Historical Background

A meningioma is, in many ways, the soul of neurosurgery. The progress in meningioma treatment mirrors advances in neurosurgery, and advancements in neurosurgery are put to maximal use to improve the treatment of meningiomas.¹ A historical account of meningiomas and their surgical management highlights that “meningiomas have left their mark, in the form of hyperostosis, on human skulls as far back as prehistoric times,”² and authors began to document the recognition of these tumors as pathologic entities starting in the 17th century.

In the 18th and 19th centuries meningiomas were diagnosed during a patient’s life only if they caused changes in the overlying skull that could be appreciated through inspection or palpation. Only a few attempts were made to remove these lesions surgically, and few were beneficial to the patient. Of 13 such operations performed between 1743 and 1896 whose outcome was specified, 9 ended in the patient’s death.²

In 1864 John Cleland, professor of anatomy in Glasgow, “set forth with prescience … the view that two tumours which he had found in the dissecting room, one of them arising from the cribriform plate and the other from the right frontal region adjacent to the superior longitudinal sinus, took their origin from the arachnoid rather than the dura. He observed that in structure they resembled the Pacchionian granulations in a number of points.”³ In 1915 Cushing and Weed⁴ reasserted Cleland’s opinion that meningiomas derived from arachnoid cell clusters.

The sequential nomenclature of meningiomas has included fungoid tumor, sarcoma, cylindroma, endothelioma, and fibroma.^2,5 Cushing proposed the term meningothelioma in an effort to describe these tumors according to the tissue involved. He avoided a histiogenic name because the cellular composition of this tumor was still in dispute; he also avoided a place name because of the various sites of distribution of meningiomas. Later, Cushing opted for the term meningioma. In his Cavendish lecture in 1922, he reported on 85 cases of meningioma.³ In 1938 Cushing and Eisenhardt⁶ published Meningiomas: Their Classification, Regional Behaviour, Life History, and Surgical End Results, in which they reported in detail the cases of 313 patients encountered between 1903 and 1932. The interest in meningiomas has not declined since then. In 1922 Cushing wrote: “There is to day nothing in the whole realm of surgery more gratifying than the successful removal of a meningioma with subsequent perfect functional recovery.”³ These words are still true more than 80 years later.

Pathology

Meningioma’s cell of origin is believed to be the arachnoid cap cell. The arachnoid villi protrude into the venous sinuses. The venous endothelium is in contact with all or a portion of the arachnoid villi. In the latter case, these cells are referred to as arachnoid cap cells. The rest of the granulation is covered by a fibrous capsule. Arachnoid villi are most numerous in the area of the superior sagittal sinus, followed by the cavernous sinus, tuberculum sellae, lamina cribrosa, foramen magnum, and torcular Herophili. Arachnoid granulations and pacchionian bodies are larger and more pronounced versions of arachnoid villi (Fig. 131-1).

FIGURE 131-1 Semidiagrammatic representation of the arachnoid granulation shows its relation to the superior sagittal sinus (venous sinus) and the continuity of its layers and spaces with those of the meninges on the surface of the brain. The individual layers or cell clusters are not intended to be proportional but are drawn to emphasize the cytoarchitectural features of the granulation. SAS, subarachnoid space.

(From Haines DE, Frederickson RG. The meninges. In: Al-Mefty O, ed. Meningiomas. New York: Raven Press; 1991:9.)

The 2000 World Health Organization (WHO) classification of tumors of the nervous system lists meningiomas under the heading “tumours of the meninges” and the subheading “tumours of meningothelial cells.”⁷ WHO recognizes three grades based on pathologic criteria and the risk of recurrence and aggressive growth (Table 131-1).

TABLE 131-1 Meningioma Grades

Adapted from Louis DN, Scheithauer BW, Budka H, et al. Meningiomas. In: Kleihues P, Cavenee WK, eds. Pathology and Genetics of Tumours of the Nervous System. Lyon, France: IARC Press; 2000:176-180.

Meningiomas are usually globular, encapsulated tumors. They are attached to the dura and compress the underlying brain without invading it. Even though invasion of the dura and dural sinuses is common, meningiomas are usually easily separated from the pia mater. The cleavage plane, however, may not encompass the whole surface of the tumor.⁸ Meningiomas may also occur as a flattened sheath of tumor, taking the shape of the underlying bone. This so-called meningioma en plaque is more common in the area of the sphenoid bone. Seldom, meningiomas occur in a location where dural attachment cannot be shown (e.g., intraventricular, intraparenchymal). The cut surface of a meningioma is pale and translucent or homogeneous and reddish brown, depending on the degree of vascularity. A gritty consistency is common. After fixation of a specimen, a whorled pattern may be apparent on the cut surface. Intratumoral hemorrhage is rare, and necrosis is generally absent.

