Skull Base Surgery of the Posterior Fossa

201

circulation aneurysms, posterior circulation
aneurysms should be considered for treatment if
they occur in patients with a history of SAH, if
they exhibit growth, if they are present in a
patient with a family history of SAH, if the
patient is symptomatic and the symptoms could
be attributed to the aneurysm, and if they are of a
size that place them at significant risk for rupture
based on the ISUIA data. Factors such as patient
age, sex, comorbidities, patient wishes, and other
risk factors should be taken into account when
deciding whether the patient should undergo
treatment and the type of treatment that would
best suit the patient.

Posterior Fossa Arteriovenous

Malformations

Incidence of Arteriovenous
Malformations of the Posterior Fossa

AVMs constitute 2% of all hemorrhagic strokes
[ 26 ]. They are ten times less common than aneu-
rysms but cause 38% of all intracerebral hemor-
rhages in patients between 15 and 45 years old,
and they are a disproportionate cause of morbid-
ity and mortality [ 27 – 29 ]. Posterior fossa AVMs
represent 7–15% of all intracranial AVMs, with
cerebellar AVMs being the most common sub-
type (75–82%) [ 30 , 31 ]. Brainstem AVMs consti-
tute 12.5–23% of cases [ 30 , 31 ]. The mean age at

presentation for all AVMs is 32.8 ± 15 years, and

the mean age of patients with AVMs in the poste-

rior fossa is 42 years [ 32 , 33 ]. There is no gender

predilection.

Presentation of Arteriovenous

Malformations of the Posterior Fossa

Unlike supratentorial AVMs, AVMs in the poste-

rior fossa rarely present with seizures. Instead,

these lesions are more likely to present with hem-

orrhage (60–86% of cases compared to 34–55%

for supratentorial AVMs) [ 33 – 35 ]. Hemorrhage

from AVMs in the posterior fossa may be sub-

arachnoid, intraventricular, or intraparenchymal.

Hemorrhage into the ventricular space can result

in hydrocephalus.

The second most common presentation of

posterior fossa AVMs is progressive neurological

deficit (seen in nearly 1/3 of cases) [ 34 ]. These

symptoms may be due to ischemia, mass effect,

or hydrocephalus. Less common presentations

include cranial nerve palsies, gait instability, cer-

ebellar symptoms, hemiparesis, and headache.

Natural History of Arteriovenous

Malformations of the Posterior Fossa

As discussed above, posterior fossa AVMs are

more likely to present with hemorrhage than their

Table 14.1 Relationship between size and location of aneurysms and the annual and cumulative risk of rupture after
5 years

Aneurysm location

Aneurysm size (% of aneurysms)

<7 mm group 1a <7 mm group 2b 7–12 mm 13–24 mm ≥25 mm

Cavernous carotid

artery (n = 210)

0 0 0 3.0 6.4

AC/MC/IC (n = 1037) 0 1.5 2.6 14.5 40

Post-P comm

(n = 445)

2.5 3.4 14.5 18.4 50

Data from International Study of Unruptured Intracranial Aneurysms Investigators [ 21 ]
AC/MC/IC, anterior communicating artery or anterior cerebral artery, middle cerebral artery, internal carotid artery (not
cavernous carotid artery); Post-P comm, posterior cerebral arterial system, vertebrobasilar, or posterior communicating
artery.
aGroup 1, no history of SAH; group 2, previous SAH from another aneurysm
bThe data were not provided in the original article because for lesions larger than 7 mm the presence of previous SAH
made no difference in bleeding rate
Adapted from Lancet 2003;362:103–110

14 Microsurgical Management of Posterior Fossa Vascular Lesions