Surgical Management of Midline Anterior Skull Base Meningiomas

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Chapter 34 Surgical Management of Midline Anterior Skull Base Meningiomas

Surgical Anatomy

Meningiomas arising in the midline of the anterior fossa are generally separated in the more ventral olfactory groove meningiomas and the more dorsal planum sphenoidale and tuberculum sellae meningiomas. Olfactory groove meningiomas arise over the cribriform plate of the ethmoid bone and the area of the frontosphenoid suture. Those tumors may grow symmetrically around the crista galli and thus may involve any part of the planum of the sphenoid bone or extend predominantly to one side. They occurred with a frequency of less than 6% in our series of 1200 meningiomas. Of all anterior skull base meningiomas, 22% were pure olfactory groove meningiomas. Of these, 7% had at least one additional meningioma at a different location. Planum sphenoidale/tuberculum sellae meningiomas arise from the roof of the sphenoid sinus and the tuberculum sellae, which is an area between the optic nerves and the anterior clinoid processes belonging to the frontal part of the middle cranial fossa. The tuberculum sellae is located between the chiasmatic grooves and on either side at the optic foramen, which transmits the optic nerve and ophthalmic artery to the orbit. Behind the optic foramen, the anterior clinoid process is directed posteriorly and medially and attaches to the tentorium cerebelli. These structures are frequently overgrown by these types of meningiomas, as are the posteriorly located dural folds of the sella turcica and the lateral adjacent cavernous sinus (Fig. 34-1). Planum sphenoidale/tuberculum sellae meningiomas occurred at rates similar to those of olfactory groove meningiomas in our series: less than 6% of all intracranial meningiomas but 21% of anterior skull base meningiomas.

The planum sphenoidale and tuberculum sellae are part of the sphenoid bone. The former is a dorsal extension beyond the ethmoid bone and part of the anterior cranial base; the latter belongs to the middle cranial base. Whereas planum sphenoidale meningiomas usually push the optic nerves dorsally and caudally, tuberculum sellae meningiomas lead to an upward bulging of these structures. However, it is often difficult to clearly separate these tumors simply based on their bony covering. Rather, their relationship to the optic nerves and chiasm can distinguish these tumors as to their most likely origin. Both entities might grow between, around, and beyond the optic nerves. Depending on the exact extension, tuberculum sellae tumors are usually approached either frontolaterally or strictly laterally from the pterion using an angle that allows viewing posterior to the optic nerves.

Branches of the ethmoidal, meningeal, and ophthalmic arteries enter through the midline of the base of the skull and constitute the primary blood supply of those tumors. In smaller tumors, the A2 segments of the anterior cerebral arteries usually are not involved in the tumor capsule but rather are separated from the tumor by a rim of cerebral tissue and arachnoid. However, in large tumors, these and additional segments, e.g., the frontopolar or other small branches originating from the anterior cerebral arteries, may adhere to the posterior and superior tumor capsule. They should be meticulously coagulated and separated from the capsule to avoid postoperative bleeding.

The olfactory nerves either are displaced laterally on the lower surface of the tumor or are adherent, compressed, or even not visible while diffusely spread within the tumor capsule. Preservation of these nerves should be attempted in small tumors, resulting in displacement of one or both nerves. Once the olfactory nerves are compressed by large tumors or even tightly involved in the tumor capsule, it is difficult to preserve them, and this becomes almost impossible when the tumor has a broad attachment to the dura and infiltrates adjacent, often hypertrophic bone. In addition, in large tumors, the optic nerves and chiasm may be displaced downward and posteriorly, which is in contrast to tumors originating from the sella region.

Clinical Presentation

Olfactory groove meningiomas are on average larger than meningiomas at different locations in our series. This is most likely due to the relative lack of focal symptoms at the frontal base with smaller meningiomas. For large tumors, the slow growth rate allows surrounding tissues to adapt. Many symptoms are difficult to localize neurotopically, and the initial consultations of family and physicians often tend toward interpretation of these as functional personality changes rather than focal cerebral symptoms. Personality changes, such as apathy and akinesia, can be common when the tumors grow to larger size; in our series, this was found in up to 13% of patients. Onset of these symptoms is gradual, and they may not be observed early in their course. Other common symptoms include headache and visual deficits, both of which were more frequent in frontal meningiomas than in any other type of meningioma in our series. Because the optic nerves and chiasm are compressed superiorly by the tumor, an inferior visual defect was most common in up to one third of our patients. The Foster-Kennedy syndrome of unilateral optic atrophy and contralateral papilledema, although originally described in olfactory groove meningiomas,1 occurred in only a small number of patients.

Double vision is a rare symptom, occurring in less than 6% of patients. In our series, smelling disorders up to anosmia were apparent in 64.5% of the patients that were diagnosed based purely on the routine preoperative workup. Only 7.1% were completely anosmic preoperatively. Interestingly, anosmia is not an important symptom for most patients, most likely because it develops slowly. In Cushing and Eisenhardt’s series, the sense of smell was the primary symptom in only 3 of the 29 patients.2 Bakay reported that even if anosmia was apparent, it was not the leading symptom.3

Epilepsy was less frequent, occurring in 12% of cases, compared to all other supratentorial sites and was only lower, occurring in 8% of cases, in medial sphenoid wing tumors. Planum sphenoidale and tuberculum sellae meningiomas encompassed a further 21% of frontobasal meningiomas in our series. They presented more frequently with visual pathway symptoms than did olfactory groove meningiomas and less frequently with disorders of smell. Symptomatic epilepsy was also rare in these more dorsal tumors and occurred only in very large lesions.

