The physiological basis of concussion: 50 years later

Insights from Rodent Models for Improving Bench-to-Bedside Translation in Traumatic Brain Injury

Road accidents, domestic falls, and persons associated with sports and military services exhibited the concussion or contusion type of traumatic brain injury (TBI) that resulted in chronic traumatic encephalopathy. In some instances, these complex neurological aberrations pose severe brain damage and devastating long-term neurological sequelae. Several preclinical (rat and mouse) TBI models simulate the clinical TBI endophenotypes. Moreover, many investigational neuroprotective candidates showed promising effects in these models; however, the therapeutic success of these screening candidates has been discouraging at various stages of clinical trials. Thus, a correct selection of screening model that recapitulates the clinical neurobiology and endophenotypes of concussion or contusion is essential. Herein, we summarize the advantages and caveats of different preclinical models adopted for TBI research. We suggest that an accurate selection of experimental TBI models may improve the translational viability of the investigational entity.

What Happens in TBI? A Wide Talk on Animal Models and Future Perspective

Traumatic brain injury (TBI) is a global healthcare concern and a leading cause of death. The most common causes of TBI include road accidents, sports injuries, violence in warzones, and falls. TBI induces neuronal cell death independent of age, gender, and genetic background. TBI survivor patients often experience long-term behavioral changes like cognitive and emotional changes. TBI affects social activity, reducing the quality and duration of life. Over the last 40 years, several rodent models have been developed to mimic different clinical outcomes of human TBI for a better understanding of pathophysiology and to check the efficacy of drugs used for TBI. However, promising neuroprotective approaches that have been used preclinically have been found to be less beneficial in clinical trials. So, there is an urgent need to find a suitable animal model for establishing a new therapeutic intervention useful for TBI. In this review, we have demonstrated the etiology of TBI and post- TBI social life alteration, and also discussed various preclinical TBI models of rodents, zebrafish, and drosophila.

Clinical relevance of midline fluid percussion brain injury: Acute deficits, chronic morbidities and the utility of biomarkers

BACKGROUND

After 30 years of characterisation and implementation, fluid percussion injury (FPI) is firmly recognised as one of the best-characterised reproducible and clinically relevant models of TBI, encompassing concussion through diffuse axonal injury (DAI). Depending on the specific injury parameters (e.g. injury site, mechanical force), FPI can model diffuse TBI with or without a focal component and may be designated as mild-to-severe according to the chosen mechanical forces and resulting acute neurological responses. Among FPI models, midline FPI may best represent clinical diffuse TBI, because of the acute behavioural deficits, the transition to late-onset behavioural morbidities and the absence of gross histopathology.

REVIEW

The goal here was to review acute and chronic physiological and behavioural deficits and morbidities associated with diffuse TBI induced by midline FPI. In the absence of neurodegenerative sequelae associated with focal injury, there is a need for biomarkers in the diagnostic, prognostic, predictive and therapeutic approaches to evaluate outcomes from TBI.

CONCLUSIONS

The current literature suggests that midline FPI offers a clinically-relevant, validated model of diffuse TBI to investigators wishing to evaluate novel therapeutic strategies in the treatment of TBI and the utility of biomarkers in the delivery of healthcare to patients with brain injury.

Animal models of traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of mortality and morbidity both in civilian life and on the battlefield worldwide. Survivors of TBI frequently experience long-term disabling changes in cognition, sensorimotor function and personality. Over the past three decades, animal models have been developed to replicate the various aspects of human TBI, to better understand the underlying pathophysiology and to explore potential treatments. Nevertheless, promising neuroprotective drugs that were identified as being effective in animal TBI models have all failed in Phase II or Phase III clinical trials. This failure in clinical translation of preclinical studies highlights a compelling need to revisit the current status of animal models of TBI and therapeutic strategies.

