Diffuse axonal injuries: pathophysiology and imaging

Structural dissociation of attentional control and memory in adults with and without mild traumatic brain injury

Memory and attentional control impairments are the two most common forms of dysfunction following mild traumatic brain injury (TBI) and lead to significant morbidity in patients, yet these functions are thought to be supported by different brain networks. This 3 T magnetic resonance diffusion tensor imaging (DTI) study investigates whether microstructural integrity of white matter, as measured by fractional anisotropy (FA) within a small set of individually localized regions of interest (ROIs), is associated with these cognitive domains in normal adults and adults with mild TBI. Results in a sample of 23 normal controls reveal a significant correlation between attentional control and FA within a ROI in the left hemisphere anterior corona radiata. Furthermore, the controls demonstrate a correlation between memory performance and FA in a ROI placed in the uncinate fasciculus. Next, to examine whether these relationships are found in the pathological ranges of attention, memory and microstructural white matter integrity associated with mild TBI, these analyses were applied to a group of 43 mild TBI patients. Results, which generally demonstrated a wider range of attention, memory and FA scores, replicated the correlation between attentional control and FA in left hemisphere anterior corona radiata, as well as the correlation between memory performance and FA in the uncinate fasciculus. These two sets of brain-behaviour relationships were highly specific, as shown by a lack of correlation between attention and uncinate fasciculus FA and the lack of correlation between memory performance and anterior corona radiata FA. Furthermore, a 'correlational double dissociation' was demonstrated to exist between two distinct frontal structures independently associated with attention and memory, respectively, via a series of multiple regression analyses in both normal controls and adults with mild TBI. The results of the multiple regression analyses provide direct evidence that tract-specific variation in microstructural white matter integrity among normal controls and among mild TBI patients can account for much of the variation in performance in specific cognitive domains. More generally, such findings suggest that diffusion anisotropy measurement can be used as a quantitative biomarker for neurocognitive function and dysfunction.

[Diagnostic imaging of traumatic brain injury]

Traumatic brain injury (TBI)--either in isolation or within the context of multiple trauma--is a major cause of disability and death in young adults. The prognosis depends not only on the extent and localization of traumatic lesions, but also on the promptness of surgical intervention if indicated. The following article presents diagnostic imaging strategies in the acute and sub-acute phases of head injury, discussing their relevance with regard to various clinical situations. In addition to standard CT and MRI techniques, the use of other methods such as perfusion measurements, magnetic resonance spectroscopy or diffusion tensor imaging is briefly discussed. By means of these relatively new techniques it is possible to visualize not only structural changes but also gain information relating to functional and metabolic aspects of traumatic brain injury.

MR imaging in the detection of diffuse axonal injury with mild traumatic brain injury

PURPOSE

To evaluate the occurrence and distribution of mild traumatic brain injury (MTBI) caused by diffuse axonal injury (DAI) using magnetic resonance (MR) imaging and to attempt to correlate MR findings with post-concussion symptoms (PCS).

PATIENTS AND METHODS

Forty MTBI patients (mean age: 32.5 years) with normal cranial computed tomography (CT) findings were examined with standard MR protocol including T(1)-weighted, T(2)-weighted, fluid attenuated inversion recovery (FLAIR), gradient echo (GRE) and diffusion-weighted (DW) sequences. MR imaging was performed within 24 hours of injury. The lesions were classified as DAI based on their location and morphologic appearance.

RESULTS

In MR imaging of five (12.5%) of the patients, the lesions compatible with DAI were observed. Four patients (10%) had the foci of low signal intensity compatible with hemorrhagic shear injury on the GRE sequence, and five (12.5%) patients had high signal intensity on FLAIR and DW sequence.

CONCLUSION

MR imaging can be helpful in revealing DAI lesions in patients with normal CT scan findings after MTBI. FLAIR, GRE and DW sequences are superior to conventional spin-echo images in detecting DAI lesions.

Extent of microstructural white matter injury in postconcussive syndrome correlates with impaired cognitive reaction time: a 3T diffusion tensor imaging study of mild traumatic brain injury

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) may be a useful index of microstructural changes implicated in diffuse axonal injury (DAI) linked to persistent postconcussive symptoms, especially in mild traumatic brain injury (TBI), for which conventional MR imaging techniques may lack sensitivity. We hypothesized that for mild TBI, DTI measures of DAI would correlate with impairments in reaction time, whereas the number of focal lesions on conventional 3T MR imaging would not.

