Treating Traumatic Brain Injuries with Electroceuticals: Implications for the Neuroanatomy of Consciousness

: According to the Centers for Disease Control and Prevention (CDC), traumatic brain injury (TBI) is the leading cause of loss of consciousness, long-term disability, and death in children and young adults (age 1 to 44). Currently, there are no United States Food and Drug Administration (FDA) approved pharmacological treatments for post-TBI regeneration and recovery, particularly related to permanent disability and level of consciousness. In some cases, long-term disorders of consciousness (DoC) exist, including the vegetative state/unresponsive wakefulness syndrome (VS/UWS) characterized by the exhibition of reﬂexive behaviors only or a minimally conscious state (MCS) with few purposeful movements and reﬂexive behaviors. Electroceuticals, including non-invasive brain stimulation (NIBS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS) have proved efﬁcacious in some patients with TBI and DoC. In this review, we examine how electroceuticals have improved our understanding of the neuroanatomy of consciousness. However, the level of improvements in general arousal or basic bodily and visual pursuit that constitute clinically meaningful recovery on the Coma Recovery Scale-Revised (CRS-R) remain undeﬁned. Nevertheless, these advancements demonstrate the importance of the vagal nerve, thalamus, reticular activating system, and cortico-striatal-thalamic-cortical loop in the process of consciousness recovery.


Traumatic Brain Injury and Consciousness
Traumatic brain injury (TBI) is defined as an injury caused by an external force that results in the disruption of normal brain function. In the United States, between 2016-2017, there were approximately 451,000 cases of TBI that resulted in hospitalization. The most common mechanisms of injury contributing to TBI were unintentional falls and motor vehicle crashes [1]. Following a severe TBI, disorders of consciousness (DoC) are common sequela [2,3]. Clinical features correlated with prognosis include age and severity of the TBI [2,[4][5][6][7]. In several studies, there is an inverse correlation between the probability of recovering from a DoC and the duration after the injury [5,6,8]; however, some recovery has been observed in patients years after the initial injury [3,9]. The integrity and function of various neural structures and their relationship to consciousness are crucial for predicting outcomes and treating patients [10,11].
In this review, we define consciousness, severe DoCs, and describe clinical screening mechanisms and clinically relevant scoring systems utilized for diagnosing severe DoCs. Next, we address current pharmaceuticals and electroceuticals described in case studies and/or evaluated in clinical trials that are used to treat severe DoCs. Finally, we discuss how and/or evaluated in clinical trials that are used to treat severe DoCs. Finally, how these advanced clinical scoring systems, pharmaceuticals, and electroce peutics have contributed to our knowledge of the neuroanatomy of conscious

Consciousness
Consciousness, in its most basic sense, is defined as being awake and re stimuli. The systems in the brain responsible for consciousness mediate sens memory, and emotional functions that give rise to one's perceptions and emot Levels of consciousness are generally assessed via three parameters: alertness and attention. Alertness requires function of the ascending reticular activa (ARAS) circuit involved in the sleep-wakefulness cycle and enables the indi receptive to stimuli [5,6,14]. Awareness requires function of sensory and mo regions and circuits to enable perception of and response to stimuli [15,16]. A quires those same circuits and regions plus processing in the frontoparietal co dala, and hippocampus which give rise to perceptions and feelings experie individual ( Figure 1) [17]. The ARAS is composed of a n neurons connecting the reticular formation, hypothalamus, and thalamus, which have projections to various cortical regions. A variety of stimuli including visual and somat (pain, touch, and temperature) excite the reticular activating system, generating arous tions to the sensory and motor cortex are requisite for awareness. Additional connecti frontoparietal cortex, amygdala, and hippocampus contribute to attention.

