198
height of the basilar bifurcation relative to the
bony anatomy of the clivus is of important clini-
cal consideration because the level of the bifurca-
tion can render approaches to the basilar apex
challenging in some cases.
The BA gives rise to three paired vessels:
these include the AICAs, SCAs, and PCAs.
The AICAs arise from the lower half of the
BA, most commonly from a single trunk
(Fig. 14.4) [ 5 ]. There are four segments to the
AICA: the anterior pontine (a1), the lateral
pontine (a2), the flocculopeduncular (a3), and
the cortical (a4) segments. The a1 extends
from its origin to the midpoint of the inferior
olive. The a2 extends from the inferior olive to
the flocculus and consists of multiple named
branches, including the labyrinthine, the subar-
cuate, the cerebellosubarcuate, and recurrent
perforating arteries. The a3 extends from the
flocculus to the cerebellopontine fissure. The
a4 constitutes the vessel distal to the cerebel-
lopontine fissure.
The SCAs originate from the BA proximal
to or within 2.5 mm of the PCAs (Fig. 14.5)
[ 6 ]. The SCA is divided into four segments: the
anterior pontomesencephalic (s1), the lateral
pontomesencephalic (s2), the cerebellomesen-
cephalic (s3), and the cortical (s4) segments.
The s1 extends from the origin to the anterolat-
eral margin of the brainstem. The s2 extends
from the anterolateral margin of the brainstem
to the cerebellomesencephalic fissure. The s3
is the portion that resides within the cerebello-
mesencephalic fissure. The s4 is the portion of
the vessel distal to the cerebellomesencephalic
fissure.
The PCAs constitute the termination of the
BA (Fig. 14.6) [ 7 ]. The PCAs have comple-
mentary calibers with their associated poste-
rior communicating arteries (PCoA), a remnant
of their development in the embryo (i.e., the
caliber of the PCA and PCoA will vary but
complement each other to serve the vascular
territory). The PCA also consists of four named
segments, with branches of clinical impor-
tance. The P1 segment is the portion of the ves-
sel from where it branches from the BA to its
point of insertion on the PCoA. This portion of
the PCA is rich in thalamoperforating arteries,
which if injured can result in devastating
effects, particularly during surgical treatment
of basilar apex aneurysms. The P2 segment is
further divided into an anterior (P2a) and a
posterior (P2p) segment. The P2a segment is
the portion of the PCA from the insertion of the
PCoA to the posterior border of the peduncle.
It gives rise to the peduncular perforating arter-
ies, the thalamogeniculate arteries, the medial
posterior choroidal artery, the anterior tempo-
ral artery, and the hippocampal arteries. TheFig. 14.4 Illustration
showing the anatomy of
the anterior inferior
cerebellar artery
(segments a1-a4) and its
relationship to adjacent
neural structures (Used
with permission from
Barrow Neurological
Institute, Phoenix,
Arizona)
M.Y.S. Kalani and R.F. Spetzler