Concussion Supplement

The Pathophysiology of ConcussionStefano Signoretti, MD, PhD, Giuseppe Lazzarino, PhD, Barbara Tavazzi, PhD,

Roberto Vagnozzi, MD, PhD

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359-S368

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Clinical Bio-ome, Rome,

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Abstract: Concussion is defined as a biomechanically induced brain injury charby the absence of gross anatomic lesions. Early and late clinical symptoms, iimpairments of memory and attention, headache, and alteration of mental statusresult of neuronal dysfunction mostly caused by functional rather than structurmalities. The mechanical insult initiates a complex cascade of metabolic events leperturbation of delicate neuronal homeostatic balances. Starting from neurotoxicgetic metabolism disturbance caused by the initial mitochondrial dysfunction seethe main biochemical explanation for most postconcussive signs and symptoms.more, concussed cells enter a peculiar state of vulnerability, and if a second concsustained while they are in this state, they may be irreversibly damaged by the occuswelling. This condition of concussion-induced brain vulnerability is the basic paiology of the second impact syndrome. N-acetylaspartate, a brain-specific corepresentative of neuronal metabolic wellness, is proving a valid surrogate markpost-traumatic biochemical damage, and its utility in monitoring the recoveraforementioned “functional” disturbance as a concussion marker is emerging, beceasily detectable through proton magnetic resonance spectroscopy.

PM R 2011;3:S

INTRODUCTION

Concussion is the most common form of traumatic brain injury (TBI) worldwideEuropean countries, approximately 235 people per 100,000 are admitted annuahospital after TBI, 80% of which are classified as belonging in the mild TBI (mTBI)[3,4]. This phenomenon mirrors U.S. figures, in which approximately 1.5-8 millioexperience a TBI each year and, among those requiring hospitalization, a proportionfrom 75%-90% are classified as “mildly” injured or “concussed” [1]. Although the iof mTBI is relatively high, death from this type of trauma appears to be very low (100,000/year), and only 0.2% of all patients with mTBI who visit emergency dep(EDs) will die as a direct result of this injury [4].

Supported by the absence of structural lesions on traditional neuroimaging, a gebroadly accepted view is that mTBI is indeed a very frequent entity but is not a verinjury, leading only to transient disturbances, and that no intervention other thantion typically is required [5-10]. However, according to a recent report revealingdiagnosis of an intracranial hematoma in such patients was made with a median dehours [11], the quality of the “observation” that mildly injured patients receive whhospital is of utmost concern. In the United States, it has been found that neuobservations were documented in only 50% of patients admitted with a mild he[11], and in Europe, patients with mTBI historically have been observed on nonwards by nurses and doctors not experienced in neurological observation. Thewhether to perform imaging tests, observe, or discharge a patient with mTBI is omany challenges of the concussive injury, whose early and late symptoms and seqube under-reported by patients and underestimated by physicians [6-11].

The label “mild” in mTBI does not reflect the severity of the underlying metabphysiologic processes, if not even the potential clinical manifestations. The worimplies the general absence of overt structural brain damage. However, long be

typically reported recovery interval of 1 week to 3 months, at least 15% of persons w

PM&R © 2011 by the American Academy of P1934-1482/11/$36.00

Printed in U.S.A.

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S.S. Division of Neurosurgery, DNeurosciences Head and NeckCamillo Hospital, Rome, ItalyDisclosure: nothing to disclose

G.L. Department of Biology, Geovironmental Sciences, Division otry and Molecular Biology, Univenia, Catania, ItalyDisclosure: nothing to disclose

B.T. Institute of Biochemistry andchemistry, Catholic University of RItalyDisclosure: nothing to disclose

R.V. Department of Neuroscienceof Rome “Tor Vergata,” Via MRome 00133, Italy Address cor

ith ato R.V.; e-mail: vagnozzi@uniroma2.itDisclosure: nothing to disclose

hysical Medicine and RehabilitationVol. 3, S359-S368, October 2011

DOI: 10.1016/j.pmrj.2011.07.018S359

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S360 Signoretti et al PATHOPHYSIOLOGY OF CONCUSSION

history of mTBI continue to see their primary care pbecause of persistent problems [12-16]. In additionhealth care professionals frequently become involvcare of persons with mTBI, including family practicians, behavioral psychologists, clinical psychologiropsychologists, neurologists, psychiatrists, neuro-mologists, neurosurgeons, physiatrists, nurses, occutherapists, and physical therapists.

Awareness of the potential of a high level of disabmTBI is increasing. The provision of comprehensive dand treatment services could bring great benefits to patotherwise would spend prolonged periods off work odent on others. Yet considerable confusion and incostill exists in defining and understanding the pathophythis type of trauma [17,18]. The following review repreauthors’ effort to piece together the current conceptmost recent findings about the complex basic physiololying the injury processes of this particular type of braiand to emphasize the nuances involved in conductingin this area.