Disorders of Consciousness
Consciousness can be disrupted by pharmacological agents such as anest brain injury. In both cases, there is a lack of subjective experience. Numerou can cause disorders of consciousness including: TBI, hypoxic-ischemic ence from cardiac arrest, ischemic stroke, hemorrhage (intracerebral, subdural, ep arachnoid), seizures, toxic-metabolic insults, and metabolic abnormalities. C TBIs, several of these etiologies including ischemic stroke and hypoxic-ischem alopathy follow predictable patterns which allow for improved prognostic Figure 1. Diagram of the ARAS and cortical projections. The ARAS is composed of a network of neurons connecting the reticular formation, hypothalamus, and thalamus, which have widespread projections to various cortical regions. A variety of stimuli including visual and somatosensory (pain, touch, and temperature) excite the reticular activating system, generating arousal. Projections to the sensory and motor cortex are requisite for awareness. Additional connections to the frontoparietal cortex, amygdala, and hippocampus contribute to attention.

Disorders of Consciousness
Consciousness can be disrupted by pharmacological agents such as anesthetics or by brain injury. In both cases, there is a lack of subjective experience. Numerous etiologies can cause disorders of consciousness including: TBI, hypoxic-ischemic encephalopathy from cardiac arrest, ischemic stroke, hemorrhage (intracerebral, subdural, epidural, subarachnoid), seizures, toxic-metabolic insults, and metabolic abnormalities. Compared to TBIs, several of these etiologies including ischemic stroke and hypoxic-ischemic encephalopathy follow predictable patterns which allow for improved prognostication [3,18]. DoCs are generally classified as acute (within the first 28 days) or chronic (persistent) [19,20].
Five levels of DoCs are generally used within the clinical setting: brain death, coma, vegetative state/unresponsive wakefulness syndrome (VS/UWS), minimally conscious state minus (MCS-), and minimally conscious state plus (MCS+) ( Table 1). Accurate prognostication is crucial, because withdrawal of life-sustaining therapies is the leading cause of death for patients with acute TBI [21,22]. Both brain death and coma are acute diagnoses, with coma generally lasting no more than two to three weeks [20,23]. Brain death is the irreversible cessation of clinical brain functions, including the capacity to regulate respiratory and vegetative function, which is diagnosed using a series of tests known as the brain death examination. For children, this examination is performed twice before withdrawal of life sustaining therapies [23][24][25]. Coma is clinically defined as the complete absence of arousal or awareness [26], although some patients have described experienced awareness during the comatose state upon recovery. Unless the ARAS is severely injured, function generally returns within two to three weeks, at which time the vegetative systems that control the sleep-wake cycle, breathing, digestion, and basic motor reflexes begin functioning [27]. This clinical presentation is the VS/UWS, wherein the patient is alert but is not capable of attention or awareness [20]. Clinically, the VS/UWS is considered persistent one month after diagnosis [5][6][7]. Unlike coma and VS/UWS, the minimally conscious state (MCS) often includes impaired awareness and attention, as well as inconsistent responses that are consciously driven [9]. The first clinical signs to occur are generally visual pursuit and command following [11]. This category is further subdivided into without language (MCS-), or with language including command-following, intelligible verbalization, and/or intentional communication (MCS+) [4,[28][29][30].

