Subdural Hematoma

Anatomy

The brain is protected inside the skull by three separate layers of tissue (meninges). The innermost layer, the pia mater, is a thin and delicate membrane that lies on the surface of the brain. The second layer, the arachnoid mater, covers the brain and pia mater but does not follow the contour of the involutions of the brain. The outermost layer, the dura mater, provides a thicker and tougher layer of protection.

These layers define three potential spaces for blood to collect. The epidural space, between the skull and the dura; the subdural space, between the dura and the arachnoid layer; and the subarachnoid space, between the arachnoid and pia layers—each with their own potential sources of hemorrhage. The pia mater is too closely adhered to the brain and too fragile to act as a barrier for blood, and therefore, there is no potential space between the pia and the brain for a hemorrhage to form.

Mechanism of Injury

A network of veins traverses the space between the surface of the brain and the dura. These veins, the bridging veins, can tear if the contents of the skull experience sudden changes in velocity. Blood leaking from the bridging veins then collects in the subdural space, creating a hematoma. The size of the hematoma and the speed with which it expands depend primarily on the number and size of the tears in the bridging veins. Given that the blood in the bridging veins is coming from the venous side of the circulatory system and is therefore under less pressure, subdural hematomas typically expand at a much lower rate than hematomas that are formed from arterial blood, such as epidural hematomas. The expanding subdural hematoma increases the intracranial pressure and can lead to damage of the underlying brain.

Subtypes

Subdural hematomas are often classified based on their acuity into acute, subacute, and chronic subtypes. Acute subdural hematomas are extremely [Page 1417]dangerous and frequently lethal without quick surgical intervention. Symptoms develop quickly, and mortality rates are estimated between 60% and 80%. Subacute subdural hematomas become symptomatic over several hours to days and carry a better prognosis. Chronic subdural hematomas develop over days to several weeks and are common in elderly individuals. Frequently, they are only mildly symptomatic or without symptoms completely. In these cases, the bleeding is self-limited, and no surgery or acute intervention is required. In the athletic population, a subdural hematoma caused by a physical impact is more likely to present as an acute or subacute subtype.

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