Update on protein biomarkers in traumatic brain injury with emphasis on clinical use in adults and...

REVIEW ARTICLE

Update on protein biomarkers in traumatic brain injurywith emphasis on clinical use in adults and pediatrics

Erzsébet Kövesdi & János Lückl & Péter Bukovics & Orsolya Farkas & József Pál &Endre Czeiter & Dóra Szellár & Tamás Dóczi & Sámuel Komoly & András Büki

Received: 5 December 2008 /Accepted: 10 July 2009# Springer-Verlag 2009

AbstractPurpose This review summarizes protein biomarkers inmild and severe traumatic brain injury in adults andchildren and presents a strategy for conducting rationallydesigned clinical studies on biomarkers in head trauma.Methods We performed an electronic search of the NationalLibrary of Medicine’s MEDLINE and Biomedical Libraryof University of Pennsylvania database in March 2008using a search heading of traumatic head injury and proteinbiomarkers. The search was focused especially on proteindegradation products (spectrin breakdown product, c-tau,amyloid-β1–42) in the last 10 years, but recent data on“classical” markers (S-100B, neuron-specific enolase, etc.)were also examined.Results We identified 85 articles focusing on clinical use ofbiomarkers; 58 articles were prospective cohort studies withinjury and/or outcome assessment.

Conclusions We conclude that only S-100B in severetraumatic brain injury has consistently demonstrated theability to predict injury and outcome in adults. The numberof studies with protein degradation products is insufficientespecially in the pediatric care. Cohort studies with well-defined end points and further neuroproteomic search forbiomarkers in mild injury should be triggered. Aftercritically reviewing the study designs, we found that largehomogenous patient populations, consistent injury, andoutcome measures prospectively determined cutoff values,and a combined use of different predictors should beconsidered in future studies.

Keywords Review .Mild . Severe . Pediatric .

Traumatic brain injury . Biomarkers . Outcome

Introduction

Epidemiological and socioeconomic significance

Traumatic brain injury (TBI) is the leading cause of mortalityin the active/working population and a major cause ofdisability in people under age 35 in Europe and in the USA.Population studies in the USA suggest that the yearlyincidence of TBI is between 180 and 250 per 100,000 [27].Approximately 1,600,000 head-injured patients receivehospital care in Europe, producing a brain injury rate of235/100,000 per year and causing as many as 66,000 deathsper year [156]. The leading causes of TBI are trafficaccidents, falls, violence, and sport-related injury. Thedemographic groups at high risk for TBI include males andindividuals living in regions characterized by socioeconomicdeprivation [27]. Unfortunately, TBI is also the leading causeof death for children and young adults. In children less than

E. Kövesdi : P. Bukovics :O. Farkas : J. Pál : E. Czeiter :D. Szellár : T. Dóczi :A. Büki (*)Department of Neurosurgery, University of Pécs,Rét u. 2.,7623 Pécs, Hungarye-mail: [email protected]

J. LücklDepartment of Neurology, University of Pennsylvania,Philadelphia, PA, USA

P. BukovicsNeurobiology Research Groupof the Hungarian Academy of Sciences,Pécs, Hungary

S. KomolyDepartment of Neurology, University of Pécs,Pécs, Hungary

Acta NeurochirDOI 10.1007/s00701-009-0463-6

2 years of age, inflicted TBI—often referred to as shakenbaby syndrome—is a leading cause of death and disabilityand accounts for up to 25% of all brain injuries [35, 84]. Inhighly motorized countries such as the USA, the totalmedical costs for TBI patients was 60 billion USD in2000, while the cost of acute care and rehabilitation for newcases was nine to ten billion USD [52]. The average cost(Euro 2004) per inpatient care associated with severe TBI inthe most developed European countries is around 6,000 Euro[12] with the costs due to initial hospitalization being only asmall part of total costs. A modest estimate of total lifetimecosts per TBI case in the US is around $200,000. Accordingto some estimates, TBI-caused serious socioeconomic con-sequences will become the third most common cause ofdeath globally by the year 2020 [102].

Current methods of outcome prediction in severe TBI

In the last two decades, the diagnostic instruments for theprediction of severity or the potential outcome of headinjuries have hardly changed. Especially in the case ofdiffuse brain injuries, the information provided by theroutine diagnostic instruments is insufficient for measuringthe extent and severity of such injuries. Great expectationspreceded the introduction of the Glasgow Coma Scale(GCS). The clinical utility of the GCS is limited, however,by the application of therapeutic guidelines that are basedon sedation [4, 55, 141]. The cranial computed tomography(CT) scan image shows several elements regarding theextent and prospective outcome of the injury (volume offocal lesion, ventricular compression, midline dislocation,compression of basal cistern). However, the adequacy ofCT scanning as a diagnostic instrument, especially in thecase of diffuse injuries, is limited due to its low sensitivityand its poor specificity. Therefore, the ability of CT topredict outcome is compromised. Although magneticresonance imaging (MRI) of the brain, especially T1 watermaps, diffusion-weighted imaging, and diffusion-tensorimaging, provide information on the extent of diffuseaxonal injuries, the high cost of magnetic resonance (MR)scanners, the time needed for scanning, and the requirementfor special equipment (MR-compatible respirator, monitor,etc.) have hampered the widespread application of thismodality in the routine treatment of acute head injuries[6, 50, 51, 73, 93, 113, 145, 157].

Among the routine data gathered in the process ofneurointensive monitoring (intracranial pressure (ICP),mean arterial blood pressure, cerebral perfusion pressure(CPP)) gathered in the process of neurointensive monitor-ing, ICP can best indicate the severity of primary injury.However, these parameters primarily reflect the secondaryinjuries caused by hypoperfusion and are only appropriatefor prognosis in the situation of extremely abnormal results

[58, 62, 138]. The most widely used functional outcomemeasure in TBI trials is the five-point Glasgow OutcomeScale (GOS). It is frequently used as the “dichotomizedGOS” of “good” and “bad” outcome. As such, the GOSbecomes a two-point outcome measure with reducedsensitivity and is not able to detect subtle improvementsin patient neuropsychological status. Outcome measuressuch as the Disability Rating Scale or the American BrainInjury Consortium Neuropsychological Test Battery [160]may offer better sensitivity but suffer from administrativedifficulties and interobserver variability.

