As mentioned previously, TBI is structurally characterized by a primary injury resulting from a direct or indirect biomechanical force on the brain matter itself (). Based on the above definition, a practical and preventive solution for reducing primary injury would be to improve the use of protective head-gear during high-risk activities where TBI has a high likelihood of occurrence. In connection with the primary injury, TBI is associated with several secondary events that occur with some delay and often have an extended duration after the primary impact, thus allowing a large window of opportunity for therapeutic interventions. We have provided a timeline of events in , summarizing the current data available from both clinical and experimental models of TBI. As depicted in , the underlying mechanisms accounting for behavioral changes in the first days and weeks after brain trauma are various and may range from: changes in cerebral blood-flow (CBF) associated with hypo-metabolism, increased intracranial pressure, edema formation and brain swelling, as well as inflammation and related events at the molecular level such oxidative stress, excitotoxicity, apoptosis, and neuropathology [38
]. These changes are observed in the pediatric and adult populations and in experimental models with diverse levels of severity (), although very little data exists for direct comparisons of pathophysiological cascades between the pediatric and adult populations. As briefly summarized in and throughout the manuscript, younger individuals are more vulnerable to TBI and have more severe outcomes following injury than adults as underlined by mortality risk, neurobehavioral function, cerebral blood flow and edema.
Significant changes in CBF can lead to either decreased or increased ischemic or oligemic levels, depending on injury severity, and on the time and the anatomical location of the CBF measurement [57
]. Clinical and experimental data indicate that the most profound reductions in CBF are found in and around contusion injuries. On the other hand, diffuse and mild injuries result in very low reductions or even increases in blood flow, at least at initial time points following TBI [41
]. Several works highlight modifications not only in CBF, but impaired cerebral autoregulation in adults [63
] and in the pediatric population [64
] with young age being a significant predictor of CBF dysregulation [67
]. In children, this impaired cerebral autoregulation is further associated with overall poor outcome [64
], which is confirmed by studies in animal models such as the juvenile piglet with a fluid percussion injury (FPI) [70
There are several possible molecular explanations within the NVU for TBI-related changes to CBF. Alterations to endothelin-1, decreased nitric oxide (NO) levels, cyclic guanosine monophosphate, cyclic adenosine monophosphate, and changes in K+ channel activation can affect tone of the cerebral vasculature and therefore the cerebral autoregulatory capacity [71
]. NO signaling has been well demonstrated as a regulator of vascular tonus in the periphery and central nervous system [75
], and this molecule is synthesized from L-arginine by endothelial, neuronal, or inducible NO-synthases (NOS) [76
]. Following TBI, the activity of endothelial NOS (eNOS) increases briefly for a few minutes, then decreases to ~50% of baseline for 7 days before it’s levels normalize [77
]. These decreases of constitutive NOS activity may contribute to altered CBF and cerebral autoregulation. Therapeutically, it may be possible to compensate for the decrease in NO after TBI by administration of NO donors like sodium nitroprusside (SNP), and thus improve CBF and cerebral autoregulation. Indeed, administration of SNP prevented the reduction of CBF, but could not reverse autoregulatory impairment during hypotension after FPI in the juvenile piglet model [79
]. Additionally, while eNOS activity remains suppressed for up to 7 days, simultaneous increases occur for inducible NOS (iNOS) expression and activity in neurons, macrophages, neutrophils, astrocytes, and oligodendrocytes, reaching peak levels between 4h and 48h after injury [78
]. Unfortunately, up-regulation of iNOS results in a harmful increase of tissue NO [78
], well known to contribute to neuroinflammation, apoptosis, excitotoxicity, energy depletion, and uncoupling of NOS with subsequent production of reactive oxygen species (ROS) [83
]. These findings illustrate the complexity of the role of NO in TBI pathophysiology, and the challenges for remedial applications of NO therapies.