MATERIALS AND METHODS

Thirty-four adult patients with mild TBI with persistent symptoms were assessed for DAI by quantifying traumatic microhemorrhages detected on a conventional set of T2*-weighted gradient-echo images and by DTI measures of fractional anisotropy (FA) within a set of a priori regions of interest. FA values 2.5 SDs below the region average, based on a group of 26 healthy control adults, were coded as exhibiting DAI.

RESULTS

DTI measures revealed several predominant regions of damage including the anterior corona radiata (41% of the patients), uncinate fasciculus (29%), genu of the corpus callosum (21%), inferior longitudinal fasciculus (21%), and cingulum bundle (18%). The number of damaged white matter structures as quantified by DTI was significantly correlated with mean reaction time on a simple cognitive task (r = 0.49, P = .012). In contradistinction, the number of traumatic microhemorrhages was uncorrelated with reaction time (r = -0.08, P = .71).

CONCLUSION

Microstructural white matter lesions detected by DTI correlate with persistent cognitive deficits in mild TBI, even in populations in which conventional measures do not. DTI measures may thus contribute additional diagnostic information related to DAI.

Low signal intensity and increased anisotropy on magnetic resonance imaging in the white matter lesion after head trauma: Unrecognized findings of diffuse axonal injury

We report on a four-year-old girl with head trauma caused by a motor vehicle accident. She presented with delirium, oculomotor palsy and ptosis in her left eye, left hemiparesis, and pyramidal signs in all extremities. Computed tomography on the day of admission showed diffuse cerebral edema with right-sided predominance. Magnetic resonance images on day 3 of admission showed lesions of diffuse axonal injury and contusion in the corpus callosum and right occipital and bilateral temporal lobes. There was a low-intensity lesion in the white matter of the right hemisphere on T2-weighted images, fluid-attenuated inversion recovery, T2()-weighted images, apparent diffusion coefficient maps and diffusion-weighted images. This low-intensity lesion disappeared by day 7, and a transient brain atrophy in the right hemisphere appeared on day 28. The low signal intensity in the cerebral white matter was apparently different from that associated with contusion and typical diffuse axonal injury, and might represent a late-onset accumulation of non-heme iron and free radicals in the white matter after head trauma.

Imaging after brain injury

Head injury remains an important cause of death and disability in young adults. This review will discuss the role of structural imaging using computed tomography (CT) and magnetic resonance imaging (MRI) and physiological imaging using CT perfusion, 131Xe CT, MRI and spectroscopy (MRS), single photon emission computed tomography, and positron emission tomography (PET) in the assessment, management, and prediction of outcome after head injury. CT allows rapid assessment of brain pathology which ensures patients who require urgent surgical intervention receive appropriate care. Although MRI provides greater spatial resolution, particularly within the posterior fossa and deep white matter, a complete assessment of the burden of injury requires imaging of cerebral physiology. Physiological imaging techniques can only provide 'snap shots' of physiology within the injured brain, but they can be repeated, and such data can be used to assess the impact of therapeutic interventions. Perfusion imaging based on CT techniques (xenon CT and CT perfusion) can be implemented easily in most hospital centres, and provide quantitative perfusion data in addition to structural images. PET imaging provides unparalleled insights into cerebral physiology and pathophysiology, but is not widely available and is primarily a research tool. MR technology continues to develop and is becoming generally available. Using a complex variety of sequences, MR can provide data concerning both structural and physiological derangements. Future developments with such imaging techniques should improve understanding of the pathophysiology of brain injury and provide data that should improve management and prediction of functional outcome.

Magnetic Resonance Imaging of Diffuse Axonal Injury: Quantitative Assessment of White Matter Lesion Volume

Diffuse axonal injury (DAI) is a common mechanism of traumatic brain injury (TBI) for which there is no well-accepted anatomic measures of injury severity. The present study aims to quantitatively assess DAI by measuring white matter lesion volume visible in fluid-attenuated inversion recovery (FLAIR) weighted images and to determine whether higher lesion volumes are associated with unfavorable functional outcome 6 months after injury. Twenty-four patients who experienced moderate to severe TBI without extra-axial or major cortical contusions were included in this study. Lesion volume was assessed by quantifying areas of hyperintensities in the white matter utilizing digitized FLAIR images. Two independent raters processed the magnetic resonance (MR) images and determined the total DAI volume. Functional outcome was assessed at 6 months after injury using the Glasgow Outcome Scale-Extended (GOSE). Interclass correlation analyses showed very high interrater reliability for each measure between the two raters (Interclass Correlation Coefficient = 0.95, p <or= 0.001). Total DAI volume was significantly, although modestly, correlated to GOSE (r = -0.453, p = 0.034). White matter lesion volume resulting from DAI can be quantitatively and reliably assessed from standard FLAIR-weighted MRIs. Patients with greater DAI volume have poorer functional outcomes. These methods may be useful in stratifying injury severity and for the assessment of DAI-directed therapies.

