Assessment of motor cortex excitability and inhibition during a cognitive task in individuals with concussion

Recovery of clinical, cognitive and cortical activity measures following mild traumatic brain injury (mTBI): A longitudinal investigation

The mechanisms that underpin recovery following mild traumatic brain injury (mTBI) remain poorly understood. Identifying neurophysiological markers and their functional significance is necessary to develop diagnostic and prognostic indicators of recovery. The current study assessed 30 participants in the subacute phase of mTBI (10-31 days post-injury) and 28 demographically matched controls. Participants also completed 3 month (mTBI: N = 21, control: N = 25) and 6 month (mTBI: N = 15, control: N = 25) follow up sessions to track recovery. At each time point, a battery of clinical, cognitive, and neurophysiological assessments was completed. Neurophysiological measures included resting-state electroencephalography (EEG) and transcranial magnetic stimulation combined with EEG (TMS-EEG). Outcome measures were analysed using mixed linear models (MLM). Group differences in mood, post-concussion symptoms and resting-state EEG resolved by 3 months, and recovery was maintained at 6 months. On TMS-EEG derived neurophysiological measures of cortical reactivity, group differences ameliorated at 3 months but re-emerged at 6 months, while on measures of fatigue, group differences persisted across all time points. Persistent neurophysiological changes and greater fatigue in the absence of measurable cognitive impairment may suggest the impact of mTBI on neuronal communication may leads to increased neural effort to maintain efficient function. Neurophysiological measures to track recovery may help identify both temporally optimal windows and therapeutic targets for the development of new treatments in mTBI.

An Evaluation of the Emerging Techniques in Sports-Related Concussion

SUMMARY

Sports-related concussion is now in public awareness more than ever before. Investigations into underlying pathophysiology and methods of assessment have correspondingly increased at an exponential rate. In this review, we aim to highlight some of the evidence supporting emerging techniques in the fields of neurophysiology, neuroimaging, vestibular, oculomotor, autonomics, head sensor, and accelerometer technology in the setting of the current standard: clinical diagnosis and management. In summary, the evidence we reviewed suggests that (1) head impact sensors and accelerometers may detect possible concussions that would not otherwise receive evaluation; (2) clinical diagnosis may be aided by sideline vestibular, oculomotor, and portable EEG techniques; (3) clinical decisions on return-to-play eligibility are currently not sensitive at capturing the neurometabolic, cerebrovascular, neurophysiologic, and microstructural changes that biomarkers have consistently detected days and weeks after clinical clearance. Such biomarkers include heart rate variability, quantitative electroencephalography, as well as functional, metabolic, and microstructural neuroimaging. The current challenge is overcoming the lack of consistency and replicability of any one particular technique to reach consensus.

Corticospinal Excitability and Inhibition Are Not Different between Concussed Males and Females

It has been consistently demonstrated that females report greater numbers of concussions in sex-comparable sports and take longer to recover from concussive symptoms than males. However, it is unknown if the neurophysiological consequences of concussion may contribute to these sex differences in concussion symptoms and recovery. The purpose of this study was to examine potential sex-related differences in neurophysiology in healthy and concussed individuals. Twenty-one (nine F) concussed individuals (20.9 ± 4.1 years; CONC) and twenty-one age-, sex-, height-, weight-, and activity-matched controls (21.2 ± 4.2 years; CONT) participated in the study. The CONC group reported to the lab within 72 h, 1-week, and 2-weeks post-injury and the CONT group followed a similar measurement schedule. Using transcranial magnetic stimulation, motor evoked potential (MEP) amplitude and cortical silent period (CSP) duration were measured from the first dorsal interosseous muscle to assess corticospinal excitability and inhibition, respectively. There were no significant differences across time (p ≥ 0.13) or between the CONC and CONT group in MEP amplitude (p = 0.72) or CSP duration (p = 0.54). Overall, males (119.08 ± 29.91 ms) had significantly longer CSP durations compared with females (101.24 ± 33.43 ms), indicating greater corticospinal inhibition in males, regardless of injury status (p = 0.04). An important and novel finding of this study was the lack of differences in these neurophysiological measures between males and females following concussion. To our knowledge, this is the first study to document greater corticospinal inhibition in males compared with females.

Longer Neurophysiological vs. Clinical Recovery Following Sport Concussion

Objectives: The objective of this study was to assess if injury-related alterations in the Sport Concussion Assessment Tool-5 (SCAT5) are matched by changes in transcranial magnetic stimulation-derived intracortical inhibition. We hypothesised that neurophysiological measures would take longer to return to normal than recovery assessed by the SCAT5 following sport related concussion (SRC).

Methods: Thirteen male contact sport athletes (20.5 ± 4.5 years), who reported a concussion were recruited from local Rugby and American football clubs. Participants were tested at 4 timepoints throughout the concussion recovery period: within 24 h of concussion (day 0), and at 7, 9, and 11 days after concussion. All participants completed the SCAT5 and underwent TMS to assess cortical silent period duration (CSp), a measure of intracortical inhibition.

