arachnoid. Our general principle is to “leave the arachnoid with the patient” and “take the tumor from the patient.” Intraoperative monitoring is intended to aid the neurosurgeon in preserving neurological function. Somatosensory evoked potentials provide a measure of ascending pathways within the surgical field, whereas electromyographic recordings in the sternocleidomastoid muscle and tongue reflect 11th and 12th cranial nerve activity, respectively. If either of these modalities demonstrates a change, then the surgeon is alerted to a potentially threatening maneuver and may pursue a different manner of dissection. Although we have found electromyographic monitoring of the 11th cranial nerve useful, stimulation of the 12 cranial nerve occasionally will cause protrusion of the tongue, which, if not returned to position by the anesthesia staff, can lead to postoperative tongue swelling. There is insufficient evidence to support the use of routine evoked potential monitoring in this location. Changes, if noted, are always noted after the event has occurred, and if retraction is minimized, they rarely change intraoperative management. Currently, these modalities have not gained absolute clinical acceptance; their use instead is based on surgeon preference.
Surgical Approaches to Foramen Magnum Meningiomas The FM can be approached via anterior, lateral, and posterior approaches. Each approach serves an important function and each was developed to deal with specific
problems. The anterior transoral approach to the FM is rarely conducted to reach intradural lesions, such as meningiomas, because of problems with dural repair, risk of CSF leakage, and meningitis. Debate about FM meningioma resection primarily involves the posterior suboccipital craniectomy and posterolateral approaches, which necessitate drilling of the occipital condyle (Fig. 31.6). We limit our discussion to these approaches. To simplify understanding of approaches to this region, we use the terms suboccipital craniotomy and transcondylar approach. Both require laminectomy, although the transcondylar is more commonly associated with mobilization of the VA from its lateral attachments to widen the surgical corridor. Terms such as far lateral and extreme lateral have only conjured up confusion and in our opinion should be avoided.
31 Foramen Magnum Meningiomas
Fig. 31.5 (A) Artist’s depiction of the surgeon’s intraoperative view of the anterolateral foramen magnum meningioma. Note that most of the tumor is anterior to the dentate ligament and by definition is classified as anterolateral. The spinal component of nerve XI is on the surgeon’s side of the tumor, and care should be taken not to injure it or mistake it for a leaf of the dentate ligament. (B) Intraoperative photograph demonstrating pathological displacement of anatomical structures of an anterolateral foramen magnum meningioma. A standard suboccipital craniectomy was performed without resection of the condyle. Tumor (*) is noted through intact arachnoid to the left of the rostral spinal cord and is draped with various nerve rootlets (arrows) and blood vessels. CH, cerebellar hemisphere; CT, cerebellar tonsil; FM, rim of foramen magnum; C2, C2 segmental nerve root; DT, dentate ligament; PICA, post inferior cerebral artery; VA, vertebral artery. Used with permission from Boulton MR, Cusimano MD. Foramen magnum meningiomas: concepts, classifications, and nuances. Neurosurg Focus 2003;14(6):e10.
Suboccipital Craniotomy Patient position: prone, head flexed on neck, neck kept neutral Lateral decubitus, head turned 20 to 30 degrees toward floor Craniotomy: suboccipital With or without C1 laminectomy Suboccipital craniotomy, or craniectomy, with or without cervical laminectomy, represents the classic approach to the FM meningiomas and is familiar to most neurosurgeons. For posteriorly situated lesions, we place the patient prone. The
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