Pathogenesis and Pathophysiology. A subdural hematoma (SDH) results from venous bleeding after blunt head trauma, which causes brain motion within the skull, shearing off the bridging veins between the surface of the brain and adjacent dural venous sinuses. The blood leaks and collects slowly, forming a hematoma in the subdural space.y An SDH may be absorbed spontaneously or may form an encapsulated and liquefied hematoma. After about 2 weeks, membranes form around the hematoma. The outer membrane is thicker and more vascular than the inner membrane. The center of the encapsulated SDH liquefies and may enlarge due to repeated bleeding from the vascular outer membrane, assuming a progressively larger biconvex lens shape. '791
Epidemiology and Risk Factors. Acute SDHs develop after severe head trauma and carry a poor prognosis. The mortality rate of a treated acute SDH is roughly 50 percent. A chronic SDH usually develops after minor trauma, typically in an elderly person under anticoagulation or in an alcoholic individual with some degree of brain atrophy. Because an atrophic brain cannot tamponade a beginning hematoma, bleeding frequently continues, causing headaches, behavioral changes, altered level of consciousness, or a focal neurological deficit such as hemiparesis. y At times, subdural hematomas develop without a history of trauma. Patients treated with anticoagulants and those with bleeding disorders may develop spontaneous subdural bleeding. Occasionally, subdural collections develop after lumbar puncture. In other patients head trauma is forgotten or considered too inconsequential to mention. In many instances, a fall causes retrograde amnesia and the patient may not have been fully aware of the injury.
Clinical Features and Associated Disorders. The three most common findings in patients with subdural hematomas are headache, decreased level of alertness, and abnormalities of cortical function. Headache is usually ipsilateral to the hematoma and may be worse at night. Drowsiness and decreased alertness reflect an increase in intracranial pressure. There is often slight weakness, hyperreflexia, and Babinski sign contralateral to the hematoma. Slight aphasia may develop in patients with left-sided hematomas and neglect of the right side of space occurs in patients with right subdural hematomas. Usually, the neurological abnormalities are soft and seldom are as profound deficits as those that occur in patients with large hemisphere infarcts or intracerebral hematomas. Seizures may occur and probably indicate some contusion of the underlying brain tissue as the hematoma enlarges, headache worsens, and the level of consciousness often decreases. An ipsilateral Babinski sign or elements of an ipsilateral third nerve palsy, or both, may develop and indicate midbrain compression.
Differential Diagnosis and Evaluation. The diagnosis is usually obvious in patients with head injury. The insidious development of symptoms, especially in patients who provide no history of trauma can readily mimic brain tumor or abscess. Brain infarcts and hematomas usually present with more acute onset symptoms and signs and more severe focal deficits. Neuroimaging is now indispensable for accurate diagnosis. On a CT scan, an acute SDH appears as a sickle-shaped, hyperdense lesion over the outer surface of the brain lying against the inner surface of the skull and dura. A subacute SDH appears isodense in relation to the brain, making diagnosis difficult. During this acute period, a T1-weighted MRI is very helpful and shows a high signal intensity lesion in the subdural space. A chronic SDH appears hypodense on CT scans.y
Management. If an SDH is not recognized and is left untreated, it may cause a severe neurological deficit or death. SDHs should be surgically evacuated. The prognosis is good and primarily related to the degree of associated brain injury. y In older patients and in those with brain atrophy, re-expansion of the compressed brain may be delayed. Because subdural bleeding may recur, a drain must be left in for days and the patient must be watched carefully for continued bleeding.
Prognosis and Future Perspectives. Recovery is usually excellent in patients in whom subdural hematomas are recognized and treated. Small subdural hematomas probably are more common than recognized and may heal spontaneously without medical treatment. Because subdural hematomas represent a very treatable cause of mental abnormalities and neurological signs in elderly populations prone to falls, improved screening of these patients would be an important public health advance.
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