DEFINING CONCUSSION

Although concussion certainly is blended into the vastmTBI, by definition, concussion should be consideredand distinct entity because not all cases of mTBI“concussive”; thus the 2 terms refer to different constshould not be used interchangeably [19]. That beingauthors understand the common synonymous accemTBI and concussion. During the past decade, concubeen considered by 3 international consensus confewhich it has been defined and redefined by a panel ountil finally and unanimously the following statemreached: “Concussion is a complex pathophysiologicaaffecting the brain, induced by traumatic biomechanic[19-21].

Given this general and propaedeutical definitioncommon features were added by the consensus panelexplain the nature of this peculiar brain injury [19].concussion typically results in the rapid onset of shimpairment of neurologic function, which resolves sously. Postconcussive symptoms may be prolonged ipercentage of cases, but the acute clinical symptomreflect a functional disturbance rather than a structurwhich usually is confirmed by the absence of abnormstandard neuroimaging studies. Finally, concussion mnot involve loss of consciousness.

Notwithstanding such a comprehensive, well-desigmultifunctional definition, a certain degree of confuexists regarding the compelling pathomechanisms thagered by the mechanical insult and that unfold theredominant theory that diffuse axonal injury (DAI) isneuropathological process behind concussion is prov

weak or, at best, inconclusive, given the current literature a

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the fact that neuronal injury inherent to mTBI improfew lasting clinical sequelae in the vast majority ofClinically, concussion still can be considered as the milof the spectrum continuum that is DAI. A large body oand experimental evidence suggests that such a dcourse based on temporal neuronal dysfunction is an iconsequence of complex biochemical and neurochemcade mechanisms that are directly and immediately trigtraumatic insult.

The distinction between DAI and concussion is notheoretical, and from a biomechanical perspective,acknowledged. According to Ommaya et al [22], rotatioacceleration must exceed the threshold of 12,500 rad/sa mild DAI, whereas for moderate and severe DAI, 15,5and 18,000 rad/s2 are required, respectively. Althouauthors determined that a rotational acceleration of 90is capable of generating DAI [23], it recently has beenthat much smaller head acceleration values ranging frto 5500 rad/s2 are needed to provoke a concussion [2

THE MECHANICAL INSULT AND THE“IGNITION” OF THE NEUROCHEMICALCASCADES

Concussive head injury causes the brain to experienchanical “shake,” by virtue of the action of the acceleradeceleration forces transmitted to the head immediatelyimpact, initiating a complex cascade of subsequent neuical and neurometabolic events.

The sudden stretching of the neuronal and axonbranes initiates an indiscriminate flux of ions throuously regulated ion channels and transient physical mdefects [25,26]. This process is followed by a widespreaof a multitude of neurotransmitters, particularly eamino acids (EAAs) [27,28], resulting in further chneuronal ionic homeostasis. Among the EAAs, glutamthe pivotal role by binding to the kainite, N-methyl-d-aand D-amino-3-hydroxy-5-methyl-4-isoxazolepropioionic channels. N-methyl-d-aspartate receptor activatsponsible for a further depolarization, ultimately cainflux of calcium ions into the cells.

The essential point of this post-traumatic ionicderangement is mitochondrial calcium overloadingwhich is responsible for inducing changes of innbrane permeability with consequent malfunctioningpling of oxidative phosphorylation, and finally, oswelling [32,33]. As suggested by experiments in wmitochondrial capacity to catalyze the tetravalent rof molecular oxygen through the electron transpoappears compromised, dysfunctional mitochondriathe main intracellular source of reactive oxygen(ROS) [34-36], inducing a phenomenon known as ostress. The occurrence of an overproduction of ROS

nd the physiologic capacity of the cell to scavenge them progres-

ses wy impnly chealedndete

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S361PM&R Vol. 3, Iss. 10S2, 2011

sively leads to a decrease of antioxidant cell defenconsequent irreversible modification of biologicalltant macromolecules. ROS-mediated damage, maiacterized by the onset of lipid peroxidation, is revmeasuring tissue malondialdehyde, a compound uable during normal conditions [37].

As clearly demonstrated in bench studies, this evevery rapidly, starting 1 minute after trauma and pers24-48 hours after injury [37]. Once the lipid perreaction chain is initiated, it spontaneously propagcauses significant ascorbate depletion, explained eithdirect oxidizing action of ROS on ascorbate or by its uredox cycling of �-tocopherol (vitamin E), which rthe only membrane-bound lipid soluble compound cbreaking lipid peroxidation reaction chain [38].

