assessed after HD-tDCS targeting the primary motor cortex (M1) and/or the dorsolateral prefrontal cortex (DLPFC). Eighty-one (40 females) subjects were randomly assigned to 1 of 4 anodal HD-tDCS protocols (20 minutes) applied on 3 consecutive days: Sham-tDCS, DLPFC-tDCS, M1-tDCS, and DLPFC&M1-tDCS (simultaneous transcranial direct current stimulation [tDCS] of DLPFC and M1). Subjects and experimenter were blinded to the tDCS protocols. The somatosensory sensitivity were assessed each day, before and after each tDCS by detection and pain thresholds to thermal and mechanical skin stimulation, vibration detection thresholds, and pressure pain thresholds. Subjects were effectively blinded to the protocol, with no significant difference in rates of whether they received real or placebo tDCS between the 4 groups. Compared with the Sham-tDCS, none of the active HD-tDCS protocols caused significant changes in detection or pain thresholds. Independent of tDCS protocols, pain and detection thresholds except vibration detection were increased immediately after the first tDCS protocol compared with baseline (P < 0.05). Overall, the active stimulation protocols were not able to induce significant modulation of the somatosensory thresholds in this healthy population compared with sham-tDCS. Unrelated to the HD-tDCS protocol, a decreased sensitivity was found after the first intervention, indicating a placebo effect or possible habituation to the quantitative sensory testing assessments. These findings add to the increasing literature of null findings in the modulatory effects of HD-tDCS on the healthy somatosensory system.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

Healthy Pain-Free Individuals with a History of Distal Radius Fracture Demonstrate an Expanded Distribution of Experimental Referred Pain Toward the Wrist

OBJECTIVE

Nociception caused by injuries may sensitize central mechanisms causing expanded pain areas. After recovery, the status of such pain distribution and sensitivity mechanisms is unknown. The present study investigated whether individuals who have fully recovered from a distal radius fracture demonstrate increased pain sensitivity and expanded distribution of pressure-induced pain.

DESIGN

Cross-sectional single-blinded study.

SETTING

Clinical setting.

SUBJECTS

Twenty-three pain-free individuals with a history of painful distal radius fracture and 22 nonfractured, age/gender-matched controls participated in two experimental sessions (day 0, day 1) 24 hours apart.

METHODS

Pressure pain thresholds (PPTs) were recorded bilaterally at the extensor carpi radialis longus (ECRL), infraspinatus, and gastrocnemius muscles. Spatial distribution of pain was assessed following 60-second painful pressure stimulation at the ECRL (bilateral) and the infraspinatus muscles on the fractured or dominant side. Participants drew pain areas on a body map. After day 0 assessments, prolonged pain was induced by eccentric exercise of wrist extensors on the fractured/dominant side.

RESULTS

Compared with controls, pressure-induced ECRL pain in the fracture group referred more frequently toward the distal forearm (P < 0.005) on day 0. Both groups showed larger pain areas on day 1 compared with day 0 (P < 0.005), although the fracture group showed a larger relative change between days (P < 0.005). The fracture group showed larger pain areas on the fracture side compared with the contralateral side on both days (P < 0.005).

CONCLUSIONS

Prolonged pain and recovered prior painful injuries like fractures may sensitize pain mechanisms manifested as expanded pain distribution. Pressure-induced referred pain can be a simple pain biomarker for clinical use.

Neurophysiological responses and adaptation following repeated bouts of maximal lengthening contractions in young and older adults

A bout of maximal lengthening contractions is known to produce muscle damage, but confers protection against subsequent damaging bouts, with both tending to be lower in older adults. Neural factors contribute to this adaptation, but the role of the corticospinal pathway remains unclear. Twelve young (27 ± 5 yr) and 11 older adults (66 ± 4 yr) performed two bouts of 60 maximal lengthening dorsiflexions 2 weeks apart. Neuromuscular responses were measured preexercise, immediately postexercise, and at 24 and 72 h following both bouts. The initial bout resulted in prolonged reductions in maximal voluntary torque (MVC; immediately postexercise onward, P < 0.001) and increased creatine kinase (from 24 h onward, P = 0.001), with both responses being attenuated following the second bout ( P < 0.015), demonstrating adaptation. Smaller reductions in MVC following both bouts occurred in older adults ( P = 0.005). Intracortical facilitation showed no changes ( P ≥ 0.245). Motor-evoked potentials increased 24 and 72 h postexercise in young ( P ≤ 0.038). Torque variability ( P ≤ 0.041) and H-reflex size ( P = 0.024) increased, while short-interval intracortical inhibition (SICI; P = 0.019) and the silent period duration (SP) decreased ( P = 0.001) in both groups immediately postexercise. The SP decrease was smaller following the second bout ( P = 0.021), and there was an association between the change in SICI and reduction in MVC 24 h postexercise in young adults ( R = −0.47, P = 0.036). Changes in neurophysiological responses were mostly limited to immediately postexercise, suggesting a modest role in adaptation. In young adults, neural inhibitory changes are linked to the extent of MVC reduction, possibly mediated by the muscle damage–related afferent feedback. Older adults incurred less muscle damage, which has implications for exercise prescription.

NEW & NOTEWORTHY This is the first study to have collectively assessed the role of corticospinal, spinal, and intracortical activity in muscle damage attenuation following repeated bouts of exercise in young and older adults. Lower levels of muscle damage in older adults are not related to their neurophysiological responses. Neural inhibition transiently changed, which might be related to the extent of muscle damage; however, the role of processes along the corticospinal pathway in the adaptive response is limited.