Approach Considerations
An emergent computed tomography (CT) scan of the head needs to be performed when an acute subdural hematoma is suspected. It should be obtained immediately after the patient is stabilized using standard Advanced Trauma Life Support (ATLS) guidelines. CT scanning is also the initial imaging modality of choice for chronic subdural hematoma.
The trauma team and neurosurgeon must determine quickly which lesions warrant immediate evacuation, and CT is the imaging modality of choice to facilitate this decision. Modern CT devices can produce appropriate images in about 5 minutes, and the scans are highly sensitive to acute blood. Although magnetic resonance imaging (MRI) is superior for demonstrating the size of an acute subdural hematoma and its effect on the brain, noncontrast head CT is the primary means of making a diagnosis and suffices for immediate management purposes.
A worsening of the Glasgow Coma Scale by 2 or more points should prompt repeat imaging in salvageable patients. A cervical spine radiograph series is important in evaluating the possibility of concomitant cervical spine fracture.
Initial blood tests include the following:
- Complete blood count
- Hemoglobin or hematocrit
- Coagulation profile
- Basic metabolic panel
- Type and screen/cross-match
In addition, drug and alcohol screenings may be important for correlating the neurologic examination with the imaging studies.
Detection of electrolyte abnormalities is important because they can exacerbate brain injury and therefore require correction in a timely manner. For example, hyponatremia (5-12% estimated incidence in patients with head injury) can potentiate brain edema and cause seizures.
Coagulation Profile
Coagulation profiles are particularly important for patients taking anticoagulants and for alcoholics, who may have an associated coagulopathy placing them at high risk for subdural hematoma. Their altered mental status may be from the hematoma rather than ethanol.
In addition, the prevalence of coagulation abnormalities has long been recognized as unusually high in patients with head injuries. These abnormalities are believed to result from the release of thromboplastic materials by damaged brain tissue.
Stein et al showed that the presence of coagulopathy and the development of delayed brain injury are strongly associated. In a review of 253 patients with head injury who required serial CT scans, the risk of developing a delayed brain insult as seen on CT scan increased from 31% in patients with coagulation study findings within reference range to almost 85% in patients with abnormal findings on prothrombin time (PT), activated partial thromboplastin time (aPTT), or platelet count.[30]
Subdural hematomas themselves were associated with disease progression; 26 of 35 patients with subdural hematoma had expansion of their hematoma or a delayed brain injury seen on a follow-up CT scan. Therefore, all patients with head injury should have at least a basic coagulation panel (PT, aPTT, and platelet count). Fresh frozen plasma or platelets should be given as needed. However, awaiting the results of these studies should not delay emergency surgery.
Computed Tomography
On noncontrast CT scan, an acute subdural hematoma appears as a hyperdense (white), crescent-shaped mass between the inner table of the skull and the surface of the cerebral hemisphere (see the images below). Subdural hematomas are concave toward the brain and unlimited by suture lines, as opposed to epidural hematomas, which are convex toward the brain and restricted by suture lines. Rarely, a subdural hematoma appears lens shaped (ie, more like an epidural hematoma). Acute subdural hematomas are usually unilateral.
Acute right-sided subdural hematoma associated with significant midline shift (ie, subfalcine herniation) shown on CT scan. 
An acute subdural hematoma (SDH) as a complication of a craniotomy. Note the significant mass effect with midline shift. Acute subdural hematomas occur most commonly over the cerebral convexity in the parietal region (see the image below). The second most common site is above the tentorium cerebelli.
Acute subdural hematoma. Note the bright (white) image properties of the blood on this noncontrast cranial CT scan. Note also the midline shift. Image courtesy of J. Stephen Huff, MD Subdural hematomas are relatively uncommon in the posterior fossa since the cerebellum undergoes little movement, which is protective of its bridging cortical veins. Subdural hematomas that do occur in that location are usually a result of parenchymal cerebellar injury.
A small acute subdural hematoma may be difficult to appreciate because of the appearance of the overlying skull. Use of the bone window setting may aid in discrimination.
A subdural hematoma may also be located along the falx cerebri (ie, interhemispheric), along the tentorium, or in the posterior fossa. Interhemispheric subdural hematoma causes the falx cerebri to appear thickened and irregular and often is associated with child abuse.[13]
All or part of an acute subdural hematoma may appear hypodense or isodense to brain if the patient’s hematocrit is low, if the clot is hyperacute (eg, < 1 hour old), if the subdural space contains active bleeding, if coagulopathy is present, or if the CSF is creating a dilutional effect.