Because of the “inhomogeneity” of the brain and thecomplex nature of TBI, an injury might have effects onGCS and/or GOC score that are disproportionate toexpected biochemical changes. For example, an injury toa small number of critical neurons (e.g., in the brainstem)might have major impact on consciousness but release arelatively small amount of biomarkers. Conversely, majorinjury to a relatively noneloquent part of the brain mayexplain a high concentration of biomarkers and a goodoutcome. Repeated injury can also complicate the relation-ships between GCS and acute biomarker release. Long-termoutcome is affected by factors such as chronic stress,medical care, and the rehabilitation and psychosocialenvironment in which the patient recovers.

It should be also noted that the correlations of GCSand GOS scores with biomarkers in cerebral spinal fluid(CSF) are different in younger (age<4 year versus age>4 year) and abused patients [148]. This difference may beexplained by a propensity towards repetitive trauma,delayed presentations, and a possible association withadditional mechanisms of damage such as diffuse axonalinjury [17].

Biomarkers: expectations and benefits

While in certain medical fields there is a wide range ofserum markers available for analysis, those who specializein the morbidity of the central nervous system do not have awide choice of serum markers with adequate specificity andsensitivity. This is especially disadvantageous since special-ists who treat head injuries could significantly profit fromthe benefits provided by biomarkers. In the case of patientswith mild head injury without prominent neurologicaldeficits, it would advance the establishment of a diagnosis,by indicating whether tissue has been damaged. Thisinformation would help the physician decide if the patientshould be observed, if it is necessary to obtain CT or MRIscans and, if so, when such examinations should take place.The application of biomarkers could lead to expeditiousdiagnosis in the case of sedated, unconscious, or polytrau-matized patients even before the application of neuro-imaging techniques.

E. Kövesdi et al.

In the process of clinical therapeutic examinations, theresults of laboratory tests may help elucidate the mecha-nism of injury and, in the determination of a therapeuticwindow, could serve as a cost-saving end point. Suchlaboratory test results could have a significant prognosticvalue in the severely injured with regard to rehabilitationand to the mild/moderate injured with respect to health carebenefits, sick allowance, and physical activity (sport) afterthe injury.

More recently, expectations regarding biomarkers havechanged. Biomarkers should be traceable in blood andshould be proportional to the mechanical impact and theextent of the injury. Their specificity is as important as theirsensitivity. They should appear rapidly in the blood andshow a well-defined distribution in time. It is also importantthat they should reflect the difference between sex and agegroups and preferably should indicate the pathophysiologicmechanism of CNS injury of the central nervous system.The appearance of markers in the blood and/or in the CSFmay indicate which cell population or which domain of thecells has been affected by the injury. For example, thepresence of neuron-specific enolase indicates injury tothe neurons, whereas the glial fibrillary acidic protein(GFAP) denotes the involvement of the glia. The detectionof tau and spectrin protein breakdown products in the bloodand/or in the CSF indicates axonal damages [32, 48].

The aim of the study

Although, there is an increasing demand for proteinbiomarkers in the management of TBI patients and theanimal/clinical research has boomed in this field over thepast 10 years, no single brain-specific biomarker has yetbeen unanimously established for traumatic brain injury inthe routine clinical practice. The importance of this issue isindicated by the fact that even the latest version of theguidelines for the management of severe traumatic braininjury [25] does not include any recommendations for theclinical use of biomarkers. The present review is summa-rizing our current knowledge on protein biomarkers withrespect to their predictive ability on primary and secondaryend points in order to suggest possible candidates forfurther intensive multicenter investigation. This reviewserves also as a critical analysis of the diverse studydesigns and gives an overview on the progression in thisfield including the novel protein degradation biomarkers.

Methods

We conducted a systematic review of published primaryresearch on biochemical markers of traumatic brain injury.We initially performed an electronic search of the National

Library of Medicine’s MEDLINE and Biomedical Libraryof University of Pennsylvania database in March 2008using a search heading of traumatic head injury and proteinbiomarkers. The search was focused especially on proteindegradation products (spectrin breakdown product (SBDP),c-tau, amyloid-β (Aβ)1–42), but recent data on “classical”markers (S-100B, neuron-specific enolase (NSE), etc.) werealso examined. The initial search included only the last10 years. We then manually searched reference lists for allarticles and reviews related to this field in the medicalliterature. Three authors (E.K., J.L., and O.F.) independent-ly assessed all potentially eligible studies and discussedthem with all the other authors. Studies concerning theclinical use of serum markers for traumatic brain injurywere considered primarily and reviewed in detail. Weidentified 85 articles focusing on clinical use of biomarkers;58 articles were prospective cohort studies with injury and/or outcome assessment. This is not a comprehensive reviewof the basic research and experimental studies in this field,but significant data from selected studies are presented asbackground information.

Results

Below, we review the protein biomarkers, describing theresults of human studies and of their correlation with theseverity of head injury and functional outcome.

S-100 beta protein

S-100 proteins are calcium-binding proteins with smallmolecular weight (20 kDa) and with three known subtypescomposed of a dimeric combination of the α- and β-chain[9]. S-100Β (ββ; 10–12 kDa) is brain specific. This proteincan be found in the cytoplasm of astroglia and Schwanncells but also in nonnervous cells such as adipocytes,chondrocytes, and melanoma cells (Table 1) [179]. It has animportant role in regulating the protein kinase C phosphor-ylation of growth-associated protein 43, which is involvedin axonal growth and synaptogenesis during development,synaptic remodeling, and long-term potentiation. S-100Binteracts with and stabilizes microtubule-associated proteinssuch as tau and microtubule-associated protein 2 (MAP-2)[168]. Low concentrations of S-100B in the extracellularspace act on glial and neuronal cells as a growth-differentiating factor but at higher concentrations induceapoptosis [47].

Animal experiments

After severe controlled cortical impact (CCI) injury in the rat,significantly raised serum S-100B levels (0.84–0.93 μg/L)

Update on protein biomarkers in traumatic brain injury

have been detected up to 24 h postinjury. Hardemark andcolleagues [63] observed a peak in the level of S-100B at7.5 h in CSF with a weight drop model. Serum levels aftermild, moderate, and severe injury did not appear to bestatistically significantly different [135]. It should also benoted that a significant twofold increase of S-100B has beenobserved in serum of fasting rats, without changes in its CSFcontent [105].