The distribution of intracranial meningiomas is approximately as follows: convexity (35%), parasagittal (20%), sphenoid ridge (20%), intraventricular (5%), tuberculum sellae (3%), infratentorial (13%), and others (4%). In this chapter we briefly mention the characteristics of the meningothelial (syncytial), transitional, and fibroblastic varieties. These meningiomas mimic the appearance of normal arachnoid villi in their ultrastructural features seen on electron microscopy: prominent interdigitation of the plasma membrane, abundant cytoplasmic intermediate filaments (10 nm) immunocytochemically consistent with vimentin, frequent hemidesmosomes to which the intermediate filaments anchor, and focal intercellular deposits of electron-dense granular material. Arachnoid and meningioma cells are connected by epithelial cadherins (E-cadherins), which are Ca²⁺-dependent adhesion molecules, and both express glutathione-independent prostaglandin D₂ synthase.⁹

Histologically, meningothelial (syncytial) meningiomas are characterized by densely packed cells arranged in sheets with no clearly discernible cytoplasmic borders (Fig. 131-2). Microscopically, they mimic normal arachnoid cells. Whorls can be found but are not prominent. Mineralized whorls, containing calcium apatite and collagen, are called psammoma bodies (from the Greek word psammos, meaning “sand”). Distinctive features of meningiomas include intranuclear cytoplasmic pseudoinclusions, in which an invaginated cytoplasmic remnant occupies the interior of the nucleus and displaces the nuclear chromatin. Another useful feature is the presence of so-called Orphan Annie’s eye nuclei—target-like nuclei with central clearing and peripheral margination of the chromatin.

FIGURE 131-2 Hematoxylin-eosin staining shows the histologic pattern of meningothelial meningiomas and polygonal cells with ill-defined cytoplasm.

Microscopically, fibroblastic (fibrous) meningiomas reveal multilaminated sheets of interlacing, elongated spindle cells. The intervening stroma is composed of reticulin fibers and collagen (Fig. 131-3). Transitional meningiomas represent a combination of the meningothelial and fibroblastic types. Characteristically, cellular whorls are seen, separated by elongated spindle cells (Fig. 131-4). Variations in meningioma histology may reflect mutations at separate genetic loci, in that the loss of heterozygosity on chromosome 22 is much more common in fibroblastic than in meningothelial variants.¹⁰

FIGURE 131-3 Hematoxylin-eosin staining shows a fibroblastic meningioma formed of sheets of elongated meningothelial cells.

FIGURE 131-4 Hematoxylin-eosin staining shows a transitional meningioma with cellular whorls and psammoma bodies.

Many other variants of meningioma have been reported, including psammomatous, angiomatous, microcystic (humid), xanthomatous, lipoblastic, myxoid (myxomatous), osteoblastic, chondroblastic, secretory,¹¹ melanotic, lymphofollicular, chordoid,¹² hemangiopericytic, oncocytic,¹³ and papillary meningiomas. Not all these terms for variants are currently in use.

It is beyond the scope of this chapter to discuss all these variants; however, it is important to briefly discuss the so-called hemangiopericytic variety. Sometimes meningiomas are composed partly or totally of small cells that grow focally in a hemangiopericytic pattern. The biologic behavior of this variant has not been well characterized. It is important to differentiate these so-called hemangiopericytic meningiomas (of meningothelial origin) from true hemangiopericytomas, which are mesenchymal tumors of nonmeningothelial origin. True hemangiopericytomas of the meninges are similar to those occurring in other parts of the body. Their behavior is characterized by early recurrence and a tendency to metastasize.

Atypical meningioma is associated with a higher rate of recurrence and aggressive growth. The criteria used to diagnose atypical meningioma are independent of meningioma subtype. Atypical meningioma exhibits increased mitotic activity or three or more of the following features: increased cellularity, small cells with a high nucleus-to-cytoplasm ratio, prominent nucleoli, uninterrupted patternless or sheetlike growth, and foci of spontaneous or geographic necrosis. For this variant, increased mitotic activity has been defined as four or more mitoses per 10 high-power fields (Fig. 131-5).⁷

FIGURE 131-5 Hematoxylin-eosin staining shows an atypical meningioma. A mitotic figure is seen in the middle.

The exact definition of anaplastic and malignant meningiomas is still open to discussion.¹⁴ One feature undoubtedly labels a meningioma as malignant: distant extraneural metastasis.¹⁵ The most common sites for metastasis are the liver, lungs, pleura, and lymph nodes. Frank parenchymal invasion of the underlying brain also carries an ominous prognosis. Anaplastic meningioma is a meningioma exhibiting histologic features of frank malignancy far in excess of the abnormalities present in atypical meningiomas. Such features include obviously malignant cytology (e.g., an appearance similar to sarcoma, carcinoma, or melanoma) or a high mitotic index (≥20 mitoses/10 high-power fields) (Fig. 131-6).