CBF changes after TBI may also be related to changes in basic properties of the cerebral vasculature and decreased CBF early after injury is a common signature of TBI in adults and juveniles. For example, TBI in juvenile animals impaired N-methyl-D-aspartate dependent pial dilation and reversed it to a vasoconstriction, partly mediated by tissue type plasminogen activator (tPA) through the activation of the mitogen activated protein kinase (MAPK) family including JNK and ERK [79
]. In a cortical contusion model, decreased CBF was characterized by lack of perfusion in the core within minutes of injury [41
] indicating that high reduction in CBF close to the impact site often reaches an ischemic threshold. Alternatively, other models showed a widespread reduction in CBF involving the entire brain, without reaching ischemic values in most cases, and with recovery over time [60
]. In addition to dysfunction in the larger blood vessels, several studies have shown vasoconstriction, compression of microvessels by swollen astrocytes in the NVU, and obstruction of microvessels by microthromboses that may be responsible for peri-contusional ischemia. A recent study elegantly showed that immediate post-TBI decreases in peri-contusional blood flow were not caused by arteriolar vasoconstriction, but rather by injury-induced formation of microthrombi in 33% of arterioles and by rolling leukocytes and platelet activation in 70% of venules [87
Alterations in CBF may contribute to and be exacerbated by secondary injury, as decreased blood supply is associated with reduced energy metabolism in the brain tissue of several TBI models [88
] (). During the first week after TBI, glucose metabolism is likewise impaired in adults and juveniles both in the clinic and laboratory models. For example, poor neurological outcome was associated with increased lactate, measured by proton spectroscopy, in infants and children 6 and 9 days after closed head injury [88
]. In juvenile rats, a time course of brain metabolites also revealed global increases in lactate (in both ipsi and contralateral hemispheres to the injury) at 4h until 24h after TBI [91
]. Although some debate exists concerning the interplay of glucose versus lactate post-injury, increased lactate can result from increased glycolysis, a consequence of the decreased CBF described above. Altogether, these data show that TBI-induced changes in basic neurovascular properties can lead to widespread functional damage with visible consequences on tissue integrity.
As observed in stroke, TBI is associated with an inflammatory response in the NVU involving leukocyte accumulation in the brain tissue. More specifically in TBI, leukocyte infiltration is observed at 12 hours until several days after the injury. However, leukocyte recruitment is preceded by the activation of pro-inflammatory cytokines secreted by the glial cells, such as tumor necrosis α (TNFα), interleukin 1β (IL1β) and interleukin 6 (IL6). These cytokines activate the expression of specific leukocyte adhesion factors on the endothelial cells, such as intercellular adhesion molecules (ICAMs) and E-selectin [93
]. In addition, these pro-inflammatory cytokines may trigger chemokine synthesis, such as interleukin 8 (IL8) and macrophage inflammatory proteins (MIPs), which serve as chemo-attractive molecules contributing to the migration of leukocytes from the blood stream into the central nervous system [93
]. Thus, the intracerebral accumulation of leukocytes is considered part of the secondary injury cascade following the disruption of the BBB and vasogenic edema.
Inflammatory responses after TBI can contribute to oxidative stress, as leukocytes themselves can produce free radicals such as superoxide dismutase (SOD) and nitric oxide [93
]. In turn, free radicals induce lipid peroxidation and damage several cell types in the NVU, including the endothelial cells of the cerebral microvasculature. While leukocyte inhibition is beneficial toward improved neurological outcome for stroke [93
], a similar treatment is not beneficial for TBI and must be evaluated in the context of differences between the nature of the primary injury. While there are many similarities between stroke and TBI at the molecular level, TBI often involves the local destruction of the blood vessels and presence of bleeding which contrasts with cerebral blood flow and reperfusion patterns during stroke.
