Patient cases can help illustrate the common locations of brain aneurysms, and help you effectively search for brain aneurysms. First, let’s review the basics of how aneurysms are named!
How brain aneurysms are named
Saccular versus fusiform aneurysms
Nearly all brain aneurysms are saccular, meaning that they extend off the side of an artery. This is why these spherical protrusions can be closed off using a spring clip in surgery, or coils in an angiography suite, without occluding the normal artery.
Less common are fusiform aneurysms; these are characterized by dilation of the entire circumference of the artery. Since most cannot be clipped or coiled, fusiform aneurysms are more difficult to treat than saccular aneurysms and sometimes require occlusion of the affected artery.
Aneurysms are named after the vessel they arise from or the nearest branch artery. For example, an aneurysm arising from the carotid artery within the cavernous sinus is called a cavernous carotid aneurysm.
What are the common locations for brain aneurysms?
There are six common locations of brain aneurysms that may be the cause of a subarachnoid hemorrhage:
- Anterior communicating artery (ACoA)
- Posterior communicating artery (PCoA)
- Basilar artery
- Posterior inferior cerebellar artery (PICA)
- Middle cerebral artery (MCA) bifurcation
- Distal carotid artery apex
Since the anterior communicating artery and posterior communicating artery are the most common sites for aneurysm formation, you should start your search there when looking for a brain aneurysm on CT. Most aneurysms arise near the circle of Willis, the MCA bifurcation, and PICA origin.
Anterior communicating artery (ACoA) aneurysms
Let’s discuss the most common aneurysm site, the ACoA. You may be able to detect an ACoA aneurysm on a computed tomography angiography (CTA) scan. Volume-rendered 3D images can be helpful for aneurysm detection when reviewing a CTA scan.
But, remember that volume-rendered CTA scans are displayed as if you were looking from the top of the head and not from the feet of the patient (as axial CT scans are displayed).
Posterior communicating artery (PCoA) aneurysms
The second location where you’ll commonly find brain aneurysms is the PCoA. You should be able to locate the posterior communicating arteries that connect the internal carotid artery with the posterior circulation in patients with a complete circle of Willis. The posterior cerebral arteries can be seen curving around the midbrain on CTA and magnetic resonance angiography (MRA).
Let’s take a look at a case
When a 45-year-old patient presented with a severe headache and sensitivity to bright light (e.g., photophobia), a CT was obtained in the emergency room. The CT demonstrated a diffuse subarachnoid hemorrhage filling the interpeduncular cistern, the ambient cisterns, and the Sylvian fissures. Additional imaging was ordered to determine if a ruptured aneurysm was the source of the hemorrhage.
A volume-rendered CTA scan revealed a saccular aneurysm arising near the patient’s posterior communicating artery. This was confirmed on the digital subtraction angiography (DSA) examination.
One way you can be sure on DSA that this is an aneurysm—and not a vascular loop—is that the aneurysm should have a similar attenuation as the parent blood vessel. This differs from vascular loops, which should have higher attenuation on DSA. This effect is due to summation of attenuation of the overlapping contrast-filled arteries.
Basilar artery aneurysms
The third location that’s important to assess for brain aneurysms is at the basilar tip. Sometimes, the aneurysm itself is visible on CT! In our next patient case, an unruptured basilar tip aneurysm was identified on a patient’s non-contrast CT scan. Based on its size and location at the basilar tip, treatment was suggested and a DSA was obtained.
When sufficiently large, these aneurysms may be evident on non-contrast CT and should not be mistaken for a parenchymal hemorrhage.
Posterior inferior cerebellar artery (PICA) aneurysms
Brain aneurysms can also be commonly found at the origin of the PICA. Our next case involves a patient who presented with headaches which were found to be related to a subarachnoid hemorrhage.
A posterior inferior artery aneurysm can be seen on this patient’s CTA scan. However, you would need the entire stack of images from the CTA to recognize that this is an aneurysm!
The aneurysm and its relationship with the PICA are better demonstrated on the maximum intensity projection image from the CTA scan and on a catheter angiography scan.
Middle cerebral artery (MCA) aneurysms
Another common location for brain aneurysms is the MCA. In some patients, you cannot detect abnormalities by just comparing the patient’s left side to the right side—since they can be symmetrically abnormal. In this patient, who presented with a diffuse (but roughly symmetrical) subarachnoid hemorrhage, the CTA revealed an aneurysm arising from the left MCA that was confirmed on a DSA (Fig. 8).
Despite the aneurysm arising on the patient’s left, the hemorrhage was symmetrical in the Sylvian fissures on an axial CT scan. While the distribution of a subarachnoid hemorrhage will occasionally help you find the aneurysm, do not depend on it as a reliable indicator for the bleeding site in patients with more than one aneurysm.
Nearly all ruptured aneurysms are found in adults, and the peak age for rupture is 49 years of age. Fusiform aneurysms are frequently secondary to atherosclerosis in adult patients and usually occur in the basilar artery or cavernous segment of the carotid.
However, you may see MCA fusiform aneurysms in young patients. When middle cerebral artery fusiform aneurysms occur in young patients, they have been called dysplastic aneurysms since their etiology and behavior can differ from the usual adult saccular aneurysm.
Distal carotid artery aneurysms
Brain aneurysms are also commonly found in the distal carotid artery, especially where it branches into the anterior and middle cerebral arteries, called the carotid apex. In the case of a patient with an unruptured fusiform aneurysm of the distal right carotid artery, a volume-rendered image from CTA confirmed that it arose in the distal internal carotid artery itself.
The aneurysm’s location was just distal to the ophthalmic artery, where the entire circumference of the carotid was involved. When aneurysms arise distal to the cavernous segment, the rupture of the aneurysm will result in subarachnoid hemorrhaging. While cavernous aneurysms can cause symptoms from cranial nerve compression or dural irritation, they are not the cause of a subarachnoid hemorrhage.
Aneurysms rarely arise from the distal segment of a cerebral artery
Nearly all aneurysms, both fusiform and saccular, appear on the proximal portion of the cerebral arteries. However, you may eventually encounter a patient with subarachnoid or parenchymal hemorrhages that are the result of an aneurysm arising from the distal segment of a cerebral artery.
In our next patient case, the ruptured aneurysm was found beyond the circle of Willis and was unusual in both shape and location.
Distal aneurysms may be secondary to an infection caused by trauma, vasculitis, septic emboli from the heart (also called mycotic aneurysms), or tumor emboli from the heart. Distal aneurysms require further evaluation to properly determine their etiology.
So, when assessing a brain scan for aneurysms, start your search at the most common sites for aneurysm formation, such as the anterior and posterior communicating artery origins. Remember to search thoroughly, since more than one aneurysm is detected in up to 20% of cases!
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