Mild head injury in adults

The median plasma concentration of S-100B in blood in200 healthy blood donors between 18 and 65 years of age is0.05 μg/L. There is no gender difference, but plasmaconcentrations of S-100Β do decrease slightly with age, butthis is not statistically significant [170]. Serum S-100Βlevels over 0.5 μg/L are considered pathologic after minorhead injury [76]. However, De Kruijk and colleagues [39]reported a mean S-100B level of 0.25 μg/L as significantlyhigher than control. S-100Β clears rapidly from the bloodwith an estimated mean half-life of 97 min following mildhead injury (n=14) [161] and 2.2 h after cardiopulmonarybypass [24]. In a study of 182 patients with mild headinjury (GCS scores of 13 to 15), CT scans were obtained atadmission, and the occurrence of postconcussion symptomswas evaluated 3 months after injury according to theRivermead Postconcussion Symptoms Questionnaire(RPSQ). An increased serum level of S-100B was detectedin 38% of the patients. The accuracy for intracranial injuryevident by CT scan was 0.66, and the negative predictivevalue of an undetectable serum level was 0.99, indicating

that nondetectable S-100B protein levels in serum predict anormal CT scan (if the blood sample is collected early afterhead injury). A trend (but nonsignificant correlation) wasalso observed toward an increased frequency of postcon-cussion symptoms among patients with detectable serumlevels [77]. Similarly, not only S-100B but also S-100A1Bfailed to show diagnostic validity and correlation withcognitive impairment (RPSQ) [38, 40]. Another studysuggests that serum S-100B levels after mild TBI are notpredictive of neuropsychological performance in the early(<1 month) phase of recovery [153]. Herrmann andcolleagues reported a cutoff value of 0.12 μg/L whichsignificantly divided subgroups (69 patients) with mild andsevere injuries, but they did not find a correlation betweensigns of intracranial pathology by CT or MRI scans andS-100B levels [67]. In a study of patients with mild (n=29)and severe (n=14) head injury, a significant difference wasfound in serum S-100B between the two subgroups. Themean level of S-100B in mild injury was 0.77 μg/Lalthough this did not correlate with the early (3 month)and late (24 month) outcome (Jennet and Bond) but it didwith the cerebral pathological finding in CT (MarshallClassification) scan [103]. Unfortunately, the statisticalanalysis was not performed separately on the mildly injuredgroup. In contrast to the previously mentioned studies,however, the level of S-100B shortly after trauma showed asignificant correlation with the 1-year disability in a logisticregression model [152] and with a short-term unfavorableoutcome (return to work within a week) at a cutoff level of0.15 μg/L [155]. Savola and colleague [139] found thatS-100B protein is a sensitive (92%) but not a specific(41%) predictor of postconcussive symptoms. A favorableoutcome was associated with the level of 1.2 μg/L in apatient study with mild and severe injury [136]. Animportant question in the management of patients withminor head trauma is whether the level of a biomarker cansupport the selection of patients for CT scanning. In aprospective multicenter study (n=1,309) with an S-100Bcutoff value of 0.10 μg/L, the patients with positive CTfindings were identified with a sensitivity level of 99% and aspecificity level of 30% [19]. Another multicenter study (n=226) reported similar results (95% sensitivity, 30% specific-ity) at 0.10 μg/L cutoff limit [101]. It is worth noting thatusing creatine kinase (CK) as a correcting factor and a cutoffthat maximizes sensitivity (>90%) can improve the predic-tive value of the initial CT scan from 75% to 96% [8].

Severe head injury in adults

A correlation between S-100B and primary end points (GCS,CT) in patients with severe head injury has been reported inseveral studies [68, 85, 127, 128]. Ample evidence has beenaccumulated in severe TBI, showing that S-100B also

Table 1 Characteristics of biomarkers

Biomarker Source innervoussystem

Extracerebral source Molecularweight (kDa)

S-100B Glia Adipocytes,chondrocytes,melanoma cells,epidermalLangerhans cells

20

GFAP Glia _ 52

NSE Neuron Peripheralneuroendocrinecells, certain tumors

78

Alpha II-spectrin

Neuron, glia _ 280

SBDPs Neuron, glia _ 150, 145, 120

Cleaved tau Neuron, glia _ 30–50

Amyloid-β1–42

Neuron, glia _ 4.5

ApoE Glia, neuron _ 34

GFAP glial fibrillary acidic protein, NSE neuron-specific enolase,SBDPs spectrin breakdown products, ApoE apolipoprotein E

E. Kövesdi et al.

correlates well with outcome [67, 85, 90, 127, 172, 173].These studies report mean peak S-100B serum levels rangingbetween 0.3 and 1.6 μg/L in patients with good outcomes(GOS score of 4–5) and between 1.1 and 4.9 μg/L in thosewith bad outcomes (GOS score of 1–3) with the cutoff valuepredicting unfavorable outcome between 2.0 and 2.5 μg/L.The highest S-100B serum levels are usually detected in thefirst blood sample after a severe head injury. In patients thatdo not survive, the serum level of S-100B remained elevatedfor days, but in patients that did survive the level of S-100Bdecreased within the first 36 h after moderate TBI [118].There is a correlation between the peak value of S-100B, thelowest cerebral perfusion pressure, the highest ICP scores,and the age of the patients [127]. If the postinjury CT scandemonstrates intracranial pathology, then the serum level israised in more than 90% of the patients [133]. The earlyserum level of S-100Β correlates significantly with the extentof the contusion and may indicate the extension of diffusebrain damage [127]. The marker also peaks in CSF 6 h afterinjury and may reflect primary brain damage [66].Themeasurement of S-100Β level in the CSF is more reliablethan that from serum [162]. In serum, it is detectable for upto ten consecutive days [127], whereas it is elevated in CSFfollowing injury for only 5 days [68]. Secondary elevationsof serum S-100Β protein levels occur in approximately halfof the patients with an unfavorable outcome [127]. MeanS-100B levels as an independent parameter associated withsecondary complications has been also reported [163].

Patients with multiple traumas

Several conditions appear to lessen the diagnostic value ofS-100B. It is well known that after inserting a ventriculardrain or during hydrocephalus the serum level of S-100Βincreases in the CSF [10]. This protein has also beenobserved in adipocytes, in skin, and in chondrocytes andmyocytes, although in much lesser quantities [114, 115].During multiple traumas, patients without TBI haveincreased serum S-100B levels, but 6 h after injury thelevel of S-100Β decreases [2]. Thus, early serum sampling(0–6 h after injury) should be avoided because the observedincrease in S-100Β is probably due to extracerebral sourcesand does not reflect evolving brain injury. This result issupported by the work of da Rocha and coworkers, as theirfirst examination time point was 6–24 h (mean time 10.9 h)after injury, and they did not observe differences in S-100Blevels between patients with isolated head injury andpolytrauma victims with severe TBI [37].

