John M. Barkley
L. Anne Hayman
Pedro J. Diaz-Marchan
Imaging technology changes quickly; thus it is advantageous to learn pathology in an organ-based fashion. No matter how the technology changes, the pathology remains the same. With the basic knowledge of pathophysiology, one can learn and adapt to how a particular condition appears within a myriad of changing imaging modalities. Imaging of patients with traumatic brain injury in the acute setting often begins with a noncontrast CT of the brain (6).
All of the available data including history, physical exam, mechanism of injury, and vital signs are used in conjunction with the imaging evaluation to triage these patients into surgical or medical management.
If surgery will be needed, decisions as to the timing of surgery are also made based on the clinical presentation and imaging results. These acute injuries may require early neurosurgical intervention and a timely interpretation of the imaging findings is advantageous in the emergency room setting (11).
When interpreting an emergent CT scan of the brain, particular attention should be paid to the midline structures, presence of shift, ventricles, basal cisterns, the presence of herniation, cerebral convexities, and mass effect (12). The skull of infants and young children has open sutures and fontanelles; thus there is more plasticity of the cranial vault, should pathology develop.
The adult brain exists in a closed box, the densely ossified skull that does not yield to intracranial pathology. Skull fractures, particularly depressed fractures often herald underlying brain injuries. Another advantage of CT is the exsquisite bony detail it offers. Subtle, nondisplaced fractures in the plane of the scan may go undetected; however, these are felt by many researchers to be clinically insignificant (3,13).
CT allows one to characterize fractures, the extent of bony injury and the sequelae of these fractures to the underlying brain parenchyma. Air present within the skull or brain is termed pneumocephalus. The presence of pneumocephalus often heralds the presence of a fracture, usually communicating with air in the sinuses or outside of the skull. It is imperative to recognize fractures involving neurologically exquisite areas such as the orbit or the temporal bone.
Temporal bone fractures, depending on type may lead to cerebrospinal fluid otorrhea, conductive hearing loss, sensorineural hearing loss or facial nerve paralysis. Other fractures such as nondisplaced calvareal fractures may be managed conservatively.
There is a finite amount of space within the adult skull. If something occurs within the skull to occupy the space, it occurs at the expense of the brain that is soft and very pliable. In response to a space-occupying intracranial lesion, the brain may become deformed, swollen or edematous. If the mass effect within the skull is severe enough, a herniation syndrome may result. The brain is made up of gray and white matter. The gray matter consists of the cerebral cortex, cerebellar cortex, and deep nuclei.
The white matter consists of a network of axons—connections and tracts between these nuclei. Injuries involving the brain itself—the gray or white matter of the brain parenchyma, are called intra-axial injuries. The extra-axial space, on the other hand, is outside of the brain parenchyma. The brain is separated from the inner table of the calvarium by several layers of tissue called meninges.
The meninges are composed of different layers of connective tissue. The dura is the outer layer that is firmly attached to the inner table and periosteum of the skull. The arachnoid exists between the dura and the pia. The pia is a thin lining that covers the brain and spinal cord. It follows the cortex of the brain and lines each gyrus, while dipping into the intervening sulci. The subarachnoid space is the space bounded by the arachnoid layer and pia. It contains cerebrospinal fluid and blood vessels. There are several areas within the brain where the subarachnoid space expands to create cisterns. Abnormalities that affect the subarachnoid space may extend into these cisterns (14). The pia and arachnoid comprise the leptomeninges. Injuries outside of the brain parenchyma are called extra-axial injuries.
The brain and spinal cord are bathed in cerebrospinal fluid (CSF). This fluid is also present in the ventricles and cisterns of the brain. There are two lateral ventricles, with frontal, temporal, and occipital horns. The third and fourth ventricles are unpaired, midline structures. The lateral ventricles communicate with the third ventricle through the paired foramina of Monro.
The third ventricle communicates with the fourth ventricle—which is in the posterior fossa of the brain through the cerebral aqueduct. In the normal state, the CSF volume remains relatively constant, although it is constantly in flux. A homeostasis is reached where the amount of CSF produced in the choroids plexus is the same as the amount of CSF being absorbed into the venous sinuses. Cerebral capillaries form an impermeable blood-brain barrier and the capillaries and ependymal cells form a blood-CSF barrier (14).
These barriers may break down in pathological conditions such as trauma, infection or tumor. Injuries within the ventricles or CSF spaces, while inside the confines of the brain, are actually outside the brain parenchyma and thus are extraaxial injuries. Sometimes, it is difficult to determine whether a lesion is intra-axial or extra-axial, however there are signs on imaging that help to define the location of lesions.
In the sections that follow, several types of traumatic brain injury will be discussed. The injuries are organized according to location. Injuries involving the brain parenchyma, or intra-axial space will be discussed first, followed by a discussion of extra-axial manifestations of traumatic brain injury.
Original: Brain Injury Medicine. Principles and Practice