FIGURE 131-6 Hematoxylin-eosin staining shows a meningioma with obvious malignant cytology and a very high mitotic index.

Immunohistochemistry helps in the diagnosis of meningiomas. The test for epithelial membrane antigen (EMA) is positive in 80% of meningiomas. The results of S-100 staining are quite variable. Meningiomas also express markers for fibroblasts (vimentin) and epithelial cells (EMA and cytokeratins). Antileu 7, an antibody positive in schwannomas, is uniformly negative in meningiomas. Even though results of glial fibrillary acidic protein (GFAP) stains are negative in meningiomas, a few cases of GFAP-positive meningiomas have been reported in the world literature.¹⁶ Syncytial and transitional meningiomas express E-cadherin.¹⁷ Another use of immunohistochemistry lies in differentiating atypical meningiomas from similar but pathologically distinct tumors, such as secretory meningioma from metastatic carcinoma.¹¹

The nonhistologic diagnosis of meningiomas is being developed. In this technique, biopsy specimens are treated with perchloric acid and analyzed with high-resolution ¹H magnetic resonance spectroscopy and automatic amino acid analysis with ionic exchange chromatography. This method can accurately differentiate between meningiomas and other tumors involving the brain (e.g., high- and low-grade astrocytoma, medulloblastoma, metastasis, neurinoma, and oligodendroglioma).¹⁸

Hyperostosis is a characteristic finding in meningiomas, especially in en plaque meningiomas. In most cases, histologic studies of hyperostotic bone reveal tumor cells in the diploë and haversian canals (Fig. 131-7).¹⁹

FIGURE 131-7 Meningioma cells are invading a portion of hyperostotic bone.

Al-Mefty and coworkers²⁰ documented that meningiomas also go through tumor progression, which is defined by irreversible changes in tumor characteristics reflecting the sequential appearance of a genetically altered subpopulation of cells with the new characteristics.²¹ In some instances, the properties of advanced malignany may be established before the neoplasm reaches macroscopic size; in other cases, well-differentiated and slow-growing tumors may persist for years before shifting to more aggressive behavior. The progression of a tumor is explained by the model of clonal evolution: tumor development is initiated by a single cell carrying a mutation (the mutation model) that gives it a select growth advantage. The distinction between malignant transformation and de novo malignant tumor was addressed with a recent study demonstrating differences in clinical behavior, hormone receptor status, proliferative indices, and cytogenetic profile between these two subgroups of meningioma.²²

Extensive efforts have been made to determine the clinical behavior and malignant potential of meningiomas. These efforts have focused on different aspects of meningioma pathology: histology, labeling, karyotype and genetics, radiology, and hormone receptors. Some histologic features portend aggressive behavior. These features include hypercellularity, loss of architecture, nuclear pleomorphism, increased mitotic index, focal necrosis, hypervascularity, hemosiderin deposition, and small cell formation.¹⁴ Labeling techniques have been devised to quantify the mitotic rate of meningiomas and therefore predict their behavior. Bromodeoxyuridine must be injected intravenously shortly before tumor removal, and the surgical specimen must be fixed in 70% ethanol before being embedded in paraffin. Bromodeoxyuridine labeling allows the examiner to determine the percentage of cells in the S phase of mitosis. An alternative is immunohistochemical staining for proliferating cell nuclear antigen (Fig. 131-8). Fresh specimens can also be labeled for the monoclonal antibody Ki-67²³ or MIB-1, which can be done on any paraffin-embedded specimen. A high labeling index indicated by either method denotes a more aggressive tumor.^24,25 However, an elevated Ki-67 labeling index can paradoxically be seen in tumors that were previously irradiated.²⁶ Immunoreactivity for transforming growth factor-α has also been correlated with an increased level of malignant behavior.²⁴

FIGURE 131-8 Immunohistochemical staining of a meningioma for proliferating cell nuclear antigen. Note the high labeling index, which portends aggressive behavior.

