They also occur in the posterior cranial fossa, and near the falx cerebri and tentorium cerebelli. Unlike epidural hematomas, which cannot expand past the sutures of the skull, subdural hematomas can expand along the inside of the skull, creating a concave shape that follows the curve of the brain, stopping only at dural reflections like the tentorium cerebelli and falx cerebri.On a CT scan, subdural hematomas are classically crescent-shaped, with a concave surface away from the skull. However, they can have a convex appearance, especially in the early stages of bleeding. This may cause difficulty in distinguishing between subdural and epidural hemorrhages. A more reliable indicator of subdural hemorrhage is its involvement of a larger portion of the cerebral hemisphere. Subdural blood can also be seen as a layering density along the tentorium cerebelli. This can be a chronic, stable process, since the feeding system is low-pressure. In such cases, subtle signs of bleeding—such as effacement of sulci or medial displacement of the junction between gray matter and white matter—may be apparent. Fresh subdural bleeding is hyperdense, but becomes more hypodense over time due to dissolution of cellular elements. After 3–14 days, the bleeding becomes isodense with brain tissue and may therefore be missed. Subsequently, it will become more hypodense than brain tissue. Classification
Subdural hematomas are classified as acute, subacute, or chronic, depending on the speed of their onset.Acute bleeds often develop after high-speed acceleration or deceleration injuries. They are most severe if associated with cerebral contusions.
Infants, too, have larger subdural spaces and are more predisposed to subdural bleeds than are young adults. It is often claimed that subdural hematoma is a common finding in shaken baby syndrome, although there is no science to support this. In juveniles, an arachnoid cyst is a risk factor for subdural hematoma.Other risk factors include taking blood thinners (anticoagulants), long-term excessive alcohol consumption, dementia, and cerebrospinal fluid leaks. Pathophysiology
Acute
Acute subdural hematoma is usually caused by external trauma that creates tension in the wall of a bridging vein as it passes between the arachnoid and dural layers of the brains lining—i.e., the subdural space. The circumferential arrangement of collagen surrounding the vein makes it susceptible to such tearing.Intracerebral hemorrhage and ruptured cortical vessels (blood vessels on the surface of the brain) can also cause subdural hematoma. In these cases, blood usually accumulates between the two layers of the dura mater. This can cause ischemic brain damage by two mechanisms: one, pressure on the cortical blood vessels, and two, vasoconstriction due to the substances released from the hematoma, which causes further ischemia by restricting blood flow to the brain. When the brain is denied adequate blood flow, a biochemical cascade known as the ischemic cascade is unleashed, and may ultimately lead to brain cell death.Subdural hematomas grow continually larger as a result of the pressure they place on the brain: As intracranial pressure rises, blood is squeezed into the dural venous sinuses, raising the dural venous pressure and resulting in more bleeding from the ruptured bridging veins.