Although no full explanation has been found, aphenomenon occurring during the onset of oxidativethe significant depletion of the nicotinic coenzyme pooa condition that jeopardizes all the oxidoreductiveincluding those related to the cell energy supply. Possibanisms for this phenomenon are the hydroxyl radicalhydrolysis of the N-glycosidic bond of reduced nicoadenine dinucleotide (phosphate) and the activation odized form of the enzyme nicotinamide adenine dinglycohydrolase [42]. Both mechanisms cause the hydnicotinic coenzymes and give rise to the same end prodis, adenosine diphosphate (ADP)-ribose(P) and nicoExperimental evidence showed that methylenetetrahyreductase can be subject to direct ROS attack and suirreversible degradation of a consistent amount of thecoenzymes [43,44].

To re-establish pretrauma ionic balance, the Na1/Ksine triphosphate (ATP)-dependent pumps must wormaximal capacities, and a high level of glucose oxurgently required to satisfy this sudden increased enmand. Under normal aerobic conditions and correct mdrial functioning, most of glucose consumption is cooxygen consumption, thus optimizing ATP generatioever, damaged by the calcium overloading and underattacks from ROS, most of these oxidoreductive reacimpaired, and the mitochondria cannot maintain thphosphorylating capacity. This scenario results in adecrease of all metabolites representative of the cell enesuch as high-energy phosphates (eg, ATP and gtriphosphate). This phenomenon is mirrored by a proincrease of their dephosphorylated products (ie, ADPsine monophosphate, guanosine diphosphate, gmonophosphate, nucleosides, and oxypurines). Particteresting are the increases of xanthine (5�) and uric awhich strongly suggests the activation of xanthine ophenomenon that perpetuates the vicious cycle of ROStion via the catalytic mechanism of this enzyme [45].

Thus it happens that during the time of maximu

request, the concussion-induced transient mitochond

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malfunctioning causes an imbalance between ATP cotion and production, a condition that obligates nework overtime via the more rapid, but less efficient,independent glycolysis. The uncoupling between oxglucose consumption and the yet unfulfilled energyment explain the paradoxical temporary increase inglucose consumption, notwithstanding a period ometabolic depression. In fact, local cerebral metabofor glucose are documented to increase by 46% abovlevels within the first 30 minutes after injury andfrom 30 minutes to 4 hours [46-50].

The overall evidence from these studies demonstrthe traumatic insult is directly responsible for sudchemical changes, beginning immediately after injleads to subsequent depression of brain energy metEven if it is considered a “mild” form of TBI, concable to cause profound biochemical changes, withdifference being that the described modificationsreversible [51]. As recently reported, the metabolicment and the post-mTBI “energy crisis” are cochiefly responsible for the compromised synapticand the subsequent cognitive deficits [52].

A SURROGATE MARKER OFPOSTCONCUSSIVE BRAIN DAMAGE:N-ACETYLASPARTATE

When proton magnetic resonance spectroscopy (1H-Mapplied to the human brain, it was evident that the moinent proton signal, detectable after having suppressedton signal of water, was that of a metabolite knowacetylaspartate (NAA). Subsequently, NAA becamereliable molecular marker for the imaging of several pain brain 1H-MRS studies. These findings captured theof the general neurosciences, dramatically acceleratingof research into the neurochemistry and neurobiolmolecule indeed definable as “unique” [53].

Although the exact biochemical role of this coremains to be fully established, brain NAA was fconcentrations hundreds-fold higher than in nonsystem tissues and therefore was considered a brainmetabolite and an in vivo marker of neuronal[53,54]. A decrease in NAA has been observedneurological diseases that cause neuronal and axonaeration, such as tumors, epilepsy, dementia, stroke,multiple sclerosis, and many leukoencephalopathiversely, the only known pathologic state characteridramatic increase in cerebral NAA is an autosomal-genetic leukodystrophy (Canavan disease) causedsynthesis of a defective form of the enzyme responthe NAA degradation (N-acetyl-asparto-acylaseMore generally, any major central nervous systeminvolving either direct neuronal and/or axonal dam

rial ondary hypoxic-ischemic, or toxic insult will result in abnor-

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S362 Signoretti et al PATHOPHYSIOLOGY OF CONCUSSION

malities of NAA homeostasis. In the field of TBI, hovery innovative hypothesis seemed more fascinatinothers, according to which NAA reduction was belieproportional to the severity of trauma [55].