Head injury in children

The S-100B level in healthy children (mean of 8.5 years)has a mean of 0.3 μg/L and is inversely correlated with age

[151]. The mean of S-100B is 1.1 μg/L in neonates atdelivery [95]. Diagnosis of inflicted TBI (iTBI) is oftendifficult in children because they present nonspecificsymptoms [43] with an unremarkable physical examination[65, 99]. Berger and colleagues report [16] that, in childrenunder 1 year of age arriving at the emergency departmentwith nonspecific symptoms but with suspicion of TBI,S-100B was neither sensitive nor specific for iTBI, and anincreased level was measured in 90% of patients withnonbrain injury. A significantly longer time to peak serumS-100Β has been reported in children in iTBI versusnoninflicted TBI (nTBI) under 3 years with varying levelsof severity of injury; functional and cognitive tests showedsignificant differences between groups with respect tooutcome [11]. In a study of patients (n=100) after iTBIand nTBI, the sensitivity and specificity of the initialS-100Β for identifying TBI were 77% and 72%, respec-tively [14]. In a clinical analysis of S-100Β in youngchildren between ages of 2 months and 9 years (n=35) aftersevere nTBI and severe iTBI, the peak CSF concentrationwas markedly increased (1.67 ng/mL) in TBI versus thecontrol group [17]. They observed a single peak 27 h afterboth nTBI and iTBI. The mean concentration, peakconcentration, and the time to peak were not associatedwith the mechanism of injury (nTBI vs iTBI). Forty-fivechildren aged 0–13 with varying degrees of severity (mildto severe) and type (inflicted and noninflicted) of headinjury were enrolled into a study where the investigatorsfound a correlation between injury by CT and serumS-100B (mean 0.39 pg/mL), but only a marginally“significant” (p=0.052) correlation was found betweenGCS and the serum S-100B [18]. A study by Spinella andcolleagues evaluated 27 children (mean of 6.7 years) withTBI and an intracranial lesion. Serum S-100B concentra-tions were measured at admission, and outcome wasevaluated using the Pediatric Cerebral Performance Cate-gory (PCPC) score at hospital discharge and 6 months afterinjury. There was a significant difference in S-100Bconcentration between subjects with good (PCPC≤3) andpoor outcome (PCP≥4) 6 months postinjury. Using a levelof 2.0 ng/mL as a cutoff value to predict poor outcome, thesensitivity was 86%, and the specificity was 95% [151]. Arecent study reported that S-100B concentrations in CSFshowed a significant inverse correlation with the initialGCS and 6-month GOS scores in children (n=88) withsevere (GCS≤8) nTBI over 4 years of age, while nocorrelation was found for patients less than 4 years old orvictims of iTBI [148]. However, Berger and colleaguesshowed that the mean concentrations of S-100B weresignificantly greater than normal control concentrations inchildren with mild and severe TBI (n=152), and there wasa significant correlation between outcome and serummarker in children both over and under 4 years [15].


Glial fibrillary acidic protein

A glial fibrillary acidic protein (52 kDa) is a monomericintermediate filament and represents the major part of theastroglial cytoskeleton (Table 1) [98]. GFAP is highlyspecific to the central nervous system [96] and is releasedinto the extracellular space in the event of cell damage.Serum GFAP might be a useful marker for various types ofbrain damage in neurodegenerative disorders [7] and instroke [68].

Animal experiments

A time-dependent release of GFAP into serum has beenreported after severe cortical impact injury in rat. Thehighest GFAP levels (0.079 μg/L) have been measured 1 hafter trauma with biomarker levels significantly elevated inthe early phase [174]. However, GFAP is not correlatedwith severity of trauma.


Serum GFAP levels over 0.033 μg/L are considered path-ological [96]. In a human study, the highest serum GFAPlevel was detected during the first days after severe TBI andthen decreased gradually starting on day 3. Patients withunfavorable outcome had a significantly higher maximalserum GFAP value in the acute phase compared to patientswith favorable outcome. Not only the peak concentrationbut also serum GFAP levels on days 1–8 and 11–14 weresignificantly higher in patients with unfavorable outcome.Those patients with serum GFAP levels over 15 μg/L died[109]. Similarly, it was found by another group that serumlevels of GFAP were higher in nonsurvivors thansurvivors, and this was a predictive marker for mortality.The relationship of GFAP to the Marshall Classification ofcerebral CT showed that GFAP was lower in DiffuseInjury II (cisterns present with midline shift of 0–0.5 cmand/or no focal lesion of >25 mL) than in Diffuse InjuryIV (swelling, midline shift of >0.5 cm, no lesion of>25 mL). Additionally, GFAP was lower in Diffuse InjuryII than in nonevacuated mass lesions. GFAP was associ-ated not only with death versus good to moderateoutcome, but with ICP, CPP, severe disability, andvegetative state versus good to moderate outcome aswell [118].


No significant difference was found between the highestserum GFAP levels in patients with isolated brain injurycompared to multiple-trauma patients with brain injury[109]. In addition, Pelinka and colleagues [119] showed

that GFAP remained completely normal in polytraumatizedpatients without TBI.

Head injury in children and mild head injury in adults

Unfortunately, there is no information available on the levelof GFAP either in children or in mild injury.

Neuron-specific enolase

NSE is a glycolytic enzyme with a molecular weight of78 kDa and a biological half-life of 48 h (Table 1) [143]. Itis functionally active as a heterodimer assembled from α,β, and γ subunits [54]. The γ–γ isoform is specific forneurons, while the α–γ isoform is specific for neuroendo-crine cells [89]. This protein is passively released into theextracellular space only under pathological conditionsduring cell destruction. Levels are higher in the CSF thenin the serum since CSF is anatomically coupled to thebrain, and peripheral sources of the markers have difficultycrossing the blood–brain barrier (BBB).

Animal experiments

In a rat study, the high serum level is correlated with theseverity of the impact and demonstrated a well-defineddistribution in time. The serum concentration level rose toits peak 6 h after the injury [171].


Unfortunately, there are little data on serum NSE level inmild or moderate head-injured patients. Median NSEconcentration is only slightly higher in patients (n=104)with mild injury (9.8 μg/L) than in controls [39]. Afterreviewing four publications on mixed patient populations(mild and severe TBI) [45, 134, 150, 176], Ingebrigtsen andcolleagues concluded that, although the specificity of NSEfor brain is high, its sensitivity is inadequate in mild TBI[75]. A recent study revealed significantly higher serumNSE levels in a group with severe concussion (hospitaliza-tion>1 day) compared to a group of diffuse axonal injury(n=15) [164].


