I Hit My Head

Slipped on a pickle getting out of bed,
Tried white water rafting but swam instead,
Cage fighting with my Grandpa Fred,
Oh, the many ways I’ve hit my head.
—Anonymous

At some point in their lives, I would bet nearly everyone gets a good knock on the head. Certainly my toddler can check that box more than once. Some of these patients visit the emergency room, but a smaller subset head (no pun intended) to the radiology department for CT scan. This adds up to quite a few head CTs each day around the world.

What precisely is a radiologist looking for on a head CT? Why do we use CT instead of, say, MRI?

…One fell off and bumped his head

Head injury seems oddly overrepresented in nursery rhymes, doesn’t it? Let’s assume that Mrs. Monkey took her little one to the emergency department. How does the ED doctor decide whether a CT is needed?

In short, it’s controversial. Sometimes the need for CT is obvious: a nail is protruding from your skull, or a heavy piano fell on your head. Symptoms such as persistent loss of consciousness or seizure should always be imaged, too. In cases of minor head injury or lack of significant symptoms, the need to image is often in question. Numerous criteria exist to help determine whether to get a CT. I’ve presented a few select criteria below, but please remember that these are by no means agreed upon by everyone in the medical community.

• Old or young age: Older than 60 or infants.
• Level of consciousness: the Glasgow coma scale measures how responsive someone is to their environment, by evaluating eye opening, verbal response, and motor response, and giving a score for each. A perfect scores includes spontaneous eye opening; knowledge of who they are, the date, and where they are; and the ability to follow commands. Those with less-than-perfect responses might need a head CT.
• Amnesia: what was I saying?
• Vomiting: self explanatory
• Headache: ditto
• Alcohol or drug intoxication: People under the influence are more likely to do dumb stuff, but the presence of alcohol or drugs in the bloodstream can potentially worsen the effects of some types of brain injuries. Also, chronic substance abuse can result in brain shrinkage, which increases risk for certain injuries, particularly subdural hematoma. Personally, the worst brain injury and bleeding I have ever seen occurred in a drunk individual who fell off a bar stool.

As always, there are exceptions. In rare cases of seemingly mild trauma with no symptoms, intracranial hemorrhage can be present on head CT.

The unhappy trio of head injury

With blunt (non-penetrating) head trauma, we primarily look for three serious sequelae: intracranial hemorrhage, fracture, and brain contusion. CT is best suited to initially evaluate head trauma for a few reasons:

• it is fast, taking seconds rather than the minimum of twenty minutes with brain MRI, and can be interpreted more quickly by the radiologist
• it is good for detecting all three pathologies, with fracture seen much better compared to MRI, and hemorrhage and contusion also well seen
• it is less expensive then MRI, important given the high number of CTs performed each day for head trauma

As usual, a crash course (pun intended) in relevant head anatomy is in order. Evolution saw fit to protect our squishy, delicate brains with a hard outer bony skull. Beneath our skulls and on top of our brains are three sandwiched layers of tissue collectively known as the meninges (you might be familiar with the term meningitis, an inflammation of the meninges).

The outermost layer of the meninges, just beneath the skull, is called the dura mater. The thick, tough dura mater is another layer of protection for your brain. Deep to the dura mater is the arachnoid mater, named for its spiderweb-like projections that extend below into the cerebrospinal fluid (CSF), the fluid surrounding and cushioning the brain. The primary job of the arachnoid mater is to absorb the CSF back into the bloodstream. Finally, the pia mater is a thin membrane that lies directly on the surface of the brain, and extends into every nook and cranny of the deep folds of brain tissue.

Intracranial hemorrhage: Location location location

Intracranial hemorrhage means bleeding that occurs inside your skull. Bleeding is categorized based on its relation to the layers of the meninges, and location is often related to the type of injury and the prognosis. A quick note on terminology: hemorrhage and hematoma are often used interchangeably, but hematoma suggests the blood has formed a defined collection, whereas hemorrhage is a generic term for any kind of bleeding.

