Dear Rusty,
I'm a surgery resident (MD) and can give you some information, however
this answer is no substitute for direct medical evaluation, advice,
and treatment.
Let me first give some background on basilar tip aneurysms:
The basilar artery is formed by the union of the left and right
vertebral arteries in the region of the cervical spine. The basilar
artery then divides as it travels up into the brain into the posterior
cerebral arteries (PCA), which give off branches called the posterior
communicating arteries (PComm). The left and right PComm?s and a
portion of the PCA?s form the back or posterior portion of the Circle
of Willis. The Circle of Willis is a ring of blood vessels deep in
the brain, connected to the internal carotid arteries, which can
supply collateral blood to the brain if one branch is occluded.
A good diagram of the blood vessels can be found here, with a typical
basilar aneurysm shown at the point where the basilar artery
bifurcates:
http://www.wsiat.on.ca/images/Fig_1_aneurysm.gif
The view in this diagram is from below and somewhat in front of the
brain. Also, the aneurysm shown in this diagram is a vertebro-basilar
aneurysm, not the most common type of basilar artery aneurysm. A true
tip of basilar aneurysm can be seen in this angiogram (at the arrow):
http://brainavm.uhnres.utoronto.ca/images/case/aneurysm/PreAng1.jpg
http://brainavm.uhnres.utoronto.ca/case_examples/case_ANEURYSMS_ST.htm
The most common type of basilar artery aneurysm, and the one I will
focus on, is the basilar bifurcation (more commonly known as a ?tip of
the basilar aneurysm?). This type of aneurysm occurs where the
basilar artery splits into the left and right PCA?s. It is most
common type of aneurysm found in the posterior circulation (the back
portion of the blood supply to the brain). They comprise ~5% of all
intracranial aneurysms. These aneurysms were thought to be inoperable
until 1961, when a neurosurgeon (Drake) reported 4 cases. Larger
series of patients have been reported since, and treatment of these
aneurysms is a standard part of neurosurgery practice today.
Basilar tip aneurysms usually present with subarachnoid hemorrhage
(bleeding in the brain). More rarely, the aneurysm can enlarge prior
to hemorrhage to the point where it puts pressure on the optic chiasm
(the place where the optic nerves meet and the nerve fibers in them
are redistributed to the occipital lobe visual centers). This can
cause a visual problem (or defect) known as bitemporal hemianopsia.
This is a loss of vision to the left (in the left eye) and to the
right (in the right eye). An example: hold a hand over the right eye
and a person can?t see things on the left, but can still see things on
the right with the open left eye. The opposite is the case in the
other eye. These symptoms are the same as those found in patients
with pituitary tumors that are large enough to compress the optic
chiasm, because the pituitary, located in the sella turcica deep in
the brain, sits just below the optic chiasm. Another symptom
sometimes seen is a palsy (malfunction) of the third cranial nerve (CN
III), known as the oculomotor nerve. This occurs due to pressure on
CN III as it exits from the interpeduncular fossa at the base of the
brain.
CN III controls motion of the eyeball (everything except two muscles,
the lateral rectus and superior oblique), and is accompanied by the
parasympathetic autonomic innervation to the pupil. Pressure on CN
III is seen in many conditions, and results in an eye that points down
and out (due to weakened innervation to the muscles of the eye) to the
side and a pupil that is dilated (due to weakened parasympathetic
input).
The treatment of these aneurysms usually involves waiting 10-14 days
to let the brain ?cool down,? since they usually present to medical
attention after a leak (a hemorrhage in the brain). Timing of surgery
is, however, controversial and depends on the specific configuration
of the aneurysm and any anatomic anomalies that may be present in the
individual. The precise anatomy is usually determined by angiogram
(shooting dye into the bloodstream and taking x-rays). The surgery
itself involves placing a small clip across the neck (base) of the
aneurysm to isolate the swollen aneurysm sack from the normal blood
vessels. Aneurysms can also sometimes be coiled from within the blood
vessel using Guglielmi coils. You can read more about this method of
treatment here:
http://www.cjns.org/26augtoc/endovascular.html
The Mayfield Clinic has a very good discussion of all of the various
treatments for aneurysms, with good figures:
http://www.mayfieldclinic.com/PE-AneurUn.htm
In terms of outcome from surgery, mortality is ~5%, morbidity
(sequelae from surgery) is about 12%.
