Measurements and models of cerebral function in the severely injured brain

What do we know about the risks of developing dementia after traumatic brain injury?

Traumatic brain injury (TBI) is a risk factor for the later development of dementia, but although the evidence dates back to the early 20^th century, the nature of any association and its mechanistic pathways remain unclear. There has been greater focus on this subject over recent years, in part because of increasing reports around sports related TBIs, especially in the USA. Differences in research methods and clinical sampling remain the primary reason for the variable findings, although there is clearly increased prevalence of neurodegenerative disorders in general. Duration of follow up, definition of both TBI and dementia, and differences in the extent to which other dementia risk factors are controlled, as well as concerns about medical record accuracy are all issues yet to be resolved in TBI research, as is an absence pathological evidence. In addition, TBI has been reported to initiate a cascade of pathological processes related to several neurodegenerative disorders, and as such, it is likely that the risks vary between individuals. Given the evidence that dementia risk may increase with injury severity and frequency, a detailed account of age and type of injury, as well as lifetime TBI exposure is essential to document in future studies, and further longitudinal research with biomarker assessments are needed.

The Impact of Traumatic Brain Injury on Later Life: Effects on Normal Aging and Neurodegenerative Diseases

The acute and chronic effects of traumatic brain injury (TBI) have been widely described; however, there is limited knowledge on how a TBI sustained during early adulthood or mid-adulthood will influence aging. Epidemiological studies have explored whether TBI poses a risk for dementia and other neurodegenerative diseases associated with aging. We will discuss the influence of TBI and resulting medical comorbidities such as endocrine, sleep, and inflammatory disturbances on age-related gray and white matter changes and cognitive decline. Post mortem studies examining amyloid, tau, and other proteins will be discussed within the context of neurodegenerative diseases and chronic traumatic encephalopathy. The data support the suggestion that pathological changes triggered by an earlier TBI will have an influence on normal aging processes and will interact with neurodegenerative disease processes rather than the development of a specific disease, such as Alzheimer's or Parkinson's. Chronic neurophysiologic change after TBI may have detrimental effects on neurodegenerative disease.

The Role of Neuroimaging Techniques in Establishing Diagnosis, Prognosis and Therapy in Disorders of Consciousness

Non-communicative brain damaged patients raise important clinical and scientific issues. Here, we review three major pathological disorders of consciousness: coma, the unresponsive wakefulness syndrome and the minimally conscious state. A number of clinical studies highlight the difficulty in making a correct diagnosis in patients with disorders of consciousness based only on behavioral examinations. The increasing use of neuroimaging techniques allows improving clinical characterization of these patients. Recent neuroimaging studies using positron emission tomography, functional magnetic resonance imaging, electroencephalography and transcranial magnetic stimulation can help assess diagnosis, prognosis, and therapeutic treatment. These techniques, using resting state, passive and active paradigms, also highlight possible dissociations between consciousness and responsiveness, and are facilitating a more accurate understanding of brain function in this challenging population.

Multimodal imaging of recovery of functional networks associated with reversal of paradoxical herniation after cranioplasty

Cranioplasty following decompressive craniectomy is reported to result in improved blood flow, cerebral metabolism, and concomitant neurological recovery. We used multimodal functional imaging technology to study a patient with marked neurological recovery after cranioplasty. Resting-state networks and auditory responses obtained with functional MRI and cerebral metabolism obtained with PET before and after cranioplasty revealed significant functional changes that were correlated with the subject's neurological recovery. Our results suggest a link between recovery of behavior, cerebral metabolism, and resting-state networks following cranioplasty.

More dead than dead: perceptions of persons in the persistent vegetative state

Patients in persistent vegetative state (PVS) may be biologically alive, but these experiments indicate that people see PVS as a state curiously more dead than dead. Experiment 1 found that PVS patients were perceived to have less mental capacity than the dead. Experiment 2 explained this effect as an outgrowth of afterlife beliefs, and the tendency to focus on the bodies of PVS patients at the expense of their minds. Experiment 3 found that PVS is also perceived as "worse" than death: people deem early death better than being in PVS. These studies suggest that people perceive the minds of PVS patients as less valuable than those of the dead - ironically, this effect is especially robust for those high in religiosity.

Reaching across the abyss: recent advances in functional magnetic resonance imaging and their potential relevance to disorders of consciousness

