Meningiomas

while maintaining the arachnoidal layer intact between the brain and the tumor. As patties are being placed sequentially around the tumor, they are used to gently strip the arachnoid from the tumor capsule, covering the brain and arachnoid together, thereby protecting the brain from surgical trauma. As the remaining tumor capsule is brought into the surgeon's view, any adjacent cranial nerves are carefully dissected, and exposed blood vessels on the capsule surface are thoroughly inspected. Only tumor-feeding vessels are obliterated, preserving and dissecting free those transit vessels that are either passing through the depth of tumor or adherent to the tumor surface. Portions of tumor capsule thus devascularized and completely dissected from the surrounding neu-rovasculature are further removed in segments. These alternating sequential steps of internal decompression, extracapsular dissection and removal of devascularized capsule are repeated until the entire tumor is removed.

For meningiomas in the clival, petroclival or cerebellopontine-angle regions, the surgeon must analyze the pre-operative MRI scan carefully. First, evidence of surrounding edema in the brainstem noted on T2-weighted scan must be appreciated prior to surgery as this indicates disruption of the arachnoidal layer and the blood-brain barrier. This implies that the surgical plane between the brainstem and tumor may have been obliterated and, therefore, that aggressive resection off the brainstem should be avoided. Second, the basilar artery location in relation to the tumor and brainstem must be noted. Although rare, if the tumor is located between the brainstem and basilar artery or completely encases the artery, this indicates that all the perforating branches of the basilar artery are stretched and course through the tumor. In this situation, an attempt at aggressive tumor removal is likely to result in a brainstem infarct. When the basilar artery is abutting directly on the brainstem, aggressive tumor removal off the brainstem is possible.

During extracapsular dissection, as a rule, no artery or arterial branch is sacrificed except when the vessel is definitely confirmed to be a tumor feeder. Commonly, loops of vessels may be encased by the tumor or may course onto the capsule surface and become adherent. In these situations, the surgeon may initially misinterpret these vessels as tumor feeders. Before con cluding that a vessel is a tumor feeder and therefore amenable to obliteration, the afferent and efferent course of the vessel must be fully appreciated. It is very rare for meningiomas to have feeders directly from main intracranial arterial trunks. Therefore, no vessels coming directly off the ICA (in tuberculum sella or clinoidal tumors), basilar artery (in petroclival- and cere-bellopontine-angle tumors) or vertebral artery (in foramen magnum meningiomas) should be coagulated. If any appreciable vasospasm occurs while dissecting tumor off arteries, small pieces of gelfoam soaked in papaverine applied directly onto the vessel readily reverse the spasm.

In removing the tumor from cranial nerves, especially the optic nerve, fine vessels feeding the nerves must be preserved. The optic chiasm and intradural optic nerve have main feeders on the inferior surface, and therefore removal of large tumors involving the subchiasmatic and suboptic space must be done carefully so as to preserve these fine vessels. Again, the preserved arachnoid around the cranial nerves facilitates tumor removal and reduces risks of intraoperative neurovascular injury.

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