Schwechheimer and coworkers²⁷ studied the expression of E-cadherin in a variety of brain tumors. Only choroid plexus papillomas (five of five) and meningiomas showed E-cadherin expression. In benign meningiomas, positive E-cadherin immunoreactivity was found regardless of the histomorphologic subtype or the tumor’s invasion into dura, bone, brain, or muscle. In contrast, E-cadherin was absent from most morphologically malignant meningiomas. In recurrent meningiomas, E-cadherin expression was identical to that in the primary neoplasm except in cases of malignant progression, in which the malignant recurrent tumor was negative for E-cadherin. In two cases of metastasizing meningiomas, no E-cadherin immunoreactivity was found in the primary tumors or in their metastases.²⁷

SPARC, a secreted, extracellular matrix–associated protein implicated in the modulation of cell adhesion and migration, was investigated as an aid to assess the invasiveness of meningiomas. It was not expressed in 9 benign meningiomas and highly expressed in 20 invasive tumors, regardless of grade. SPARC can be used to assess the invasiveness of histologically benign meningiomas in the absence of a tumor-brain interface.²⁸ Kitange and colleagues²⁹ studied the immunohistochemical expression of Ets-1 transcription factor and the urokinase-type plasminogen activator (u-PA) in meningiomas. The Ets-1 transcription factor is thought to play an important role in the invasive process of tumor cells through the induction of u-PA. A significant difference was observed between Ets-1 and u-PA expression in benign and high-grade meningiomas.²⁹ Complex karyotypes increase progressively from benign (34%) to atypical (45%) to anaplastic (70%) meningiomas.³⁰ The absence of progesterone receptors also correlates with poor outcome.³¹

Multiple meningiomas are defined as two or more meningiomas appearing simultaneously or sequentially in the same patient.³² The reporting of multiple meningiomas has increased with the advent of new imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI). The incidence of multiple meningiomas ranges from 1% to 16% of all meningiomas. Between 60% and 90% of patients with multiple meningiomas are women. Multiple meningiomas occur in association with neurofibromatosis type 2. They have also been described in families with no evidence of neurofibromatosis.³³ Multiple meningiomas may be secondary to a recurrence at the edge of the surgical resection or to postoperative seeding through the cerebrospinal fluid. This possibility is supported by the fact that recurrent meningiomas were found to be clonal with respect to the primary tumors.³⁴ Intraosseous meningiomas and extraneuraxial meningiomas are uncommon. All the reported intraosseous meningiomas have been in cranial bones. Extraneuraxial meningiomas can involve the orbit, paranasal sinuses, and nasopharynx. Sixteen percent of reported primary extraneuraxial meningiomas occurred in the skin and subcutis; others have been reported in the lungs,^35,36 mediastinum, and adrenal gland.

Tumors of the central nervous sysytem may metastasize to a primary intracranial tumor. Three fourths of these metastases target meningiomas, even though meningiomas represent only 20% of intracranial tumors. There are probably many reasons for this propensity, including the fact that patients with meningiomas, which are slow-growing tumors, are at greater risk for metastasis than patients with other brain tumors. Other factors may be the increased vascularity of meningiomas and the peculiar microenvironment of these tumors. Abscesses may also form within meningiomas.^37,38

Epidemiology

When discussing the epidemiology of meningiomas, a distinction must be made between studies dealing with only a limited population (hospital based) and those dealing with the population at large. A distinction must also be made between series including only operated cases and those including autopsies, and between series including metastases and those dealing exclusively with primary intracranial tumors. In their 1938 review Cushing and Eisenhardt⁶ found that meningiomas constituted 13.4% of Cushing’s series of intracranial tumors. In a population-based clinical study performed in Manitoba from 1980 through 1985, 22% of primary intracranial tumors were meningiomas.³⁹ The occurrence of meningiomas in the general population varies from 2.3 cases per 100,000 people during their life span to 5.5 per 100,000 if autopsy data are included.^39,40 In the Manitoba study, the incidence rates for meningioma were 1.5 and 3.1 per 100,000 for males and females, respectively.⁴⁰

A study performed in Rochester, Minnesota, covering the years 1935 through 1977 revealed the following distribution of primary brain tumors: gliomas 35%, meningiomas 40%, and pituitary adenomas 13% when data from postmortem examinations were included; gliomas 43%, meningiomas 21%, and pituitary adenomas 17% when postmortem data were excluded.³⁷ These data emphasize the fact that many meningiomas remain clinically silent and do not require surgery. Staneczek and Jänisch⁴¹ reported on 8119 new cases of meningioma diagnosed in the former German Democratic Republic between 1961 and 1986. Approximately one half of all these tumors were first discovered at autopsy. The crude annual incidence of meningiomas in this study was 1.85 per 100,000 in the general population. In 1985 Walker and coworkers⁴² reported on the epidemiology of brain tumors in the United States by surveying 166 hospitals. Among 13,720 patients with pathologically confirmed primary intracranial tumors seen during 1973 and 1974, 58% had gliomas, 20% had meningiomas, and 14% had pituitary adenomas.

The incidence of meningiomas increases with age. A slight dip after the eighth decade may be explained by several factors, including a less aggressive surgical approach in the elderly. Another factor is the failure to realize that even though the total number of cases in the ninth decade is less than in earlier decades, the incidence of meningiomas in this age group is actually higher. A meningioma may be found incidentally, and the surgeon must then weigh the risks and benefits of surgery against the natural history of the disease.^43,44 As Rengachary and Suskind⁴⁵ have written, “Some die from meningiomas, others with them. A neurosurgeon’s role is to recognize these two sets of populations and give the benefit of surgery to those who need it and spare those who do not.”

