In this video, from our Brain CT Essentials course, you will learn how to recognize and correctly localize subdural, epidural, and subarachnoid hemorrhages on computed tomography (CT) imaging.
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The finding of blood outside of the arteries and veins within the skull is abnormal and important to recognize on CT because it can be both symptomatic and a sign of significant underlying disease. In this Medmastery lesson, I will review its typical appearance and how to identify its location on CT scan. Let's begin with some basic anatomy.
If we were to drill down from the skin, through the skull to the surface of the brain, once we got beyond the skull, we would encounter a dense fibrous layer called the dura, then a much finer membrane called the arachnoid, and then a layer called the pia that covers the surface of the brain. These layers offer three potential spaces for blood to collect.
The epidural space, which lies outside of the dura and just beneath the skull, the subdural space that lies between the dura and the arachnoid layers, and the subarachnoid space that lies between the arachnoid and the pia. So when a patient has a subdural hematoma, that means below the dura, just as a submarine is below the water, there's hemorrhage lying beneath the dura and outside of the arachnoid.
Subdural hematomas are usually result of significant head trauma, but occasionally appear after seemingly trivial fall or injury in older patients. And keep in mind that subdural hematomas can be difficult to see on axial imaging, when the subdural is at the top or bottom of the skull. It is for this reason that both axial and coronal images should be reviewed separately in all trauma patients.
For example, in this trauma patient that tentorial subdural hematoma is much more evident on the coronal scan than on the axial scan. Normally the tentorium is poorly seen, and when it is visible, it should be pencil line thin and symmetric left to right. The thickness of the right tentorium when compared to the left supports the diagnosis of a traumatic tentorial subdural hematoma.
And this case illustrates the benefit in reviewing coronal imaging, since the abnormality is just more conspicuous. The acute subdural hematoma will appear with an attenuation intermediate between that of the brain cortex and the skull, because clotted blood has a higher attenuation than normal brain.
But, you may need to appropriately window the CT scans just to see a subdural. Notice how the inner margin of the blood collection follows the curve of the inner table of the skull. The shape of a subdural reminds me of a crescent moon. This is the typical contour of blood in the subdural space.
That same subdural hematoma, but seen here one month later, appears of low attenuation when compared to the brain, as a result of the breakdown of red blood cells and the influx of water. This is due to the normal evolution of blood products in the subdural space. In this phase It is called a chronic subdural hematoma.
So now we know that an acute subdural hematoma has a higher attenuation than normal brain due to clot retraction. And at about four to six weeks after an injury, it will appear of lower attenuation than brain due to break down of the blood products. Logically, that means that at some point in time, usually about two to three weeks after the injury, the subdural hematoma will have precisely the same attenuation as a normal brain.
In that phase it is called an isodense subdural. And these can be very difficult to perceive on CT imaging. For example, this patient's CT scan shows subtle high attenuation along the right hemisphere indicating blood in the subdural space. But if you keep looking, you should notice that the cortex appears unusually thick on the other side of the brain.
The MR scan of the same patient demonstrates the patient has bilateral subdural hematomas. The reason the one on the left is so very difficult to see is that it has nearly the same attenuation with the brain, again an isodense subdural hematoma. One visual clue that can be helpful for determining whether the patient has a chronic subdural hematoma, versus a white subarachnoid space is the location of the cortical veins.
These are indicated on this enhanced CT scan by the arrows. They will be displaced away from the inner table of the skull by a chronic subdural hematoma since the subdural resides between the cortical veins and the dura. While the right cortical veins are particularly evident on this enhanced CT scan, large cortical veins are usually visible on noncontrast CT scans as well.
Epidural hematomas are less common than subdural hematomas and are usually seen in association with a skull fracture. These are the result of blood accumulating at a relatively high pressure so that it pulls the dura away from the inner table of the skull. Epidural hematomas can enlarge rapidly, since they are most often the result of an arterial injury secondary to a skull fracture.
The shape of these collections appears different from that of a subdural hematoma - since they have a biconvex or lens shape, rather than the crescent moon shape seen in the other case. In this case, an epidural hematoma is indicated by an arrow that is seen along with a skull fracture. In this location the epidural hematoma is the result of bleeding from the middle meningeal artery that courses along the skull lateral to the temporal lobe.
When epidural hematomas enlarge rapidly there can be significant displacement and compression of the adjacent brain. The brain epidural hematoma interface in this case is indicated by arrows. The resulting brain displacement is evident when compared to the other side of the brain. Symptomatic mass effect from an epidural hematoma may require emergency brain surgery to drain this collection.
The image on the right shows that this patient's brain appears normal after surgery, which is typical after timely surgery for an epidural hematoma. This differs from patients that require surgery for traumatic subdural hematomas since in those patients there's frequently a traumatic injury at the brain as well.
When blood collects between the arachnoid layer and the brain, it's called a sub, again below, subarachnoid hemorrhage. The appearance of this type of intracranial bleeding differs from both epidural and subdural hematomas because the blood in the subarachnoid space flows freely around the brain and conforms to the shape of the cortical sulci.
This is well demonstrated in this patient where there's blood in the central sulci and along the right frontal lobe of the brain. Trauma is a very common cause of subarachnoid hemorrhage, but it may also be seen as a result of a ruptured intracranial aneurysm. Detection of a small amount of subarachnoid hemorrhage requires careful evaluation on all reconstructions of the CT scan, and it may be helpful to review thinner sections than the usual five millimeter to increase its conspicuity.
The finding of subarachnoid hemorrhage on CT that is out of proportion to the magnitude of the trauma would suggest the possibility of an underlying ruptured aneurysm. On this CT scan obtained after an unwitnessed accident, note the subtle high attenuation between the left and right frontal lobes indicated by the arrow on your left.
The right image shows a small amount of subarachnoid hemorrhage in the interpeduncular cistern. Hemorrhage there takes on the cisterns triangular shape, so it is worthwhile to look carefully for blood in the interpeduncular cistern whenever a subarachnoid hemorrhage is suspected. In this case in anterior communicating aneurism as indicated by the arrow was demonstrated on a CT angiogram.
This is also called a CTA, and then confirm on a digital subtraction angiogram or DSA. For that technique, a catheter is navigated through the arterial system using a leg or arm access to the arterial system and contrast is injected, in this case within the internal carotid artery to demonstrate the fine details of the vessel contour and the aneurism.
In patients with equivocal findings on CT, consider reviewing the thinnest possible reconstructions of the CT scan data. Another option is to obtain an MR scan, and carefully review the FLAIR images since they often demonstrate subtle abnormalities of the subarachnoid space. Notice that the right funnel subarachnoid hemorrhage in this case that is faintly visible on the CT scan on your left, are much more evident on the FLAIR scan on your right, since the blood will appear white, rather than the CSF which is normally black on FLAIR scan.