136
in 1986 for the treatment of lesions of the verte-
brobasilar system [ 13 ]. A similar technique was
then described in 1988 for the resection of ante-
rior foramen magnum pathology [ 12 ]. Since
those early descriptions, many variations on the
far- lateral approach have been described with
differing degrees of mastoidectomy, condylec-
tomy, the extent of the cervical laminectomy, and
mobilization of the vertebral artery depending on
the specifics of the case [ 5 , 21 ]. In 1991, six vari-
ations of the dorsolateral approach for meningi-
oma resection were reported: transfacetal,
retrocondylar, partial transcondylar, complete
transcondylar, extreme-lateral transjugular, and
transtubercular [ 17 ].
Classification
Foramen magnum meningiomas may be classi-
fied based on their compartment of development
(intradural or extradural), their relation to the
vertebral artery (below, above, or on both sides),
and the location of their dural attachments (pos-
terior, lateral, or anterior) [ 8 ]. FMMs are consid-
ered anterior if the origin is bilateral with respect
to the anterior midline (Fig. 10.1), lateral if the
origin is between midline and the dentate liga-
ment, and posterior if the origin is posterior to the
dentate ligament. Extradural meningiomas at the
foramen magnum are less common than intradu-
ral tumors, but complete resection is more chal-
lenging due to their invasive nature [ 8 , 15 ]. In
tumors that originate beneath the vertebral artery,
the cranial nerves are displaced cranially and
posteriorly. However, in tumors that originate
above the vertebral artery, the location of the cra-
nial nerves is variable, and great care must be
taken to avoid injury during tumor resection [ 15 ].
Preoperative Assessment
Preoperative workup is aimed at determining
the best surgical approach and proximity to
nearby structures in order to accurately assess
surgical risks. CT is the best tool for assessing
bony anatomy in regard to hyperostosis and cal-
cifications, as well as allowing for preoperative
determination of the surgical corridor and the
degree of bony removal required. In the event of
significant bony erosion by the tumor, the
patient may require surgical fusion to prevent
instability at the craniocervical junction. MRI
remains the imaging modality of choice for
assessing soft tissues, including the origin of the
tumor and the involvement of the critical neuro-
vascular structures located nearby [ 19 ].
Many advocate vascular imaging (CTA,
MRA, or conventional angiography) preopera-
tively to evaluate arterial feeders, venous
drainage, and the extent of vascular involve-
ment. In particular, defining the V3 and V4
segments of the vertebral artery along with the
origin of the PICA will aid in operative plan-
ning and the avoidance of complications [ 8 ].
Identification and analysis of vessels that are
encased in tumor are particularly important.
The presence of significant stenosis is sugges-
tive of tumor invasion of the adventitia. In
these cases, conventional angiography allows
the surgeon to determine if vessel sacrifice is a
feasible option based on the results of balloon
occlusion test and the presence or absence of
collaterals [ 4 ].Surgical Considerations
Patients presenting with FMMs are usually con-
sidered for surgery if the lesion is symptomatic,
has experienced growth, or is causing mass effect
on the brainstem. In these patients, radiosurgery
is considered difficult due to the absence of a
plane between the tumor and the brainstem and
the likelihood of ongoing compression of the
brainstem.
As most foramen magnum meningiomas are
located ventrally, surgical resection via a far-
lateral or extreme-lateral approach may be uti-
lized (see chapter on the far-lateral approach).
For midline posterior tumors that do not cross theA.M. Richardson et al.