By measuring whole-brain levels of NAA via higmance liquid chromatography [56] in 3 different levperimental, closed, and diffuse TBI (mild, moderatevere), it was clearly demonstrated that at 48 hours aftereduction in NAA correlated to the severity of the insuing spontaneous recovery with lower levels of traumaversible decrease in the others [57]. The findings aconsistent with long-term behavioral observation ininjured with the same model of mTBI, showing ondifferences with sham-injured animals, with the maences being present 1 day after injury and showing cimprovement over time [58]. All these bench datasupported the indication for a potential role of NAA ifying neuronal damage and predicting neuropsycholocome after TBI [59] and being of high clinical relevanceuse of 1H-MRS allow to measure NAA noninvasivel[59,60].

The finding of recovery in the “concussed” aniplied that the process leading to the reversible NAA rwas attributable to transient biochemical changessimply to cell death. Similar to the previously dbiochemical changes, the striking finding was agaipidity of the onset of significant NAA reduction, ideearly as 2 hours after injury, with the lowest valuesat 15 hours after impact (�46% compared withvalues). Spontaneous recovery was observed to occu48-96 hours, but that took place only in mildly injuBeyond showing the profound TBI-induced modifiNAA homeostasis, this finding clearly demonstradifferent levels of “physical” injury correlated withlevels and kinetics of “biochemical” damage, whicversible in mTBI and irreversible in severe TBI (sTB

Substantial evidence exists that NAA synthesis taexclusively in neuronal mitochondria, that it is strictneuronal energy metabolism, and that the distributern of NAA closely parallels the distribution of “reactivity.” For an overview of the data supporting agetic role for NAA in neurons, see Moffet et al [53]

A close linear relationship has been demonstratedthe efficacy of ATP synthesis and the ability to synthe[60,61]. NAA synthesis is indeed an energy-requirindependent on the availability and the energy of hydacetyl coenzyme A (CoA) used as the acetyl group donacetylation reaction of aspartate catalyzed by aspartate-transferase. It is fundamental to understand that whCoA is used for NAA synthesis, there is an indirect higcost to the cell. In fact, because acetyl CoA will notcitric acid cycle (Krebs’ cycle), a decrease will occproduction of reducing equivalents (3 reduced nico

adenine dinucleotide and 1 reduced flavin adenine dinuc

r, angbe

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otide) as the fuel for the electron transport chain. Itclearly evidenced that in the general post-traumatic mderangement, acetyl CoA homeostasis is affected by grainjury, following a pattern very similar to those obsboth ATP and NAA [62]. Therefore in metabolic condlow ATP availability, when all of the pathways and cvoted to energy supply are operating at their maximawith the aim of replenishing ATP levels, acetyl CoA waccessible for NAA synthesis. Only when the ATP defifully restored acetyl CoA will become available to be shthe NAA “production” pathway. It also should be rechigh ATP concentrations are able to inhibit the activitysynthase, which is the enzyme of the Krebs’ cycle, usiCoA to synthesize citric acid [63]. With this in mind, aconcentration can be seen as an indirect marker of pmatic metabolic energy impairment.

According to these concepts, it is evident that if Nrecovering after an mTBI, the concussive biochemrangement (involving more complex pathways thaNAA homeostasis [64]) cannot be considered to beThus NAA embodies a biochemical surrogate mmonitor the overall cerebral metabolic status, and itthat under conditions of reduced NAA, although thefunctional, they are still experiencing energetic imb

POSTCONCUSSIVE BRAIN VULNERABI

The basic pathophysiological paths explored thus far hified some aspects of this particular clinical entity, suthat even if concussion is considered a form of mTBInot to be considered as “mild” as the name would suggthe exception of the almost always punctual reversibithe modifications induced, it probably is not prudent tadjective “mild” when referring to a traumatic eventhave such consequences to the fundamental metabenergy states of neuronal cells. However, while allbiochemical modifications are scientifically interestmight appear, at a first glance, of negligible clinical ucause they are all spontaneously and fully reversible.

Despite this reversibility, a reasonable body ofclearly demonstrates that the “concussed” brain cells upeculiar state of “vulnerability,” during which time iftain a second, typically nonlethal insult in a closeproximity, they would suffer irreversible damage[65,66]. In the preclinical setting, this period of timewell defined in duration thanks to high reproducibilclosed-head rat model of mTBI that has been used tostrate biochemically the concept of vulnerability [62,6nally proposed by Hovda et al [65]. In other words, coninduced pathophysiologic conditions, mainly manifenergetic metabolic perturbations, make the brain mortible to severe and irreversible cellular injury by a seconof modest entity, creating a disproportion between th

le- severity and the subsequent cerebral damage.

focused, a

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S363PM&R Vol. 3, Iss. 10S2, 2011