Most studies show a predominance of meningiomas in women. The female-to-male ratio is about 2 : 1. In the Manitoba study, the female predominance was noticeable in patients only after the fifth decade.³⁹ This pattern reflects meningnioma’s lack of predominance in female children.

Meningiomas constitute 1% to 4% of all brain tumors in children (younger than 18 years). They are extremely uncommon in infancy. The average age at presentation is 11.6 years, compared with 6.3 years for all other pediatric brain tumors. Several features distinguish meningiomas in children from their adult counterparts. There is an equal incidence of the tumor between girls and boys, but a male predominance (71%) has been reported among infants. Meningiomas in children may arise in unusual sites. Eleven percent of meningiomas in children are intraventricular, compared with 3.9% in adults. They may be multiple (23%) or have a cystic component (23%). They are often associated with neurofibromatosis (23% to 41%)⁴⁶ and have no dural attachments in up to 13% of cases.⁴⁷

Etiology

Irradiation

In 1953 Mann and colleagues⁵⁴ were the first to report a radiation-induced meningioma. The patient, a 6-year old girl, received 6500 rad after resection of an optic nerve glioma. A meningioma was diagnosed 4 years later within the radiotherapy field. There is no doubt that radiation injury is a factor in the development of meningiomas. In 1909 Adamson described a protocol for irradiation of the scalp to treat tinea capitis (ringworm). The method, referred to as the Kienböck-Adamson technique, delivers 450 to 850 rad to the scalp and between 70 and 175 rad to the surface of the brain. The protocol was widely used from 1900 until 1960, when griseofulvin was introduced. Modan and coworkers⁵⁵ carried out a statistical analysis of 11,000 children and found that meningiomas were four times more common in irradiated patients than in the control group. Although the mean age at diagnosis in the general population is 58 years, it is 45 years in the low-dose radiation group and 31 years in the high-dose radiation group. Even though the female predominance of intracranial meningiomas in the general population is less apparent (and may even be reversed) in the irradiated group, this pattern may be caused by a bias inherent in the population of patients irradiated for tinea capitis. Radiation-induced meningiomas have also been reported in patients who underwent high-dose radiotherapy (Fig. 131-9).⁵⁶ The age of patients at presentation with meningioma and the latency period of radiation-induced meningiomas are dose related. These tumors are more aggressive and are certain to recur, have a higher histopathologic grade, and are associated with complex cytogenetic aberrations, particulary involving 1p and 6q.⁵⁷

FIGURE 131-9 MRI of a radiation-induced meningioma in the left cavernous sinus after 20 years of radiation therapy for craniopharyngioma.

Several studies are being carried out to determine whether the use of mobile phones increases the risk of developing brain neoplasms. A study published in 2001 failed to find such a correlation, but the matter is not settled.^58,59

Genetic Aspects

The genetic aspects of meningiomas have been studied intensively during the past decade, but the final chapter of this research remains to be written. The detailed genetics of meningiomas are beyond the scope of this chapter, but it is possible to succinctly summarize the findings that most researchers agree on. Genetic alterations in the long arm of chromosome 22 play an essential role in the development of meningiomas. Monosomy of chromosome 22 has been observed in up to 50% of patients with meningiomas (Fig. 131-10). The meningioma chromosomal region has been localized to the center of the long arm of chromosome 22 in bands 22q12.3-qter. This is the same area that harbors the neurofibromatosis type 2 (NF2) tumor suppressor gene. This region also harbors SIS, the platelet-derived growth factor (PDGF)-β locus that is homologous to the SIS oncogene. Meningiomas may result from mutation in a tumor suppressor gene or from the SIS oncogene, both located on the long arm of chromosome 22. Whether the tumor suppressor gene responsible for meningiomas is the same NF2 tumor suppressor gene or distinct from it is still subject to debate. Evidence shows, however, that other gene alterations on chromosome 22 may give rise to meningiomas. With the addition of the rhabdoid variety of meningiomas to the WHO classification, it is important to note that a gene involved in the pathogenesis of rhabdoid tumors (the INI1 [SMARCB1/hSNF5] gene) is also mutated in a subset of meningiomas. The INI1 gene may be a second tumor suppressor gene on chromosome 22 important for the genesis of meningiomas.⁶⁴

FIGURE 131-10 Spectral karyotyping demonstrates monosomy of chromosome 22 in a meningioma.

