How to identify common causes of subarachnoid hemorrhages on computed tomography (CT) scans of the brain

Subarachnoid hemorrhages are frequently noted on computed tomography (CT) scans of patients after head trauma. It is important to consider this finding carefully since a subarachnoid hemorrhage can contribute to the cause of the traumatic event rather than its consequence.

A reasonable approach is to first consider if the extent of the subarachnoid hemorrhage corresponds with the magnitude of the trauma. This is usually based on history and the physical examination as well as CT signs of head trauma (e.g., scalp swelling and fractures). Traumatic subarachnoid hemorrhages are frequently limited to a few sulci over the convexities of the brain, but patterns of hemorrhage alone are not sufficient to predict the cause.

In addition to considering the severity of the trauma, the nature of the traumatic injury is also an important predictive factor. For example, a subarachnoid hemorrhage on the brain CT scan of a pedestrian struck by a car while crossing the street is most likely traumatic.

But, a diffuse subarachnoid hemorrhage in a 50-year-old patient who was found unconscious after an unwitnessed, minor car accident should lead you to consider that the hemorrhage could be secondary to an underlying ruptured aneurysm—rather than trauma.

Let’s consider four of the most common causes of subarachnoid hemorrhages on a brain CT scan:

1. Vascular causes
2. Trauma
3. Pseudoaneurysm
4. Penetrating injury

Whenever there is little evidence of substantial head injury, it may be worthwhile to obtain a computed tomography angiography (CTA) to look for a vascular cause of a subarachnoid hemorrhage.

Case 1: A subarachnoid hemorrhage with no vascular cause

In our first case, the patient was noted to have a subarachnoid hemorrhage on CT after a motor vehicle collision. Because of poor history about the nature of this patient’s accident, and no evidence of a significant head injury, this patient had a CTA with volume reconstructions to look for a vascular cause of the hemorrhage. The CTA was negative with no visible vascular cause and no further imaging was recommended.

Case 2: A subarachnoid hemorrhage with a vascular cause

In another case, a 45-year-old woman was examined after being found unconscious off the road in a car that had hit a tree. The event was unwitnessed, and she had no memory of the accident (which is common in many cases of head trauma). Her brain CT scan revealed a diffuse subarachnoid hemorrhage and an incidental colloid cyst.

Based on the extent of the hemorrhaging on her brain CT scan, vascular imaging was obtained. Her digital subtraction angiogram (DSA) demonstrated an anterior communicating artery (ACoA) aneurysm. Essentially, her acute subarachnoid hemorrhage was the result of a ruptured aneurysm that most likely caused the accident!

Case 3: A subarachnoid hemorrhage due to trauma

Brain CT imaging from another patient after a car accident revealed scattered subarachnoid hemorrhages in the cortical sulci as well as several hemorrhages typical to traumatic brain injuries. These included hemorrhages in the splenium of the corpus callosum and in the left frontal lobe.

In a case like this, unless there is compelling clinical history suggesting otherwise, the imaging is entirely consistent with the diagnosis of a subarachnoid hemorrhage secondary to trauma.

Even when you see uncommon patterns of hemorrhages, it’s important not to jump to concluding that the cause is due to trauma. Don’t dismiss the possibility that the hemorrhaging is due to a ruptured aneurysm.

For example, if you note a subarachnoid hemorrhage filling the basilar cisterns accompanied by a parenchymal hemorrhage, you shouldn’t rule out the possibility that both the hemorrhages could have been caused by a ruptured aneurysm.

So, keep looking! In some cases, you may find other abnormalities associated with aneurysmal subarachnoid hemorrhages. For example, if the temporal horns are dilated, it is a sign of communicating hydrocephalus, which is not commonly encountered with traumatic subarachnoid hemorrhages…but it is frequently associated with aneurysmal causes!

A few times in your career, you may see cases where there was a rupture of a pseudoaneurysm caused by a traumatic injury to an arterial wall. A pseudoaneurysm differs from a typical aneurysm because it has fewer layers in its wall.

Case 4: A subarachnoid hemorrhage due to traumatic pseudoaneurysm

For example, a patient was found unconscious in her car after going off the road at night at a high velocity. Her brain CT scan revealed an unusual pattern of subarachnoid hemorrhages along with the pericallosal cistern above the corpus callosum, rather than the typical traumatic hemorrhage that lies within the corpus callosum. A CTA was ordered while the patient was still in the emergency room.

Sagittal reconstructions of the patient’s CTA revealed a small pseudoaneurysm of the distal anterior cerebral artery, which was confirmed on DSA. The change in appearance between the CTA and DSA suggested there was an enlargement of the pseudoaneurysm in the interval.

This particular injury of the anterior cerebral artery has been reported after trauma. It’s thought to be the result of an injury to the pericallosal artery wall as a result of a vascular injury as it strikes the falx during trauma.

Case 5: A subarachnoid hemorrhage due to a penetrating injury

Also rare (but worth considering) are hemorrhages and pseudoaneurysms due to penetrating injury of the skull. For example, a patient had a CT scan after a knife entered the skull that resulted in a parenchymal hemorrhage and diffuse subarachnoid hemorrhage filling the suprasellar and interpeduncular cisterns.

A DSA, in this case, revealed that the cause of the subarachnoid hemorrhage was a middle cerebral artery pseudoaneurysm that was caused by the penetrating injury.

These last two unusual cases illustrate that a subarachnoid hemorrhage in a patient after trauma is not always caused by just trauma alone or ruptured aneurysm alone.

On rare occasions, the subarachnoid hemorrhage may be caused by a ruptured pseudoaneurysm that is formed at the time of the trauma.

In the emergency room, a CTA is a valuable and noninvasive test. It can be very helpful in establishing a diagnosis—especially in cases when uncertainty regarding the cause of a subarachnoid hemorrhage persists.

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