An aneurysm that has not yet leaked or ruptured carries a risk of
rupture of ~1%/year, however the actual value is not known with
certainty. Patients with multiple aneurysms have about a three times
greater risk of bleeding. The most important factor determining risk
of rupture is aneurysm diameter. Risk of rupture for a diameter, 10mm
is 0.05% per year. For diameters > 10mm, the risk is 1% per year. I
should also note that aneurysms are not static ? they increase in size
when followed on serial head scans. In patients who have aneurysms
clipped prior to rupture, the mortality is ~2.3% at 30 days after
surgery and 3.8% at 1 year. Looking at all the studies together (a
meta-analysis), the overall mortality was 2.6%. Morbidity was mild in
~5% of cases, moderate to severe in 6%. For comparison, the 3 month
mortality for a patient who presents with a hemorrhage is 55%. Rate
of serious morbidity is 15%.
In the case you describe, the aneurysm existed, and then some trauma
occurred. It should be noted that there is a class of ?traumatic
aneurysms? that comprise <1% of all aneurysms. These are ?false?
aneurysms, in that they usually arise from penetrating trauma (gunshot
wounds to the head, etc.) or blood vessel traction occurring with
closed head trauma. These usually occur in the distal anterior
cerebral artery or in the distal cortical artery. They can also
involve the internal carotid at various points. These are almost
always associated with skull fractures involving the base of the
skull. The aneurysm at the tip of the basilar is not likely to be of
this class, and is very unlikely to have been caused by trauma alone.
A summary of studies can be found at the AANS (American Assoc. of
Neurosurgeons):
http://www.aans.org/education/journal/neurosurgical/jan00/8-1-4.asp
A case report of a basilar aneurysm caused by trauma is discussed here:
http://www.ajnr.org/cgi/reprint/17/2/283.pdf
For the situation of a pre-existing aneurysm growing after a trauma is
certainly possible. Patients come in all the time with traumatic
subarachnoid hemorrhages due to tearing of blood vessels in the brain.
The aneurysm has thinner walls than the blood vessels from which it
arises and is more prone to rupture. That said, it is also more prone
to grow, however it is unlikely to make a ?jump? in size after a
trauma. This could happen, however, if the trauma is such that the
blood pressure (more specifically the MAP, or mean arterial pressure)
is elevated for a period of time, putting more pressure on the
internal wall of the aneurysm. It?s a fine line though ? in these
situations, the aneurysm is more likely to rupture than grow by a
significant amount. For this reasons, patients at risk for aneurysmal
rupture need to have their blood pressure under control, particularly
after trauma. There have been no studies done to formally measure how
much trauma or repeated trauma might increase the rate of aneurysm
growth, likely because such a study would be nearly impossible to
perform and the likelihood of rupture is highly dependent on the exact
details of the trauma, so such results could not be generalized to all
patients with aneurysms who go through trauma.
The paper below discusses risk factors for increased rate of aneurysm growth:
http://stroke.ahajournals.org/cgi/content/full/32/2/485
http://stroke.ahajournals.org/cgi/reprint/32/2/485.pdf
The biggest risk factor found in this study was cigarette smoking,
which led to a growth rate of 0.5mm/yr for smokers vs. 0.13mm/yr for
non-smokers.
Some more general information on aneurysms can be found at eMedicine:
http://www.emedicine.com/NEURO/topic503.htm
or at the American Association of Family Physicians:
http://www.aafp.org/afp/20010615/practice.html
I hope this has given you more information about basilar aneurysms and
answers your question. Please feel free to ask for any clarification.
-welte-ga |
