A subdural hemorrhage (SDH), is a form of traumatic brain injury in which blood collects between the dura (the outer protective covering of the brain) and the arachnoid (the middle layer of the meninges). Unlike in epidural hematomas, which are usually caused by tears in arteries, subdural bleeding usually results from tears in veins that cross the subdural space. This bleeding often separates the dura and the arachnoid layers. Subdural hemorrhages may cause an increase in intracranial pressure (ICP), which can cause compression of and damage to delicate brain tissue. Acute subdural hematoma (ASDH) has a high mortality rate and is a severe medical emergency. As such, it has become a recurring plot device on current medical dramas such as the television series House.
Causes
Subdural hematomas are most often caused by head injury, when rapidly changing velocities within the skull may stretch and tear small bridging veins. Subdural hematomas due to head injury are described as traumatic. Much more common than epidural hemorrhages, subdural hemorrhages generally result from shearing injuries due to rotational or linear forces (University of Vermont; Wagner, 2004).
Features
Most of the time, subdural hematomas occur around the tops and sides of the frontal and parietal lobes (University of Vermont; Wagner, 2004). They also occur in the posterior fossa, and near the falx cerebri and tentorium (Wagner, 2004). Unlike epidural hematomas, which cannot expand past the sutures of the skull, subdural hematomas can expand along the inside of the skull, creating a convex shape that follows the curve of the brain, stopping only at the dural reflections like the tentorium and falx cerebri.
On a CT scan, subdural hematomas have a crescentic shape, with a concave surface away from the skull. Unlike epidural hematomas, subdural bleeds can cross skull sutures, so they can spread along the inside of the skull. 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. A chronic bleed can be the same density as brain tissue (called isodense to brain), meaning that it will show up on CT scan as the same shade as brain tissue, potentially obscuring the finding.
Pathophysiology
Collected blood from the subdural bleed may draw in water due to osmosis, causing it to expand, which may compress brain tissue and cause new bleeds by tearing other blood vessels (Downie, 2001). The collected blood may even develop its own membrane (McCaffrey, 2001).
In some subdural bleeds, the arachnoid layer of the meninges is torn, and cerebrospinal fluid (CSF) and blood both expand in the intracranial space, increasing pressure (University of Vermont).
Substances that cause vasoconstriction may be released from the collected material in a subdural hematoma, causing further ischemia under the site by restricting blood flow to the brain (Graham and Gennareli, 2000). 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.
The body gradually reabsorbs the clot and replaces it with granulation tissue.
Treatment
It is important that a patient receive medical assessment, including a complete neurological examination, after any head trauma. A CT scan or MRI scan will usually detect significant subdural hematomas.
Treatment of a subdural hematoma depends on its size and rate of growth. Small subdural hematomas can be managed by careful monitoring until the body heals itself. Large or symptomatic hematomas require a craniotomy, the surgical opening of the skull. A surgeon then opens the dura, removes the blood clot with suction or irrigation, and identifies and controls sites of bleeding. Postoperative complications include increased intracranial pressure, brain edema, new or recurrent bleeding, infection, and seizure.
Prevention
In addition to avoiding risk factors if possible, following safety precautions and wearing hard hats, helmets, and seat belts can prevent serious head injuries.