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Aneurysms resemble bubbles or focal dilation of arteries that
occur at weak points of the artery wall. The likely reason for their formation
is an interaction of many factors. These factors include genetic
predisposition, the anatomy of the artery and its branches, “wear-and-tear” on
the wall of the arteries due to blood flow and artery disease. Other factors
such as hypertension or smoking may contribute to the formation of aneurysms.
Aneurysms occur at points where there is the greatest stress on the artery
walls (where arteries branch or change direction). The arteries that are
involved supply blood to the brain and are situated within the space around the
brain known as the subarachnoid space. This space contains the cerebrospinal
fluid (CSF).
Aneurysms can lead to trouble if they rupture, if they increase in size
(compressing surrounding brain or nerves) or if solid material from within
breaks off and travels downstream to block an artery causing
a stroke. The most common cause for trouble is haemorrhage (bleeding)
caused by aneurysm rupture.
It is uncommon to diagnose an aneurysm before it has ruptured and most people
with aneurysms are unaware that they have an aneurysm until it bursts.
What happens when an aneurysm ruptures?
The most common complaint when an aneurysm bursts is a sudden
onset (usually explosive onset) of severe headache. Although other serious
problems can immediately follow (eg. sudden death in 10% of cases,
unconsciousness or a loss of body function) there may be no other associated
complaints. It is very important to recognise that the headache is serious as a
second more dangerous bleed may quickly follow.
What is happening at the time of rupture is that a hole has formed in the
aneurysm allowing blood from the artery to rush out under pressure (the artery
pressure in the brain is about 16x that of the brain) into the space around the
brain (subarachnoid haemorrhage) or into the brain itself (intracerebral
haemorrhage). If a temporary seal is not quickly formed by blood clot the
haemorrhage will prove to be fatal.
In Australia, more than 1,600 aneurysms rupture each year. Of all those that
are known to have a subarachnoid haemorrhage 39% have died within 4 weeks. This
figure may even be higher as studies have shown that if people with known
unruptured aneurysms are followed until they burst closer to 66% die. The cause
for death or disability can be due to:
1 The rupture itself. As the bleeding is under considerable pressure it can do
local damage to brain tissue or it can reduce the flow of blood to much of the
brain (as the pressure normally driving the blood through the arteries and into
the vein can be negated by the high pressure after a subarachnoid haemorrhage).
2 The aneurysm can re-bleed and again cause the above damage. The chance of
re-bleeding are particularly high in the first 24 hours but if a haemorrhage
has gone undetected then there is a 50% chance that the aneurysm will bleed
again with 6 months. When a re-bleed occurs 50% of people die even when they
are in hospital at the time of the re-bleed.
3 Vasospasm. The blood clot bathes the arteries supplying blood to the brain.
When the red blood cells within this clot start to die and break-up they
release substances that irritate the arteries causing them to narrow. It takes
several days for this process to commence after a haemorrhage and it takes one to
two weeks before the narrowing reaches its maximum. This narrowing is known as
vasospasm. If the narrowing is sufficiently severe the blood flow restriction
to the brain may cause a stroke (a disorder of brain cell death causing a loss
of function due to a lack of blood flow in this case). The toxic substances to
the arteries eventually wash away and the danger period for vasospasm has
mostly gone by three weeks.
4 Hydrocephalus. This is known as “fluid on the brain.” Because the blood clot
sits in the space normally occupied by the flowing cerebrospinal fluid (CSF)
this flow of CSF may be impaired. The solid blood clot acts like a dam
obstructing the out flow of CSF which continues to be produced in the
ventricles (the spaces in the middle of the brain where CSF is produced). The
ventricles expand as CSF builds up and put pressure on the brain ultimately
causing damage.
Are there any warning signs that an aneurysm may be present?
Usually not. However, occasionally aneurysms are sufficiently
large or close enough to the pain sensitive covering of the brain to cause
headache. Occasionally the aneurysm can press on nerves to the eye causing
impaired vision, double vision, large pupil or drooping eye-lid. Other problems
of the brain may prove to be due to an aneurysm but few of these complaints are
specific for aneurysms.
Who gets aneurysms?
Aneurysms can occur at any age but it is unlikely that aneurysms
are present at birth. Normally they develop after birth and in some cases, may
be present for only months or weeks before rupturing. However many aneurysms
appear to remain stable for many years and not all aneurysms rupture. Aneurysms
occur in both men and women. Rupturing appears to be most common from the age
of 40 to 70 but has also been seen at any age.
Usually no health problems are likely to alert a person that an aneurysm may be
present. However, if there are family members with intracranial aneurysms there
is a slightly higher risk that other family members may have an aneurysm. If an
aneurysm has been treated in the past the chance of developing a new aneurysm
in the same person is increased and may be as high as 20%. Occasionally, other
health problems are associated with aneurysms (eg. Polycystic kidneys,
coarctation of the aorta and disorders of collagen).
What is the risk for an aneurysm bursting?
Aneurysm location, size and shape all play a role in the potential
for aneurysms to rupture. Although it is known that the majority of aneurysms
that have ruptured are less than 10 mm in size it is different if an aneurysm
is found by chance. If an aneurysm has developed symptoms there is a risk that
this may be due to the sac of the aneurysm expanding with the wall of the
aneurysm becoming thinner predisposing to rupture. It is important in such cases
to consider treatment quickly.
In an aneurysm that has been found incidentally size is important in
influencing the risk of bleeding. Over the next 5 years after diagnosis an
average aneurysm between 2 and 6 mm in size has a risk of bleeding between 1
and 2%, an aneurysm between 7 and 9 mm size has a five year risk of bleeding of
6%, an aneurysm between 10 and 24 mm in size has a five year risk of bleeding
of 11% and larger aneurysms have a 28% chance of bleeding. These risks vary for
certain aneurysm locations or shapes and thus an individual assessment of the
risks requires expert evaluation.
Aneurysms at certain locations (such as the basilar artery and the posterior
communicating artery) are more likely to bleed at a smaller size. In addition,
there is growing evidence that an irregularly shaped aneurysm is more likely to
bleed at a smaller size.
How are aneurysms detected?
Aneurysms can be detected with a high degree of accuracy with both
CT scans and MRI scans. However, the radiologist doing the test must be aware
of the need to look for aneurysms as both these extremely low risk procedures
(proving that there are no iodine allergies in the case of CT scan and no
implanted metal or pacemakers in the case of MRI) require a special sequencing
of the study. These are called CTA (in the case of CT scans) and MRA (in the
case of MRI). The “A” in each of these tests stands for “angiography.” MRA and
CTA can be easily performed without requiring admission to hospital.
The gold standard test is the digital subtraction angiogram (DSA). This test
does require admission to hospital and does carry a small risk (depending on
the risk factors of the patient and the experience of the radiologist. This
risk is between 1 in 1,000 and 1 in 100. This test should not be done without
being informed of the risks, expectations of how the results will be used and
awareness of alternate investigations. Despite the greater risk and
inconvenience of DSA, the detailed pictures that can be obtained (particularly
with 3-dimensional angiography) may be necessary in order to establish the best
plan of action.
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