The neurophysiology of concussion

Cerebral concussion is both the most common and most puzzling type of traumatic brain injury (TBI). It is normally produced by acceleration (or deceleration) of the head and is characterized by a sudden brief impairment of consciousness, paralysis of reflex activity and loss of memory. It has long been acknowledged that one of the most worthwhile techniques for studying the acute pathophysiology of concussion is by the recording of neurophysiological activity such as the electroencephalogram (EEG) and sensory evoked potentials (EPs) from experimental animals. In the first parts of this review, the majority of such studies conducted during the past half century are critically reviewed. When potential methodological flaws and limitations such as anesthetic protocols, infliction of multiple blows and delay in onset of recordings were taken into account, two general principles could be adduced. First, the immediate post-concussive EEG was excitatory or epileptiform in nature. Second, the cortical EP waveform was totally lost during this period. In the second parts of this review, five theories of concussion which have been prominent during the past century are summarized and supportive evidence assessed. These are the vascular, reticular, centripetal, pontine cholinergic and convulsive hypotheses. It is concluded that only the convulsive theory is readily compatible with the neurophysiological data and can provide a totally viable explanation for concussion. The chief tenet of the convulsive theory is that since the symptoms of concussion bear a strong resemblance to those of a generalized epileptic seizure, then it is a reasonable assumption that similar pathobiological processes underlie them both. Further, it is demonstrated that EPs and EEGs recorded acutely following concussive trauma are indeed the same or similar to those obtained following the induction of a state of generalized seizure activity (GSA). According to the present incarnation of the convulsive theory, the energy imparted to the brain by the sudden mechanical loading of the head may generate turbulent rotatory and other movements of the cerebral hemispheres and so increase the chances of a tissue-deforming collision or impact between the cortex and the boney walls of the skull. In this conception, loss of consciousness is not orchestrated by disruption or interference with the function of the brainstem reticular activating system. Rather, it is due to functional deafferentation of the cortex as a consequence of diffuse mechanically-induced depolarization and synchronized discharge of cortical neurons. A convulsive theory can also explain traumatic amnesia, autonomic disturbances and the miscellaneous collection of symptoms of the post-concussion syndrome more adequately than any of its rivals. In addition, the symptoms of minor concussion (a.k.a. being stunned, dinged, or dazed) are often strikingly similar to minor epilepsy such as petit mal. The relevance of the convulsive theory to a number of associated problems is also discussed. These include the relationship between concussion and more serious types of closed head injury, the utility of animal models of severe brain trauma, the etiology of the cognitive deficits which may linger long after a concussive injury, the use of concussive (captive bolt) techniques to stun farm animals prior to slaughter and the question of why some animals (such as the woodpecker) can tolerate massive accelerative forces without being knocked out.

Neurophysiological and behavioral concomitants of mild brain injury in collegiate athletes

OBJECTIVES

There is still limited understanding regarding the effect of mild brain injury (MBI) on normal functioning of the human brain with respect to motor control and coordination. To our knowledge, no research exists on how both the accuracy of force production and underlying neurophysiological concomitants are interactively affected by MBI. The aim of this study is to provide empirical evidence that there are at least transient functional changes in the brain associated with motor control and coordination in collegiate athletes suffering from MBI as reflected in alterations of force trajectory patterns and electroencephalogram (EEG) potentials both in time and frequency domains.

METHODS

Comparisons of the performance and concomitant EEG waveforms both in time and frequency domains of 6 collegiate athletes with MBI and 6 normal subjects in a series of isometric force production tasks were made. The traditional averaging techniques to obtain the slow-wave movement-related potentials (MRP) and Morlet wavelet transform to obtain EEG time-frequency (TF) profiles associated with task performance were used. Subjects performed isometric force production tasks when the level of nominal force was experimentally manipulated. EEG recordings from the frontal-central areas were analyzed with respect to the accuracy of force production during the ramp phase.

RESULTS

Behaviorally, the accuracy of force trajectory performance was considerably impaired in MBI subjects even when the amount of task force was only increased from 25 to 50% maximum voluntary contraction (MVC) within a given subject. Electro-cortically, impaired performance in MBI subjects was associated with alterations in EEG waveforms, amplitude of MRP and TF profiles of EEG.

CONCLUSIONS

Both behavioral and electro-cortical data of control subjects generally were comparable with those from subjects with MBI when small amounts of force were regulated. However, differences become apparent as the amount of task force production was increased. Overall our findings identify the presence of transient functional changes in the brain associated with motor control and coordination in subjects suffering from MBI.