Several studies in animals in which investigatorson mTBI-induced dysfunction have been publishcurrent data support the concept of transient bioand physiologic alterations that may be exacerbatepeated mild injuries within specific time windows oability [62,64,67]. In a rat weight-drop experimformed by applying a new and easily reproducible prsimulate a “second impact” condition, it was clearlystrated that levels of NAA, ATP, and the ATP/Adecreased significantly when measured 2 days afterconcussion (Figure 1). Maximal metabolic abnowere seen when the occurrence of 2 mild injurseparated by a 3-day interval; in fact, the metabolmalities in these animals were similar to those occursTBI. In a follow-up study, similar perturbations weto persist as late as 7 days after double impact, inprolonged metabolic effects from repeat mTBI inmodel [62]. Similar data were reported by Laurer eta histopathology study in which they described thtant cumulative effects of 2 episodes of mTBI (24 houin mice, which led to pronounced cellular damage cwith animals that sustained only a single trauma. Thconcluded that although the brain was not morphodamaged after a single concussive insult, its vulnerasecond concussive impact was dangerously increas

According to Hovda et al [65] and Doberstein emetabolic alterations can persist for days after con

Figure 1. Concentrations of N-acetylaspartate (Ny-axis) and adenosine triphosphate (ATP) (right y-axtermined by high-performance liquid chromatograpwhole brains of rats subjected to repeat diffuse mild tbrain injuries (TBIs) (spaced by 3 or 5 days) or single d(mild TBI or severe TBI). Control subjects were sham-oanimals, ie, animals receiving anesthesia and surgicdures out of injury. Each histogram is the mean of 6 ansignificant differences were demonstrated when rating a single mild TBI and rats sustaining a repeatspaced by 5 days were compared. Similarly, no diwere observed when rats sustaining a single severe TBsustaining a repeat mild TBI spaced by 3 days were co*P � .05 versus control subjects.

creating no morphological damage but representing

edndcalre-er-er-l ton-tiotediesereor-terndingmein

or-rt)ed

orsllyo a

9],on,

pathological basis of the brain’s vulnerability. All thprovide the experimental demonstration of the exmetabolic nature of “brain vulnerability” after mTBIa unique contribution to the complex biochemicalunderlying the clinical scenario of a repeated cotrauma, sometimes leading to catastrophic brain inj

To explain the differences between the underlyibolic dysfunction occurring after a concussion anoccurring after an sTBI, it is again necessary to use beand consider the degree of the NAA and ATP rewhich is approximately 20% and 50%, respectivMore importantly, the ADP concentration is onlyincreased after mTBI but is found to be substancreased by 35% after sTBI [70]. Despite the significreduction by one fifth, if the insult is “mild,” the mdria are not yet irreversibly damaged and still psufficient phosphorylating capacity (ie, a modest dethe ATP/ADP ratio, which is a very good index to evamitochondrial phosphorylating activity) to allow sous complete ATP restoration, which was fulfilledproximately 5 days in the aforementioned experimOn the contrary, the 35% increase in ADP found afsevere levels of injury indicates a profoundly differetion with an altered capacity of mitochondria to supcell energy requirements in terms of ATP synthesisfound decrease in the ATP/ADP ratio).

If after a first mild injury a second concussioncells in the condition of recovering from the initial anperfectly reversible energetic failure, it will causemitochondrial malfunctioning, leading to the sameible energetic failure observed in severe injury. Thusthat occur too close in temporal proximity can simeffects of a single severe injury. The key biochemicathe vulnerable brain lies in the incomplete resolutioinitially reversible energetic crisis triggered by the fir

The foremost clinical implication of these experimeis that within days after injury, the metabolic effectscussions can be dangerously additive. This informatinot be surprising; however, similar human data regardmetabolites currently are not available. The second cliplication of this notion is again remarkable becausedifficult to establish how long the aforementioned pbrain vulnerability will last and when the occurrence oftrauma would be uneventful.

THE SECOND IMPACT SYNDROME ANHYPOTHESIS OF “THE PERFECT STORM

A handful of previously published cases have reppatients (mostly involved in sports-related activitiwhile still having symptoms from a previous heaexperienced a second injury that unexpectedly andictably led to sustained intracranial hypertension a

lefte-

theticTBIede-

Noin-TBIes

atsed.

the strophic outcomes [71]. This entity, also known as the sec-

astrop

[78],gly r

ncidenbsequg fromed bynot coof br[82,8

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cationublish2 diffu, 4, anr the lprogr

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S364 Signoretti et al PATHOPHYSIOLOGY OF CONCUSSION

ond impact syndrome (SIS), is the occurrence of catcerebral edema after mTBI/concussion [72-77].

Skepticism about this entity notwithstandingmajor concern about SIS is that it is an exceedinclinical condition when compared with the overall iof concussion, even though an elevated risk for sumTBI exists among persons who are still recoverinprevious one [79-81]. The topic is further complicatfact that the resolution of clinical symptoms mightcide with the “closure” of the temporal windowmetabolic imbalance “opened” by the first traumaThus the question of whether the brain had fully rfrom the first concussive injury while experiencingond one remains unanswered.

