Introduction
Background
Subdural hematomas are 1 of the 3 types of extra-axial intracranial hemorrhages (along with subarachnoid and epidural hemorrhages) and usually occur as a result of trauma. Deceleration injuries are often the cause of subdural bleeding from rupturing of veins via a shearing mechanism. Other entities, such as child abuse and ventricular decompression, also can result in subdural bleeding, and spontaneous hemorrhages may occur in patients receiving anticoagulants or patients with a coagulopathy condition. Compression of a dural sinus does not directly cause a subdural hematoma, although compression may result in a venous infarction.
Some subdural hematomas are clinically silent, whereas others cause symptoms as a result of mass effect on the adjacent brain. Some hematomas can grow large enough to result in herniation of cerebral tissue. Before computed tomography (CT) scanning and magnetic resonance imaging (MRI) technology, subdural hematomas were diagnosed only on the basis of this mass effect, which was depicted as displacement of the blood vessels on angiograms or as a calcified pituitary gland on skull radiographs. The advent of CT scan and MRI studies has made the diagnosis of even small hemorrhages routine (see Image 1).
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Pathophysiology
The meninges are composed of the dura mater, arachnoid mater, and pia mater. A space potentially exists between the arachnoid and dura (termed the subdural space), which, unlike the epidural space, is not confined by the cranial sutures. Bridging veins cross this space, extending from the cortical surface into the dural sinuses. Bleeding from these veins may occur as a result of a shearing injury (either by rotational or linear forces) along the plane of the subdural space and traumatic stretching of the veins, which can occur from rapid ventricular decompression.Because the subdural space is not limited by the cranial sutures, blood can spread along the entire hemisphere and into the hemispheric fissure, limited only by the dural reflections (see Image 2). This ability of blood to spread relatively unchecked results in a crescent-shaped subdural hemorrhage rather than the biconvex or football shape of most epidural hematomas.
In larger, more clinically important subdural hematomas, the etiology of the bleeding is often an adjacent cortical laceration or shearing of cortical vessels. Patients with these hematomas have a worse prognosis; some studies report mortality rates in excess of 80%.
Most subdural hematomas occur along the cerebral convexities. Less common locations include the posterior fossa and along the interhemispheric fissure and tentorium (see Image 3).
In children, subdural hematomas occurring along the posterior interhemispheric fissure and the tentorium have been described as common findings following violent nonaccidental shaking (ie, shaken baby syndrome). Although these hematomas are not pathognomonic for child abuse, they should always suggest the possibility of abuse. It should be noted, however, that in children with benign external hydrocephalus, subdural hematomas and retinal hemorrhages may occur following even minor trauma. In addition, a recent study by Tung et al (2006) revealed no statistically significant difference in the incidence of interhemispheric subdural hematomas in accidental trauma relative to nonaccidental trauma in children, raising the possibility that such injuries are not a reliable method for detecting child abuse. The authors did note, however, a higher incidence of mixed-density subdural hematomas in nonaccidental trauma.
Although the vast majority of subdural hematomas result from trauma, spontaneous subdural hematomas can be seen in elderly individuals, those receiving anticoagulation, or patients with intracranial hypotension. Subdural hematomas after lumbar puncture, epidural injection, and puncture of spinal meningeal cysts have been reported. Patients with polycystic kidney disease and Prader-Willi syndrome also have an increased risk for spontaneous subdural hematomas.
Rare occurrences of subdural hematomas resulting from ruptured aneurysms (see Image 4) or dural arteriovenous malformations have been described. These diagnoses should be considered in cases of unexplained subdural hematomas, particularly recurrent subdural hematomas, which may be caused by intermittent bleeding from a dural arteriovenous malformation.
Another rare cause of chronic subdural hematomas is an arachnoid cyst. The hematoma may be preceded by cyst rupture and formation of a subdural hygroma with subsequent bleeding into the space.
Subdural hematomas are more common in the elderly and in infants because both groups have a larger subarachnoid space than young adults. The larger subarachnoid space allows for more movement between the brain and dura, predisposing these populations to the formation of subdural hematomas.
Frequency
United States
The frequency varies because many subdural hematomas are caused by trauma. One study of chronic subdural hematomas showed that this type of hematoma occurs at a rate of about 1 case per 10,000 population.
Mortality/Morbidity
- In patients with small subdural hematomas ( <1 cm in diameter), the prognosis is good. One study showed that 78% of patients undergoing burr-hole evacuation of chronic subdural hematomas had a good or complete recovery.
- Clinical outcome in patients with acute subdural hematomas is not as good because of the presence of other injuries and because of selection bias. In patients with subdural hemorrhages large enough to warrant surgery, mortality exceeds 50% and is related to the size of the hematoma and the amount of midline brain shift. However, the most important feature used to predict the outcome is the absence or presence of adjacent parenchymal contusion.
- Mortality in patients with large subdural hematomas causing mass effect on adjacent brain tissue is improved if the hemorrhage is treated surgically within 4 hours after injury. Although patients with large subdural hematomas do better when the hematoma is evacuated within the first 4 hours, a longer wait does not necessarily result in death.
Age
Subdural hematomas may occur at any age as a result of various types of trauma. In children, shaken baby syndrome usually occurs before the age of 1 year. In older persons, a subdural hematoma is more likely related to a fall. Both the elderly and infants are predisposed toward subdural hematomas because of their relatively large subarachnoid spaces.
Anatomy
See Pathophysiology.
Presentation
Subdural hematomas may be clinically silent when small and discovered only when imaging of the brain is performed as part of the workup for trauma. Some patients may complain of headache or dizziness when an isolated subdural hematoma is present. When the hematoma is larger, symptoms usually result from mass effect on the brain tissue or from adjacent parenchymal injuries. Decreased mental status, unsteady gait, headache, deviated gaze, and respiratory depression may be presenting symptoms.
Preferred Examination
CT scanning is usually the first evaluation in patients with suspected acute subdural hematoma because CT scans depict acute hemorrhage and skull fractures well, they are relatively fast to obtain, and CT scanning is more readily available than MRI. Smaller hemorrhages may be missed on CT scans, and in the nonacute setting, MRI is the study of choice because of its high sensitivity and specificity.
Limitations of Techniques
CT scanning may fail to depict small hemorrhages because of the similarity in attenuation between blood and adjacent bone and because of streak artifacts in the posterior fossa and inferior middle cranial fossa. MRI aids in the detection of small hematomas because of its multiplanar capabilities.
Differential Diagnoses
Arachnoid Cyst
Epidural Hematoma
Other Problems to Be Considered
Brain atrophy
More on Subdural Hematoma |
 Overview: Subdural Hematoma |
| Imaging: Subdural Hematoma |
| Follow-up: Subdural Hematoma |
| Multimedia: Subdural Hematoma |
| References |
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Further Reading
Keywords
SDH, subdural hemorrhage, subdural bleed, brain hemorrhage, intracranial hemorrhage, extraaxial hemorrhage, extra-axial hemorrhage, intracranial hemorrhage, cranial bleeding, head trauma, brain injury, brain trauma, acute subdural hematoma, subacute subdural hematoma, chronic subdural hematoma, shaken baby syndrome, shaken-baby syndrome, child abuse