Diagnosing Disorders of Consciousness
Diagnosing DoCs after traumatic brain injury is crucial for appropriate treatment. The process begins with a standard neurological examination assessing consciousness; response to auditory, visual, and tactile stimulation; and assessment of pupillary and corneal reflexes. Additional clinical screens [31] can include computed tomography (CT) perfusion to assess brain death [23], diffusion tensor tractography (DTT) to evaluate the ARAS in the live human brain [32], functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) responses to detect higher order cortical function [33][34][35], and positron emission tomography (PET) and fMRI to identify brain activity in individuals diagnosed as unresponsive [19,36].
Several clinical scoring systems are used to determine levels of consciousness and disability (Table 2) [37]. The Coma Recovery Scale-Revised (CRS-R) is the gold-standard behavioral assessment, with a modified version for pediatric patients [38,39]. It consists of six categories that assess arousal as well as auditory, visual, motor, and communication functions. The score range is 0 to 23, with higher scores associated with higher function. Scores do not directly correspond to DoC diagnoses, but certain responses are associated with MCS−, MCS+, and emergence from MCS (Table 3) [4,8,40]. The Glasgow Coma Scale (GCS) is also used in a number of studies addressed in this review [2,23,32,33]. This scale consists of three components: eye opening, motor, and verbal responses with scores ranging from 3-15 (Table 2). There is substantial overlap in scores between DoCs, thus scores do not directly correspond to DoC diagnoses. A newer version, The Glasgow Outcome Scale Extended-Revised (GOSE-R) has been proposed to address difficulties in separating out MCS− and MCS+ [2,32,41]. Due to the time and training requirements for the CRS-R, the Simplified Evaluation of CONsciousness Disorders (SECONDs) was recently developed to provide a similar evaluation in~5 min, which enables easy adaptation to emergency and critical care settings. SECONDs evaluates six mandatory items and two conditional items and provides a score ranging from 0-8 that corresponds with the patient's DoC diagnoses (Tables 2 and 3) [42,43]. The Disability Rating Scale (DRS) assesses eight items with scores ranging from 0-29, with 12-21 corresponding with MCS and 22-29 with VS/UWS and coma (Tables 2 and 3) [2,4,9,29,44,45]. Notably, there are numerous other assessments which evaluate similar properties not covered in this review [37].

Pharmaceuticals
Currently, there are no United States Food and Drug Administration (FDA)-approved pharmaceuticals for the treatment of TBI. Pathologically, TBI follows a biphasic pattern consisting of the primary structural injury followed by a secondary injury cascade. Primary injuries include cerebral contusion, blood vessel damage, blood brain barrier disruption, axonal shearing, and neuronal apoptosis. The secondary injury cascade includes inflammation, edema, changes in cerebral circulation, glutamate toxicity, mitochondrial dysfunction, and increased reactive oxygen species (ROS) production. In 2019, the FDA fast-tracked Neu-roSTAT (cyclosporine; NeuroVive Pharmaceutical) for the treatment of moderate-to-severe TBI by inhibiting mitochondrial permeability transition pore (mPTP), which is indicated in the secondary injury cascade [46]. Notably, if approved, this treatment will only be effective during the early acute injury phase. Amantadine is safe and has shown some accelerated recovery in studies of acute and subacute DoCs due to TBI [47][48][49], but with variable effectiveness [50,51]. Like amantadine, apomorphine has effects on the dopaminergic system. A case study and a pilot study with apomorphine described a spontaneous awakening phenomenon [52,53], and a controlled study has been proposed to evaluate its safety and effectiveness [54]. During the chronic phase post-TBI, case reports on zolpidem have indicated a paradoxical ability to improve consciousness [55,56]. An EEG study suggested it acts on cortical, striatal, and thalamic neuronal populations to potentially produce this spontaneous awakening phenomenon [57]. Controlled studies have shown zolpidem to be safe, but again showed variable effectiveness [58][59][60][61]. Lorazepam was also shown to be safe, albeit ineffective in a single study for TBI, but showed some effectiveness for patients with anoxic brain injury [61]. Together, these results suggest potential pharmaceutical options for acute, subacute, and chronic DoCs, with reasonable safety profiles but variable effectiveness.

Electroceuticals
Electroceuticals utilize electrical impulses, the nervous system's primary language, to treat disease. In general, electroceuticals consist of a power source that provides electrical stimulation to electrodes, which then deliver these impulses to targeted cells or tissues [62]. A number of electroceuticals currently have therapeutic uses including pacemakers; cochlear implants [63]; vagus nerve stimulation for treatment of epileptic seizures and depression [64]; and deep brain stimulation for Parkinson's disease, epilepsy, and other neurological disorders [65]. Although the first attempt at using electroceuticals to treat DoCs was in 1968 [66], progress was slow. With recent advances in imaging and assessment, a limited set of studies have evaluated electroceuticals for treating patients with TBI-induced DoCs [67].

