Transbasal/Transcranial Microsurgical Approaches to Anterior Fossa Meningiomas

19 Transbasal/Transcranial Microsurgical Approaches to Anterior Fossa Meningiomas

Benedicto Oscar Colli and Guilherme Gozzoli Podolsky Gondim

Abstract

The objective of this chapter is to do a comprehensive review of the transcranial microsurgical approaches for meningiomas of the anterior cranial fossa, including the clinical manifestations, diagnosis, surgical treatment, and clinical outcome of patients harboring these tumors.

Meningiomas of the tuberculum sellae are clinically differentiated from meningiomas of the olfactory groove. The main radiological characteristics of these tumors in the CT, CT angiography, MRI, and MRA are outlined, as well as the role of these examinations in the preoperative assessment and follow-up. The eventual necessity of digital subtraction angiography is also discussed.

In the surgical treatment the indications for surgery and the choice of the approach for different locations of the tumors are discussed and the technical steps of the bifrontal approach with removal of one orbital rim, the fronto-orbital approach, and the subcranial approach are presented in detail illustrated with operative photos. These technical aspects include prevention of surgical complications.

A review of the literature regarding the clinical outcome, the recurrence rate, the morbidity, and the mortality after transcranial microsurgical treatment is provided.

Keywords: Keywords: anterior fossa meningiomas, tuberculum sellae, olfactory groove, transcranial approaches

19.1 Introduction

Anterior cranial fossa meningiomas are related to the development of the neurosurgery. The first intracranial tumor removed by Durante, in 1885, was an olfactory groove meningioma (OGM).1 Cushing (1927) introduced the electrocautery in the removal of an OGM. Cushing and Eisenhardt resected the first tuberculum sellae meningioma (TSM) in 1916, and, they included a chapter about suprasellar meningioma and OGM in their book on meningiomas in 1938.² ^, ³

In the anterior cranial fossa, meningiomas originate mainly from the crista galli, olfactory groove, planum sphenoidale, and tuberculum, diaphragma, and dorsum sellae. Eventually they can have a lateral origin, characterizing the anterior fossa floor meningiomas.⁴ Despite their origin being apart by few centimeters, meningiomas originating from the crista galli to the fronto-sphenoidal suture should be considered true OGM and should be differentiated from those originating in the planum sphenoidale meningioma (PSM)/TSM because they are clinically and radiologically different and their treatments require different surgical strategies.³ ^, ⁵ Due to the difficulties in the definition of specific clinical presentations, Al-Mefty and Smith² classified TSM as those originating in the anterior aspect of the suprasellar region (tuberculum itself, limbus of the sphenoid, chiasmatic sulcus, and diaphragm sellae), although they refer that diaphragm sellae should be considered as a distinct entity.

19.2 Incidence

TSM and OGM are the most frequent in the anterior cranial fossa. TSM accounts for 5.5 to 10% of intracranial meningiomas, and OGM occur in 4 to 11% of cases in major clinical series.³ ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸

19.3 Clinical Features

Although closely related in their origins, the OGM and TSM present as distinct and clearly defined clinical-pathological entities from each other and the sphenoid wing meningiomas.

19.3.1 Tuberculum Sellae Meningiomas

Due to its origin in the tuberculum sellae, these tumors displace the chiasm upwards and backwards and cause early visual symptoms. In general, they present with insidious, progressive, and asymmetrical visual deficits, starting with unilateral visual acuity decrease or concentric reduction of one visual field, followed by bilateral compromise.⁴ ^, ⁹ Common findings upon neuro-ophthalmological examination are asymmetrical and incongruent visual field alterations, primary optic atrophy with central scotoma and asymmetrical bitemporal visual field compromise, and exceptionally, symmetrical loss of the bitemporal visual field.² The visual compromise may increase during pregnancy and decrease after delivery, suggesting the hormone dependency of those lesions. Papillary edema is infrequent and occurs later.² ^, ⁸ Headache, mainly located in the frontal and orbital/retroorbital regions, is the second most common symptom in frequency (25 to 76.2%).² ^, ⁸ ^, ⁹ ^, ¹⁰ Mental disturbances (memory loss, personality changes, and anxiety) occurs in 4.8 to 11.9% of the patients⁸ ^, ¹⁰ ^, ¹¹; seizures may be the initial clinical manifestation in 2.8 to 3.6%⁴ ^, ⁸ ^, ¹¹ and motor deficits can be observed in 14.8%.² ^, ⁸ TSM growing in the posterior direction can compress the hypothalamus and pituitary stalk. Preoperative signs of pituitary insufficiency are rare; however, panhypopituitarism, hyperprolactinemia, galactorrhea, hypothyroidism, and hypogonadism secondary to hypothalamic disturbances have been reported.² ^, ¹² ^, ¹³ Anosmia is an uncommon complaint, but it is found on examination in 11% of patients.⁸