Serum levels above 7–10 μg/L are considered abnormal[108, 128]. NSE in serum peaks within 12 h after injury anddecreases during the subsequent hours. Secondary increasesof NSE have been seen in a few patients with poor or fataloutcomes [94, 172]. A statistically significant elevation wasobserved in serum (mean 12.8 μg/L) and CSF (mean7.8 μg/L) levels in patients after severe traumatic brain

E. Kövesdi et al.

injury (n=51), but only CSF levels showed a correlationwith GCS [134]. In a similar study, serum NSE levels werecorrelated significantly with the injury severity score andCT findings and were significantly higher in nonsurvivors(>21.7 μg/L) and in patients with poor outcome 6 monthspostinjury [166]. A significant correlation between serumNSE and GCS and 3-month outcome has also been reported(n=41) [140]. They associated the secondary increase in themarker with a secondary insult such as hypoxia orhypotension; this increase predicted unfavorable outcome.


Although NSE has the advantage of being neuron specific,it has been established that in the case of both patients andrats with multiple traumas, during hemolysis, hemorrhagicshock, after open femur fracture, and renal or kidneyfailure, it had increased and the raise degree is similar withand without TBI [116, 117]. In patients with multipletraumas but without TBI, the level of NSE increases withinthe first 48 h after the trauma then returns to normal [116].Also, during certain peripheral disorders of the centralnervous system (polyneuropathia, myopathia), an elevatedNSE level can be measured in the CSF [53].


NSE shows different patterns after iTBI (child abuse)compared to nTBI (accidental) in children. In iTBI, theserum levels of both NSE and S-100B show a delayedincrease but of similar magnitude [14]. The sensitivity andspecificity of the initial NSE were 71% and 64%,respectively. Likewise, a significantly longer time to peakserum NSE has been reported in children with iTBIcompared to nTBI in children under 3 years and withvarying severity of injury [11]. In infants who are atincreased risk for iTBI, both CSF and serum NSEconcentrations (77% sensitivity, 66% specificity) have thepotential to be used as screening tests [16]. Neither NSEnor S-100B in CSF however correlate with initial GCS and6-month GOS scores in children under 4 years with iTBI[148]. The same study reported that NSE concentrations inCSF show a significant inverse correlation with the initialGCS and 6-month GOS scores in children with severe(GCS≤8) nTBI over 4 years of age (n=88), while nocorrelation was found for patients less than 4 years. Serialserum sampling of NSE in children (n=152) with mild(61%) and severe (30%) TBI shows that higher concen-trations are associated with worse outcome (GOS andextended GOS (GOSE) Peds). However, the initial andpeak NSE concentrations (mean 29.1 ng/mL) showed astronger correlation with outcome in children less than4 years than those >4 years of age [15]. Another study

(n=86, age 0–18) found that serum NSE level of 21.2 ng/dL was 86% sensitive and 74% specific in predicting pooroutcome (GCS=5 vs GCS<5)[5].

Spectrin and its breakdown products (SBDPs)

Alpha II-spectrin is expressed in brain and other non-erythroid tissues. The nonerythroid alpha II-spectrin(280 kDa) is the main component of the cortical cytoskel-eton and a substrate for the calcium-activated cysteineproteases, such as calpain and caspase-3 [167]. It is foundprimarily in presynaptic terminals or in the subaxolemmalcompartments of the axon [42, 61, 175]. Calpain-mediatedcleavage of intact spectrin (280 kDa) results in 150- and145-kDa fragments specific for calpain [64], whereas thecaspase-3-specific products are linked to 150- and 120-kDafragments (Table 1) [167].

Animal experiments

Using antibodies targeting alpha II-spectrin breakdownproducts, numerous studies have demonstrated that non-erythroid alpha II-spectrin and SBDPs are elevated in vitroafter mechanical injury [26, 122], as well as in CSF andbrain following experimental TBI [79, 106, 123, 137]. Boththe direct and indirect inhibitors of calpain (cyclosporine A,MDL-28170, PACAP) inhibit the proteolytic breakdown ofspectrin [29–31, 120]. Following controlled cortical impact,traumatic brain injury in the rat the level of alpha II-spectrindecreased in brain tissue and increased in CSF from 24 to72 h after injury [121]. Significant elevation of calpain-specific SBDPs level in the CSF within 24–72 h followingthe injury appeared to be correlated with the intensity of theimpact and the size of the lesion. In a similar injury model,the level of SBDPs are increased significantly with injuryseverity both in the ipsilateral cortex and the CSF withSBDP levels peaking in the CSF 2 h after the trauma [131].The injury severity and the levels of SBDPs correlated withthe size of the lesion as evaluated by T-2 weighted magneticresonance. The level of SBDPs in the ipsilateral cortex hasbeen increased slowly in the first 6 h after TBI. Decreasedmotor performance (Rotarod) test correlated with injuryseverity and lesion size 1–5 h after TBI.


In severely head-injured patients (GCS<9) with elevatedICP, CSF has been analyzed for spectrin and SBDPs levelsthrough a daily examination. In serum and CSF, a well-defined time distribution has been observed. Spectrin(280 kDa) and SBDPs (120, 145, 150 kDa) peak on thesecond and third day. Elevated serum levels were observedin patients with high intracranial pressure but without TBI,


although these serum levels were significantly lower thanSBDP levels in patients with TBI. Linear regressionanalysis does not reveal a significant correlation betweenthe accumulation of various SBDPs and injury severity(GCS) and outcome (GOS), most probably due to therelatively low number of patients [48]. CSF alpha ΙΙ-spectrin and SBDP150 are significantly increased inpatients with severe head injury (GCS≤8) examined at alltime points (6–96 h) and correlate well with outcome(dichotomized GOS) [34]. Pineda and colleagues show thatin patients with severe head trauma (GCS≤8) the level ofSBDP150, SBDP145, and SBDP120 in CSF peaks 6 h afterthe injury with SBDP150 and SBDP145 remaining signif-icantly elevated for 24 and 72 h, respectively, andSBDP120 remains elevated for at least 5 days postinjury.The elevation of SBDP145/150 in CSF correlates signifi-cantly with the initial severity of injury by CT and the 6-month outcome (GOS), while no similar correlation wasfound for SBDP120. The 12 h postinjury mean values ofSBDP145/150 in CSF were the strongest predictors ofseverity of outcome [124].

Head injury in children and mild injury in adults

Unfortunately, there are no data available in mild ormoderate head injury in adults such as in child TBI. Theobservation in animals with mild/moderate head trauma ispromising for the human, but monitoring of the above-mentioned groups is needed.

