Subdural Hematoma

Excerpt

A subdural hematoma forms because of an accumulation of blood under the dura mater, one of the protective layers to the brain tissue under the calvarium. The understanding of subdural hematoma relies on the knowledge of neuroanatomical sheets covering the brain. The brain is the central repository of delicate neural tissue. This network of neurons and neuronal connective tissue is prone to injury without the protective layers, starting with the scalp and the bony structures of the skull. The brain finds its protection under the skull by the meninges that comprise three layers.

The inner surface of the skull toward the brain includes:

First, there is a leather-like structure called the dura mater, derived from the neural crest, adhering to the periosteum and facing the other meningeal structure, the arachnoid mater. The arachnoid mater lies under the dura mater (middle meningeal layer) forming many villi piercing through the dura with bridging veins acting as one-way valves to drain the neural tissue lying underneath the last meningeal layer called the pia mater. These so-called bridging veins may rupture when direct opposing forces rupture their thin walls, releasing blood under the dura mater forming a subdural hematoma.

When there is a larger space between the dura mater and the brain, as seen in the young, growing brain or an aging brain (because of contraction), the cerebrospinal fluid (CSF) flows between bridging veins occupying a larger space. In this context, the structure stretches bridging veins and renders them prone to rupture. A small extravasation may resolve spontaneously. A larger bleed may augment the distance between the bridging veins and trigger an extensive amount of blood layering around the brain, slowly augmenting subdural space, decreasing the space of the brain leading to herniation of the cerebral structures.