Subarachnoid Hemorrhage:
When a Brain Aneurysm Bleeds

Gary E Cordingley, MD, PhD
Although most headaches -- even severe ones -- are due to benign factors,
when a headache is due to a bleeding aneurysm it is a medical emergency.
Spontaneous subarachnoid (pronounced sub-uh-RACK-noid) hemorrhage is rightfully the most feared
cause of sudden headache. Usually due to rupture of aneurysms (abnormal, balloon-like outpouchings of
arteries) located near the base of the brain, subarachnoid hemorrhages involve bleeding into the space
between the brain and its surrounding membrane, known as the meninges. A traumatic blow to the head
can also cause subarachnoid hemorrhage, but this is a completely unrelated process and is not the
subject of this essay.

About 10% of people with spontaneous subarachnoid hemorrhages die before they even get to a hospital
and over a third die within the first four weeks following the bleed. Survivors can have significant
impairments due to brain damage.

And while the effects of the initial bleed are bad enough, in the following few weeks individuals with
subarachnoid hemorrhage can suffer additional, serious complications. One complication is that the
aneurysm responsible for the initial hemorrhage can bleed a second time and cause even more damage.
This occurs in 4% of cases within the first 24 hours and there is another 1.2% chance of re-bleeding each
day thereafter for the first two weeks. Thus, without treatment 20% of cases have a second hemorrhage
within the first two weeks.

The other serious complication is that the blood deposited in the subarachnoid space can cause otherwise
healthy arteries passing through this space to go into spasm. The spasm decreases blood-flow to the
parts of the brain ordinarily nourished by these arteries and thereby inflicts additional damage. Or, said
another way, a blocked artery causes a new stroke, this time of the non-bleeding type. For reasons that
are not entirely understood, these spasms of the arteries do not occur within the first few days after the
initial hemorrhage. Instead, they typically develop after a delay of 4-9 days.

What can be done to reduce these complications? In the case of blood-vessel spasm, the best treatment
is a preventive one. Administering a drug called nimodipine (prononounced nye-MO-dih-peen)
intravenously makes spasming less likely to occur. But in order to prevent the other major complication, re-
bleeding, the best treatments are those which physically stabilize the aneurysm. In one such procedure, a
surgeon places a metal clip across the aneurysm where it joins the otherwise normal artery. An alternative
surgery is to wrap the outside of the aneurysm with surgical gauze or plastic sheeting. A newer procedure
involves filling the aneurysm with tiny metal coils inserted via a flexible catheter snaked through the

How can one tell if a particular headache is caused by a bleeding aneurysm? It can be a tough call, but
certain features make a ruptured aneurysm more likely. First, a headache due to a ruptured aneurysm is
typically of very abrupt onset (often described as a “thunderclap”) and is classically the worst headache of
one’s life. In people who already have recurrent severe headaches from other causes, the headache due
to a ruptured aneurysm might feel different from the more usual attacks.

Medical evaluation of patients with ruptured aneurysms can turn up additional clues, like a stiffened neck
or changes in the backs of the eyes made visible through an ophthalmoscope. Of course, if the patient is
drowsy or confused, this might suggest that something serious is going on, as would any new impairment
in the ability to move the eyes, an arm or a leg. A computed tomographic (CT) scan of the head performed
within the first 24 hours is very sensitive in detecting a hemorrhage, but if the scan is delayed it is less
able to detect the bleed. A lumbar puncture (also known as a spinal tap) always detects subarachnoid
hemorrhage even when it is a few days old, but if the needle causes bleeding by piercing a blood-vessel
on its way to the subarachnoid space, the test might give the false impression that a subarachnoid
hemorrhage occurred when it hadn’t.

After discovery of subarachnoid hemorrhage, the next round of testing focuses on where exactly the
bleeding occurred. While in over two-thirds of the cases it originates from ruptured aneurysms, other
potential sources include tangles of abnormal blood-vessels known as arteriovenous malformations or
from bleeds within the brain tissue that secondarily leak into the subarachnoid space. The managing
physician can order any of three tests to image the blood vessels themselves and pinpoint the source of

The oldest test--still considered the gold-standard--is known as an arteriogram or, alternatively, an
angiogram. An arteriogram is considered an “invasive” test because the doctor must slide a long, flexible
catheter through the arterial system (which is under much higher pressure than the veins) so that dye
infused through the catheter will enter the arteries in question. Two newer tests are “non-invasive,”
though, in truth, they often involve an infusion into a vein. One is magnetic resonance arteriography
(MRA) which is performed with the help of an MRI-scanner. The other is computed tomographic
arteriography (CT-A) which is performed with the help of a CT-scanner. While the non-invasive tests are
getting better all the time, they still occasionally miss aneurysms otherwise visible on arteriograms.

Apart from identifying the bleeding aneurysms, these tests can detect additional aneurysms, when
present. About 20% of people experiencing a ruptured aneurysm have one or more co-existing,
unruptured aneurysms.

Subarachnoid hemorrhages occur annually in about 10 people out of 100,000. This computes to a 0.01%
rate of annual occurrence. Contrast this figure with the 12% of the adult population who have migraine
(most of whom have at least one severe headache per year) and it is apparent that the vast majority of
severe headaches are not due to ruptured aneurysms. But the concern about missing a ruptured
aneurysm means that many people without subarachnoid hemorrhage must receive tests in order to
diagnose the few who have it.

What causes aneurysms in the first place? More than one factor is involved. First, there can be an inborn
weakening of the artery’s wall. When the wall subsequently deteriorates in ways that can be accelerated
by hypertension and smoking, an aneurysm can form.

Actually, aneurysms affecting the brain’s arteries are fairly common. Autopsy and arteriogram studies
indicate that about 1-4% of the general population have them. This is many more people than have
subarachnoid hemorrhages, so a logical conclusion is that most people with aneurysms go through their
entire lifetimes without having symptoms. Studies show that aneurysms less than 5 millimeters (0.2 inches)
in diameter have a very low rate of rupture, while aneurysms greater than 10 millimeters (0.4 inches) in
diameter have a significant risk of bleeding.

Do ruptured aneurysms run in families? A 2005 report from the Scottish Aneurysm Study Group showed a
slight tendency for this trait to be shared by close relatives. The 10-year risk for subarachnoid
hemorrhage in first-degree relatives (parents, siblings and children) was 1.2%. The risk was even lower in
more distant relatives. In families with two affected first-degree relatives there was a trend toward higher
risk. The authors felt that most relatives of patients suffering subarachnoid hemorrhages have low risk of
future hemorrhages, and that routine screening of family members is inappropriate unless there are
multiple affected individuals in the same family.

(C) 2005 by Gary Cordingley