Cleaved tau protein

Microtubule-associated tau proteins (MAP-tau) are locatedprimarily in the axonal compartment [20, 86]. Functionally,MAP-tau binds to axonal microtubules and results in theformation of microtubule bundles, which are importantstructural elements of the axonal cytoskeleton and criticalelements of axoplasmatic flow of proteins between axonterminal and cell body [91]. Tau proteins contain severalphosphorylational sites whose pathological hyperphos-phorylation plays an important role in the pathogenesisof neurodegenerative disorders [28, 146]. Map-tau isexpressed from a single gene that undergoes alternatesplicing and results from alternate splicing of MAP-taumRNA to six tau isoforms with molecular masses from 48to 68 kDa [60]. TBI in humans results in the proteolyticcleavage of these six isoforms; that is the reason forreferring to the cleaved forms as cleaved tau (c-tau) [49,178]. The molecular weight of c-tau in humans after TBIis 30–50 kDa (Table 1), but in studies with rats theproteolytic cleavage results in 40–56-kDa proteins [57].The homology between human and rat c-tau is more than90% [178].

Animal experiments

Neuronal damage after TBI results in a rapid loss of axonalmicrotubules and MAPs which are common features ofhead trauma [125, 126]. In experiments with rats afterdifferent severity of CCI injury, the level of brain c-tau inthe cortex and hippocampus increases with the severity ofinjury. C-tau levels increase as early as 6 h after TBI andpeaking 168 h after injury. In addition, serum c-tau levelsare significantly increased 6 h after TBI but not at later timepoints [57].


A pilot study of 35 patients with mild (GCS≥13) headinjury did not show significant correlation between serumlevels of c-tau (mean 4.85 ng/mL) and 3-month postcon-cussive syndrome. Using the lowest detectable level ascutoff, the sensitivity and specificity of this marker in mildTBI are 43.8% and 71.4%, respectively [8]. Kavalci andcolleagues showed elevated serum tau (18.39 pg/mL) levelsin patients (n=55) with mild injury (GCS≥13) andintracranial pathology by CT, but this increase was notsignificantly different from the serum levels obtained inpatients (n=33) with mild injury but without intracranialpathology. Statistical analysis also did not reveal asignificant difference between sex, age, mechanism oftrauma, and GCS score either [81]. In a similar study of60 patients with mild injury, mean serum tau (188 pg/mL)levels were not significantly higher compared to the controlgroup. However, a significant difference was foundbetween high-risk and low-risk patients. There were nosignificant correlations found between serum tau levels andpostconcussive syndrome [33].


Zemlan and colleagues demonstrated in human studies thatthe level of c-tau in the CSF increased by approximately40,000 times during the first 24 h after severe injurycompared to elevations in other neurological disorders andin healthy control group [177]. C-tau levels steadily declinedduring the first 3 days after brain injury. Initial c-tau levelscorrelated significantly with the subsequent intracranialpressure. This study found also a high correlation betweeninitial c-tau level and clinical outcome (dichotomized GOS)demonstrating a sensitivity of prediction of 92% and aspecificity of 94%. The c-tau cutoff value for predictingbetter outcome (GOS 1–3) was 1,600 ng/mL. In a pilot studyof adult patients with closed head injury, the initial c-taulevel of more than zero was significantly correlated with agreater chance of intracranial injury on the initial head CTand a poor outcome (disabled or death) [147]. Patients

E. Kövesdi et al.

(n=39) with severe TBI were included in a study where asignificantly higher ventricular CSF (vCSF) were found inTBI patients compared to control. A correlation was foundbetween initial c-tau and the 1-year GOSE. A vCSF total taulevel (>2.12 pg/mL) on days 2 to 3 discriminated betweendead and alive (sensitivity of 100% and a specificity of 81%)and the tau level of >702 pg/mL discriminated between bad(GOSE 1 to 4) and good (GOSE 5 to 8) outcome (sensitivityof 83% and a specificity of 69%) [111].


Unfortunately, there are no data available on c-tau level inchildren with TBI.

Amyloid-β1–42

The amyloid precursor protein (APP) is a well-known celladhesion protein observed in larger quantities in synapticmembranes. The Aβ1–40 (4.3 kDa) and Aβ1–42 (4.5 kDa)with 40- and 42-amino-acid lengths are the breakdownproducts of one of the two alternative proteolytic cleavagepathways of APP (Table 1) [59, 158]. Caspase-cleaved APPand Aβ accumulate and colocalize with activated caspase-3and other APP-cleaving enzymes in axons and neuronalsoma after TBI [36, 154]. An increase of activated caspase-3 and Aβ could create a continuous cycle of neuronalinjury after TBI and contribute to the initiation ofAlzheimer's disease (AD)-like pathology over a prolongedperiod of time. In certain neurodegenerative diseases suchas in the Creutzfeld–Jacob disease [112], amyotrophiclateral sclerosis [149], and multiple system atrophy [69],low Aβ CSF levels can be observed. However, there areinconsistencies in the AD literature regarding the level ofthis in the CSF [78, 80, 165]. It should be also noted thatsevere head trauma is a recognized risk factor for AD, as itmore than doubles the risk for developing AD dementia[132, 144].

Animal experiments

An immunohistochemical study in pigs after inertial braininjury revealed the accumulation of amyloid precursorproteins, and Aβ peptides colocalized with β-secretaseprimarily in the swollen ends of disconnected axons [36].Similarly, an increased expression of β-secretase has beenfound in rat hippocampus and cortex after TBI [21]. Innontransgenic mice expressing human Aβ peptide, thebrain tissue level of Aβ1–42 peaks 3 h after controlledcortical injury and remains above sham level for 6 to 12 h.Following a slow secondary increase between 12 and 72 h,the level remains elevated [1]. Unfortunately, data on CSFlevels of Aβ peptides in animal experiments are lacking.


