29a complex of two to three veins that drain the lat-
eral cerebellar surface before converging to insert
into the petrosal sinus [ 7 ]. These veins may lie
adjacent to CN V or may enter the tentorium sep-
arate from CN V, effectively tethering the lateral
cerebellar hemisphere [ 7 ]. The middle neurovas-
cular complex consists of the AICA, middle cer-
ebellar peduncle, and cranial nerves VI–VIII. CN
VII arises from the brainstem at the level of the
pontomedullary junction 1–2 mm ventral to the
vestibulocochlear entry point (Fig. 2.3). The
facial and vestibulocochlear nerves course
together as they travel laterally to the internal
acoustic meatus. AICA typically forms a loop
just below the CN VII–VIII complex with laby-
rinthine, recurrent perforating and subarcuate
branches arising from this loop to course with
CN VIII into the internal acoustic meatus [ 9 – 12 ].
The lateral recess of the fourth ventricle transi-
tions into the foramen of Luschka, which is situ-
ated posteroinferior to the junction of CNs
VII–VIII with the brainstem and is often not
visualized, but may be identified by a tuft of cho-
roid plexus protruding into the CP angle [ 11 ].
The flocculus, which projects from the lateral
recess, forms a bulge of cerebellar tissue sitting
superficial to CNs VII–VIII and adjacent to the
choroid plexus [ 11 ].
The lower complex consists of CNs IX–XI,
the inferior cerebellar peduncle, and the PICA
[ 10 ]. CNs IX–XI arise as rootlets along the pos-
terior edge of the medullary olive in the groove
of the postolivary sulcus (Fig. 2.3). CN IX is
typically one or two rootlets arising from the
upper medulla just caudal to CN VII, whereas
CN X comprises a line of tightly packed root-
lets just inferior to this. The rootlets of CN XI
are more widely separated, running from the
lower two- thirds of the olive to the upper cervi-
cal cord, its cranial roots often being difficult to
distinguish from vagal fibers. CNs IX–XI exit
at the jugular foramen with the glossopharyn-
geal exiting more anteromedially at the pars
nervosa and CNs X–XI exiting posterolaterally
at the larger pars vascularis. CN XII exits the
medulla along the anterior margin of the caudal
olive, its roots running anterolaterally to reach
the hypoglossal canal. Both branches of PICA
and the vertebral artery may pass through or
contact rootlets of the lower cranial nerves [ 10 ].Surgical Technique
PositioningThe three-quarter prone position is our position
of choice because it maximizes the working
space between the patient’s head and ipsilateral
shoulder, thus providing adequate working space
for the surgical corridor (Fig. 2.4a, b) [ 13 ].
Several other positions have been described for a
retrosigmoid approach, each with its own relative
advantages and disadvantages, including the sit-
ting position [ 14 ], supine position with the head
maximally rotated to the contralateral side [ 7 ], or
the lateral position with the head turned 90° [ 15 ].
A series of stepwise maneuvers are under-
taken to place the patient in this position. If uti-
lized, a vacuum positioner [i.e., “beanbag”] is
prepositioned on the bed. Alternatively, pillows
and bolsters may be used to support the torso. A
minimum of four surgical personnel are required
to safely perform these maneuvers, with the neu-
rosurgeon typically in control of the head, assis-
tants at each side controlling shoulders and hips,
and one assistant in control of the legs and feet.
Our preference is to pin the head with the skullFig. 2.3 Cadaveric specimen demonstrating the right CN
VII–VIII complex and CNs IX, X, and XI as they exit the
brainstem
2 Retrosigmoid Craniotomy and Its Variants