200
Posterior Circulation Aneurysms
Incidence of Aneurysms
of the Posterior Circulation
One to five percent of individuals harbor cerebral
aneurysms [ 8 ]. Aneurysms of the posterior circu-
lation constitute 15–20% of all intracranial aneu-
rysms [ 9 ]. The most common location of posterior
circulation aneurysms is the BA apex (49–72%
of cases), followed by aneurysms of the VA and
PICA (which constitute 18–20% of all infratento-
rial aneurysms). Less commonly, aneurysms
arise from the SCA (7–9%), the basilar trunk
(2–8%), or the vertebrobasilar junction (9–13%)
[ 10 – 16 ]. The mean age of patients harboring
aneurysms is between 50.5 and 53.9 years old,
and 60–70% of these patients are women [ 10 –
16 ]. Most posterior circulation aneurysms are
small (≤ 11 mm; 47–63%), followed by large
aneurysms (12–24 mm; 25–40%) and giant aneu-
rysms (≥ 25 mm; 6–9%). Women are also more
likely to have multiple aneurysms [ 17 ].
Presentation of Aneurysms
of the Posterior Circulation
The three most common presentations of poste-
rior circulation aneurysms include subarachnoid
hemorrhage (SAH), mass effect, and ischemia
[ 18 , 19 ]. Posterior circulation aneurysms com-
monly present with sudden onset of headache
from SAH. SAH is the presenting symptom in
55–81% of cases [ 18 – 20 ]. Because of the prox-
imity of the vasculature to the brainstem, espe-
cially the proximity of the VA to the medulla,
these patients may present with sudden onset of
loss of consciousness, respiratory or cardiac
arrest, or lower cranial nerve dysfunction from
aneurysm rupture [ 20 ]. Large or giant aneurysms
may become symptomatic due to mass effect on
the brainstem or the lower cranial nerves or due
to hydrocephalus caused by compression of the
fourth ventricle. Fusiform or dissecting aneu-
rysms can present with ischemic episodes, likely
caused by thromboembolic phenomena resulting
in hemiparesis. Smaller aneurysms can be dis-
covered incidentally during imaging workup for
other symptoms. In general, 22–45% of posterior
circulation aneurysms are unruptured when dis-
covered [ 10 – 16 ]. Due to the proximity of the
PCA and SCA to the third cranial nerve, patients
with aneurysms arising from these vessels may
present with a partial or complete third cranial
nerve palsy. Alternatively, the proximity of the
SCA to the fifth cranial nerve may result in tri-
geminal neuralgia as a presenting symptom.
Patients with AICA aneurysms may present with
facial weakness or hearing loss.Natural History of Aneurysms
of the Posterior CirculationAneurysms of the posterior circulation have a
more aggressive course and natural history than
their counterparts in the anterior circulation [ 21 ,
22 ]. The International Study of Unruptured
Intracranial Aneurysms (ISUIA) found that
regardless of size, aneurysms of the posterior cir-
culation had a higher rate of rupture over a 5-year
period than aneurysms of the cavernous sinus or
other anterior circulation locations [ 21 ]
(Table 14.1). The outcomes for patients with rup-
tured aneurysms treated conservatively are poor,
with rebleeding rates approaching 40% at
4 weeks and mortality from a repeat hemorrhage
approaching 50% [ 23 – 25 ]. The common risk fac-
tors for development and rupture of aneurysms in
the posterior circulation include female sex,
increasing age, family history of aneurysms,
tobacco use, connective tissue disorders, hyper-
tension, and prior history of SAH.Indications for Interventions
for Aneurysms of the Posterior
CirculationThe decision to intervene and treat any lesion
depends on the natural history of the lesion, the
risk associated with treatment versus observa-
tion, the experience of the surgeon, intrinsic
patient factors (such as health and life
expectancy), and the wishes of the patient and
family. Because of the aggressive nature of pos-
terior circulation aneurysms compared to anteriorM.Y.S. Kalani and R.F. Spetzler