Most brain aneurysms are thought to be congenital, meaning people are likely born with them. A weak point in the blood vessel wall allows an outpouching which can stay dormant or burst. About 5% of people have an aneurysm, however many are unaware since symptoms may not develop until rupture. Warning signs are often subtle and rare. Most aneurysms are small… about 1/8th inch. Those that rupture tend to be about a ¼ inch in size, so it is thought that aneurysms may grow prior to rupture.
Bleeding from an aneurysm leads to death about 50% of the time. Of those who survive, about 66% suffer some permanent neurologic deficit. Therefore rupture is a big deal which we try to avoid.
If a patient has a family history or incidentally finds an aneurysm, it can be treated upfront or followed to hopefully avoid the significant risk of bleeding. But are there any other ways to “predict” an aneurysm in patients without the risk factors or an existing diagnosis? Is there predictability for aneurysm growth in those previously diagnosed?
A new study has suggested a link between variations in the size of brain arteries and the likelihood of a cerebral aneurysm and growth.
The brain is supplied by four main arteries, and the blood flow is asynchronous. However intrinsic properties of the vessels help maintain continuous and smooth perfusion, minimizing high fluctuations in pressures.
Aneurysms tend to form at certain locations. One is along the segment connecting the two hemispheres’ arteries… right-brain arteries connecting to left-brain arteries. This is called the anterior communicating artery, connecting the right and left anterior cerebral arteries. The other is the middle cerebral artery. (Of course, there are less common locations elsewhere)
In this study, an anterior communicating artery aneurysm was 80% more likely when there was an asymmetric difference in the diameter between the left and right anterior cerebral arteries. Therefore aneurysm enlargement could be caused by altered hemodynamics from the asymmetric size of the right and left anterior cerebral artery segments.
What does this mean?
Aneurysms less than 2-3 millimeters can be missed by common imaging. Even in 2021, the sensitivity is only so good. Therefore in patients with no family history or predisposing risk of aneurysm, yet have asymmetric in their anterior cerebral arteries, a non-visualized aneurysm may be present and could grow in time. This patient may need follow-up surveillance, despite no aneurysm currently being observed.
Additionally, it may not only be a missed anterior communicating artery aneurysm in that patient. Fluctuations in blood flow can occur in the middle cerebral artery when there are asymmetric anterior cerebral arteries. There’s an 85% incidence of internal carotid and middle cerebral artery aneurysms in these patients.
Overall, symmetrically sized anterior cerebral artery segments seem to be crucial to regulating the flow of overall brain circulation. Symmetry dampens the peak cerebral blood pressure and reduces the incidence of aneurysm growth. So if you have similar-appearing arteries in one spot, your likelihood of having an aneurysm (or have the aneurysm grow) is low. If you have a wonky artery, there may be a hidden aneurysm that could use surveillance.
Burlakoti A et al. Relationship between cerebral aneurysms and variations in the cerebral basal arterial network: a morphometric cross-sectional study in Computed Tomography Angiograms from a neuro interventional unit. BMJ Open. 2021