Indications and Preoperative Considerations
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Meningiomas represent 34% of all primary brain tumors, originate from the arachnoid cells in the brain and spine and are histologically non-cancerous lesions.
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Anterior midline meningiomas can be classified according to their location ( Figure 31.1 ) into three different groups:
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Olfactory groove meningiomas
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Planum sphenoidale meningiomas
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Tuberculum sellae meningiomas.
Figure 31.1
Classification of the midline anterior skull base meningiomas.
Modified from Drake, R.L., 2008. Gray’s Atlas of Anatomy, Churchill Livingstone, Elsevier, Edinburgh.
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Olfactory groove meningiomas arise from the cribriform plate of the ethmoid bone and correspond to up to 22% of all skull base meningiomas. They may grow symmetrically around the crista galli or extend predominantly to one side.
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Planum sphenoidale and tuberculum sellae meningiomas arise from the roof of the sphenoid sinus and at the tuberculum sellae between the optic nerves.
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Olfactory nerves are displaced either laterally or are adhered and compressed within the tumor capsule. Planum sphenoidale meningiomas usually push optic nerves dorsally and caudally, while tuberculum sellae meningiomas lead to an upward bulging of the optic nerves and chiasm ( Figure 31.2 ).
Figure 31.2
Midline anterior skull base meningiomas localization: (A) olfactory groove; (B) planum sphenoidale (the optic nerves are usually pushed dorsal and caudally); (C) tuberculum sellae (the optic nerves are usually pushed upwards).
© A. Quiñones-Hinojosa.
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Branches from the ophthalmic artery, anterior ethmoidal arteries and small meningeal arteries provide most of the arterial supply for midline anterior skull base meningiomas.
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Symptoms include:
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Personality changes, such as apathy and akinesia: secondary to compression of the basal frontal lobes.
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Smelling disorders: up to 7.1% of these patients may present with anosmia and it is more frequent in olfactory groove meningiomas. Lack of smell is not a frequent clinical complaint, but might be diagnosed during neurological examination. The patient usually complains of change in taste.
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The Foster–Kennedy syndrome, classically related to olfactory groove meningiomas, is an unusual clinical finding nowadays. It is defined as an association between unilateral optic atrophy because of direct compression of the optic nerve by the tumor, with contralateral papilledema due to the increased ICP, and anosmia.
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Other symptoms may include headache, seizures and visual deficits.
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Preoperative imaging evaluation ( Figure 31.3 ) consists of:
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CT scan for analysis of bone (hyperostosis) and relation of the lesion with the planum sphenoidale, tuberculum sellae, medial sphenoid wing and sellar floor.
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MRI for evaluation of the size, location and relation of the lesion with the optic nerves and chiasm, as well as relation to the internal carotid arteries and its branches and basal frontal lobes.
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CT angiography, magnetic resonance angiovenography (MRA/MRV) and conventional angiography are useful for preoperative evaluation of arterial feeders and surrounding draining veins.
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Preoperative embolization, although controversial, may be helpful in cases of large lesions (>5 cm) in order to reduce the arterial tumor supply prior to surgery.
Figure 31.3
(A) Sagittal view of head CT showing the normal regional anatomy. (B) Anatomic aspect with tumor.
© A. Quiñones-Hinojosa.
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Management of midline anterior fossa meningiomas includes clinical follow-up, surgery and radiosurgery/radiotherapy.
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Surgery is indicated for patients with:
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Significant enlargement of the lesion compared to previous scans
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Development of neurologic symptoms
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Presence of marked surrounding vasogenic edema.
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Asymptomatic patients with small lesions and no edema may be clinically followed with routine MRI scans.
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Radiosurgery/radiotherapy is considered for treatment of recurrent lesions and as adjuvant treatment for grades II and III meningiomas.
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Surgical Procedure
Pterional, orbitozygomatic, bifrontal and supraorbital approaches are useful for the resection of midline anterior skull base meningiomas.
Pterional/Orbitozygomatic Approach ( Figure 31.4 )
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Unilateral approaches are considered for resection of small and medium-sized tuberculum and planum meningiomas. They are especially useful for tumors with prominently unilateral growth without extensive dural attachment onto the contralateral side and if the contralateral olfactory nerve is expected to have a clear plane of dissection.
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Positioning, skin incision and craniotomy are performed as described in other chapters (see Chapters 18 and 19 ).
Intradural Dissection and Tumor Resection
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The sylvian fissure and the basal cisterns (opticocarotid and suprachiasmatic) can be opened in order to drain CSF for brain relaxation.
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The sylvian and lateral subfrontal routes expose the ipsilateral optic nerve and carotid artery lateral to the tumor at the beginning of the surgery ( Figure 31.4 ). Tracing the ipsilateral carotid artery, the anterior cerebral artery, the anterior communicating artery and the contralateral A1 segment of the anterior cerebral artery can be identified. Early identification of major neurovascular structures is one of the main advantages of this approach.
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Superficial coagulation and shrinkage of the tumor is then done with bipolar electrocautery to obtain tissue for diagnosis and/or to begin to devascularize the tumor at the skull base.
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Coagulation of the basal surface of the tumor and its basal feeders devascularizes the tumor and decreases the bleeding that can obscure the surgical field during the surgery.
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Debulking is performed with use of aspirators, dissectors and ultrasonic aspirators. This reduces the tumor volume and facilitates manipulation of the lesion while decreasing the manipulation of the brain and allowing less use of brain retraction. The risk of extensive bleeding is the main limitation at this step of the procedure and can be decreased by the initial devascularization.
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Dissection of the tumor borders away from its attachments to the contralateral optic and olfactory nerves and internal carotid artery is carefully done with microdissectors and microscissors under high magnification with microscopic visualization. Care must be exercised when manipulating or coagulating the optic apparatus, especially when the optic nerve is atrophic and has been thinned by the tumor. Manipulation of the nerve or coagulation of small vessels that may be supplying it and the tumor at the same time can cause postoperative visual deficits (temporary or permanent).
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A gentle pulling of the lesion to expose its interface with the basal frontal lobe allows continuous stepwise resection of the lesion.
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Arteries are usually surrounded by a thickened arachnoid. A continuous evaluation of the dissection plane around the tumor is important in order to avoid vascular injuries.
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Once the tumor has been freed from the neurovascular structures surrounding it, the dural attachment is excised or coagulated and the tumor is removed en bloc if possible.
Bifrontal Approach ( Figure 31.5 )
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This approach is especially useful in patients with large anterior fossa tumors. With minimal or no brain retraction, it exposes the anterior cranial fossa and main arterial tumor feeders while providing access to the anterior and lateral sides of the lesion. In addition, it allows drilling of hyperostosis at the cribriform plate area, planum sphenoidale and tuberculum sellae, and optic nerve unroofing when necessary.
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Some disadvantages include: frontal sinuses are usually opened, swollen brain may herniate into the craniotomy and the ligation and division of the anterior portion of the superior sagittal sinus that is performed during dural opening may increase the risk of venous lesions and consequently may produce brain edema ( Figure 31.6 ). In addition, important structures such as the optic nerves and carotid arteries are exposed only after the tumor has been partially removed.