Disorders of consciousness (DOC) raise profound scientific, clinical, ethical, and philosophical issues. Growing knowledge on fundamental principles of brain organization in healthy individuals offers new opportunities for a better understanding of residual brain function in DOCs. We here discuss new perspectives derived from a recently proposed scheme of brain organization underlying consciousness in healthy individuals. In this scheme, thalamo-cortical networks can be divided into two, often antagonistic, global systems: (i) a system of externally oriented, sensory-motor networks (the "extrinsic" system); and (ii) a system of inward-oriented networks (the "intrinsic" or default system). According to this framework, four distinct mental states would be possible that could be relevant for understanding DOCs. In normal healthy volunteers and locked-in syndrome patients, a state of high functionality of both the extrinsic and intrinsic or default systems is expected--associated with full awareness of environment and self. In this case, mental imagery tasks combined with fMRI can be used to detect covert awareness in patients that are unable to communicate. According to the framework, two complementary states of system imbalance are also possible, in which one system is in a hyperfunctional state, while the other is hypoactive. Extrinsic system hyperfunction is expected to lead to a state of total sensory-motor "absorption" or "lost self." In contrast, intrinsic or default system hyperfunction is expected to lead to a state of complete detachment from the external world. A state where both extrinsic and intrinsic systems are hypofunctional is predicted to lead to markedly impaired consciousness as seen in DOCs. Finally, we review the potential use of ultra-slow fluctuations in BOLD signal as a tool for assessing the functional integrity of extrinsic and intrinsic systems during "resting state" fMRI acquisitions. In particular, we discuss the potential provided by assessment of these slow spontaneous BOLD fluctuations as a novel tool in assessing the cognitive state and chances of recovery from brain pathologies underlying DOCs.

The neural correlates of consciousness: an update

This review examines recent advances in the study of brain correlates of consciousness. First, we briefly discuss some useful distinctions between consciousness and other brain functions. We then examine what has been learned by studying global changes in the level of consciousness, such as sleep, anesthesia, and seizures. Next we consider some of the most common paradigms used to study the neural correlates for specific conscious percepts and examine what recent findings say about the role of different brain regions in giving rise to consciousness for that percept. Then we discuss dynamic aspects of neural activity, such as sustained versus phasic activity, feedforward versus reentrant activity, and the role of neural synchronization. Finally, we briefly consider how a theoretical analysis of the fundamental properties of consciousness can usefully complement neurobiological studies.

The predictive validity of a brief inpatient neuropsychologic battery for persons with traumatic brain injury

OBJECTIVE

To examine the predictive validity of a brief neuropsychologic test battery consisting of the Galveston Orientation and Amnesia Test, the California Verbal Learning Test-II, Trail-Making Test (TMT), Symbol Digit Modalities Test, grooved pegboard, phonemic and categorical word generation tasks, the Wechsler Test of Adult Reading (WTAR), and the Wisconsin Card Sorting Test-64 relative to functional outcome at 1 year in persons with traumatic brain injury.

DESIGN

Inception cohort study. Follow-up period of 12 months.

SETTING

Seven Traumatic Brain Injury Model System centers. Neuropsychologic testing was conducted during the acute inpatient rehabilitation stay and functional outcome measures were obtained at 1-year outpatient follow-up.

PARTICIPANTS

Adults (N=174) who met criteria for admission to inpatient brain injury rehabilitation.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

FIM instrument, Disability Rating Scale, Supervision Rating Scale, Satisfaction With Life Scale (SWLS), and Glasgow Outcome Scale-Extended.

RESULTS

Multiple regression analyses revealed that performance on the neuropsychologic test battery was predictive of outcome at 1 year postinjury for all outcome measures, except FIM motor scores and the SWLS. Cognitive performance using this battery was found to predict 1-year outcomes above and beyond functional variables and injury severity variables collected during inpatient rehabilitation, thereby indicating incremental validity for this test battery. Individual tests that were found to be significant predictors of 1-year outcomes included the WTAR and TMT part B.

CONCLUSIONS

These findings support the clinical utility and ecological validity of this battery with respect to level of disability, functional independence, and supervision required.

Brain perfusion and VEP reactivity in occipital and parietal areas are associated to recovery from hypoxic vegetative state

Patients in a vegetative state (VS) show a spontaneous wake-sleep-cycle but no evidence of awareness, of interaction with the environment, voluntary action, and language comprehension. The neuropathological conditions underlying VS are still not fully understood. In this retrospective study we focused on VS due to hypoxia and used SPECT, VEP and event related potentials (N100, N200, MMN, and P300) to assess differences between a group of patients moving into a permanent VS (n=13) and a group recovering from VS (n=8). The two groups were matched for age, gender, duration of illness, and on the coma remission scale at admission. The patient groups differed in global uptake of (99m)Tc-ethylencysteine dimer (being reduced in non-recovered VS patients to 2/3 of the recovered group), and in presence of VEP and N100 (recovered patients always had a present VEP and N100). Moreover, analysis of uptake in specific brain areas showed that the recovered group had a higher perfusion in the visual cortex and in the precuneus, whereas no differences were found in the frontal pole and more ventral parts of the brain. Statistical testing revealed a strong association between occipital and parietal perfusion and the presence of a VEP, but no specific results for the N100. We conclude that occipital and parietal lobe perfusion and rudimentary vision may be critical characteristics distinguishing between VS and patients recovered from VS. Although this may just reflect haemodynamics during hypoxia leading to differences in severity of VS, it also may be regarded as a functional precondition for orientation towards stimuli and therefore for conscious actions in general.