19.3.2 Olfactory Groove Meningiomas

OGM arises in a central area in the junction of the cribriform plate with the planum sphenoidale. Due to its location, the first structure to be compromised is the olfactory nerve and anosmia is an early sign of these tumors. Although the unilateral or bilateral olfactory loss is not usually reported, its frequency increases significantly (49 to 100%) when actively searched in the history and examination.¹ ^, ⁵ ^, ⁶ ^, ⁸ ^, ¹⁴ ^, ¹⁵ ^, ¹⁶ Foster-Kennedy syndrome (primary optic atrophy and central scotoma in the same side of the lesion, with contralateral papillary edema), described as a result from several frontobasal lesions such as meningiomas, abscess, and frontobasal gliomas, is characteristic of the OGM but, however, not so frequent in those tumors.³ ^, ⁴ ^, ⁶ ^, ¹⁶ ^, ¹⁷ ^, ¹⁸

In general OGMs only cause clinical signs when they are large and the most frequent are psychiatric disturbances in 36 to 66.2%, headache in 29.5 to 48%, and visual loss in 42.5 to 58.1%, occurring simultaneously or in isolation.1 ^, ⁴ ^, ⁶ ^, ⁸ ^, ¹⁴ ^, ¹⁵ ^, ¹⁷ ^, ¹⁸ ^, ¹⁹ Less frequent manifestations were apathy, epileptic seizure, vertigo, gait disturbance, nausea, tinnitus, exhaustion, aphasia, and exophthalmos.⁶ ^, ⁸ ^, ¹⁵ ^, ¹⁷ ^, ¹⁸ Tumors that grow in the anterior direction are more inclined to cause high intracranial pressure and papilledema, and tumors that, less commonly, grow in the posterior direction cause late visual compromise due to effect over the optical nerves and chiasm.

19.3.3 Anterior Fossa Floor Meningiomas

Due to their lateral origin, these tumors raise symptoms only when they reach larger volumes. The signs and symptoms result more frequently from their mass effect and constitute mental status compromise, signs of raised intracranial pressure, and seizures. Eventually those tumors may compress the optic pathways and cause visual compromise, usually unilateral.

19.4 Diagnostic Imaging

19.4.1 Skull Radiography

The importance of the plain X-ray of the skull diminished markedly after CT became available. Some findings observed in the skull radiography are typical and, when present, are sufficient to establish the diagnosis of TSM or OGM. These tumors usually cause dense hyperostosis of the tuberculum sellae, seen in the anterior-posterior and lateral incidences. TSM or OGM may present hyperostosis with a brush-like aspect in the implantation site, perpendicular to the skull base. Another characteristic finding is the upward bulging of the superior cortical bone of the posterior ethmoidal cells and anterior portion of the sphenoid sinus, generally associated with hyperostosis of the planum sphenoidale, or tuberculum sellae (Fig. 19.1).

Fig. 19.1 Plain X-ray films of patients with frontobasal meningiomas. (a) Frontal incidence showing a superior convexity, rather than a concavity, of the region of the planum sphenoidale with cortical thickening from hyperostosis. (b) Hyperostosis of the planum sphenoidale and of the olfactory groove and a coarse intracranial calcification above the floor of the anterior fossa.

19.4.2 Digital Subtraction Angiography of the Internal Carotid Artery

The use of digital subtraction angiography (DSA) of the internal carotid artery (ICA) in the preoperative assessment of the TSM and OGM became less frequent for analysis of the relationship of the tumor with the ICA and of the anterior cerebral artery (ACA) and its branches, since this can be evaluated by MRI, MRA, and three-dimensional CT angiographic reconstructions. Those tumors present the following angiographic characteristics: elevation of the ACA and its branches (Fig. 19.2); centered or displaced (round deviation) ACA in relation to midline; carotid siphon generally displaced downwards (closed siphon); and posterior displacement of the bifurcation of the ICA. When the ophthalmic arteries are enlarged, it is possible to see its ethmoidal branches in anastomosis with meningeal branches through the orbital roof ( Fig. 19.2b). The involvement of major arteries by the tumor may be observed in the angiographies through localized and irregular narrowing in the vessel walls and occasionally by vessel obstructions. TSM generally presents homogenous contrast enhancement which leads to an exact estimation of the tumor dimensions. Exceptionally, DSA may be necessary to clarify vessel involvement by tumor or to perform balloon occlusion test to identify collateral circulation.