Once again, laboratory data have provided clarifisome of these complex matters. In a recently pweight-drop experiment, rats were subjected tomTBIs, with the second mTBI delivered after 1, 2, 3days, and then all animals were killed 48 hours afteimpact. Notably, mitochondrial-related changessively worsened with the time between concussiondays apart, when the metabolic abnormalities were sthose occurring after a single sTBI [62]. In this mwith this experimental timeline, the third day aftewas the point when the cell’s energy-dependentprocesses were at their maximal intensity. Howevreproducibility of the model allowed to establish theof the window of vulnerability in the rat, this caaffirmed in the case of human beings in which the vuncontrolled variables render each impact differanother. This concept was clearly developed by GHovda [66], who showed that each physiologic pmodified by a concussion has its own time frame,head injury can be very different from the next. Tthey concluded that it is difficult to definitively stateduration of vulnerability to a second injury [66]. Rour studies in concussed athletes strongly corroborknowledge [82,83]. In fact, while it was clear that no50 concussed athletes recovered NAA concentratio30 days post-impact, it was also evident that the timnormalization was not identical in each subject, thuing impossible to define the time of brain vulnerabthe same degree of certainty found in animals [62,64other hand, it was also clearly demonstrated that noconcussed patients had clearance of post-concussivsymptoms faster than NAA normalization, ie, disturbrain metabolism lasted much longer than gross clin[82,83], when post-concussive symptoms are persweeks after concussion. In an as yet unpublished adescribe a group of 6 doubly concussed athletes in wpost-concussive syndrome persisted up to 2 moninjury (Vagnozzi et al., 2011, submitted). Even instricted group of patients, recovery of NAA occurr

later (75 to 120 days), once again suggesting that rescue

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brain metabolism does not correlate with self-reportance of post-concussive symptoms. Therefore a secpact occurring at this stage had the most profounbecause of the minimal “metabolic buffering capcounteract the known early changes reinitiated bymTBI. With their biochemical homeostasis not yetlished, the ionic imbalance will prevail and massiveswelling will take place [84,85].

The reason why SIS is, fortunately, an extremcondition is probably because it represents a sort ofstorm,” an extremely random and hardly predictabtion generated by the odd combination of the severinitial concussion, the time interval between the 2and the metabolic state of the brain at the time of thconcussion.

The results of a recent pilot study performed in asingly and doubly concussed athletes who were exam1H-MRS for their NAA cerebral content at differpoints after concussive events demonstrated that thery of brain metabolism is not linearly related to timethis study, athletes who experienced a second cobetween the 10th and the 13th day after the first innot have SIS, nor did they demonstrate signs of sTever, they all had a significant delay in both symptlution and NAA normalization [82]. In other woeffects of the second concussion were not fatal, but tsomehow not proportionate to the entity of thesive insult. Most likely, the second concussionwhen the brain cells were completing recovery ofmetabolic functions, and thus it only produced acumulative effect with moderate worsening of thepictures. Thus it is conceivable to infer that it isinterval between the 2 concussions that drives theand metabolic evolution.

It is our belief that SIS should not be solely consan “all-or-none” phenomenon and should not be lithose instances that result in death from malignantThe concept of SIS should be extended to includother occurrences in which a disproportion betwseverity of the second injury and the concussivefeatures (ie, intensity and/or time of resolution) ormetabolic changes (ie, extent of NAA decrease andin its normalization) is clearly observed. The degretype of SIS will depend on which phase of the mrecovery the brain is in at the time of the second con

UNDERSTANDING THE DEGREE OFMILDNESS OF AN mTBI: CHANGES INGENE EXPRESSION

With use of the same model of experimental repestudies from our laboratories [62] demonstrated thwas an effect of the time interval between concus

of ASPA gene expression. A progressive increase in the messen-

was omals treinjuracidanimaA var

d rate

s to h2 distipendl permow fr

ly, miith cof reve

mTBI,vulneions wbsequhesisincremTBIlnerab

he steaAA oadapt

phenosyntheltimat

ollabos stud,000

f celluoxyribthat aftable c99 gennts. Tthen to

ctureal tracommd stre, intenhangein genpend-dommolo

ssocia

EIF4G2-up-regu-poptotic(CCL2),

culoviralfamily,

interact-amily 4,

nd afterpal cell

hanisms.ere dif-

er severeer mTBI,

clinicalated andcorrobo-y” cellu-oxygen

the com-the bio-tly, not

d” whenat all themust be

a secondral win-

us post-ssion canrain. Heho had

ead hit aonsistentrding toa func-such as

89].ntuition,nly somesuggest-ces may

he meta-

t measur-it until

rward inetabolic

S365PM&R Vol. 3, Iss. 10S2, 2011

ger ribonucleic acid transcript of the ASPA geneserved, again with a maximum 4-fold increase in anisustained the 2 injuries 3 days apart [62]. Animalspast 5 days had values of messenger ribonucleicASPA comparable with those recorded in controlBased on these data, it appears that TBI-induced NAtions may not be attributable simply to a decreaseNAA biosynthesis.