Non-Invasive Brain Stimulation
Non-invasive brain stimulation (NIBS) utilizes an externally applied electrical current to stimulate brain activity. There are two main types of NIBS, transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS). tDCS delivers a weak, continuous current flowing from an anode to a cathode (Figure 2A). One uncontrolled study showed improvements in the CRS-R up to a year after tDCS therapy [68], which was followed by similar improvements in a subgroup of patients using the CRS-R in a controlled double-blind study [69]. A retrospective analysis showed that tDCS responsive patients show preserved gray matter and metabolic activity in the dorso-lateral prefrontal cortex, medial-prefrontal cortex, precuneus, and thalamus, which are all areas involved in consciousness [70]. These improvements with tDCS were not consistent across all studies [71,72]. TMS delivers magnetic impulses generated by the flow of current through a coil placed on the scalp ( Figure 2B). In DoCs, TMS has been used for assessing levels of consciousness [73]. Repetitive TMS (rTMS) has shown minor EEG changes and a single case report of behavioral improvement [74]. Similar improvements were also observed in a pilot trial in patients with chronic DoCs treated with rTMS and amantadine [75]. The most significant adverse effect associated with rTMS is induction of seizures [76]. Importantly, a study showed that tDCS and TMS are safe and well-tolerated in children with TBI [77].

Vagus Nerve Stimulation
The vagus nerve is comprised of autonomic sensory and motor fibers that control some motor movements in the mouth, the gag reflex, heart rate, and gastrointestinal peristalsis. Vagus nerve stimulation (VNS) utilizes a device surgically implanted under the skin (generally on the chest), and the electrode is threaded up connecting to the left vagus nerve ( Figure 2C). Notably, the right vagus nerve is not utilized because of autonomic connections to the heart. One case study demonstrated CRS-R improvements from VS/UWS to MCS 15 years post-severe TBI [78]. A study utilizing non-invasive transcutaneous VNS (tVNS) was safe but showed few clinically relevant effects [79].

Deep Brain Stimulation
Deep brain stimulation (DBS) utilizes electrodes implanted into subcortical areas of the brain that are controlled by a pacemaker-like device surgically implanted under the skin (generally on the chest; Figure 2D). DBS procedures have utilized a number of different electrode placements, all of which correspond anatomically with projections of the ARAS [67]. Two out of five patients showed improvements following DBS therapy, with the responding patients showing an increase in medial cortex activity [80]. Although promising results have been seen in a number of small uncontrolled studies, controlled and randomized studies are necessary to determine therapeutic response to DBS in DoC following TBI [81]. Spontaneous recovery from TBI-induced DoCs generally occurs within the first 12 months post-TBI [5,6]. However, DBS therapy outcomes are often confounded by overlap in the timeframe for spontaneous recovery from DoCs, as well as a lack of controlled and randomized studies [82]. Recently, a DBS device was developed that can also continuously record electrophysiological responses to track objective effects in the patient rather than just correlating with the patient's subjective experience [65].
NeuroSci 2021, 2, FOR PEER REVIEW Figure 2. Diagram of electroceuticals used in patients with severe DoCs. (A) tDCS utilizes two electrodes that are placed on the scalp (an anode and a cathode) that are connected via wires to a power supply. Together, this makes a complete circuit through which a constant, low (1-2 milliampere; mA), electrical current is delivered for a set amount of time. (B) rTMS utilizes the flow of electric current through a magnetic coil placed over the scalp, generating a rapidly changing mag netic field that induces an electrical current. (C) VNS employs a pulse generator implanted under the skin that is connected by a wire to an electrode that is on the left vagus nerve. (D) DBS employs a pulse generator implanted under the skin that is connected by a wire to an electrode generally implanted into the thalamus.

