Skull Base Surgery of the Posterior Fossa

187

Paramedian and Retrosigmoid

Approaches for Lateral and Far

Anterolateral Lesions

For lateral lesions, a retrosigmoid or paramedian
bony exposure will provide maximum access to
the tumor, allow sufficient working space for
resection, and reduce unnecessary retraction on
the cerebellum. Patients are positioned in the lat-
eral oblique position for paramedian and retrosi-
gmoidal approaches. If iMRI will be needed for
successful completion of the tumor removal, a
prone position with mild head turn might be a
reasonable compromise and still allow for para-
median approach.
In the lateral oblique position, axillary rolls
are used to support the downward arm. The
upward arm is supported with pillows, and the
superior portion of the table should be elevated
10–15° above horizontal. The head is then rotated
into a flexed position with the vertex shifted
toward the floor. Again, attention to the place-
ment of the Mayfield pins—with the single pin

placed on the superior side and the double pins

placed in the most dependent area of the cra-

nium—to securely stabilize the patient is critical.

Careful thought about pin placement and

Mayfield attachment is necessary to avoid line-

of- sight or instrument interference during the

microsurgical portion of the procedure.

For a paramedian approach to a lateralized,

hemispheric lesion, a vertical, linear incision is

made beginning approximately 5 cm above and

2 cm medial to the mastoid notch. The incision is

extended inferiorly approximately 4 cm below

the level of the mastoid notch (Fig. 13.6b). Soft-

tissue dissection should preserve the paraspinal

musculature as much as possible, while hemosta-

sis is maintained by using cautery. Depending on

the level of the planned craniotomy, the vertebral

artery may be identified and carefully avoided.

The craniotomy can be extended above, or situ-

ated below, the transverse sinus, and care should

be taken to avoid injury to these structures during

approach. The dura is opened such that the bases

of the dural flaps are on the transverse and/or

Fig. 13.6 Patient positioning and surgical approaches for
lesions of the posterior fossa. Solid lines represent planned
incision points. Dashed lines represent planned craniot-
omy/craniectomy areas. (a) Positioning, incision, and cra-
niotomy/craniectomy for straight prone positioning for a
midline or medial hemispheric lesion. Deeper midline
lesions can be accessed through this approach, and the
craniectomy may be extended inferiorly as far as the fora-
men magnum, often including C1 laminectomy for greater
inferior exposure. Superior lesions of the cerebellar hemi-
spheres may be accessible through this approach as well
but necessitate tentorial retraction or transtentorial dissec-
tion. The incision in these cases may be moved progres-
sively farther off of the midline to provide access to more
anteriorly oriented lesions. (b) Park bench positioning for

a lateralized lesion of the cerebellar hemisphere. Incision

and craniotomy/craniectomy are made in relation to the

mastoid notch but should be modified depending on the

exact location of the lesion. (c) Modified supine position-

ing is useful for retrosigmoid approaches to far lateralized

or anterior lesions of the cerebellum and/or brainstem.

Incision and craniectomy are made in relation to the mas-

toid notch. A linear or curved (black line) incision can be

made, depending on surgeon preference. Craniotomy/cra-

niectomy can follow a similar path, using transverse and

sigmoid sinuses as boundaries when possible. Importantly,

the approach can be altered slightly depending on the

anterolateral positioning of the lesion for maximal tumor

exposure

13 Metastasis to the Posterior Fossa