On CT, blood usually shows up as bright spot. Blood actually takes a few hours to form this clot, but patients rarely make it to the CT scanner before the clot appears. The bright appearance only occurs with new blood clot formation; blood that is unclotted or has been sitting around for a while is not as bright and can be trickier for the radiologist to notice.

Bleeding inside the skull (and outside the brain) can occur in three major locations:

• Epidural hematoma: Located between the inner layer of the skull and the dura mater, epidural hematomas are rare but potentially devastating bleeds that occur more commonly in older children and adults (but are unusual in the elderly and infants). Up to 95% are accompanied by skull fracture, which often results in arterial injury and ultimately the hematoma.
  • The classic presentation (about 50% of the time) for one of these bleeds is a so-called “lucid interval”, during which the patient is initially knocked unconscious but subsequently wakes up with minimal or no symptoms for a period of time. This sequence is dangerous because a patient will often defer medical treatment during the lucid interval, but later the bleed will cause more serious symptoms, coma, or even death. Actress Natasha Richardson died in a skiing accident as the result of an epidural hematoma, and her alleged refusal of treatment during the lucid interval.
  • Most epidural hematomas are bright on CT and shaped like a biconvex lens (similar to a football), in contrast to the crescent-shaped and more common subdural hematoma. Coma and death can result if the hematoma becomes large and pushes on the brain, what we refer to as mass effect. If the mass effect is significant enough, it can push parts of the brain into places it should not be—called herniation—and cause irreversible brain damage.
  • Due to these risks, most epidural hematomas are evacuated (sucked out) surgically. If small, doctors may choose to watch and wait, repeating a head CT to ensure the hematoma is not enlarging. Despite the seriousness of these bleeds, mortality rate is low at 5%, in part due to prompt recognition and treatment, but also the younger, otherwise-healthy patient population in which they more commonly occur.

• Subdural hematoma: If you hear that an elderly friend or relative has a head bleed, chances are it is a subdural hematoma. These bleeds occur between the dura and arachnoid mater and originate from injured veins that traverse this space. Older folks’ brains have often shrunk a bit (where did I put the dang keys, Helen?), which pulls on these veins and makes them more likely to tear. Trauma—with or without a direct blow to the head—is the most common culprit in subdural hematoma. They can occur without trauma too, especially in the setting of anticoagulation (e.g. blood thinners).
  • Now for the tricky part. I said blood clot is usually bright on CT, but this is only because it’s usually discovered hours to days after bleeding occurs. Subdural hematomas in particular can sometimes go weeks or longer before being discovered. If we get a CT anywhere from three days to about a month—the subacute period—after the initial bleeding, the hematoma can look very similar to normal brain except for its mass effect. After a month, subdural hematomas get even darker and can look like the normal CSF surrounding the brain; we call these chronic subdural hematomas. Sometimes, patients can bleed again into a chronic subdural hematoma, which shows up as a bright spot in an otherwise dark fluid collection.
  • Crescent-shaped subdural hematomas can become very large and cause similar issues with mass effect and herniation as epidural hematomas. If it is enlarging or causing symptoms, it is drained surgically. Mortality rates are unfortunately high—up to 90%—in part because older individuals (in whom these bleeds are more common) often have other serious injuries or underlying medical conditions which make recovery more difficult.
  • Subdural hematomas can also sadly occur in children. If a CT scan shows an older (chronic) subdural in a child with new areas of bleeding, red flags should be raised for the possibility of child abuse, known as nonaccidental trauma in the medical community.

• Subarachnoid hemorrhage: Deep to the arachnoid mater and immediately overlying the brain and pia mater, subarachnoid hemorrhages can slip into the brain’s nooks and crannies. Bright areas of blood clot fill these curvy, relatively small spaces when blood vessels traveling through the subarachnoid space are injured.
  • Although trauma is the most common cause of subarachnoid hemorrhage, a critical non-traumatic cause is a ruptured (bleeding) aneurysm. Aneurysm means focal enlargement of a blood-carrying artery. On routine head CT, the appearance of a bleeding aneurysm can look the same as traumatic subarachnoid hemorrhage; however, the specific location of the bleeding—i.e. near a site typical for aneurysm—can tip off the radiologist that a bleeding aneurysm may be the culprit. A special CT or MRI to look at the blood vessels can confirm this suspicion.
  • Traumatic subarachnoid hemorrhage is not infrequently accompanied by other brain injuries or even subdural/epidural hematoma. Patient outcomes are largely dependent on other intracranial trauma that may be present. If subarachnoid hemorrhage is the only abnormality, outcomes are generally good. One long-term complication specific to subarachnoid hemorrhage is problems with absorption of CSF, which can lead to hydrocephalus (i.e. “water on the brain”).