19.4.3 Computed Tomography (CT)

The CT scan of the skull is the single most sensitive examination for detection of intracranial meningiomas,20 surpassed only by MRI in some aspects of the preoperative assessment of these patients.² Noncontrast enhanced CT suggests or allows the diagnosis of the meningiomas in 6% of the cases and with contrast enhancement, this records rises up to 90%.²⁰ Tumors smaller than 1 cm, mainly those located in the parasellar, orbital, and parasagittal regions, may not be detected by CT. Without contrast injection, the meningiomas are noted as hyperdense lesions in 75% of the cases, as hypodense lesions ( Fig. 19.3a) in 14.4%,²¹ and occasionally as isodense lesion. The CT scan is more sensitive than skull radiography in detecting calcium deposits allowing detection of calcifications in 10 to 26% of the meningiomas.¹⁷ ^, ²¹ Calcifications range from small foci to diffuse areas resulting from the confluence of psammoma bodies ( Fig. 19.3a–e) and the hyperostosis areas. After contrast injection, those tumors, when not very calcified, usually show homogenous contrast enhancement, with round and clear shape, sometimes lobulated, and with a broad base of contact with the dura mater of the planum sphenoidale and tuberculum sellae. The CT shows hypodense areas around the meningiomas, which generally indicate cerebral edema or eventually cysts, in 60 to 75% of the cases.²¹ However, the meningiomas of the parasellar regions present peripheral edema less frequently (0 to 30% of the cases²⁰). The presence of peritumoral edema is related to the size of the lesion, being most frequent in the larger lesions. Three-dimensional CT angiographic reconstruction can show well the tumor and its relationship with the ICA and ACA and their branches, and also allow detection of possible vessel entrapment by the tumor mass.

Fig. 19.2 Right internal carotid artery angiography of a patient with a planum sphenoidale meningioma. (a) Frontal view showing the A1 segment of the anterior cerebral artery and its branches elevated by the tumor. (b) Lateral view showing the anterior cerebral artery branches and enlargement of the ophthalmic artery and its ethmoidal branches in anastomosis with meningeal branches through the orbital roof, providing the tumor irrigation.

Fig. 19.3 CT of the skull of patients with anterior fossa meningiomas, without contrast injection. (a, b) Large hyperdense suprasellar tumor with peripheral calcifications. (c, d) Different types of tuberculum sellae calcifications. (d, e) Coarse calcification of suprasellar meningiomas.

Fig. 19.4 MRI of a patient with meningioma of the planum sphenoidale. (a–c) T1-weigthed images with contrast enhancement showing a huge homogeneous contrast-enhancing tumor occupying all the anterior cranial fossa, invading the sphenoid sinus (b, c). (d, e) T2-weighted images showing a hyperintense peripheral line around the tumor (cerebrospinal fluid), suggesting the presence of an arachnoid plane, and hyperintensity in the frontal lobes (edema).

Fig. 19.5 MRI of a patient with meningioma of the planum sphenoidale. (a–c) T1-weigthed images with contrast enhancement showing a large tumor with homogeneous contrast enhancement occupying all the anterior cranial fossa, with hyperostosis of the planum sphenoidale. (d) T2-weighted image showing hyperintensity in the frontal lobes (edema). (e) MRA showing the anterior cerebral artery elevated by the tumor and dilatation of the ophthalmic artery whose branches are responsible for the irrigation of the tumor.

Fig. 19.6 (a–c) Contrast-enhanced MRI (T1-weighted) of a patient with a meningioma of the tuberculum sellae showing a homogeneous contrast-enhancing suprasellar tumor, with hyperostosis of the tuberculum sellae.

