Skull Base Surgery of the Posterior Fossa

202

intracranial counterparts [ 36 – 38 ]. The annual
risk of hemorrhage for all AVMs is estimated to
be between 2% and 4%, but rates as low as 1%
have been reported [ 32 , 36 , 37 , 39 – 44 ]. Each epi-
sode of hemorrhage is associated with a 15–20%
risk of significant morbidity and mortality [ 32 ,
38 , 42 – 44 ]. Mortality associated with hemor-
rhage has been reported to be as high as 67% for
patients with posterior fossa AVMs [ 45 ]. In one
of the largest data sets on the natural history of
AVMs, Hernesniemi et al. determined that poste-
rior fossa AVMs have a relative rupture risk of
3.07 compared to supratentorial AVMs [ 32 ]. The
risk of hemorrhage increases in patients with a
history of a prior hemorrhage. Ruptured AVMs
have an annual rerupture rate of 6–7%, compared
to a rate of 2–3% for previously unruptured
AVMs. The 5-year risk of rupture of a previously
ruptured AVM can approach 26%, while that of
an unruptured AVM is 10% [ 32 ]. Other features
that could increase the likelihood of hemorrhage
include the presence of flow-related and intrani-
dal aneurysms [ 46 – 48 ], venous outflow stenosis
[ 49 ], and high feeding artery pressures [ 50 , 51 ].

Indications for Interventions
for Arteriovenous Malformations
of the Posterior Fossa

Patient selection is critical in order to optimize
outcomes for AVM treatment. Although a full
discussion of patient selection is beyond the
scope of this chapter, we refer the reader to the
excellent reviews on patient selection and consid-
erations in AVM surgery that are available [ 52 –
56 ]. The Spetzler-Martin grading scale provides
a paradigm for selecting an appropriate treatment
algorithm and ascertaining the likely risk of mor-
bidity from surgical treatment [ 57 ]. In general,
patients with low-grade AVMs (Spetzler-Martin
grade I and grade II lesions) should be considered
for treatment with surgery, regardless of presen-
tation. Patients with high-grade lesions (Spetzler-
Martin grade IV and grade V lesions) whose
lesions are identified incidentally should be mon-
itored closely, although a subset may undergo
multimodality treatment with good outcomes

[ 58 ]. Patients with intermediate-grade AVMs

(Spetzler-Martin grade III lesions) should be

considered on a case-by-case basis [ 57 , 59 , 60 ].

In patients who present with hemorrhage, the sur-

geon may be forced to intervene, despite a high-

grade lesion, in order to prevent devastating

consequences from the hemorrhage [ 61 ]. In many

cases, the surgeon may evacuate the hematoma in

the acute setting to ameliorate symptoms caused

by mass effect until definitive treatment can be

considered. Patients who present with incidental

lesions that demonstrate high-grade features may

undergo selective treatment of the high-risk com-

ponent of their AVM, but selective treatment,

especially with endovascular techniques, is not

without risk [ 62 , 63 ]. Although rare, patients

with posterior fossa AVMs may present with sei-

zures due to vascular steal. In this cohort with

high-grade lesions, selective embolization may

relieve the symptoms caused by steal [ 64 ].

Brainstem and Cerebellar

Cavernous Malformations

Incidence of Cavernous

Malformations of the Brainstem

and Cerebellum

The incidence of cavernous malformations in

the general population is estimated to be on the

order of 0.5%, meaning that 1 in 200 individu-

als harbor a cavernous malformation [ 65 ].

Cavernous malformations are distributed in the

supratentorial and infratentorial compartments

in proportions roughly equal to the volume of

the brain. Supratentorial lesions constitute

60–90% [ 66 , 67 ] of all cavernous malforma-

tions, while infratentorial lesions account for

8–36% of all cases [ 68 – 72 ]. In the posterior

fossa, the brainstem [ 73 ] is the involved site in

4–35% of cases [ 67 ]. Brainstem cavernous mal-

formations account for 13% of all vascular mal-

formations of the posterior fossa [ 73 ]. The

remainder of the cavernous malformations of

the posterior fossa are located in the cerebel-

lum, and cerebellar cavernous malformations

account for 1–12% of all intracranial cavernous

M.Y.S. Kalani and R.F. Spetzler