You are in: eMedicine Specialties > Radiology > BRAIN/SPINE Diffuse Axonal InjuryArticle Last Updated: Jul 26, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Jeffrey R Wasserman, DO, Staff Physician, Department of Diagnostic Radiology, Medical College of Pennsylvania-Hahnemann University Hospital Jeffrey R Wasserman is a member of the following medical societies: American Medical Association Coauthor(s): Robert A Koenigsberg, DO, MSc, FAOCR, Director of Neuroradiology, Professor, Department of Radiology, Drexel University College of Medicine Editors: Jeffrey L Creasy, MD, Associate Professor, Associate Section Head, Division of Neuroradiology, Director, Neuroradiology Fellowship, Department of Radiology, Vanderbilt University; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Robert L DeLaPaz, MD, Director, Professor, Department of Radiology, Division of Neuroradiology, Columbia University; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; James G Smirniotopoulos, MD, Professor of Radiology, Neurology, and Biomedical Informatics, Chairman, Department of Radiology and Radiological Sciences, Uniformed Services University of the Health Sciences Author and Editor Disclosure Synonyms and related keywords: DAI, axonal shear injury, axonal shear-strain injury, traumatic brain injuries INTRODUCTIONSection 2 of 10
  BackgroundDiffuse axonal injury (DAI) is a frequent result of traumatic deceleration injuries and a frequent cause of persistent vegetative state in patients. DAI is the most significant cause of morbidity in patients with traumatic brain injuries, which most commonly result from high-speed motor vehicle accidents. DAI is a significant medical problem because of the high level of debilitation that is suffered by the patient, the stress that must be endured by the patient's family when the patient is in a persistent vegetative state, and the staggering medical cost of sustaining the patient in this state. DAI typically consists of several focal white-matter lesions measuring 1-15 mm in a characteristic distribution (see below). PathophysiologyThe pathophysiology of DAI first was described by Holbourn in 1943, using 2-dimensional gelatin molds.1 His work led to the understanding that shear injury is not induced by linear or translational forces but rather by rotational forces. Sudden acceleration-deceleration impact can produce rotational forces that affect the brain. The injury to tissue is the greatest in those areas where the density difference is the greatest. For this reason, approximately two thirds of DAI lesions occur at the gray-white matter junction. When shearing forces occur in areas of greater density differential, the axons suffer trauma; this results in edema and in axoplasmic leakage (which is most severe during the first 2 weeks following injury). The exact location of the shear-strain injury depends on the plane of rotation and is independent of the distance from the center of rotation. Conversely, the magnitude of injury depends on the following 3 factors:
The true extent of axonal injury typically is worse than that visualized using current imaging techniques. On the microscopic level, the axon may not be completely torn by the initial force, but the trauma still can produce focal alteration of the axoplasmic membrane, resulting in impairment of axoplasmic transport. This would lead to axoplasmic swelling, with the axon subsequently splitting into 2 pieces and a retraction ball—a pathologic hallmark of shearing injury—forming. The axon would then undergo wallerian degeneration. Dendritic restructuring might occur, with some regeneration possible in mild to moderate injury. Within the basal ganglia, the effect of DAI produces parenchymal atrophy brought on by shrinkage of astrocytes in the lateral and ventral nuclei, with sparing of the anterior and dorsomedial nuclei, the pulvinar, the centromedian nuclei, and the lateral geniculate bodies. Cholinergic neurons have been found to be slightly more susceptible to trauma than are neurons belonging to other neurotransmitters. Peripheral lesions usually are smaller than central lesions. The lesions typically are ovoid or elliptical, with the long axis parallel to the direction of the involved axonal tracts. A high association is seen between thalamic injury and DAI. Both silver staining and beta-amyloid precursor protein immunohistochemical staining have proven useful in the pathologic identification of DAI lesions. DAI was classically believed to represent a primary injury (occurring at the instant that the trauma occurred). It has become apparent, however, that the axoplasmic membrane alteration, transport impairment, and retraction ball formation may represent secondary (or delayed) components of the disease process. FrequencyUnited StatesDAI represents approximately one half of all intra-axial traumatic lesions. Mortality/MorbidityDAI rarely results in death. As many as 90% of patients remain in a persistent vegetative state. RaceNo racial predilection exists. SexNo sex predilection exists. AgeDAI can occur at any age. Some studies suggest that DAI may occur in utero if a pregnant woman is subjected to sufficient force. AnatomyTypically, the process is diffuse and bilateral, involving the lobar white matter at the gray-white matter interface. The corpus callosum frequently is involved, as is the dorsolateral rostral brainstem. The most commonly involved area is the frontal and temporal white matter, followed by the posterior body and splenium of the corpus callosum, as well as the caudate nuclei, thalamus, tegmentum, and internal capsule. Internal capsule lesions are associated more frequently with hemorrhage than are the other lesions and are secondary to the proximity of the lenticulostriate vessels. The following stages of involvement have been described by Adams and colleagues according to the anatomic location of the lesions4:
Clinical DetailsClassically, DAI has been considered a primary-type injury, with damage occurring at the time of the accident. Research has shown that another component of the injury comprises the secondary factors (or delayed component), since the axons are injured, secondary swelling occurs, and retraction bulbs form. Of patients with DAI, 80% demonstrate multiple areas of injury on computed tomography (CT) scans. Compared with patients who have an epidural hematoma, patients with DAI are less likely to have a lucid interval. There is little association between DAI and the presence of skull fractures; in addition, the existence of DAI has no bearing on whether a subarachnoid or subdural hemorrhage is present. Preferred ExaminationMagnetic resonance imaging (MRI) is the preferred examination for DAI (particularly with gradient-echo sequences), although CT scanning may demonstrate findings suggestive of DAI and is more practical and available. Studies have indicated that MRI can play a role in predicting the length of coma in DAI patients. Limitations of TechniquesMRI is contraindicated in patients with implanted pacemakers or certain types of metallic prostheses, as well as in patients who have metallic foreign bodies, such as bullet fragments, in their head or neck or near important vascular structures. In addition, MRI is difficult to perform on patients who have claustrophobia and on ventilator-dependent patients. DIFFERENTIALSSection 3 of 10
Brain, Multiple Sclerosis Other Problems to Be ConsideredCavernous angioma of the brain
RADIOGRAPHSection 4 of 10
FindingsNo specific findings related to DAI can be made using conventional radiography; however, other signs of head trauma can be appreciated, such as facial bone fractures or fluid levels within the paranasal sinuses. Degree of ConfidenceThe degree of confidence is low, since conventional radiography cannot demonstrate subtle soft-tissue changes. While radiographs can clearly demonstrate skull fracture, this is not helpful in DAI, since DAI is rarely associated with skull fracture. False Positives/NegativesMany false negatives are possible, since a negative skull radiograph in no way excludes a parenchymal brain injury. CT SCANSection 5 of 10
FindingsAmong patients eventually proven to have DAI, 50-80% demonstrate a normal CT scan upon presentation. Delayed CT scanning may be helpful in demonstrating edema or atrophy, which are later findings. Small petechial hemorrhages, located at the gray-white matter junction, as well as in the corpus callosum and brainstem, are characteristic of CT-scan findings in the acute setting. The following CT-scan criteria have been suggested by Wang and colleagues2:
One may also observe small focal areas of low density on CT scans; these correspond to areas of edema occurring where shearing injury took place. MRI is more sensitive in the detection of subtle soft-tissue abnormalities; however, CT scanning is more available and practical in the current medical environment and is therefore, according to Teasdale, the "mainstay of acute investigation of head injury."3 Degree of ConfidenceThe degree of confidence in CT scanning is moderate, since the only finding may be petechial hemorrhage, and fewer than 20% of patients with DAI demonstrate this finding on CT scanning alone. When petechial hemorrhages are observed with the appropriate clinical findings, the sensitivity of CT scanning in the detection of DAI is high. False Positives/NegativesAs with conventional radiographs, frequent false negatives are possible, since normal CT-scan findings are common in patients with DAI. MRISection 6 of 10
FindingsRecommended sequences include T1-weighted, T2-weighted, T2–gradient-echo, proton density–weighted, and diffusion-weighted images.
One area of research has been magnetization transfer imaging. Studies have reported that the magnetic transfer ratio has shown promise in identifying areas of injury not visible on the above MRI pulse sequences. This may allow the radiologist to appreciate a truer representation of the degree of microscopic injury. Studies have indicated that MRI can play a role in predicting the length of coma in DAI patients. The volume of white-matter lesions has been correlated to the degree of injury, as measured by MRI. MRI has also been used to quantify cerebral blood flow in damaged areas of the brain, thus predicting injury severity. Degree of ConfidenceThe degree of confidence is high, since abnormal signal in the characteristic locations, discovered in the clinical setting of recent trauma, leaves little doubt about the diagnosis of DAI. Multiple sclerosis (MS) is a progressive neurologic disorder that can involve multiple foci of white-matter signal abnormality on MRI; however, MS lesions typically are oval or oblong and are oriented in a direction perpendicular to the border of the lateral ventricles (Dawson fingers). In addition, MS lesions may involve the spinal cord, a finding not associated with DAI, and the clinical course of MS is dramatically different from that of DAI. NUCLEAR MEDICINESection 7 of 10
FindingsNuclear medicine currently has no role in the routine diagnostic workup of patients with possible DAI; however, studies have suggested that iodine-123 single-photon emission CT (SPECT) imaging demonstrates areas of hypoperfusion in areas of known injury and reveals additional areas of injury not visualized with MRI. INTERVENTIONSection 8 of 10
Medical/Legal Pitfalls
MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
Article Last Updated: Jul 26, 2007 | ||||||||||||||||||||||||||||||||||||||||||