19.4.4 Magnetic Resonance Imaging (MRI)

The absence of the artifacts caused by bone and the possibility of paramagnetic contrast enhancement of the meningiomas make the MRI the most informative examination in the diagnosis of those tumors in the skull base and in the cranial vault.²²

Meningiomas frequently show isointensity in relation to the brain both in T1- and T2-weighted sequences. Edema around the tumors may be easily detected in heavy-T2 sequences. Besides the identification of the tumor, MRI shows the displacement, the occlusion, or the entrapment of the vessels by the tumor probably better than conventional angiography.²² The injection of paramagnetic contrast (gadolinium-diethylenetriamine penta-acetic acid [DPTA]) generally enables a better visibility of the meningiomas, through an intense and homogenous enhancement and allows, with more certainty, distinction between TSM from other suprasellar lesions (Fig. 19.4, Fig. 19.5, Fig. 19.6). MRA allows identification of the relation of the tumor with skull base vessels, especially the ICA and its main branches ( Fig. 19.6b), eventually dismissing the use of conventional angiography.²² Tumors can be classified as soft when MRI T2-weighted image sequence indicates high signal intensity (contains more water) and as hard when it has low or isointense signal (contains less water) and there is an excellent correlation between the appearance on T2-weighted images and intraoperative findings.⁹

19.5 Surgical Treatment

The ideal treatment for frontobasal meningiomas, similar to meningiomas of other locations, is tumor resection, as well as resection of the involved dura mater and compromised bone. However, the decision to perform the ideal treatment for a patient with a meningioma must consider the risk/benefit ratio of the surgical procedure, based on the technical difficulties (involvement of neurovascular structures), on the technical skills of the surgeon, and on the age of the patient. Adequate surgical planning for removal of these lesions must consider several factors in order to allow the most extensive resection possible with minimum neurological compromise. CT scans are important for evaluation of the relationship of the tumor with the skull base bone and consequently for planning an extensive resection. MRI and MRA allow analyzing the size of the tumor and its relationship with the adjacent nervous tissue, and generally they are enough to evaluate tumor vascularization and the relationship with major vessels (displacement and/or involvement of the ICA, anterior communicating and anterior cerebral arteries). Based on the relations with the optic nerves, chiasm, ICA, and ACA complex, the TSM may be divided into two groups: those that are small and grows backwards, compressing laterally the optic nerves and the chiasm posterior, and those larger that may extend above and below the chiasm, displacing the ICA laterally or the ACA upwards and may adhere or encase the ACA complex.4

19.6 Surgical Approaches

Frontobasal tumors have been operated through unilateral, bifrontal, or subcranial approaches. More recently, some of these tumors have been approached through endonasal endoscopic approaches.¹ ^, ² ^, ⁴ ^, ⁵ ^, ¹⁵ ^, ²³ Several authors¹ ^, ² ^, ⁴ ^, ¹⁵ ^, ²³ prefer to approach these tumors through a right-sided lateral craniotomy, with bone resections more or less extended (frontal, frontotemporal, pterional, with or without resection of the supraorbital rim) and with subfrontal approach² ^, ⁴ ^, ⁵ ^, ²³ or along the sphenoid wing.²² In the bifrontal and subfrontal approaches, the access is performed through the interhemispheric fissure.⁵ ^, ⁷

We decide the best approach for frontobasal tumors considering mainly their location in relation to the anterior-posterior and latero-lateral axes. Tumors located in the midline and more anteriorly (OGM) are more frequently approached through a bifrontal or subcranial craniotomies. Smaller tumors, especially when located more posterior (TSM), are generally approached through frontolateral craniotomies. Paramedian tumors are approached through unilateral craniotomies. For unilateral approaches, depending on the size of the tumor, we use a right-sided frontolateral (pterional) craniotomy or fronto-orbital craniotomy with resection of the supraorbital rim. For bilateral approaches, we use a bifrontal craniotomy with uni- or bilateral resection of the supraorbital rim and, more recently, a subcranial approach with a bifrontal craniotomy performed using the anterior wall of the frontal sinus.

19.7 Preoperative Assessment

Every patient with a frontobasal meningioma, especially those arising from the tuberculum sellae, must undertake a preoperative ophthalmologic examination, in order to assess visual acuity and visual fields. An endocrinological assessment is also advised for these tumors even in the absence of clinical manifestations.

Corticosteroids must be administered a few days before surgery in order to reduce the peritumoral edema and to contribute to the improvement of the regional blood flow. When the craniotomy is initiated, infusion of mannitol must be started for brain relaxation.

