Aneurysms (brain, aortic, abdominal) – causes, symptoms, diagnosis, treatment, pathology

Aneurysms (brain, aortic, abdominal) – causes, symptoms, diagnosis, treatment, pathology

August 7, 2019 72 By Bertrand Dibbert


The word aneurysm is based off the Greek word
aneurysma, which means “dilation.” And that makes sense because aneurysms are
abnormal dilations in a blood vessel. We officially label a bulge in a blood vessel
as an aneurysm when the diameter of that bulge is about one and a half times larger than
the normal diameter of the blood vessel. Aneurysms can happen to any blood vessel in
your body, like the aorta, the femoral artery, the iliac artery, the popliteal artery, and
cerebral arteries. They can also happen in your veins too, but
those are less common because blood pressure in veins is much much lower than in the arteries. There are two major categories of aneurysms,
true aneurysms and pseudoaneurysms. In true aneurysms all the layers of the blood
vessel wall dilate together. True aneurysms that balloon out symmetrically
on all sides of the blood vessel are called fusiform aneurysms whereas asymmetrically
shaped aneurysms balloon out to one side of the blood vessel. This asymmetrical shape usually happens because
for some reason one side of the blood vessel wall has had to put up with higher blood pressure
than the rest of the vessel wall or because the wall was weaker on one side to begin with. Asymmetrical true aneurysms can be called
either “saccular” or “berry” aneurysms. You can think of pseudoaneurysms as false
aneurysms as in they’re not actually aneurysms at all. They are caused by a small hole in your blood
vessel which allows blood to leak out of the vessel and form a pool of blood that looks
like a fusiform or berry aneurysm depending on where the hole is and how big it is. The blood pools because the surrounding tissues
act as walls that contain the blood in one spot. Arterial aneurysms occur most commonly in
the aorta with about 60% of true aortic aneurysms happening in the abdominal section of the
aorta and the other 40% happening in the thoracic section. Of all the abdominal aortic aneurysms, you
can find about 95% of them just below the point where the renal arteries branch off
from the abdominal aorta, but above the aortic bifurcation. That’s because there is naturally less Elastin
in the walls of this part of the aorta than the rest of the aorta. Aneurysms are caused by a weakness in the
blood vessel wall, so anything, really anything, that causes the wall to weaken can cause an
aneurysm. When a blood vessel wall weakens, it struggles
to contain the pressure of the blood pushing against the walls, so the diameter of the
blood vessel lumen increases. On top of this, pressure on the blood vessel
walls increases as the diameter of the lumen increases thanks to LaPlace’s law, making
the aneurysm diameter even bigger, creating this cycle of continuous bulging. You’ve likely experienced this when you
blow up a balloon. The first breath into the balloon is tough,
but the next several breaths are pretty easy. This is why aneurysms usually get bigger over
time, it’s because of this positive feedback loop. So what underlying conditions might weaken
the wall of blood vessels? Well one way is if all the layers of those
blood vessel walls don’t receive oxygen. Typically the cells of the vessel walls are
able to get oxygen from blood as it flows by, but in the first section of the aorta,
where the walls are super thick and muscular in order to handle all that blood pressure
coming from the heart, they need to have their own set of blood vessels that supply the tunica
media and the tunica externa. We call these special blood vessels vasa vasorum. In hypertension, the vasa vasorum can develop
hyaline arteriolosclerosis, narrowing their lumen, causing ischemia to the aortic wall,
and we end up with atrophy of the smooth muscle of the tunica media. Overall all this weakens the aorta’s wall. The same thing can happen in blood vessels
without vasa vasorum if we have a bunch of plaque building up on the tunica intima thanks
to our friendly neighbourhood atherosclerosis. Oxygen just can’t penetrate all the vessel’s
walls. You see this type of aneurysm often in the
abdominal aorta, where blood pressure is high and the aorta walls are nourished by passing
blood. If you think about this it’s no wonder then
why both aneurysms and atherosclerosis share similar risk factors: usually people who are
male, are over the age of 60, who have hypertension, and smoke. If we look at tertiary syphilis, it affects
the vasa vasorum as syphilis causes inflammation in the tunica intima of the vasa vasorum (which
again are those blood vessels that deliver oxygen to the thick walls of larger blood
vessels), ultimately leading to a narrower lumen and restricting blood supply to the
vessel walls of the thoracic aorta, causing atrophy. By the way, the name for this vasa vasorum
inflammation is endarteritis obliterans. If you were to look at the vasa vasorum in
tertiary syphilis, you’d see the inflammation causes fibrosis and scarring on the walls
