Chronic subdural hematoma arises from bleeding which occurs in the "potential" space between the pachymeninges or dura, and the leptomeninges or arachnoid and pia and is usually said to require approximately six weeks to develop while subacute subdural hematoma are described as those that developed between two to six weeks. This is described as a "potential" space because in normal circumstances the brain and its covering of arachnoid and pia abuts directly against the dura.
With advancing age, however, atrophy of the cerebral cortex occurs and a true "subdural" space may develop. Small veins, that drain the cortex may be adherent to the dura or drain into dural tributaries are quite common. With cerebral atrophy, these small "bridging" veins traverse this now enlarged subdural space. They are prone to tear and bleed with any trauma that would apply inertial force on the brain. This may occur after a quite minor incident such as a slip or fall in an elderly person or banging of the head against a doorway or some other relatively minor and often unremembered incident. The acute bleeding usually ceases when the torn vein clots or when the pressure developed by the enlarging clot exceed the pressure of the bleeding vein. Bleeding may recur with further trauma, or for reasons that are not clearly understood the clot itself may enlarge in size as it liquefies over the ensuing several weeks following the initial incident. In some cases, it is felt that a thin membrane of neovascularization develops along the edges of the hematoma and these friable new vessels may provoke further bleeding. In any event, the clot, depending on its size, may exert mass effect on the underlying brain as well as cortical irritation and these factors may both contribute to progressive neurologic deficit.
Due to the cerebral atrophy often present in elderly patients who develop subdural hematomas, rather large subdural collections may develop with significant shift of the brain, before clear symptoms arise. The neurosurgical team is often called to evaluate such a case after a CT scan has been performed due to a history of progressive confusion, mild hemiparesis, gait disturbance with sometimes subsequent falls, etc. Older patients who are on anticoagulant medicines for cardiac conditions are particularly prone to development of subdural hematoma.
Management of the patient with subdural hematoma begins with careful neurologic evaluation and study of the CT scan. The scan will demonstrate the size of the clot and its age. More acute blood is hyperdense on CT scan and as clots liquefy over time they become isodense to brain and subsequently hypodense to eventually assume the same CT characteristics as the cerebro spinal fluid. The degree of cerebral compression and the shift of the midline, taken in conjunction with the patients neurologic evaluation determines the acuity of surgery. Among the most common surgical procedures utilized with a variable degree of success are burr holes, craniotomy, shunting of the subdural space, and twist drill craniostomy with or without closed system drainage. Clinicians have also initiated medical treatment, including bed rest, steroids, and osmotic diuretics in various combinations. The many drawbacks of medical therapy, however, preclude its wide acceptance and are not routinely used.
At Neurology and Neurosurgical Services we have utilized the twist drill craniostomy and closed system drainage which allows the evacuation of subdural hematoma to be performed through a small opening in the skull under local anesthesia. Only rarely is placement of burr hole required and full craniotomy has been virtually eliminated. This procedure has the benefit of being performed under local anesthesia and consists of making a small hole into the skull through a 1 cm incision in the scalp placed directly over the area of the hematoma. After the drill hole has been placed, the dura is lacerated and a thin flexible catheter is placed into the area of the liquid clot. The catheter is secured to the scalp and a dressing placed. The patient is then monitored in the intensive care unit and the clot allowed to drain through the tube over the next 24 hours.
Care of these patients involves maintenance of a flat position in bed for these 24 hours to allow for slow expansion of the brain to "push" the liquefied clot out. The catheter is generally removed the morning after the procedure at the bedside and in many instances patients are discharged shortly thereafter.
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