Detection of an isodense subdural hematoma may require a high index of suspicion; subtle changes in the appearance or position of the cortical sulci may be found. Contrast-enhanced CT or MRI may help to better define the lesion. Interestingly, isodense subdural hematomas may be either hypointense or hyperintense on T2-weighted MRI; this may be a clue to the underlying pathophysiology.[31]
Some degree of midline shift should be present with moderate or large subdural hematomas. Suspect a contralateral mass when midline shift is absent. If midline shift seems excessive, suspect underlying cerebral edema.
CT in subacute subdural hematoma
In the second and third weeks (the subacute phase), subdural hematomas become isodense (with respect to the brain) and more difficult to appreciate on a noncontrast head CT scan (see the images below). Also, subacute subdural hematomas often become lens-shaped and can be confused with an epidural hematoma.
Subacute subdural hematoma. The crescent-shaped clot is less white than on CT scan of acute subdural hematoma. In spite of the large clot volume, this patient was awake and ambulatory. Image courtesy of J. Stephen Huff, MD. 
An isodense subdural hematoma (SDH). Note that no sulcal markings are below the inner table of the skull on the right side. This hematoma has scattered areas of hyperdense, or acute, blood within it. For this reason, either contrast-enhanced CT or MRI should be considered for imaging 48-72 hours after head injury. On contrast-enhanced CT scans, cortical veins over the cerebral surface are opacified and help delineate the lesion. On T1-weighted MR images, subacute lesions are hyperdense.
CT in chronic subdural hematoma
In the chronic phase, the lesion becomes hypodense and is easy to appreciate on a noncontrast head CT scan. About 20% of chronic subdural hematomas are bilateral, however, and this may prevent midline shift, thereby making the subdural hematoma harder to detect. See the image below. Despite this caveat, CT scan still supersedes MRI because of its reliability, shorter study time, and lower cost.
Bilateral chronic subdural hematomas shown on CT scan. Midline shift is absent because of bilateral mass effect. Subdural hematoma is bilateral in 20% of patients with chronic subdural hematoma. Often, a chronic subdural hematoma appears as a heterogeneously dense lesion indicative of recurrent bleeding with a fluid level between the acute (hyperdense) and chronic (hypodense) components of the hematoma (see the image below).
Chronic subdural hematomas (SDHs) are commonly bilateral and have areas of acute bleeding, which result in heterogeneous densities. Note the lack of midline shift due to the presence of bilateral hematomas. On a contrast-enhanced CT scan, the chronic subdural hematoma membrane enhances to varying degrees, depending on numerous factors. Sometimes, a contrast-enhanced scan shows evidence of an underlying cause, such as a tumor or vascular lesion (eg, in patients with acute but nontraumatic subdural hematoma).
Although the distinction between subacute and chronic is an arbitrary one, it can be important. Chronic subdural hematomas have a liquid consistency, typically resembling crankcase oil, and can be drained through burr holes. The consistency of subacute subdural hematomas might be too thick for burr-hole drainage and might require craniotomy.
Other Studies
Magnetic resonance imaging
MRI is less useful than CT in diagnosing an acute subdural hematoma because of the increased time needed to obtain the study and the inability to use metallic objects that are needed to resuscitate patients with trauma (eg, most ventilators) in the scanning environment.
MRI can be a useful study to evaluate associated parenchymal brain injury and predict prognosis, but only after stabilizing and treating any life-threatening lesions. MRI is more sensitive for detecting nonhemorrhagic brain lesions, contusions, and diffuse axonal injury.[32]
An MRI is helpful in imaging chronic subdural hematoma when CT scans are difficult to interpret (eg, when suspecting an isodense hematoma). MRI may be particularly helpful in diagnosing bilateral chronic subdural hematoma because a midline shift may not be apparent on CT scan.
Histologic findings
Acute subdural hematomas usually contain both liquid and clotted blood. Intact erythrocytes are usually found within the clot. Associated skull fractures and underlying focal traumatic parenchymal damage are often present.
Fibroblastic membranes form on the dural side and arachnoid side of the chronic subdural hematomas, with the dural neomembrane being more vascular. The neomembrane consists of many capillaries, intact and lysed erythrocytes, hemosiderin-laden macrophages, and granulation tissue.
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