Neuroanatomy of Consciousness
The neuroanatomy of consciousness is comprised of cortical and subcortical ne works that enable alertness, awareness, and attention. The ARAS connects the reticula formation with thalamic nuclei, hypothalamus, and cerebral cortex. These connection control the sleep-wakefulness cycle and are considered a core component of consciousnes [32], which is why the thalamus/ARAS is a primary target for DBS [67]. Following sever TBI, neuronal loss in the central thalamic nuclei is associated with coma, MCS, an VS/UWS diagnoses [15,16,83,84]. Ascending projections to the central thalamic nuclei pla important roles in the sleep-wakefulness system as well as the cholinergic, serotonergi and noradrenergic arousal systems of the brainstem. Descending projections connect from frontal cortical regions to the central thalamic nuclei [15,85]. Interestingly, cerebral glucos metabolism can be used to differentiate MCS-and MCS+ patients, with MCS+ patient showing higher metabolism in left cortical areas crucial for language as well as in the pos Figure 2. Diagram of electroceuticals used in patients with severe DoCs. (A) tDCS utilizes two electrodes that are placed on the scalp (an anode and a cathode) that are connected via wires to a power supply. Together, this makes a complete circuit through which a constant, low (1-2 milliampere; mA), electrical current is delivered for a set amount of time. (B) rTMS utilizes the flow of electric current through a magnetic coil placed over the scalp, generating a rapidly changing magnetic field that induces an electrical current. (C) VNS employs a pulse generator implanted under the skin that is connected by a wire to an electrode that is on the left vagus nerve. (D) DBS employs a pulse generator implanted under the skin that is connected by a wire to an electrode generally implanted into the thalamus.

Neuroanatomy of Consciousness
The neuroanatomy of consciousness is comprised of cortical and subcortical networks that enable alertness, awareness, and attention. The ARAS connects the reticular formation with thalamic nuclei, hypothalamus, and cerebral cortex. These connections control the sleep-wakefulness cycle and are considered a core component of consciousness [32], which is why the thalamus/ARAS is a primary target for DBS [67]. Following severe TBI, neuronal loss in the central thalamic nuclei is associated with coma, MCS, and VS/UWS diagnoses [15,16,83,84]. Ascending projections to the central thalamic nuclei play important roles in the sleep-wakefulness system as well as the cholinergic, serotonergic and noradrenergic arousal systems of the brainstem. Descending projections connect from frontal cortical regions to the central thalamic nuclei [15,85]. Interestingly, cerebral glucose metabolism can be used to differentiate MCS− and MCS+ patients, with MCS+ patients showing higher metabolism in left cortical areas crucial for language as well as in the posterior parietal, sensorimotor, premotor, and pre-supplementary motor cortices [30], some of which have direct cortico-thalamic projections from the central thalamus [16]. Projections from the central thalamus also innervate the striatum (caudate, putamen, and nucleus accumbens) and project onto medium spiny neurons [86], which are sensitive to dopamine deficiency [87], and may contribute to positive results seen with amantadine [47][48][49]75] and apomorphine [52,53]. Medium spiny neurons normally function to dis-inhibit the central thalamus by inhibiting the globus pallidus internus. This circuit is interrupted by severe TBI, resulting in lower output from the central thalamus. Inhibition of the globus pallidus internus by zolpidem may result in a more normal function of the central thalamus and thus explain the paradoxical observation with zolpidem treatment resulting in spontaneous awakenings [15,55,[57][58][59][60][61] (Figure 3).
NeuroSci 2021, 2, FOR PEER REVIEW of which have direct cortico-thalamic projections from the central thalamus [16]. Pr tions from the central thalamus also innervate the striatum (caudate, putamen, and cleus accumbens) and project onto medium spiny neurons [86], which are sensitive t pamine deficiency [87], and may contribute to positive results seen with amantadine 49,75] and apomorphine [52,53]. Medium spiny neurons normally function to dis-in the central thalamus by inhibiting the globus pallidus internus. This circuit is interru by severe TBI, resulting in lower output from the central thalamus. Inhibition of the bus pallidus internus by zolpidem may result in a more normal function of the ce thalamus and thus explain the paradoxical observation with zolpidem treatment resu in spontaneous awakenings [15,55,[57][58][59][60][61] (Figure 3). Figure 3. Diagram of the cortico-striatal-thalamic-cortical loop and potential impacts of apom phine, amantadine, and zolpidem on spontaneous awakenings. There are two dopaminergic p ways in this loop, the direct pathway and the indirect pathway. The direct pathway expresses citatory dopamine receptor 1 (D1), and striatal neurons project to the globus pallidus internus then to the substantia nigra pars reticulata, which then inhibits the thalamus through GABAe projections. The indirect pathway expresses the inhibitory dopamine receptor 2 (D2), and thes striatal neurons project to the globus pallidus externus and then to the subthalamic nucleus. G tamatergic neurons from the subthalamic nucleus project to the substantia nigra, which then i its the thalamus via GABAergic projections [88]. Amantadine is an indirect dopamine agonist may contribute to both the direct and indirect pathways [48]. Apomorphine is a dopamine ag with a high affinity for D2 receptors, feeding into the indirect pathway [54]. Zolpidem is a GA agonist and may inhibit the GABAergic projections reducing inhibitory tone to hypoactive ar the brain and enhancing thalamocortical pathways [57,60], Dopamine Receptor 1 (D1) and Do mine Receptor 2 (D2).