Other chromosomal abnormalities have been detected in meningiomas. Loss of heterozygosity for loci on chromosome arm 1p is relatively common in meningiomas. However, different genes are involved in different tumors, raising the possibility of several tumor suppressor genes on 1p, the inactivation of which may be important in the pathogenesis of meningiomas.⁶⁵ Reviewing cases of meningiomas after radiation treatment, Shoshan and associates⁶⁶ found that 22q deletions are far less frequent in these tumors and that other chromosomal lesions, especially the loss of 1p, possibly induced by irradiation, may be more important in the development of these tumors. Muller and colleagues⁶⁷ found a net progression of chromosome 1 abnormalities in meningiomas according to their pathologic grade; 27% of the common type, 70% of atypical, and 100% of anaplastic meningiomas had a deletion of 1p36.

Lamszus and coworkers⁶⁸ studied gene alterations in five aggressively recurring meningiomas and four malignant nonmeningothelial meningeal tumors (three undifferentiated meningeal sarcomas and one hemangiopericytoma). They stated that “a total of 40 specimens from primary tumors and multiple recurrences of the nine patients were analyzed. … Loss of hetero-zygosity (LOH) at 22q was observed in all meningioma cases at the earliest time point. … While allelic loss at 22q appears to be an early event in aggressive meningioma disease, there is a clear correlation of further deletions on chromosome arms 1p, 9q, 10q, and 14q with histopathological and clinical progression” (Fig. 131-11).⁶⁸ None of these genetic findings was present in the nonmeningiomatous meningeal tumors, indicating that “meningothelial cells have their own lineage-specific genetic pathways towards clinical malignancy.”⁶⁸ Menon and associates⁶⁹ studied 58 meningiomas and found that a loss of chromosome 22 was seen in about half of all these tumors, regardless of their malignancy; however, the most frequent chromosomal losses observed in the malignant and atypical tumors were on the long arm of chromosome 14. Common secondary aberrations include losses or deletions of chromosomes 1p, 14q, and 10q and unstable chromosome aberrations, including rings, dicentrics, and telomeric associations. Despite the analysis of several hundred tumors with cytogenetic and molecular techniques, the mechanisms involved in the progression of chromosome aberrations in meningiomas are poorly understood. Sawyer and colleagues⁷⁰ concluded from their series that the progression of chromosome aberrations in meningiomas is mediated in some respects by telomeric and centromeric instability.

FIGURE 131-11 Representative Giemsa-band karyotype of a malignant meningioma. The most common aberration observed in meningiomas is monosomy for chromosome 22. Loss of chromosome 14, structural aberrations resulting in the loss of 1p, and deletion of 6q are found in higher grade tumors.

The role of the NF2 gene in the genesis of meningiomas deserves further analysis. The NF2 gene codes for a protein called merlin (moesin-ezrin-radixin-like protein). Merlin, also called schwannomin, has striking similarity to several proteins involved in the linkage of cytoskeletal components and proteins in the cell membrane. The alterations in this gene were found in four primary cultures of meningiomas (three of the four patients had a positive family history of neurofibromatosis).⁷¹ A low level of merlin seems to be associated with the development of a subset of meningiomas, usually caused by an alteration in the NF2 gene. Kimura and coworkers⁷² advanced an alternative explanation of the role of merlin in the pathogenesis of meningiomas. They found that even though the loss of merlin leads to the development of meningioma, low levels of this protein may result from a mutation in the NF2 gene or to a high turnover of merlin. The latter mechanism may be mediated by calpain, a calcium-dependent neutral cysteine protease, which leads to the degradation of merlin in these tumors.⁷² As its name implies, merlin is a member of the protein 4.1 family of membrane-associated proteins, which also includes ezrin, radixin, and moesin. Another protein 4.1 gene, DAL1, located on chromosome 18p11.3, was identified by Gutmann and associates.⁷³ These investigators showed a loss of DAL1 in 60% of sporadic meningiomas. They concluded that “analogous to merlin, we show that DAL-1 loss is an early event in meningioma tumorigenesis, suggesting that these two protein 4.1 family members are critical growth regulators in the pathogenesis of meningiomas.”⁷³ The same investigators demonstrated that merlin expression was also lost in some schwannomas and ependymomas.⁷⁴

The clonality of multiple and recurrent meningiomas has been studied. Several studies point to the possibility that recurrent and, more surprisingly, multiple concurrent meningiomas may be monoclonal in origin.^34,75

