The dura is a gray membrane of connective tissue firmly adherent to the inner surface of the skull. Arteries run along the inner surface of the dura at its point of attachment with the skull. The potential space between the skull and dura is the epidural space, with an actual space, the subdural space, between the dura and brain. The brain, in turn, is enclosed in two thin, transparent membranes: the inner pia and the outer arachnoid. The subarachnoid space is between the arachnoid and pial membranes. It and the subdural space contain fluid. The fluid in the subarachnoid space is cerebrospinal fluid, produced by the choroid plexus that reaches the subarachnoid space through the foramen of Magendi of the fourth ventricle.
Epidural hematomas are primary impact injuries. They are relatively infrequent and are seen most commonly in falls and traffic accidents. They are infrequent in the elderly and the very young (less than 2 years of age) due to strong adherence of the dura to the skull in both these age groups.
Epidural hematomas are caused by trauma to the skull and the meningeal vessels (principally the arteries) at the point of impact. At impact, the skull is bent inward, with stripping of the dura and laceration of meningeal vessels. A fracture is virtually always present at this point (90-95% of the time) in association with the epidural hematoma.
The area of avulsion from the dura is extended by arterial bleeding that strips the dura from the bone, permitting accumulation of blood. The inability of the venous system to generate sufficient pressure to strip dura from bone accounts for the infrequency of epidural hematomas following venous injury. Epidural hematomas unassociated with fractures are uncommon. These usually occur in children who have very elastic bones, such that the dura can be avulsed from the bone without fracture. In most such cases, bleeding is minor.9,19
Epidural hematomas usually have a thick, disk-shaped appearance (Figure 6.11(B,C)). They are virtually always unilateral. Most epidural hematomas result from fractures of the squamous-temporal bone, with laceration of the middle meningeal artery (Figure 6.11(A)). Thus, most epidural hematomas are in the temporal region. Less commonly, there are lacerations of the anterior and posterior meningeal arteries with frontotemporal and parieto-occipital hematomas, respectively. Epidural hematomas caused by venous bleeding are the result of injury to the diploic veins, middle meningeal veins, and dural sinuses.
Symptoms from an epidural hematoma usually occur 4-8 h after injury. There is a lucid interval prior to the development of severe symptoms in approximately one third of patients. Occasionally, an individual will develop an epidural hematoma so rapidly that death will occur in as little as 30 min.9 Death is caused by displacement of the brain with compression of the brain stem. With large fractures, there may be decompression of the epidural hematoma and survival.
Chronic epidural hematomas are rare.20 They produce their signs and symptoms over a lengthy period of time. These lesions might lie quiescent for many days and then suddenly bring about death. It is hypothesized that chronic epidurals are more commonly associated with tears of venous rather than arterial structures. However, in some cases, tears in meningeal arteries have been demonstrated. Frequently, however, no point of bleeding is found. Symptoms are usually present from the time of the trauma, though they may be very minor in nature, such as headache or nausea. Drowsiness may or may not be present. Chronic epidural hematomas may or may not be associated with fractures of the skull. They are seen most commonly in older children and young adults. This is thought to be due to the ease with which the dura can be stripped from the skull in such individuals. Generally, an epidural hematoma is considered chronic when more than 48-72 h passes from injury to identification. In some individuals, the interval has been as long as 18 days.
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