The impact of face shield use on concussions in ice hockey: a multivariate analysis

OBJECTIVE

To identify specific risk factors for concussion severity among ice hockey players wearing full face shields compared with half face shields (visors).

METHODS

A prospective cohort study was conducted during one varsity hockey season (1997-1998) with 642 male ice hockey players (median age 22 years) from 22 teams participating in the Canadian Inter-University Athletics Union. Half of the teams wore full face shields, and half wore half shields (visors) for every practice and game throughout the season. Team therapists and doctors recorded on structured forms daily injury, participation, and information on face shield use for each athlete. The main outcome measure was any traumatic brain injury requiring assessment or treatment by a team therapist or doctor, categorised by time lost from subsequent participation and compared by type of face shield worn.

RESULTS

Players who wore half face shields missed significantly more practices and games per concussion (2.4 times) than players who wore full face shields (4.07 sessions (95% confidence interval (CI) 3.48 to 4.74) v 1.71 sessions (95% CI 1.32 to 2.18) respectively). Significantly more playing time was lost by players wearing half shields during practices and games, and did not depend on whether the athletes were forwards or defence, rookies or veterans, or whether the concussions were new or recurrent. In addition, players who wore half face shields and no mouthguards at the time of concussion missed significantly more playing time (5.57 sessions per concussion; 95% CI 4.40 to 6.95) than players who wore half shields and mouthguards (2.76 sessions per concussion; 95% CI 2.14 to 3.55). Players who wore full face shields and mouthguards at the time of concussion lost no playing time compared with 1.80 sessions lost per concussion (95% CI 1.38 to 2.34) for players wearing full face shields and no mouthguards.

CONCLUSIONS

The use of a full face shield compared with half face shield by intercollegiate ice hockey players significantly reduced the playing time lost because of concussion, suggesting that concussion severity may be reduced by the use of a full face shield.

Recommendations for grading of concussion in athletes

Mild sports-related concussions, in which there is no loss of consciousness, account for >75% of all sports-related brain injury. Universal agreement on concussion definition and severity grading does not exist. Grading systems represent expertise of clinicians and researchers yet scientific evidence is lacking. Most used loss of consciousness and post-traumatic amnesia as markers for grading concussion. Although in severe head injury these parameters may have been proven important for prognosis, no study has done the same for sport-related concussion. Post-concussion symptoms are often the main features to help in the diagnosis of concussion in sport. Neuropsychological testing is meant to help physicians and health professionals to have objective indices of some of the neurocognitive symptoms. It is the challenge of physicians, therapists and coaches involved in the care of athletes to know the symptoms of concussion, recognise them when they occur and apply basic neuropsychological testing to help detect this injury. It is, therefore, recommended to be familiar with one grading system and use it consistently, even though it may not be scientifically validated. Then good clinical judgement and the ability to recognise post-concussion signs and symptoms will assure that an athlete never returns to play while symptomatic.

Evidence-based review of sport-related concussion: basic science

The evidence base for sport-related concussive brain injury is reviewed in this paper. In the past, pathophysiological understanding of this common condition has been extrapolated from studies of severe brain trauma. More recent scientific study demonstrates that this approach is unsatisfactory, and the clinical features of concussion represent a predominantly functional brain injury rather than manifest by structural or neuropathological damage. Such understanding of this condition remains incomplete at this stage.

Current concepts. Concussion in sports

This is a special report of the findings of the Concussion Workshop, sponsored by the AOSSM in Chicago in December 1997. Here follows a listing of the members of the workshop: Julian Bailes, MD, American Association of Neurological Surgeons; Arthur Boland, MD, AOSSM; Charles Burke III, MD, National Hockey League; Robert Cantu, MD, American College of Sports Medicine; Letha “Etty” Griffin, MD, National Collegiate Athletic Association; David Hovda, PhD, Neuroscientist, UCLA School of Medicine; Mary Lloyd Ireland, MD, American Academy of Orthopaedic Surgeons; James Kelly, MD, American Academy of Neurology; Greg Landry, MD, American Academy of Pediatrics; Mark Lovell, PhD, Neuropsychology Specialist, Henry Ford Health Systems; James Mathews, MD, American College of Emergency Physicians; Michael McCrea, PhD, Neuropsychology Specialist, Waukesha Memorial Hospital; Douglas McKeag, MD, American Medical Society for Sports Medicine; Dennis Miller, ATC, National Athletic Trainers Association; Jeffrey Minkoff, MD, AOSSM; Stephen Papadopoulus, MD, Congress of Neurological Surgeons; Elliott Pellman, MD, National Football League; Richard Quincy, MS, PT, ATC, Sports Physical Therapy, El Pomar Sports Center; Herbert Ross, DO, American Osteopathic Academy of Sports Medicine; Bryan Smith, MD, National Collegiate Athletic Association; and Edward Wojtys, MD, Workshop Chairman, AOSSM.

