Traumatic brain injury (TBI) accounts for more than two million hospital visits each year in the United States alone. Mild TBI commonly resulting from car accidents, sporting events, or falls represent approximately 80% of all TBI cases1. Mild TBI is considered the ‘silent disease’ as patients often experience no overt symptoms in the days and months following the initial insult, but can develop serious TBI-related complications later in life2. Moreover, blast-induced mild TBI is prevalent among military service-members, and has been associated with chronic CNS dysfunction3,4,5,6. Due to the rising incidence of blast-related mild TBI7,8, preclinical modeling of neurobiological and pathophysiological processes associated with mild TBI has thus become a focus in the development of novel therapeutic interventions for TBI.
Historically, TBI research has primarily focused on severe forms of neurotrauma, despite the relatively lower number of severe human TBI cases. Preclinical rodent models for severe human TBI have been developed, including the controlled cortical impact (CCI)9,10 and fluid percussion injury (FPI)11 models, which are both well established to produce reliable pathophysiological effects12,13. These models have laid the groundwork for what is known today about neuroinflammation, neurodegeneration, and neuronal repair in TBI. Although considerable knowledge of the pathophysiology of TBI has been developed, there are currently no effective, FDA-approved treatments available for TBI.
More recently, the focus of TBI research has been broadened to include a wider spectrum of TBI-related pathologies with the ultimate goal of developing effective therapeutic interventions. Nevertheless, few preclinical models for mild TBI have been established that have shown measurable effects, and only a small number of studies have investigated the mild TBI spectrum2,14,15. As mild TBI accounts for the large majority of all TBI cases, reliable models of mild TBI are urgently needed to facilitate research into the etiology and neuropathophysiology of the human condition, in order to develop novel therapeutic strategies.
In conjunction with biomedical engineers and aerospace physicists, we have established a scalable, closed-head blast wave model for mild to moderate TBI. This preclinical rodent model has been specifically developed to investigate the effects of force dynamics, including blast waves and acceleration/deceleration movement, that are associated with human mild TBI obtained in military combat, sporting events, car accidents, and falls. As blast waves correlate with the force dynamics that cause mild TBI in humans, this model was designed to produce a consistent Friedlander waveform with an impulse, which is measured as pounds per square inch (psi)*millisecond (ms). The impulse level is scaled to fall below defined lung lethality curves for mice and rats in order to conduct preclinical investigations16,17,18. In addition, this model allows for investigation of coup and contrecoup injury due to rapid rotational forces of the animal’s head. This kind of injury is inherent to several types of clinical TBI presentations, including those observed in both military and civilian populations. Therefore, this versatile model fits a need that encompasses multiple clinical presentations of TBI.
The preclinical model presented here produces reliable and reproducible pathophysiological changes associated with clinical mild TBI as demonstrated by a number of prior studies17,19,20,21,22,23. Studies with this model showed that rats subjected to a low-intensity blast wave exhibited neuroinflammation, axonal injury, microvascular damage, biochemical changes related to neuronal injury and deficits in short-term plasticity and synaptic excitability19. However, this mild TBI model did not induce any macroscopic neuropathological changes, including tissue damage, hemorrhage, hematoma and contusion19 that have been commonly observed in studies using moderate to severe invasive TBI models10,24. Previous research19,21,22,23 has shown that this preclinical model can be used to characterize neurobiological and pathophysiological processes underlying the etiology of mild and moderate TBI17,19,20,21,22,23. This model also permits for testing of new therapeutic compounds and strategies, as well as the identification of novel, suitable targets for the development of effective TBI interventions19,21,22,23.
This model was developed to investigate effects induced by blast waves as well as rapid rotational forces on molecular, cellular and behavioral outcomes in rodents. Analogous to the blast wave model presented here, a number of preclinical models has been developed that attempt to recapitulate mild to moderate TBI using gas-driven overpressure waves2,14,17,25,26,27,28. Some of the limitations of other models include: the animal is fixed to a wire-mesh gurney and the head is immobilized upon impact; the peripheral organs are exposed to the wave in addition to the brain, which creates the confounding variables of polytrauma; and the models are large and stationary, which limits changing and adapting critical parameters to better model conditions reminiscent of human TBI.
The benefits of this bench-top, gas-driven shock tube setup are its relative low-cost for acquisition and running expenses, as well as ease of installation and use. Furthermore, the setup allows for high-throughput operation and generation of controlled reproducible blast waves and in vivo outcomes in both mice and rats. In order to control for consistent test conditions (i.e., constant blast wave and overpressure) the setup is equipped with pressure sensors. The advantages of this model for TBI include scalability of the injury severity and that mild TBI is induced using a non-invasive, closed-head procedure. Peak overpressure and subsequent brain injury increase with thicker polyester membranes in a consistent scalable manner17. The ability to scale TBI severity through membrane thickness is a useful tool to determine the level, at which specific outcome measures (e.g., neuroinflammation) become evident. Providing protective shielding for the peripheral organs, also allows focused investigation into mild TBI mechanisms by avoiding or reducing confounding variables of systemic injury, such as lung- or thoracic injury. Moreover, this setup allows selecting the direction, by which the blast wave strikes/penetrates the head (i.e., head-on, side, top or underneath) and therefore different types of TBI-inducing insults can be investigated. The standard procedure to induce mild to moderate TBI described here employs side exposure to evaluate the effects of blast wave injury in combination with coup and contrecoup injury due to rapid rotational forces. Furthermore, in order to investigate exclusively blast-induced injury, top down blast wave exposure can be employed in this model.