Conclusions
Since the first DBS therapy for TBI was attempted over 50 years ago [66], majo vancements in diagnosing and imaging have improved clinical identification of brai Figure 3. Diagram of the cortico-striatal-thalamic-cortical loop and potential impacts of apomorphine, amantadine, and zolpidem on spontaneous awakenings. There are two dopaminergic pathways in this loop, the direct pathway and the indirect pathway. The direct pathway expresses excitatory dopamine receptor 1 (D 1 ), and striatal neurons project to the globus pallidus internus and then to the substantia nigra pars reticulata, which then inhibits the thalamus through GABAergic projections. The indirect pathway expresses the inhibitory dopamine receptor 2 (D 2 ), and these striatal neurons project to the globus pallidus externus and then to the subthalamic nucleus. Glutamatergic neurons from the subthalamic nucleus project to the substantia nigra, which then inhibits the thalamus via GABAergic projections [88]. Amantadine is an indirect dopamine agonist and may contribute to both the direct and indirect pathways [48]. Apomorphine is a dopamine agonist with a high affinity for D2 receptors, feeding into the indirect pathway [54]. Zolpidem is a GABA agonist and may inhibit the GABAergic projections reducing inhibitory tone to hypoactive areas of the brain and enhancing thalamocortical pathways [57,60], Dopamine Receptor 1 (D 1 ) and Dopamine Receptor 2 (D 2 ).

Conclusions
Since the first DBS therapy for TBI was attempted over 50 years ago [66], major advancements in diagnosing and imaging have improved clinical identification of brain activity that informs our understanding of the neuroanatomy of consciousness [28,33,34,37,43]. Likewise, recent studies using electroceuticals, pharmaceuticals, or both are now evaluated with our improved diagnostics and imaging [36,[47][48][49]59,68,71,78]. Results of these studies have built upon our early understanding of the neuroanatomical structures and circuits involved in consciousness by identifying intact brain structures and their relation to DoC diagnoses and mechanisms of recovery [15,16,27]. However, small sample size, lack of appropriate controls, and therapeutics within the window of spontaneous recovery confound study interpretations [67,81,82]. Nevertheless, evidence is accumulating for recovery years after TBI-induced DoCs [3,9]. These advances emphasize the importance of diagnostic, therapeutic, and prognostic approaches to aid recovery in patients with TBI-induced DoCs.