Prediction of recovery from a post-traumatic coma state by diffusion-weighted imaging (DWI) in patients with diffuse axonal injury

INTRODUCTION

To determine whether diffusion-weighted magnetic resonance (MR) imaging findings combined with initial clinical factors indicate the depth of shearing lesions in the brain structure and therefore relate to coma duration in diffuse axonal injury (DAI).

METHODS

A total of 74 adult patients (48 male and 26 female) with DAI were examined with conventional MR imaging and diffusion-weighted MR imaging between 2 hours and 20 days after injury. Apparent diffusion coefficient (ADC) maps were obtained and the mean ADC values of each region of interest (ROI) were measured using MRI console software. The involvement of the brainstem, deep gray matter, and corpus callosum was determined for each sequence separately as well as for the combination of all sequences. The correlations between MR imaging findings indicating the presence of apparent brain injury combined with initial clinical factors were determined.

RESULTS

Clinical characteristics, such as initial score on the Glasgow coma scale (GCS), age and number of all lesions, and ADC scores were predictive of the duration of coma.

CONCLUSION

It was possible to predict post-traumatic coma duration in DAI from cerebral MR imaging findings combined with clinical prognostic factors in the acute to subacute stage after head injury. Age, ADC scores, GCS score and number of lesions were highly significant in predicting coma duration. The technique presented here might provide a tool for in vivo detection of DAI to allow the prediction of the coma duration during the early stages in patients with traumatic brain injury.

Differential diagnostic approach to MR imaging of white matter diseases

OBJECTIVES

The purpose of this article is to explore the magnetic resonance (MR) imaging features and the advanced MR techniques for differential diagnosis of white matter diseases of the brain.

RESULTS

Magnetic resonance imaging is exquisitely sensitive for detecting brain abnormalities. Particularly in the evaluation of white matter diseases, MR far outperforms any other imaging technique. Lesions that may be quite subtle or even invisible on computed tomography are often clearly seen on the MR scan. The MR signal characteristics of white matter lesions are similar and relatively nonspecific, but other distinguishing features are often present to assist in diagnosis, such as the pattern of the abnormality, location, signal intensities, and enhancement features. Advanced MR techniques, such as diffusion-weighted imaging, MR spectroscopy, and perfusion imaging, provide additional specificity to the diagnosis of demyelinating diseases.

CONCLUSIONS

Conventional MR images and advanced MR techniques are very helpful in distinguishing various types of white matter disease, for assessing disease burden, and for separating acute and chronic lesions.

Diffuse axonal injury in mild traumatic brain injury: a diffusion tensor imaging study

OBJECT

Diffuse axonal injury (DAI) is a major complication of traumatic brain injury (TBI) that leads to functional and psychological deficits. Although DAI is frequently underdiagnosed by conventional imaging modalities, it can be demonstrated using diffusion tensor imaging. The aim of this study was to assess the presence and extent of DAI in patients with mild TBI.

METHODS

Forty-six patients with mild TBI and 29 healthy volunteers underwent a magnetic resonance (MR) imaging protocol including: dual-spin echo, fluid-attenuated inversion recovery, T2-weighted gradient echo, and diffusion tensor imaging sequences. In 20 of the patients, MR imaging was performed at a mean of 4.05 days after injury. In the remaining 26, MR imaging was performed at a mean of 5.7 years after injury. In each case, mean diffusivity and fractional anisotropy were measured using both whole-brain histograms and regions of interest analysis. No differences in any of the histogram-derived measures were found between patients and control volunteers. Compared with controls, a significant reduction of fractional anisotropy was observed in patients' corpus callosum, internal capsule, and centrum semiovale, and there were significant increases of mean diffusivity in the corpus callosum and internal capsule. Neither histogram-derived nor regional diffusion tensor imaging metrics differed between the two groups.

CONCLUSIONS

Although mean diffusivity and fractional anisotropy abnormalities in these patients with TBI were too subtle to be detected with the whole-brain histogram analysis, they are present in brain areas that are frequent sites of DAI. Because diffusion tensor imaging changes are present at both early and late time points following injury, they may represent an early indicator and a prognostic measure of subsequent brain damage.

