The arachnoid mater is a thin, transparent layer that surrounds the brain and spinal cord. It is connected to the pia mater by web-like filaments, hence the name “arachnoid” mater (Figure 15-4D).
Generally, there is no space between the dura mater and the arachnoid mater. However, trauma to the head may stretch and rupture a bridging (cerebral) vein, resulting in bleeding into the subdural space (subdural hematoma
). Because the damaged vessel is a vein, the increase in intracranial pressure and the effect of compressing the brain is much slower when compared to an epidural hematoma, which is caused by tearing of an artery. As a result, a subdural hematoma may develop over a period of days or even a week. Enlarging the subdural space is one factor that increases the risk of a subdural hematoma. As the subdural space enlarges, the bridging veins that traverse the space travel over a wider distance, causing them to be more vulnerable to tears. As a result, infants (who have smaller brains), the elderly (whose brains atrophy with age), and alcoholics (whose brains atrophy from alcohol
use) are at increased risk of developing a subdural hematoma because of the tension of traversing vessels from the shrinking brain to the dural venous sinus. Subdural hematomas spread along the internal surface of the skull, creating a concave shape that follows the curve of the brain. The spread of blood is limited to one side of the brain due to dural reflections such as the tentorium cerebelli and falx cerebri. Contrast the spread of subdural hematomas to that of epidural hematomas that are limited in their spread due to the sutures.
A subarachnoid hemorrhage
is defined as bleeding into the subarachnoid space because of a ruptured cerebral aneurysm.