The aforementioned results allowed researcherpothesize that TBI-induced NAA decrease occurs inphases with 2 different mechanisms. Initially, indefrom the severity of injury, a change in mitochondriaability [86] causes an increased velocity of NAA outflneurons to the extracellular space. Simultaneouschondrial impairment causes a cell energy deficit wsequent diminution in NAA synthesis. In the case oible brain damage, such as single mTBI or repeatwhich the second impact occurs outside the brain’sbility “window,” recovery of mitochondrial functallow restoration of ATP homeostasis and the sunormalization in the rate of NAA efflux and biosyntNAA levels close to those of control subjects with noin ASPA expression). In single sTBI or in repeatwhich the second impact occurs within the brain vuity “window,” the profound energy crisis caused by tmitochondrial malfunctioning induces a constant Nflow toward the oligodendrocytes, which, as anmechanism, increases the expression of ASPA. Thisenon, combined with the decreased rate of NAA biocaused by persistent mitochondrial impairment, is uresponsible for the dramatic NAA depletion.

These results were immediately followed by a ctive study on transcriptomics in which the authorthe simultaneous expression of approximately 30genes whose products are involved in a variety oprocesses [87]. With the use of complementary denucleic acid microarray technology, it was reportedstretch injury to hippocampal slice cultures (as a suimodel to induce graded TBI), the expression of 9was altered in mTBI compared with control patiealtered genes in mTBI-stretched cells clustered incalled “biological process” group, which was showinvolved in the structural damage of cellular archite

Most of these genes are indeed involved in signducer activity, regulation of transcription, and cellnication. This finding indicated that even after a milinjury, as compared with a closed, diffuse mTBIactivity involving transcription and signaling excinitiated. In addition, it has been found that certainvolved in the apoptotic process, such as voltage-deanion-selective channel protein 1 (ie, VDAC1), SH3GRB2-like endophilin B1 (SH3GLB1), pleckstrin holike domain, family A, member 1 (PHDLA1), Rho-a

coiled-coil containing protein kinase 1 (ROCK1), and

b-hatedforls.ia-of

y-nctente-

omto-n-rs-in

ra-ill

ent(ie,aseinil-dyut-ivem-sisely

ra-iedratlaro-terellesheso-be

.ns-u-

tchseises

entaingy-ted

karyotic translation initiation factor 4 gamma, 2 (predicted), were down-regulated. Furthermore, anlation was seen in genes involved in the antiaprocess, such as chemokine (C-C motif) ligand 2vascular endothelial growth factor A (VEGFA), baIAP repeat-containing 3 (BIRC3), TSC22 domainmember 3 (TSC22D3), BCL2/adenovirus E1B 19-kDing protein 3 (BNIP3), and nuclear receptor subfgroup A, member 1 (NR4A1).

Most of these expression changes were only foumild stretch injury, indicating that these hippocamcultures have activated protective and repair mecThe most interesting finding was that more genes wferentially expressed after mild brain injury than aftinjury, further supporting the notion that even aftcharacterized by the absence of radiological andabnormalities, a complex cellular response is initidistinct neuronal dysfunction occurs. This findingrates previous findings that these effects are “primarlar effects not determined by local blood flow ordelivery or by any systemic factors [88].

The overall doubt that might be generated frombination of these studies on gene expression withchemical works previously cited is that, apparenmuch rationale is left to justify the adjective “mildealing with a concussive injury. It is undeniable thaforementioned changes are fully reversible, but itkept in mind that this reversibility is true only if“equally mild” TBI does not occur within the tempodow of metabolic brain vulnerability.

CLINICAL IMPLICATIONS

In the 18th century, Alexis Littre performed a famomortem examination providing evidence that concuoccur without obvious anatomic damage to the bperformed an autopsy on one particular patient wbeen rendered unconscious and died soon after his hwall. Littre detected no cerebral injury, a finding cwith the 16th-century Ambroise Pare’s notion, accowhich the symptoms of concussion “. . . reflectedtional disturbance rather than structural damagecontusion, hemorrhage or laceration of the brain” [

More than half a millennium since Pare’s first ibasic science data collected thus far have clarified oof the many aspects of this particular clinical entity,ing that short-term as well as long-term consequenvery well be overcome simply by understanding tbolic conditions of the injured brain cells.

