There are three meninges covering the brain: the dura mater, the arachnoid mater, and the pia mater.

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  The dura mater is the thickest and strongest membrane, and is firmly attached to the inner surface of the cranial bone, especially along the sutures. It contains the meningeal arteries.

  
  
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  The arachnoid mater is a thin membrane under the dura mater. Its inner surface has numerous thin trabeculae extending downward, into the subarachnoid space.

  
  
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  The pia mater is a thin membrane that covers the surface of the brain, entering the grooves and fissures.

Due to the tight adhesion of the dura mater to the inner skull, significant force is required to separate them. In contrast, separation of the dura from the subarachnoid mater can occur with relatively little force.

The middle meningeal artery arises from the external carotid artery. It enters the foramen spinosum and branches into the anterior, middle, and posterior branches with various patterns. It is a common source of bleeding in acute epidural hematomas (EDHs).

The bridging veins connect the cortical superficial veins to the sagittal sinus in the dura. They are a common source of bleeding in acute subdural hematomas (SDHs).

Acute EDH and SDH are commonly caused by blunt mechanisms (e.g. motor vehicle accident, fall, assault).

EDH develop when blood collects in the space between the inner table of the skull and the dura; SDH occur when blood collects between the dura and arachnoid.

EDHs are often due to injury to the meningeal arteries, usually the middle meningeal artery, associated with skull fractures in the temporal region. A torn dura venous sinus or bleeding from a skull fracture may also result in EDH. The hematoma is located between the inner plate of the skull and the dura mater.

Although the temporal region is the most common site for EDH, they may occur almost anywhere in the cranial cavity.

Figure 6.2

(a) Appearance of a large epidural hematoma after craniotomy. The hematoma is located between the inner plate of the skull and the dura mater.

(b) Appearance of the intact dura mater after evacuation of the epidural hematoma. Note the fracture of the skull (circle), which was the primary cause of the bleeding.

Acute SDHs is commonly caused by bleeding from brain parenchymal injury or from torn bridging veins, which connect the cortical superficial veins to the sagittal sinus in the dura. The hematoma is located between the dura mater and the arachnoid mater.

Figure 6.3

(a) Intraoperative appearance of a large subdural hematoma under the dura mater, after craniotomy and opening of the dural mater.

(b) Intraoperative appearance after evacuation of the subdural hematoma. Note the brain is exposed and edematous.

The acute EDH appears as a high-density lenticular (biconvex)-shaped lesion, often associated with an overlying skull fracture. It usually does not cross suture lines. The acute SDH appears as a crescent-shaped lesion that may cross suture lines.

Figure 6.4

(a) CT scan appearance of an acute epidural hematoma. Note the lenticular (biconvex) shape of the hematoma (arrows).

(b) CT scan appearance of an acute subdural hematoma. Note the crescent shape of the hematoma (white arrows). There is a significant midline shift (black arrow).

Patients with EDH/SDH may present with a wide array of clinical manifestations, from mild headache to coma. A classic “lucid interval” (brief loss of consciousness with recovery followed by neurological deterioration) is seen in only half of patients with acute EDH.

EDHs are typically caused by arterial sources of bleeding, and, therefore, often have a more rapid time course to neurological deterioration. Many SDHs, on the other hand, are caused by venous sources, and may accumulate and exert neurological effects via a slower progression pattern.

Emergency surgical evacuation of EDH or SDH by craniotomy is often required to prevent death and long-term functional disability.

Acute SDHs, are commonly caused by bleeding from brain parenchyma trauma or the bridging veins which cross the subdural space. The bridging veins connect the cortical superficial veins to the sagittal sinus in the dura.

Elderly people are more likely to develop SDH due to cerebral atrophy associated with increased fragility and tension of the bridging veins. On the other hand, elderly people are less likely to develop EDH because of the fibrosis and firmer attachment of the dura mater to the skull.