Before patient positioning, a large-bore venous catheter must be obtained as well as vesical catheterization. Generally, ample and systematic opening of the lateral fissure may allow sufficient cerebrospinal fluid (CSF) drainage to allow brain relaxation. Eventually, in patients with very large tumors to be resected through a bifrontal approach, the use of an external lumbar CSF drainage system, with an epidural catheter, may be useful. The catheter must remain closed and be opened only after the conclusion of the craniotomy.

19.8 Anesthesia

Anesthesia for frontobasal meningiomas follows the same principles of the anesthesia as for intracranial tumors in general and especially for skull base tumors. Essentially, one must avoid maneuvers that may increase intracranial pressure and reduce the cerebral perfusion pressure.

The arterial blood pressure must be maintained in normal levels throughout the procedure, even if temporal vessel occlusions are required.2 ^, ²⁴ In these situations, drugs that protect the brain from ischemia must be used (barbiturates, phenytoin, corticosteroids), preferably administered moments before the vessel occlusion, in case it is predictable.

19.9 Surgical Technique for the Approach of Tuberculum Sellae Meningiomas

19.9.1 Positioning

The patient is placed in the supine position with the bedrest slightly elevated in relation to the body. The slight bedrest elevation allows the surgeon to work seated and support the hands over the skull during microsurgical resection. The head is fixed in a three-pin head support, rotated 30 degrees to the left and tilted posteriorly, in such way that the zygomatic arch is the most elevated point. This tilting allows for the posterior displacement of the frontal lobe, which reduces the pression required for its retraction during surgery. Trichotomy can be restricted to a 2-cm strip along the hair line, from the tragus to the superior temporal line of the opposite side.

19.9.2 Frontotemporal Craniotomy with Resection of the Supraorbital Rim

The skin incision is curved, behind and along the hair line, from 1 cm anterior to the tragus, to the superior temporal line of the opposite side ( Fig. 19.7a). The posterior branch of the superficial temporal artery, which is normally dominant, is preserved in the posterior aspect of the incision, and the anterior branch is cut. The skin flap is folded anteriorly, with the aponeurotic galea, to the next supraorbital rim. Laterally, the superior portion of the frontal zygomatic process may be outlined through digital palpation. The pericranium is cut 1 cm above the superior temporal line, in the posterior-anterior direction, from the edge of the skin incision to the supraorbital rim ( Fig. 19.7b).

Fig. 19.7 Steps of a fronto-orbital craniotomy. (a) Head positioning, skin incision, and monitoring. Note the projection of the superficial temporal artery (dotted line). (b) Incision of the pericranium along the superior temporal line. (c) Exposure of the frontal and temporal bones. (d) Exposure of the superior orbital rim and displacement of the periorbita. (e, f) Exposure and release of the supraorbital nerve and vessels. (g) Initial burr hole on the stephanium and the fronto-orbital craniotomy. (h) Key hole behind the frontal zygomatic process showing the frontal base dura mater superiorly and the periorbita inferiorly. (i) Exposure of the temporal and frontal dura mater. (j) Exposure of the periorbita after removal of the superior and lateral orbital walls. (k, l) Bone flap to be replaced.

The external and the internal leaflet of the temporal fascia, along with the fat pad containing the branches of the facial nerve, are sectioned 1 cm above and along the zygomatic arch and folded inferiorly (above the zygomatic arch). Then the temporal muscle is detached from its origin in the temporal fossa and along the temporal fascia, above the superior temporal line and cut posteriorly parallel and behind the skin incision, and ( Fig. 19.7c–d) displaced from the temporal fossa and the zygomatic arch and pulled posteriorly, exposing all the pterional region.

The pericranium over the frontal and parietal bones should be preserved and reflected anteriorly to expose the orbital rim and to be used as a vascularized flap to cover not only the frontal sinus to be opened, but also the anterior cranial fossa floor, in case it will be necessary. The periosteal flap is folded anteriorly along with the skin flap ( Fig. 19.7c-d), until exposure of the orbital rim, leaving a lateral strip attached to the frontal and parietal bone for attachment of the temporal muscle during closure. The anterior edge of the incisura of the supraorbital foramen must be opened for releasing the supraorbital vessels and nerve, along with the periosteum ( Fig. 19.7e, f). When the anterior edge of this foramen is bony, it may be cut with a delicate osteotome. After the full release of the periosteum, the periorbita of the anterior portion of the orbital roof and lateral wall is also detached, allowing downward displacement of the periorbita.