Data reported in this summary strongly suggest thaing NAA after an initial concussion and monitoringnormalization might represent a significant step foquantifying the objective nature of postconcussive m

eu- disturbances. Because of its high concentration within neurons

onstracalizeovidine presr to “bifyingmptomnot co

lvingtly haombi

modesg the Mcente

), easyangescussimetab

recovepidly.complto valu

of theirTo have

t and tot a usefulat could

data ob-europsy-tudies totter tractt may beral func-y distur-

er a con-echanicaltoxicity,”age, en-

sion, andurrogate”thophys-l presen-eadache,sea/vom-on prob-appraisals, irrita-

ms withatic pre-

related toor school

blow toan resultation of

r equallyld showma. Theidence isation ofthe full

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. A system-Neurochir

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S366 Signoretti et al PATHOPHYSIOLOGY OF CONCUSSION

(�10 mmol/L brain water), NAA levels are easily demby 1H-MRS. This technique is based on the ability to loMR signal into a specific volume of tissue, thus prreal-time “image” of the brain neurochemistry. At thtime, 1H-MRS offers a unique opportunity to endeavologically” grade the “severity” of a concussion by quantactual metabolic dysfunction, apart from signs and sybecause often clearance of clinical disturbances doescide with full cerebral metabolic recovery.

The results of a multicenter clinical trial [83] invoconcussed athletes and 30 healthy volunteers recenbeen published and reveal that despite different ctions of magnetic field strengths (1.5 or 3.0 T) andspectrum acquisition (single- or multi-voxel) amonscanners currently in use in most neuroradiologyNAA determination represents a quick (15-minuteperform, noninvasive tool to accurately measure chcerebral biochemical damage that occur after a conPatients exhibited the most significant alteration oflite ratios at day 3 after injury and showed a gradualinitially in a slow fashion and, after day 15, more ra30 days after the injury, all subjects exhibitedrecovery, that is, having metabolite ratios similar

Figure 2. Metabolite ratios of N-acetylaspartate/containing compounds (NAA/Cr) and choline-ccompounds/creatine-containing compounds (Chhealthy control patients and concussed athletes. Hare the means of 30 healthy control patients andcussed athletes. Standard deviations are representetical bars. Data were collected in 3 different neuroradcenters with the use of either the “single-voxel” modea 3-T apparatus, the “single-voxel” mode throughapparatus or the “multivoxel” mode through a 3-T apNo differences were observed when data collectedneuroradiology centers were compared. At 3 days athe NAA/Cr ratio decreased by 17.6% and graduaered to complete normalization at 30 days. The Chodid not show any significant variation. *P � .01 with recontrol patients. **P � .01 with respect to values deterthe previous time points.

detected in control subjects (Figure 2).

tedtheg aentio-the

s,in-

40ve

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Interestingly, patients self-declared a clearancesymptoms between 3 and 15 days after concussion.a snapshot of the degree of energetic impairmenmonitor the eventual recovery curve might represenstrategy to avoid a second mTBI soon afterward thlead to a more severe injury.

Finally, the combination of metabolic regionaltained with longitudinal 1H-MRS studies, serial nchological evaluation, and diffusion tensor imaging scorrelate metabolic alteration with possible white madamage can minimize the risk of recurrent injury tharesponsible for the cumulative impairments of cerebtion and cognition, including early onset of memorbances, early depression, and even dementia.

CONCLUSIONS

Sudden and profound biochemical changes occur aftcussive trauma. These changes are activated by the minsult itself and lead to ionic disturbance, EAA “neuroinitial mitochondrial dysfunction, ROS-mediated damergy metabolism depression, alteration of gene expresultimately variation of NAA concentration, the “smarker of the dysfunctional neurons. This complex paiology represents the modern explanation of the clinicatation of concussion—a capricious combination of hdizziness, insomnia, fatigue, lethargy, uneven gait, nauiting, blurred vision, attention difficulty, concentratilems, memory problems, orientation problems, self-problems, expression and speech or language problembility, depression, anxiety, sleep disturbance, probleemotional control, loss of initiative, blunted affect, somoccupation, hyperactivity, disinhibition, or problemsemployment, marriage, relationships, and home andmanagement.

More problematically, within days after a simplethe head, this intricate biochemical derangement cin a dangerous state for the brain, generating a situmetabolic vulnerability to the point that if anothe“mild” injury were to occur, the 2 concussions wouthe biochemical equivalence of a severe brain trauimmediate clinical implication derived from this evthat trials are warranted to investigate the applic1H-MRS for measurement of NAA and to monitorrecovery of brain metabolic functions.

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