Subdural hematomas (SDH) are caused by sudden acceleration-deceleration of brain parenchyma with subsequent tearing of the bridging veins. These vessels are located beneath the dura mater and result in blood clots forming between the dura mater and the arachnoid ( Fig, 2.4.7.-8). SDHs occur in approximately 30 percent of all severe TBI.
Brains with extensive atrophy, such as the elderly and alcoholics, are more susceptible to subdural hematomas because of the relatively smaller brain volume in a larger cranial vault. Children under the age of two are also at increased risk of subdural hematomas. This is related to transfer of the force that limits the risk of sustaining epidural hematomas as discussed above. In addition, children have a larger head to body ratio than adults, which increases the overall risk of head injury.
Blood tends to collect more slowly than epidural hematomas because of its venous origin. However, unlike the epidural hematomas, SDH have more associated brain parenchyma injury so outcome is less favorable. The mortality rate averages 60 percent for patients with acute SDH, but can be significantly lowered with early surgical intervention.9
Traditionally, subdural hematomas have been classified as acute, subacute, and chronic, depending on time of presenting signs and symptoms. In acute SDH patients present between 3 and 14 days. After 2 weeks, patients are defined as having a chronic SDH. There is no specific clinical syndrome associated with a subdural hematoma. Acute cases usually present immediately after severe trauma and often, the victim is unconscious. However, chronic subdurals may present in the elderly or alcoholic with vague complaints or mental status changes. These patients often do not recall an injury. On CT scan, acute SDH are hyperdense (white), crescent-shaped lesions. Subacute SDH are isodense and more difficult to identify. A CT scan with intravenous contrast can assist in identifying a subacute SDH. A chronic SDH appears hypodense as the iron in the blood is phagocytized.
The definitive treatment of subdurals depends on the type and on associated brain injuries. Mortality and the need for surgical repair are greater for acute and subacute SDH. Chronic subdurals can sometimes be managed without surgery depending of the severity of the symptoms.
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