creating this cool to look at, but less cool to have tree-bark like texture. Other bacterial infections can also cause
aneurysms, which we label as mycotic aneurysms. The bacteria from an infection somewhere else
in the body can break off from the main infection and travel around in the blood. Usually these embolic bacteria will get stuck
in the intracranial arteries, visceral arteries, or the arteries feeding the arms and legs. There the bacteria enter and weaken the blood
vessel walls, leading to, you guessed it, an aneurysm. Some of the bacteria that commonly cause mycotic
aneurysms are, and these can be tricky to pronounce, so bare with me, Bacteroides fragilis,
Pseudomonas aeruginosa, and anything in the Salmonella species. You’ll also see mycotic aneurysms are a
complication of infective endocarditis. But it’s not limited to just bacteria. Infection from the fungi Aspergillus, Candida,
and Mucor are also a common cause of mycotic aneurysms. Genetic disorders affecting your body’s
ability to properly form connective tissue like fibrillin or collagen also weaken the
blood vessel walls, so people with Marfan’s syndrome have weakened blood vessel walls
because the elastic properties found within fibrillin is impaired. And people with Ehlers-Danlos syndrome are
more likely to develop aneurysms because their ability to form collagen proteins is disrupted. Alrighty then. So intact aneurysms are usually okay, and
don’t cause any symptoms. I say usually because the bulging blood vessel
can compress other organs or vessels around it. So if an aneurysm compressed a major vein
like the inferior or superior vena cavae, it could decrease the amount of blood returning
to the heart. Usually though the big worry with aneurysms
is the possibility that they might explode, or more like rupture, but it’s more fun
to say explode. When an aneurysm ruptures, it’s like having
a water main break. Blood is spewing out from the hole in the
blood vessel and less blood is flowing downstream to the cells that need it, which causes ischemia
in that downstream tissue. In thoracic aneurysms specifically, a very
serious complication can occur if the aneurysm is right above the aortic valve. As the aneurysm dilates, it pulls on the walls
around the aortic valve preventing the aortic valve from closing properly, and allowing
blood from within the aorta to flow back into the ventricle during ventricular diastole. This particular condition is called aortic
insufficiency, and can also cause a high-pitched brassy like cough because the left recurrent
laryngeal nerve which wraps around the aorta is stretched by that expanding diameter of
the aorta. I don’t have that audio clip for that brassy
cough sound, but it’s pretty distinct when you hear it. Moving on to the brain. If an aneurysm ruptures in the brain, blood
will pool into the subarachnoid space putting pressure on the brain tissue and irritating
the meninges, causing symptoms like a very sudden and intense headache, and the inability
for someone to flex their neck forward. Another point on the list of terrible things
aneurysms can cause is blood clots. As blood flows past the aneurysm, some of
the blood might pull over into the extra lumen space caused by the aneurysm itself. This blood isn’t being pushed along the
blood vessel as quickly as the rest of the blood, and when blood stays still for too
long, it likes to clot. Given enough time the blood clot might become
so big it blocks off the entire blood vessel, causing tissue ischemia, or it could break
into smaller pieces called emboli and get wedged in a smaller blood vessel, also causing
ischemia. As an aside, it’s kinda of nifty that the
clotting property of blood is really useful to stop blood from leaking out of our blood
vessels, but it just also happens to be really really deadly if it stops the blood from moving
altogether. Honestly blood coagulation could be the plot
for the third installment of the Speed movies. Ok, back on track, abdominal aortic aneurysms
sometimes have signs and symptoms, and sometimes they don’t, even if the aneurysm has ruptured. Really severe pain in the left flank which
includes the abdomen, chest, lower back, as well as the groin, a pulsating mass that is
in time with the heartbeat somewhere in this painful region, and hypotension occur in about
50% of aneurysms that have ruptured and are considered the classical sign of an abdominal
aortic aneurysm. Thoracic aorta aneurysms usually don’t have
any symptoms, although sometimes they can cause severe chest, back, and abdominal pain. And there you go, that’s aneurysms. Aneurysms can be asymptomatic, and are sometimes
diagnosed incidentally when an individual gets an imaging study for another reason. Usually they’re seen best on an ultrasound,
CT scan, or MRI. If an aneurysm is present and large enough
to cause risk to an individual, it can be treated surgically. All right, as a quick recap… Aneurysms are blood-filled bulges that form
in weakened areas of the blood vessel walls. Left untreated, aneurysms can burst, causing
uncontrolled internal bleeding or excessive blood clotting. Based on the size and location of an aneurysm,
surgical treatment may be necessary.