Results: After concussion CSp significantly declined from day 0 (122 ± 28 ms) to day 11 (106 ± 15 ms) [F_{(3, 33)} = 7.80, p < 0.001]. SCAT5 measures of symptom number and severity were significantly decreased [symptom number: χ(3)2 = 30.44, p < 0.01; symptom severity: χ(3)2 = 25.75, p < 0.001] between the day 0 timepoint and each of the other timepoints. SCAT5 balance errors (mBESS) decreased significantly [F_{(3, 33)} = 19.55, p < 0.001] between the day 0 timepoint and each of the other timepoints. CSp and SCAT5 recovery patterns were different. SCAT5 domains recovered faster showing no further significant changes after day 7, whilst CSp was still decreasing between days 7 and 9. Due to the small sample size we also used a Bayesian linear model to investigate the recovery of CSp and mBESS. The posterior distribution of our Bayesian model provided evidence that CSp decreased at day 7 and it continued to decrease at day 9, unlike mBESS which decreased at day 7 and then reached a plateau.

Conclusion: There are clinically important discrepancies between clinical and neurophysiological measures of concussion recovery. This finding has important implications for return to play (RTP) protocols and the prevention of complications after sport concussion.

The Neurophysiological Responses of Concussive Impacts: A Systematic Review and Meta-Analysis of Transcranial Magnetic Stimulation Studies

Aim: This systematic review and meta-analysis investigated neurophysiological responses using transcranial magnetic stimulation (TMS) following a concussion or sub-concussion.

Methods: A systematic searching of relevant databases for peer-reviewed literature quantifying motor evoked potentials from TMS between 1999 and 2019 was performed. A meta-analysis quantified pooled data for measures including motor threshold, motor latency, and motor evoked potential amplitude and for inhibitory measures such as cortical silent period duration, short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI) ratios.

Results: Fifteen articles met the inclusion criteria. The studies were arbitrarily classified into the groups, based on time post-concussion, “acute” (subjects 0–3 months post-injury, n = 8) and “post-acute” (3 months−2 years post-concussion, n = 7). A TMS quality of study checklist rated studies from moderate to high in methodological quality; however, the risk of bias analysis found that the included studies were categorised as high risk of bias, particularly for a lack of allocation concealment and blinding of participants in the methodologies. A meta-analysis showed no differences in excitability measures, apart from a decreased motor threshold that was observed in the concussed group (SMD −0.28, 95% CI −0.51 to −0.04; P = 0.02) for the post-acute time frame. Conversely, all inhibitory measures showed differences between groups. Cortical silent period duration was found to be significantly increased in the acute (SMD 1.19, 95% CI 0.58–1.81; P < 0.001) and post-acute (SMD 0.55, 95% CI 0.12–0.98; P = 0.01) time frames. The SICI (SMD −1.15, 95% CI −1.95 to −0.34; P = 0.005) and LICI (SMD −1.95, 95% CI −3.04 to −0.85; P = 0.005) ratios were reduced, inferring increased inhibition, for the post-acute time frame.

Conclusion: This systematic review and meta-analysis demonstrates that inhibitory pathways are affected in the acute period post-concussion. However, persistent alterations in cortical excitability remain, with increased intracortical inhibition. While TMS should be considered as a reliable technique to measure the functional integrity of the central nervous system, the high risk of bias and heterogeneity in data suggest that future studies should aim to incorporate standardised methodological techniques, particularly with threshold determination and stimulus intervals for paired-pulse measures.

Excitability, Inhibition, and Neurotransmitter Levels in the Motor Cortex of Symptomatic and Asymptomatic Individuals Following Mild Traumatic Brain Injury

Purpose: The purpose of this study was to determine the level of excitability and inhibition, as well as the concentrations of excitatory and inhibitory neurotransmitters, in the motor cortex of individuals with acute and chronic symptoms from mTBI.

Methods: Fifty-three individuals were assigned to one of four groups: (i) without history of mTBI (Control), (ii) within 72-h of diagnosis of mTBI (Acute), (iii) with history of mTBI and no remaining symptoms (Chronic Asymptomatic), and (iv) with chronic symptoms from mTBI, lasting at least 3 months post-injury (Chronic Symptomatic). Measures of corticospinal excitability and inhibition were obtained using transcranial magnetic stimulation (TMS). On the same day, measures of glutamate and GABA concentrations were obtained from the primary motor cortex (M1) using proton magnetic resonance spectroscopy.

Results: MEP amplitude and area were both significantly lower in the Chronic Symptomatic group compared to the Control and Chronic Asymptomatic groups (p ≤ 0.05). Intracortical inhibition was not significantly different among groups (p = 0.14). The concentration of glutamate in M1 was similar between groups (p = 0.93) while there was a trend for a lower concentration of GABA in the Chronic Symptomatic group compared to the Acute group (p = 0.06).

Conclusions: Individuals with chronic mTBI symptoms appear to have lower corticospinal excitability compared with acutely-injured individuals and asymptomatic controls, but the absence of differences in intracortical inhibition, and concentrations of excitatory and inhibitory neurotransmitters in M1 suggests that neurotransmitter changes in the human brain post-mTBI do not follow the pattern typically seen in the animal literature.