A significant decrease of Aβ1–42 in the CSF has beenobserved in patients (n=29) with severe TBI (GCS≤8)at all time points (median 4.7 days). Below a cutoff of230 pg/mL, the sensitivity of Aβ1–42 to discriminatebetween GOS 4–5 and GOS 1–3 is 100% at a specificityof 82% [56]. In another series of patients (n=13) aftersevere head injury (GCS≤8), the CSF level both of Aβ1–40

and Aβ1–42 decreases for the first 5 days after TBIcompared to noninjured controls. The decrease in Aβ1–40

concentration correlates only with the severity of injury(GCS), with no significant correlation between the Aβpeptides and outcome (GOS) [82]. It has been speculatedthat this reduction may reflect adsorption onto amyloidplaques [41, 46, 100]. However, another study reportedincreases in Aβ1–42 CSF levels (mean 1.17 ng/mg) inpatients with severe TBI (GCS<9) compared to controlsubjects. In addition, the Aβ1–40/Aβ1–42 ratio decreasesabout fivefold in trauma patients [129]. Increase in Aβ1–42

was also observed in a study of Emmerling [44]. Olssonand colleagues also found a significant stepwise increaseof Aβ1–42 in ventricular CSF reaching over 1,000% 5 daysfollowing TBI, with a mean concentration of 11–129 pg/mL. By contrast, plasma Aβ1–42 remained unchanged in thepatients (GCS≤8). Interestingly, there was no significantchange in maximal vCSF-Aβ1–42 level between apolipo-protein E (apoE) ε4 carriers and non-apoE ε4 carriers; norwas there any significant correlation between maximalvCSF Aβ1–42 and injury severity (Marshall score 1–2 vs3–4) [110]. The increased level of CSF Aβ1–42 may occuras a secondary phenomenon after TBI with axonal damageor may be related to the disruption of the blood–brainbarrier [44, 110]. The remarkable difference between Aβlevels reported by different authors might be attributable tothe different method of CSF sampling (ventricular, lumbar,or mixed) or the altered CSF circulation after TBI (thepresence of posttraumatic hydrocephalus, etc.) [23].


Data on Aβ1–42 in mild injury and pediatric TBI are lackingin the literature.

apoE

apoE (34 kDa) is a primary apolipoprotein synthesized inthe CNS (Table 1) [142]. This protein has been shown to bea neuroprotective agent acting as an antioxidant, an anti-inflammatory, and an anti-excitotoxic, with neurotrophicproperties [3, 87, 88, 92, 97, 104]. The protein has threecommon isoforms (apoE2, E3, E4), encoded by threealleles (ε2, ε3, ε4) of a single gene on chromosome


19q13.2. Various combinations of any two of the threemajor alleles give one of six possible genotypes [104].

Animal experiments

The exact function of apoE after brain injury is not wellelucidated but there is experimental evidence indicating arole in the clearance of lipid and cholesterol debris after theinsult and recycling of the lipid to injured cells undergoingrepair [169]. The relevance of these findings are supportedby the observation that apoE immunoreactivity is increasedin patients with fatal outcome after TBI, global ischemia, andherpes encephalitis [70, 71, 107], and the concentration ofapoE in the CSF of patients with neurological diseases (AD,multiple sclerosis) is less than that of control subjects [22,130]. Interestingly, the presence of the apoE ε4 allele hasbeen associated with increased Αβ deposition in the cerebralcortex and with unfavorable outcome after TBI [159]. apoEε2 appears to have a protective effect whereas apoE ε4 isconsidered to be the major risk factor in AD [72].


Kay and colleagues demonstrated in patients (n=13) withsevere head injury (GCS≤8) that the CSF level of apoEconcentration is significantly lower 2 days after injury thanin a control group and is sustained for 5 days. They did notfind any correlation between APOE genotype and injuryseverity or clinical outcome [82]. In a similar study, TBIpatients (n=27) had a fivefold decrease in CSF concentra-tion of apoE (3.7 mg/L) compared to control subjects(12.4 mg/L). No significant difference has been observedbetween patients with or without the apoE ε4 allele, andthere does not appear to be a correlation between CSF apoEconcentration, injury severity, or clinical outcome [83].


apoE as a possible protein biomarker in TBI has not beenwidely investigated so far. Very little data are available inthe literature, with articles about children and mild,moderate, and multitrauma patients lacking.

Discussion

The present review shows that there has been an increasinginterest in biochemical markers for traumatic brain injuryduring the past 10 years. Presumably, the successivefailures in finding neuroprotective compounds [160] havedrawn extra attention to new biological markers that couldserve as adequate end points both in animal and in humanstudies. At the same time during the treatment of head-injured patients, such biomarkers could provide assistancein the establishment of diagnosis, in the selection oftherapeutic intervention, and in the prediction of outcome.Since the systematic review by Ingebrigtsen et al. in 2002,several new markers have been introduced [74]. Earlier inthe research of biomarkers, enzymes (NSE, CK, lactatedehydrogenase) and structure proteins (GFAP, S-100B) hadbeen tested; more recent study have focused on degradationproducts (SBDP, c-tau, Aβ1–42) since protein breakdownproducts generated during proteolysis can be separated andidentified with the application of modern molecular biologytechniques.

The general problem is that both the animal models(severity, diffuse and focal damage, different neurotrau-matic model) and the patient population (variant patientgroup size and severity, therapeutic window, follow-upperiod, etc.) are heterogeneous, making comparison of theresults difficult.

Table 2 Summary of literature on biomarkers in mild head injury

Biomarker Number ofpublications

Primary end points(injury measures)

Secondary end points(outcome measures)

Levels in serum

S-100B 16 Conflicting (2 vs 3) Conflicting (3 vs 6) >0.25 μg/L increased

GFAP No data No data No data >0.033 μg/L increased

NSE 6 No data No data >7–10 μg/L increased

SBDPs No data No data No data No data

Cleaved tau 3 No correlation (1) No correlation (2) Mean 18.3–188 pg/mL

Amyloid-β1–42 No data No data No data No data

ApoE No data No data No data No data

The table contains the total number of reviewed publications which related to the clinical utility of the biomarkers in mild head injury. The tablealso references data on serum/CSF levels. In addition, it shows a summary of a relationships between biomarkers and both primary (GCS, CT) andsecondary (GOS, RPSQ, etc.) end points. These relationships are defined as significant correlation, no correlation, or conflicting. The number ofreferences is put into parenthesis

GFAP glial fibrillary acidic protein, NSE neuron-specific enolase, SBDPs spectrin breakdown products, ApoE apolipoprotein E

E. Kövesdi et al.

There is definitely a need for improvement especially inthe design of trials in patients with mild head injuries(Table 2). Most studies focus on S-100B and NSE.Unfortunately, all of the NSE and more than half of S-100B studies lack the measurement of primary andsecondary end points. The results with S-100B areconflicting, and the marker seems to be inadequate in mildTBI. Similarly, the results with c-tau are disappointing. CSFsampling in mild injury is considered invasive and thereforethe possible use of SBDPs, Aβ1–42, and apoE are limited inthis patient population. Both animal and clinical studiesshould be encouraged with GFAP and neuroproteomicstudies should be facilitated in search for new biomarkers inmild TBI. The exclusion criteria (alcohol consumption,stroke, trauma, or other neurological disorder in thepatient’s anamnesis) in mild TBI are not well specified.While severe TBIs are defined fairly uniformly with a GCSscore (≤8), mild TBI is defined very differently by differentinvestigators, making comparison of studies and/or meta-analysis difficult. In addition, the GCS score is a grossmeasure of injury and is insufficient to detect subtle

neurological signs (latent paresis, etc.) in mild TBI(GCS≥13). While there are a variety of validated tests tomeasure neuropsychological outcome and postconcussivesymptoms, it is difficult to directly compare them becauseof their high interobserver variability. Long-term outcomein mild head injury can be also influenced by factors suchas chronic stress, and the psychosocial environment inwhich the patient recovers. Since protein biomarkers bythemselves have failed to be good predictors, the combineduse of different markers (serum markers, radiological,clinical, etc.) should be facilitated in future studies.