The views in this report do not necessarily represent the views of the entire group comprising the Concussion Workshop Group.

A concussive-like brain injury model in mice (II): selective neuronal loss in the cortex and hippocampus

A novel concussive-like brain injury (CLBI) model characterized by transient neurobehavioral depression, short duration of brain edema, and long-lasting memory deficits has been reported in our companion paper. This was achieved by dropping a 21-g weight from a height of 25 cm onto the head of a mouse. In the present study, we examined the histopathological changes in this model. Male ddY mice were subjected to either the trauma or sham injury. Gross pathological examination of the brain 1 h posttrauma did not demonstrate subdural, subarachnoid, intraventricular, periventricular, and intraparenchymatous hemorrhage, focal lesions or contusions. Microscopic examination 24 h posttrauma with Nissl staining (cresyl violet), however, revealed a selective bilateral neuronal cell loss in the cerebral cortex and hippocampus but not in the regions of the thalamus, cerebellum, and brain stem. The characteristics of neuronal cell loss in the cortex suggested that this pathology was related in part, to the head impact dynamics, since the cell loss was noted in the central portion of the supraventricular cerebral cortex (p < 0.001), the site of the weight impact, gradually decreasing peripheral to this site, and disappearing in the areas remote from this locus. In contrast, neuronal cell loss seen in the hippocampus did not suggest that this pathology was directly associated with the impact site. Neuronal cell loss was concentrated in the pyramidal cell layer of CA2 (p < 0.01) and CA3 (p < 0.01), and a lesser degree was noted in the subfields of CA3c (p < 0.05) and the hilar region (p < 0.05) but not in the subfields of CA1 and the dentate gyrus layers. The present study characterized the histopathological change seen in the CLBI model, demonstrating the selective neuronal cell loss following weight-drop concussion in mice.

A concussive-like brain injury model in mice (I): impairment in learning and memory

The modeling of human concussive brain injury (CBI) in the laboratory has been challenging. In the present study, we developed an experimental CBI model in mice using a novel weight-drop device. Various injury levels were examined by adjusting the height of the falling weight (diameter 10 mm, length 20 cm, weight 21 g). At a height of 50 cm, the impact resulted in a mortality rate of 46.7% with a skull fracture rate of 28.6%. At a height of 25 cm, however, the impact produced a concussive-like brain injury (CLBI) to the mice without skull fracture. A series of pathophysiological and neurobehavioral responses was evaluated at this injury level. The CLBI mice lost muscle tone and righting reflex response immediately following the trauma and recovered from the latter within a short duration of 1.6 +/- 0.32 min (mean +/- SE). Brain edema formation started at 12 h, reached a maximum at 24 h and recovered 48 h. Typically edema was found in the neocortex, hippocampus, and cerebellum, but not in the brain stem. Deficits in the feeding behaviors lasted for 2 days, accompanied by lower body weight persisting for 5 days. The body weight growth rate for 24 h returned to the control levels by the third day postinjury. Learning and memory were evaluated at the end of 1-3 weeks after the trauma using a water-finding task. At 1 week, exploratory behaviors were slightly inhibited while learning and memory were profoundly impaired. Interestingly, the learning and memory deficits lasted for 2 weeks while recovering to the control levels by 3 weeks. No motor disability was found in the CLBI mice during the 3-week evaluations. These results indicate that the weight-drop impact produced graded injury to the brain, and at the injury level of 25 cm it produced a CLBI in the mice in which the characteristics of transient loss of neurobehavioral responses, short duration of brain edema, and long-lasting learning and memory deficits are similar to those of human CBI.