Jack fell down and broke his crown

Fractures of the head and especially the face are topics too complicated to cover in detail here, but I will touch on a few important issues. Most of you will know what I mean by fracture: a break in the bone. The skull (ignoring the face bones) can be divided into two contiguous structures: the skull base, where your neck connects to your head and through which many critical structures pass into and out of your brain; and the calvarium, the curved thin bones which sit on top of the skull base and extend to the top of your head. Fractures in either part can be serious and pose different complications.

On CT, a fracture looks like a dark line, a thin gap in the bone. Because of their orientation, fractures in the bones that make up the calvarium are more likely to get pushed in, what we call a depressed fracture. You would feel a divot if you run your hand over this part of your head. Never good. Most of your brain lies directly beneath the calvarium, and—as you can imagine—fracture can cause bleeding and direct injury to the brain, especially a depressed fracture.

Skull base fractures can cause damage to nearby blood vessels and important nerves called cranial nerves. Damaged blood vessels can bleed or become blocked and cause strokes. Cranial nerve injury can affect vision, hearing, smell, balance, facial muscles and sensation. Two special, dare I say fun, names are used for visible bruising that can happen in skull base fractures. “Raccoon eyes” is just as it sounds: black circles around the eyes, due to bleeding/bruising that spreads from the fractures. “Battle sign” is similar bruising behind the ears, named after Dr. William Henry Battle. Either of these signs should lead to prompt CT imaging.

Any type of skull fracture can develop the complication of a cerebrospinal fluid (CSF) leak. This cushioning fluid can leak if the fracture also injures the adjacent meninges. Most of these will heal spontaneously, but, if large or persistent, surgical repair is necessary.

A punch to the brain

Cerebral contusion—a bruise of the brain—is relatively common after blunt head trauma. Two general mechanisms account for contusion: direct blow to the brain surface with injury below the impact site, or the brain bouncing around and hitting the inside of the skull.

A direct blow can result in skull fracture and contusion to the brain beneath, known in fancy French as a coup injury. The bouncing brain can be injured in a couple of different ways. With rapid acceleration/deceleration—think car accident—the brain can strike the skull, particularly the front parts of the brain called the frontal and temporal lobes. A direct blow can also cause a brain-bouncing injury that Francophiles will love, called a contrecoup injury. This type of injury occurs on the opposite side of the site of direct impact, i.e. you hit the front of your head but your brain rebounds against the back of your skull.

Our brain reacts as you might expect when it gets beaten up: it becomes swollen and bruised. Edema—accumulation of fluid—results in swelling and appears on CT as darker areas in the brain. Our brains can develop patchy areas of bleeding if sufficiently injured, manifested on CT as bright spots of blood in these otherwise dark areas of injured brain. Follow-up imaging with MRI is quite useful in cerebral contusion to more accurately determine the extent of involvement. Prognosis depends on size of the contusion and the areas involved, as well as other associated injuries. Many patients—up to 25%—have residual symptoms that never resolve.

Time for a tylenol

Many of you have probably noticed that I did not mention concussion, perhaps the most common result of head injury, and much in the news these days. The reason for this omission is that concussion is essentially invisible on CT, and this post is focused on the role of CT in trauma. MRI, however, is emerging as an important tool in the imaging of concussion. MRI is also used to image some of the brain trauma outlined above, but—for the sake of brevity and sanity—I will leave these discussions to another day.

Despite potentially serious complications of head injury, my intention was not to encourage a CT for every little bump on the head. Clearly not all head injuries results in the acute trauma discussed above. The decision whether or not to obtain a head CT is often difficult, and should be discussed with your doctor.