Neuroimaging in traumatic brain imaging

Traumatic brain injury (TBI) is a common and potentially devastating clinical problem. Because prompt proper management of TBI sequelae can significantly alter the clinical course especially within 48 h of the injury, neuroimaging techniques have become an important part of the diagnostic work up of such patients. In the acute setting, these imaging studies can determine the presence and extent of injury and guide surgical planning and minimally invasive interventions. Neuroimaging also can be important in the chronic therapy of TBI, identifying chronic sequelae, determining prognosis, and guiding rehabilitation.

Subcortical White Matter Metabolic Changes Remote from Focal Hemorrhagic Lesions Suggest Diffuse Injury after Human Traumatic Brain Injury

OBJECTIVE:

We used positron emission tomographic studies to prospectively examine the relationship between glucose and oxidative metabolism in the subcortical white matter (WM) acutely after traumatic brain injury (TBI). The objective was to determine the nature, extent, and degree of metabolic abnormalities in subcortical brain regions remote from hemorrhagic lesions.

METHODS:

Sixteen normal volunteers and 10 TBI patients (Glasgow Coma Scale score, 4–10; age, 17–64 yr; 6 with focal and 4 with diffuse injury) were studied. Each subject underwent dynamic positron emission tomographic studies using [15O]CO, 15O2, [15O]H2O, and fluorodeoxyglucose plus a magnetic resonance imaging scan acutely after TBI. Parametric images of the metabolic rate of oxygen and metabolic rate of glucose were generated, and a molar oxygen-to-glucose utilization ratio was calculated. Data from gray matter and WM remote from hemorrhagic lesions, plus whole brain, were analyzed.

RESULTS:

There was a significant reduction in the subcortical WM oxygen-to-glucose utilization ratio after TBI compared with normal values (3.99 ± 0.77 versus 5.37 ± 1.00; P &lt; 0.01), whereas the mean cortical gray matter and whole-brain values remained unchanged. WM metabolic changes, which were diffuse throughout the hemispheres, were characterized by a reduction in the metabolic rate of oxygen without a concomitant drop in the metabolic rate of glucose.

CONCLUSION:

The extent and degree of subcortical WM metabolic abnormalities after moderate and severe TBI suggest that diffuse WM injury is a general phenomenon after such injuries. This pervasive finding may indicate that the concept of focal traumatic injury, although valid from a computed tomographic imaging standpoint, may be misleading when considering metabolic derangements associated with TBI.

Delayed presentation of diffuse axonal injury: a case report

This report highlights a case of delayed onset of diffuse axonal injury in the apparent absence of direct head trauma. This case questions the appropriate observation period for patients with signs of diffuse axonal injury presenting after a high-velocity crash.

Head trauma

The role of radiological imaging modalities in the investigation of head trauma is reviewed with reference to the mechanisms of head injury, as well as the recent guidelines for investigation produced by the National Institute for Clinical Excellence (NICE), and the Royal College of Radiologists. Lesions resulting from head trauma and the associated radiological ndings are described.

The neurophysiology of brain injury

OBJECTIVE

This article reviews the mechanisms and pathophysiology of traumatic brain injury (TBI).

METHODS

Research on the pathophysiology of diffuse and focal TBI is reviewed with an emphasis on damage that occurs at the cellular level. The mechanisms of injury are discussed in detail including the factors and time course associated with mild to severe diffuse injury as well as the pathophysiology of focal injuries. Examples of electrophysiologic procedures consistent with recent theory and research evidence are presented.

RESULTS

Acceleration/deceleration (A/D) forces rarely cause shearing of nervous tissue, but instead, initiate a pathophysiologic process with a well defined temporal progression. The injury foci are considered to be diffuse trauma to white matter with damage occurring at the superficial layers of the brain, and extending inward as A/D forces increase. Focal injuries result in primary injuries to neurons and the surrounding cerebrovasculature, with secondary damage occurring due to ischemia and a cytotoxic cascade. A subset of electrophysiologic procedures consistent with current TBI research is briefly reviewed.

CONCLUSIONS

The pathophysiology of TBI occurs over time, in a pattern consistent with the physics of injury. The development of electrophysiologic procedures designed to detect specific patterns of change related to TBI may be of most use to the neurophysiologist.

SIGNIFICANCE

This article provides an up-to-date review of the mechanisms and pathophysiology of TBI and attempts to address misconceptions in the existing literature.