The craniotomy is performed from four burr holes ( Fig. 19.7g), and the resection of the frontal portion of the bone flap is performed according to the technique recommended by Al-Mefty.² The first burr hole is drilled in the temporal bone, the second immediately in front of the coronal suture, just above the temporal superior line, the third in the midline, in the level of the supraorbital rim, and the fourth posterior to the zygomatic process of the frontal bone. The first two holes do not present special details. With the aid of a craniotome, the second hole may not be needed in young patients. However, in elderly patients, it may be necessary to avoid tear of the dura mater because of firm adherences of the dura mater to the bone. The fourth hole ( Fig. 19.7h), (keyhole or MacCarty’s keyhole), is placed in the frontosphenoidal junction, just behind the zygomatic process of the frontal bone. This hole exposes the frontal dura mater in its superior half and the periorbita in its inferior half, set apart by the orbital roof ( Fig. 19.7h). The third and fourth holes of the craniotomy are united by a cut in the orbital roof with a Gigli saw, tunneled from one hole to the other, inside the cranial vault, protecting the dura with a Gigli saw guide. During the cutting, the orbital contents are delicately displaced medially and inferiorly and protected with a spatula. The full resection of the supraorbital rim is eased by the previous transversal cut of the lateral portion of the orbital rim (zygomatic process of the frontal bone), with a sagittal saw or high-speed drill. The first three holes are united with the Gigli saw or with a craniotome. The craniotomy is extended with craniotome cuts from the temporal and retrozygomatic bone burr holes toward the pterion and the craniotomy is completed over the pterion with a small high-speed drill burr, followed by fracture of the pterion ( Fig. 19.7i–l).

After the elevation of the bone flap, the opened frontal sinus is treated. Its mucosa is removed, and the posterior wall is resected, leaving it completely open for the cranial cavity (“cranialization” of the sinus). The remaining part of the sinus is filled with cotton patties soaked in antiseptic solution, which will be removed upon closure. The pterion is broadly removed with a rongeur or with high-speed drill, completing the bone removal. The dura mater is raised close to the bone at the craniotomy edges, with nonabsorbable stitches and interposing slices of absorbable gelatin sponge underneath the bone to reduce epidural bleeding and to prevent postoperative hematomas. The dura mater incision is performed along the base of the frontal lobe from the midline to the lateral fissure, and kept close and parallel to the fissure, in the direction of the tip of the temporal lobe ( Fig. 19.7e). The incision close to the fissure is enough for the splitting of the frontal and temporal lobes and avoiding unnecessary exposure of the surface of the temporal lobe.

19.9.3 Lateral Fissure Splitting

After the dural opening, the surgical microscope is brought into the operating field and the lateral fissure dissection is initiated by cutting the arachnoid medially and along the middle cerebral veins, preserving them at the temporal lobe side, where they direct toward the cavernous sinus. The arachnoid may be opened with an arachnoid hook, blade, microscissors, and/or microforceps. Venous branches that cross the fissure must be coagulated and sectioned. The splitting of the frontal and temporal lobes generally is simple when there is no evidence of previous hemorrhage or inflammatory processes in the subarachnoid space. In the lateral aspect of the fissure (superficial), the distal branches of the middle cerebral artery (MCA) should be identified and retrograde followed toward the trunk of origin. Arterial branches do not cross the fissure from one lobe to another and thus should not be coagulated and sectioned, instead, they have to be isolated and displaced to their correct side. The MCA is followed in the proximal direction toward the ICA and, from there, the olfactory tract, optic nerve, and chiasm may be identified posterior to the tumor. In the medial aspect of the lateral fissure, usually thick arachnoid adherences may be found which require microscissors cutting for splitting the frontal from the temporal opercula. Following progression of the dissection, in the lateromedial direction, CSF drainage occurs, mainly after the opening of the carotid cistern. This CSF drainage allows a good brain relaxation and displacement of the frontal lobe from the skull base with minimum pressure exerted by the retractor. Besides, it facilitates the identification, before the tumor resection, of the ICA, the ACA complex, and the optic nerves and chiasm. When the tumor extends posteriorly, the ICA and the optic nerve may be covered by it and during the fissure splitting the tumor may be the first structure that can be identified. In this scenario, the tumor may be slightly reduced in volume through bipolar coagulation of its capsule or part of it may be resected and, starting from the MCA, the ICA may be identified.