The potential correlation of markers with injury andoutcome measures in severe head injury is promising(Table 3). The S-100B has been intensively investigatedin severe head injury. Several papers found significantcorrelation unanimously between the marker and both theinjury and the outcome. Therefore, multicenter investiga-tion and the introduction of S-100B into the clinical routineshould be considered with caution that polytrauma can alsocontribute to the higher serum levels. There are alsopromising results with GFAP, NSE, and c-tau; however,

Table 3 Summary of literature on biomarkers in severe head injury




Levels in serum/CSF*

S-100B 20 Significant correlation (4) Significant correlation (7) >2–2.5 μg/L unfavorable outcome

GFAP 5 Significant correlation (1) Significant correlation (2) >15.04 μg/L unfavorable outcome (death)

NSE 9 Significant correlation (2) Significant correlation (2) Mean 12.8 μg/L, mean 7.8 μg/L*

SBDPs 3 Conflicting (1 vs 1) Conflicting (2 vs 1) Data available only in arbitrary densitometric units

Cleaved tau 3 Significant correlation (1) Significant correlation (3) >2.12 pg/mL* unfavorable outcome (death)

Amyloid-β1–42 2 No correlation (2) Conflicting (1 vs 1) Conflicting a (2 vs 3)

ApoE 5 No correlation (2) No correlation (2) Mean 3.7 mg/L*

The table contains the total number of reviewed publications which related to the clinical utility of the biomarkers in severe head injury. The tablealso references data on serum/CSF levels. In addition, it shows a summary of a relationships between biomarkers and both primary (GCS, CT) andsecondary (GOS, GOSE, etc.) end points. These relationships are defined as significant correlation, no correlation, or conflicting. The number ofreferences is put in parenthesis

GFAP glial fibrillary acidic protein, NSE neuron-specific enolase, SBDPs spectrin breakdown products, ApoE apolipoprotein Ea Decrease vs increase

Table 4 Summary of literature on biomarkers in inflicted and noninflicted (accidental) head injury in children regardless of the severity




Levels in serum/CSF*

S-100B 7 Conflicting (1 vs 1) Significant correlation (3) >2 ng/mL poor outcome, mean 0.39 pg/mL mean, 1.67 ng/mL*

NSE 6 No data Significant correlationa (3) >21.2 ng/dL poor outcome, mean 29.1 ng/mL

The table contains the total number of reviewed publications which related to the clinical utility of the biomarkers in children with head injury.The table also references data on serum/CSF levels. In addition, it shows a summary of a relationships between biomarkers and both primary(GCS, CT) and secondary (GOS, GOSE Ped, PCPC, etc.) end points. These relationships are defined as significant correlation, no correlation, orconflicting. The number of references is put in parenthesis

NSE neuron-specific enolasea Conflicting with respect to age


this cohort studies fall behind S-100B in number andfurther investigations are needed. At the same time, the vastmajority of human studies in severe TBI are small withlimited statistical value. Another major limitation is thevery different inclusion and exclusion criteria. Although allthe patients in the studies had severe TBI (GCS<8), theage, sex, and race are often inhomogeneous. In addition,other nontraumatic neurological insults such as posttrau-matic seizures or hypoxemia, posttraumatic hydrocephalus,and other neurological disorders in the anamnesis oralterations from therapeutic guidelines can all influencethe level of the markers and the outcome. The commonpractice of dichotomizing outcome (“dead or alive”) shouldbe revised; future studies should focus more on functional“quality-of-life” measures. In many of the studies, cutoffvalues to predict outcome were determined retrospectivelyin order to maximize specificity and sensitivity. Theprospective determination of the clinically relevant cutoffvalues is preferable [13].

The number of publications related to serum biomarkersin pediatric TBI is small and has been limited to fewbiomarkers (S-100B and NSE; Table 4). Only fewpublications report significant correlation between themarkers and the outcome. Studies with other biomarkercandidates (protein degradation products, etc.) should beinitiated. The investigation of TBI in children is morecomplex than in adults. Head trauma is often a result ofchild abuse, and the complexity of iTBI imposes adifferential diagnostic burden on the physicians, andobtaining consent is also difficult. Thus, the victims ofchild abuse are less likely included in clinical trials than thevictims of accidental injury. Taking repeated samples ofblood from young children and particularly in infants mightbe difficult, and serum markers (for example, S-100B) mayshow a correlation with age. There are also inconsistenciesin the literature with respect to the relationships betweenoutcome measures and different age groups. Unfortunately,the inclusion criteria are also very often inhomogeneous inchild studies with respect to age, severity, and type ofinjury. Neurocognitive outcome studies are difficult toperform in young children, and functional deficits mayremain hidden/latent in children for years after injury [13].

In conclusion, a systematic review of the literatureindicates that intensive work needs to be done in the futureto improve the utility of markers in clinical decisionmaking. Unfortunately, only S-100B in severe TBI hasconsistently demonstrated the ability to predict injury andoutcome in adults. The most extensive work was requiredin mild TBI because of the low number of cohort studiesand the conflicting results. The number of studies withprotein degradation products is insufficient, especially inthe pediatric care. The critical analysis of the study designsreveals that large homogenous patient populations, more

consistent injury and outcome measures, prospectivelydetermined cutoff values, and a combined use of differentpredictors should be considered in future studies.

Acknowledgements This work was supported by the HungarianScience Funds (OTKAT048724/2005 and OTKA 72240). The authorswish to thank to Joel H. Greenberg Ph.D., Katalin Kariko Ph.D., andBrian Edlow M.D. for critical review of this manuscript.

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Update on protein biomarkers in traumatic brain injury with emphasis on clinical use in adults and...

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