Working an overnight the other day and had an interesting GSW present to the ED. This man was reportedly found in the driver’s seat of a car very near the entrance to the ED parking lot. GCS was 3, he was intubated shortly after arrival. He had a large stellate laceration on top of his head just left of midline. It extended from about where you’d expect the coronal suture line to be, all the way to the poterior-most portion of his occiput (kinda like the bullet skipped off the top of his head and made a big laceration). His GCS was low because of this:
This is a good example of an acute subdural hematoma from penetrating trauma. It appears the bullet damaged many of the bridging vessels and acute blood can be seen as a white layering density on the right side of the image. Remember, acute blood is hyperintense with higher Hounsfield Units (HU) similar to bone. As it matures it will eventually become darker and darker until it is less intense than brain tissue.
This CT is also a good example of mass effect. Mass effect is a term used most often in head imaging because of the limited volume of the closed cranial vault. If an extraneous volume is added to the vault it will compress or displace brain tissue thus causing “mass effect.” Mass effect is most often caused by blood, tumors, edema, or obstruction of CSF flow (1). The best way to assess for mass effect is to look at the ventricles, the falx cerebri, and the overall symmetry of the brain. In the example above one can see the lateral ventricles are displaced to the left of the image (patient’s right side) and the falx bends in that direction because of the subdural blood. Clinically this causes decreased mentation, signs of herniation, and eventual respiratory arrest due to compression of the respiratory centers of the brainstem.
How do we know this is subdural blood not epidural? Remember, subdural blood crosses suture lines, epidural blood does not. Also, subdural blood tends to easily distribute throughout the contours of the brain (causing a convex shaped collection) whereas epidural blood forms a lenticular shaped collection. Subdural blood is in the space between the dura and the arachnoid while epidural blood is between the skull and the dura. The other significant difference is that subdural blood is usually venous (sometimes can be arterial) and epidural blood is usually arterial (classically from the middle meningeal artery). Arterial and venous blood cannot be differentiated on imaging but it may be distinguishable by the timing of the patient’s clinical symptoms after trauma.
The second image has been switched to bone windows and one can appreciate the bony damage from the bullet.
There were two predominant theories about how the patient got to the ED parking lot: 1. He was shot and had time to drive to the ED while the blood collected in the subdural space. 2. He was shot in the ED parking lot. All of us reassured ourselves that is was definitely number 1, not the latter!
1. Broder JS, Preston R. ”Imaging the Head and Brain.” In: Broder JS. Diagnostic Imaging for the Emergency Physician. Elsevier Saunders, 2011.
Author: Russell Jones, MD