19.9.4 Tumor Resection

After the identification of the ICA, optic nerves, and chiasm, an autostatic retractor can be placed in the inferior surface of the frontal lobe and with a slight retraction, a space of 1.5 to 2 cm between the skull base and brain tissue is achieved, which allows partial exposure. When the tumor is small, the olfactory nerve can be identified anteriorly to the tumor and released from the adherences of the base of the frontal lobe, which allows its preservation during frontal lobe retraction.² ^, ²⁴ The exposed aspect of the tumor capsule is coagulated with bipolar forceps and cut in the base of tumor implantation, close to the dura mater, starting from the normal edge of the dura mater in the anterior clinoid or in the sphenoid wing. The sectioning is performed after systematic bipolar coagulation and for that the tumor has to be slightly displaced upward. The objective in this step is to interrupt the major vascularization of the lesion, from the posterior ethmoidal arteries and the sphenoid wing.² ^, ²² ^, ²⁴ After devascularization, the tumor is internally reduced by piecemeal resection, employing bipolar coagulation and aspiration for soft lesions and microcurettes and microscissors for hard lesions. The use of ultrasonic aspirator with an angled tip allows time saving during resection of larger lesions. With a straight tip this equipment usually requires an increased retraction of the frontal lobe to allow a safe resection.

19.9.5 Optic Nerves and Chiasm

The meningiomas usually originate and grow in the subdural space and remain outside the arachnoid. For this reason, when TSMs grow, they elevate the arachnoid floor covering the chiasmatic cistern. When the tumor fills the adjacent cisterns, it is involved by several layers of arachnoid that separate it from the arteries and nerves that are in the subarachnoid space and that constitutes the separation plane between the tumor and those structures.² ^, ²² Both optic nerves are displaced laterally and superiorly, and the chiasm is displaced upwards and backwards by the TSM. Occasionally, the tumor grows laterally and may involve the optic nerves.² ^, ⁴ ^, ²² ^, ²⁴ The lateral displacement of the optic nerves may be so remarkable that they may be found laterally to the internal carotid arteries, sometimes flattened as strips.² ^, ²⁴

After the reduction of the internal volume of the tumor, its external portion may be stripped in the posterior-anterior and inferior-superior directions (from the chiasmatic cistern), and progressively released from the optic nerves and chiasm. Dissection from the optic nerves is performed by gently displacing the tumor fragments, releasing and sectioning them with sharp dissection with microforceps, microdissectors, and microscissors.

When the tumors are large, the identification of the optic nerves may be harder; however, even in the cases of involvement of those nerves by the tumor, the arachnoid plane allows its dissection.² ^, ²⁴ This plane must be identified, and the arachnoid preserved entirely so that the dissection is performed in the plane between the tumor and the involved structure.²² Injection of intravenous sodium fluorescein after tumor exposure may help in these cases because due to contrast enhancement the tumor and the cranial nerves have different intensity expressed by different colors that can be characterized by their wavelengths through digital picture analysis.²⁵ When the tumor insinuates underneath those nerves, these portions should be resected in a piecemeal fashion between the ICA and the optic chiasm in the side of the approach and displaced toward the chiasmatic cistern and released from the optic nerve of the opposite side. The resection of the fragments from the opposite side is usually easier than on the same side of the approach.⁴ ^, ²²

The dissection of the optic nerves and chiasm must be carefully performed and with enough magnification for the identification and preservation of the vascularization of those structures, which are paramount for the maintenance of vision. Bipolar coagulation must be avoided in those structures: minor arterial bleedings may be controlled with gentle compression with hemostatic substances (absorbable hemostatic cellulose or gelatin sponge, etc.).

19.9.6 Arterial Dissection

Dissection of the ICA and of the ACA complex must be performed in the same way as the release of the optic nerves and chiasm—employing microdissection. The dissection of the ICA usually is initiated from its emergence in the intradural space through the internal ring of the dura mater, and progressively performed distally toward its bifurcation. In this path one must identify and release the ophthalmic, posterior communicating, and choroidal arteries and the anterior thalamic perforating arteries. Special care must be paid to the ophthalmic artery that usually emerges from the superior wall, near the internal ring of the dura mater and directs toward the optic foramen, under the optic nerve. Other branches usually arise from the inferior wall and are displaced laterally by the tumor and, less frequently, are involved by it. However, one must be careful to identify and isolate the branches that will irrigate the optic nerves and chiasm. When the tumor is large and identification of the proximal portion of the ICA is difficult, the dissection may be initiated from the cerebral artery, after ample splitting of the lateral fissure.