A notable consequence of TBI-induced changes in CBF and infiltration of peripheral immune cells is the formation of brain edema, which is a common feature of acquired brain injury and has a crucial impact on morbidity and mortality [94
]. A local perturbation of the brain environment usually induces regional edema [95
], which leads to expansion of brain volume. These events have a vital influence on morbidity and mortality as they increase intracranial pressure (ICP), accelerate herniation, and contribute to secondary injuries such as ischemia [96
]. Despite its complexity, brain edema has been defined as an increase in net brain water content, which leads to an increase in tissue volume [97
]. TBI in infants and children is more frequently associated with severe and widespread brain swelling than in adults [98
]. Two mechanisms may account for these age-related differences: changes of CBF post injury in the young and developmental and mechanical properties of the brain and skull [100
]. Experimental studies have suggested that post-TBI edema in the immature brain also may be related to enhanced diffusion of excitotoxic neurotransmitters, an intensified inflammatory response, and increased BBB permeability [100
In the past decade, the discovery of brain aquaporin (AQP) channels in astrocytes suggests a new hypothesis that water channels contribute substantially to edema formation and may potentially account for the exaggerated injury response seen in pediatric patients. This suggestion is supported by experimental studies which demonstrate that AQP4 expression increases with development [101
] and that brain water content is higher in the young rat compared to the adult [102
]. AQP1, AQP4, and AQP9 have been identified in the rodent brain [103
] and disordered expression of these AQPs has been found in several conditions such as stroke, trauma, brain tumors, and subarachnoid hemorrhage (SAH) [104
]. AQPs in the rodent brain are thought to play an important role in extracellular water homeostasis and sustaining normal neuronal activity [105
] and are likely involved in water movement during the formation and resolution of cerebral edema. AQP9 has been observed in astrocytes and in catecholaminergic neurons [103
] and its astrocytic expression is increased after stroke [106
] but little is known about the role of AQP9 in brain disorders [107
One of the most intensely studied brain AQPs is AQP4, which is found on astrocytic end-feet [94
]. Notably, astrocyte end-feet are a critical part of the NVU and cover ~98% of the endothelial vascular wall in the BBB [108
]. AQP4 has altered expression after trauma [109
], ischemia [106
] and human SAH [104
]. After transient cerebral ischemia in mice, we showed AQP4 expression peaked at 1h and 48h, temporally coinciding with maximal hemispheric swelling [106
]. This temporal evolution of AQP4 differs from that seen in trauma where there is an initial decrease at 48 hours, followed by an increase [109
]. In contrast, AQP9 after ischemia shows a significant induction at 24h that increases gradually, with no correlation to the degree of swelling [106
], suggesting that AQP4 but not AQP9 plays a more direct and significant role in edema formation. In adult trauma models, the role of AQP4 remains unclear. However, the absence of AQP4 in AQP4-knockout (KO) mice is protective in decreasing edema formation and lesion size in a model of spinal cord injury (SCI) [115
]. This data contrasts with a recent paper showing a better functional recovery for the wild type mice compared to the AQP4-KO animals after contusion SCI, suggesting that AQP4 plays a protective role by facilitating the clearance of excess water [116
]. Indirectly in support with this study, post TBI edema was decreased using sulforaphane, an abundant isothiocyanate present in cruciferous vegetables such as broccoli, which correlated with an increase in AQP4 expression [117
]. In contrast, progesterone administration reduced edema at 24h and 72h after TBI but was only associated with a decrease in AQP4 expression at 72h [118
]. Recently, brain AQP4 expression was decreased in vivo
after injection of small interference RNA (siRNA) against AQP4 [119
]. The next step will be to inject siRNA against AQP4 to prevent edema formation after TBI.
These reports show the multifaceted role of AQP4 in the NVU to appropriately maintain water homeostasis under normal and abnormal conditions. An important clinical question relates to the role of each AQP in edema formation and resolution profiles of different injuries and/or diseases. One reason for these discrepancies between AQP4 expression and TBI-induced edema may be due to differences in the experimental models. Overall, it appears that the presence of AQP4 is deleterious in the formation of cytotoxic edema, whereas the presence of AQP4 is important in resolution of vasogenic edema occurring after initial opening of the BBB post-injury [94
]. These findings strengthen the importance of studying AQP4 expression independently in each model and determining its precise role in edema formation, to gain a better understanding of AQP4 modulation of edema depending on injury type and developmental age.
The post-traumatic changes described in the NVU are mostly observed during the first week after injury. Additional observations during this time-point include BBB dysfunction involving a physical “opening” of the barrier shortly after TBI (see below for details). However, the evolution of these changes in NVU over a long period of time is still unknown. As mentioned in the introduction, additional evidence is also needed to show whether one TBI can induce long-term changes to the NVU that ultimately affect behavioral outcome.