Evaluation of Radiologic Studies in Planning the Operation

The standard diagnostic means for evaluation of anterior midline meningiomas such is magnetic resonance imaging (MRI), because it can delineate the mass effect relationship to other important structures such as the optic nerves, the relationship to the anterior cerebral arteries, and the elevation of the frontal lobes. However, cranial computed tomography (CT) and angiography are important adjuncts if the destruction of the anterior skull base, the infiltration of the ethmoid bone, the relationship of the major vessels, and the vascular supply are of interest.

Noncontrast CT scanning classically demonstrates a dural-based, homogeneous tumor of increased density compared to the surrounding brain, with variable mass effect and surrounding edema. Hyperostosis of the adjacent skull base is a common feature. Contrast agent administration produces dramatic homogeneous enhancement of the tumor and often reveals a dural tumor tail.

For preoperative and diagnostic evaluation, MRI is essential and provides additional information and good soft-tissue differentiation. With T1-weighted MRI, the tumor is of equivalent signal intensity compared to the surrounding brain, and T2-weighted MRI reveals that the tumor signal is slightly increased compared to the normal brain but less than that of cerebrospinal fluid. Fluid-attenuated inversion recovery and T2-weighted MRI highlight surrounding edema. With administration of gadolinium contrast medium, MRI demonstrates homogeneous tumor enhancement. It is crucial to analyze the relationship of the optic nerves and anterior cerebral arteries to the tumor capsule. Magnetic resonance angiography may provide essential information about blood supply and displaced arteries or even arteries embedded within the tumor. In correlation with CT, the extension into the osseous structures, the underlying ethmoid and sphenoid sinuses, and the foramina yields additional information. Modern imaging tools easily facilitate the coregistration of many imaging modalities to visualize the tumor from any angle and to plan the operation virtually.

In recent years, angiography generally has not been indicated unless embolization is planned. The classic angiographic appearance of a meningioma is that of increasing hypervascular tumor blush throughout the arterial phase, persisting well into the late venous phase with slow washout. Hypervascularity may complicate and lengthen the operation. Therefore, embolization may be considered, which involves the devascularization of the tumor’s blood supply through the placement of an embolic agent via a microcatheter into the feeding arteries. However, the surgeon should be aware of a considerable rate of hemorrhagic and ischemic complications when using small particles for embolization.47

General Aspects of Surgical Management

As the majority of meningiomas are benign, well-circumscribed extra-axial tumors, complete surgical removal should be the primary goal in most instances. The neurologic integrity has to be preserved as it would be in any other neurosurgical procedure. Due to the relation of anterior midline meningiomas to adjacent neurologic eloquent areas, complete tumor removal even with resection of infiltrated dura or removal of infiltrated bone might be achieved with low morbidity in most cases. However, when tumors are firmly attached to the anterior vessels or the optic chiasm, complete removal might constitute a high risk for damage of those structures. In these cases a small piece of adherent capsule might remain and is controlled by periodic MRI scans. Upon recurrence, reoperation and adjuvant radiation therapy may be considered.

The decision for surgery should not be based on age alone as long as the patient’s general health condition is stable and the patient is increasingly hampered by neurologic symptoms. Because symptoms usually occur late and at larger tumor volumes that are often increased by surrounding edema, there are rarely alternatives to surgical treatment.

On examination, the sense of smell is often compromised; however, remaining function is difficult to test and a formal olfactogram should be requested. If unaltered, the patient should be warned about the loss of this function.

Preoperatively, steroids are only applied if significant edema is present. Intraoperatively, a steroid bolus, e.g., dexamethasone of up to 24 mg, is given and subsequently weaned over a period of 3 to 7 days postoperatively. When the patient arrives in the operating room, intravenous antibiotics are given once as a prophylaxis and for one week afterward if the frontal sinus was opened.

After induction of anesthesia, the application of intravenous glycerol or 15% mannitol might be considered in a case of extensive edema.

Bifrontal Approach

General Considerations

The bifrontal approach was first described by Horsley8 and Cushing9 and was later proposed by Tönnes,10 who preserved the frontal brain tissue by a subfrontal approach.11 Many others have used the bifrontal approach for large tumors of the frontal base, such as Al-Mefty,12 Nakamura et al.,13 and Ransohoff and Nockels.14 A bifrontal craniotomy might be considered for patients with large tumors because this approach gives direct access to all sides of the tumor. Due to the wide exposure, retraction on the frontal lobes is minimal. It simultaneously allows interruption of the blood supply, preparation of the frontobasal matrix of the tumor, and concomitant decompression. There is usually no problem from the ligation of the anterior sagittal sinus. However, venous drainage should be evaluated by preoperative imaging to avoid venous congestion, and coagulation of draining veins from the anterior frontal lobe should be avoided of possible. A navigation system might be used to avoid opening the frontal sinus; however, if the frontal sinus is entered, meticulous closure of the defect should prevent any complications.

Operative Technique

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