After release of the ICA, the dissection progresses from its bifurcation toward the ACA. This vessel, along with its perforating branches, may be involved or just displaced by the tumor. The A1 segment of the ACA is usually too stretched or adhered to the tumor and frequently suffers fissures during dissection. In case of vessel damage, the fissure may be sutured with 10–0 nylon after proximal and distal occlusion with temporary clips,2 or the vessel may be involved by clips surrounding the vessel as a cuff.² A simple alternative is the involvement of the damaged vessel with a sheet of absorbable hemostatic cellulose or gelatin sponge, covered by an aponeurosis loop, maintained in place as a cuff with a clip applied parallel to vessel. Since it is a difficult and potentially risky dissection, leaving fragments of the tumor adhered to the vessels, instead of attempting full resection, can be an option occasionally.⁴ ^, ⁷ ^, ¹⁰ ^, ²⁶ TSM may also be irrigated by branches from the ACA complex and these vessels must be differentiated from perforators to the hypothalamus, Heubner recurrent artery and branches to the optic nerves, and chiasm, before coagulation and sectioning. The basilar artery is usually only displaced posteriorly by the tumor and easily released because the Liliequist membrane is intact in most patients.² ^, ²⁴

19.9.7 Pituitary Stalk

After tumor resection, between the optic nerves and in front of the chiasm usually lies a thick arachnoid (Liliequist membrane), and behind it lies the pituitary stalk. In tumors extending backwards, the stalk is usually displaced posteriorly and toward one side,² ^, ²⁴ and it may be recognized by its reddish color and its vascularization. Exceptionally, the stalk may be involved by the tumor and its preservation depends of a careful dissection that must be performed without traction of the hypothalamus.

19.9.8 Invasion of the Optic Canal and the Cavernous Sinus

When invasion of the optic canal is suspected upon neuroradiological examination, tumor removal may be performed by increasing bone removal, including the lateral orbital wall, the anterior clinoid process, and the roof of the optic canal using the technique proposed by Dolenc.²⁷ Alternatively, the optic canal can be opened intradurally after sectioning and displacement of the falciform ligament. This bone resection allows the opening the dura mater along the optic nerve, mobilization of the nerve, and resection of the intracanalicular tumor, after identification of the ophthalmic artery. Invasion of the cavernous sinus by a TSM is seldom frequent.²² Detailed description of the technique for removal of tumor invading the cavernous sinus is beyond the scope of this chapter.

19.9.9 Tumor Implantation

After gross total resection of the tumor, the dura mater above the tuberculum sellae must be removed. If total resection of the dura mater is not possible, it must be coagulated. When there is hyperostosis that may involve the tuberculum sellae, anterior clinoid process, or ethmoid bone, bony resection must be performed with a high-speed drill. When the resulting bone defect is large, it must be filled with fat and fascia lata or with a bone fragment obtained from the splitting of the posterior aspect of the bone flap or the posterior edge of the craniotomy. If the ethmoidal cells are opened, the dural defect must be repaired with a fragment of fascia or with the vascularized pericranial flap over the anterior fossa, suturing it with a few stitches to the dura mater of the sphenoid wing and to the dura of the base of the frontal. Alternatively, a fragment of the fascia lata can be used. Other homologous biological membranes (lyophilized dura mater) or heterologous (bovine pericardium) may also be used, but these substances may act as a foreign body or as a potential factor for infection perpetuation.

19.9.10 Closure

The dura mater must be sutured in a watertight fashion. When the pericranial flap is unfolded above the base of the frontal, it must be sutured to the dura mater along the supraorbital rim. If the pericranial flap graft is not used it must be unfolded above the opened frontal sinus and sutured to the dura mater close to the frontal base. The bone flap is fixed with mini-plates or stitches of 2–0 nylon, the temporal muscle is sutured to the muscle fragment left in the bone flap, and the galea and scalp are sutured in separate planes.

19.10 Surgical Technique for the Approach to Olfactory Groove Meningiomas

19.10.1 Positioning

The patient is placed in the supine position with the bedrest slightly elevated in relation to the body ( Fig. 19.8a). The head is fixed in a three-pin skull clamp, and slightly extended, allowing backwards shift of the frontal lobes and reducing traction to be applied over them during surgery. Shaving of the hair can be restricted to a 2-cm wide bizygomatic strip along the hairline.