Meningiomas and Receptors

Meningiomas may become symptomatic during pregnancy, with the symptoms abating after parturition only to reappear with the next pregnancy. Symptoms may also be exacerbated during the proliferative phase of the menstrual cycle. It is unclear whether these exacerbations result from vascular engorgement or hormonal changes. In 1979 Donnell and colleagues⁷⁶ were the first to describe the role of estrogen receptors (ERs) in meningiomas. Since then, progesterone receptors (PRs) have been identified much more consistently than ERs in meningiomas. PRs are generally thought to be present in the cytoplasm of meningiomas, but they rarely occur in the nucleus. Maxwell and coworkers⁷⁷ were unable to detect ER messenger RNA (mRNA) but found PR mRNA in 88% and androgen receptor mRNA in 66% of the meningiomas they tested. Other investigators were able to show the presence of ER-β and ER-α mRNA in meningiomas.⁷⁸ Luteinizing hormone-releasing hormone was found to increase the proliferation of meningioma cells in vitro.⁷⁹ The preponderance of PRs and the scarcity of ERs in meningiomas are well known. The expression of PR alone in meningioma signals a favorable clinical and biologic outcome. A lack of receptors or the presence of ERs in meningiomas correlates with an accumulation of qualitative and quantitative karyotype abnormalities, a higher proportional involvement of chromosomes 14 and 22 in de novo tumors, and an increasing potential for aggressive clinical behavior, progression, and recurrence of these lesions. Sex hormone receptor status should routinely be studied for its prognostic value, especially in female patients, and should be taken into account in tumor grading. The initial receptor status of a tumor may change with progression or recurrence of tumor.⁸⁰

Meningiomas exhibit a very high density of somatostatin receptors. Preferential immunoreactive staining for the sst2A subtype somatostatin receptor has been shown in meningiomas.⁸¹

Androgen and glucocorticoid receptors have been found in meningiomas. The dopamine D₁ (but not D₂) receptor has also been demonstrated in meningiomas, and there are some indications that dopamine may play a role in the proliferation of these tumors. Meningiomas are also positive for prostaglandins, most notably prostaglandin E₂.⁸² Several growth factors have been shown to stimulate meningiomas, including epidermal growth factor, fibroblast growth factor, and PDGF.

Some meningiomas are associated with high systemic levels of substances such as carcinoembryonic antigen⁸³ or prolactin. In one study, prolactin receptors were detected in 61.7% of meningiomas, whereas no prolactin binding was found in samples of normal arachnoid tissue.⁸⁴ Meningiomas may interfere with glucose metabolism by increasing insulin levels. These disturbances of the endocrine system may be explained by mechanical pressure on a regulating intracranial structure or by the secretion of specific substances that interfere with hormonal homeostasis.

Radiology

Plain radiographs reveal three characteristic findings in meningioma patients: hyperostosis, increased vascular markings, and calcification. On non–contrast-enhanced CT, meningiomas are typically isodense to slightly hyperdense compared with contiguous brain parenchyma. Calcification may be seen. Meningiomas usually enhance homogeneously and intensely. The tumor is sharply marginated and is usually broadly based against a bony structure or dural margin. A common bony manifestation is hyperostosis. About 15% of benign meningiomas have a noncharacteristic appearance, including the presence of central lucency denoting necrosis or the presence of a cystic cavity (cystic meningioma). The amount of edema surrounding a meningioma is variable. The dura mater adjacent to the attachment of a meningioma may enhance on CT or MRI after the administration of a contrast agent. These so-called dural tails were studied histologically. Although only connective tissue and vascular tissue proliferation were seen in some cases, meningioma cell nests were identified in other cases.⁸⁵

On T1-weighted MRI, 60% of meningiomas are isointense and 30% are mildly hypointense compared with gray matter. On T2-weighted images, the tumors are isointense (50%) or mildly to moderately hyperintense (40%). Hyperintensity on T2-weighted images suggests a higher water content, denoting a meningothelial meningioma, a vascular meningioma, or an aggressive meningioma. However, it is suggestive of an easily suckable tumor during surgery. Meningiomas usually enhance intensely and uniformly after the injection of gadolinium, with typical dural tail enhancement.

Angiography may be an adjunct in the preoperative assessment of some meningiomas. It enables the surgeon to assess the vascularity and vascular supply of the tumor, the feasibility of embolization, and the presence of tumor encroachment on vascular structures. Meningiomas parasitize the adjacent blood supply (Table 131-2); knowledge of the vascular supply pattern allows the surgeon to gain early control of arterial feeders during surgery. Although there are no pathognomonic angiographic features of meningiomas, some typical findings have been delineated (Table 131-3). Preoperative embolization has not been of great benefit in the surgical treatment of meningiomas.⁸⁶

TABLE 131-3 Typical Angiographic Features of Meningioma

Somatostatin receptor scintigraphy using ¹¹¹In octreotide is an extremely sensitive test for meningiomas. It can be used preoperatively when the diagnosis is not straightforward. In the postoperative period, somatostatin receptor scintigraphy can be helpful in differentiating contrast uptake from residual tumor and that from nonspecific hyperperfusion.⁸⁷

Imaging techniques are being used to determine the histologic subtype or biologic behavior of meningiomas. Results are still preliminary, but it seems that aggressive meningiomas have higher metabolic rates than benign meningiomas, with the metabolic rate gauged by positron emission tomography using ¹⁸F fluorodeoxyglucose. Other investigative groups have focused their attention on different tumoral substrates using magnetic resonance spectroscopy.

Several lesions may mimic meningiomas on radiologic studies. Superficial metastasis and a variety of neoplasms may appear similar to meningiomas on routine radiologic work-up. Two entities that merit mention are neurosarcoidosis and solitary fibrous tumors. Sarcoidosis is a granulomatous disorder of unknown origin. It may be associated with systemic manifestations and is more common in African Americans. Rarely, neurosarcoidosis is the first or sole manifestation of this disease. The intracranial disease responds well to corticosteroids, although this improvement may not be apparent preoperatively. If the neurosurgeon embarks on surgery aimed at meningioma and is faced with an unexpected tumor appearance and texture, a frozen section unveils the real pathology. Although total resection is the surgery of choice for meningiomas, safe debulking followed by corticotherapy is recommended for sarcoidosis.⁸⁸ Solitary fibrous tumors of the meninges are tumors of mesenchymal origin and not of meningothelial lineage, as meningiomas are. They can be differentiated from meningiomas on immunohistochemical and ultrastructural grounds. They are positive for CD34 and vimentin but negative for EMA and S-100.⁸⁹ Ultrastructural features of meningiomas, such as complex interdigitation of cell processes and intercellular specialized junctions, are absent. The cells show the typical appearance of fibroblasts, with proximity of banded collagen and precollagen as well as cytoplasmic, rough-surfaced endoplasmic reticulum.⁹⁰

Surgical Therapy and Tumor Recurrence

The only definitive cure for meningioma is complete surgical resection. The more complete the resection, the less chance there is of recurrence.

In 1957 Simpson⁹¹ introduced a five-grade classification of the surgical removal of meningiomas (Table 131-4). Recurrence rates for grade I are about 10%; those for grade II are twice as high. The rates of recurrence are understandably higher in the higher Simpson grades. The inclusion of an additional 2-cm dural margin has been denoted grade 0 removal.⁹² In one study, no recurrences were seen in patients with convexity meningiomas in whom grade 0 resection was achieved; the mean follow-up period was 5 years and 8 months.⁹² In 1992 Kobayashi and associates⁹³ revised the Simpson grading system from a microsurgical perspective by introducing a classification system based on the extent of microscopic resection (Table 131-5).

TABLE 131-4 Simpson Grading System

Adapted from Simpson D. The recurrence of intracranial meningiomas after surgical treatment. J Neurol Neurosurg Psychiatry. 1957;20:22.

TABLE 131-5 Modified Shinshu Grade or Okudera-Kobayashi Grade

Adapted from Kobayashi K, Okudera H, Tanaka Y. Surgical considerations on skull base meningioma. Paper presented at the First International Skull Base Congress, June 18, 1992, Hanover, Germany.

The anatomic location of a meningioma influences its rate of recurrence. Tumors that are more difficult to remove totally, such as meningiomas of the sphenoid wing, recur more often. Meningiomas that invade a dural sinus, such as parasagittal meningiomas, have a high rate of recurrence. The recurrence rates of meningiomas differ from one series to another; the highest recurrence rates (>20%) are found in patients with sphenoid wing meningiomas, followed by those with parasagittal meningiomas (8% to 24%). The recurrence rate for convexity and suprasellar meningiomas is 5% to 10%.

Although well-delineated meningiomas can be totally removed, meningiomas with flat extensions into the subdural space (10% of meningiomas) are difficult to resect completely, as are en plaque meningiomas. The risk of recurrence is increased for meningiomas with aggressive pathologic features, such as invasion of the dura or brain infiltration. Other aggressive features include a papillary or hemangiopericytic pattern. Cellular criteria portending aggressive behavior include the presence of mitoses, increased cellularity, nuclear polymorphism, and focal necrosis. A high mitotic index is also an ominous sign. In an interesting article published by Yamasaki and colleagues,⁹⁴ 54 patients with supratentorial convexity meningiomas were examined at least 3 years after surgery or until tumor recurrence. Patients with multiple meningiomas, neurofibromatosis, and atypical and anaplastic meningiomas were excluded. All patients had undergone Simpson grade I resection. The correlation between recurrence and the following factors were statistically analyzed: age, gender, tumor volume, tumor shape, bone changes, brain edema, vascular supply, histologic subtype, MIB-1 labeling index, and vascular endothelial growth factor (VEGF). High levels of expression of VEGF constituted the most useful predictor of recurrence, followed by high MIB-1 labeling index.⁹⁴ Moller and Braendstrup⁹⁵ found, however, that proliferating cell nuclear antigen and Ki-67 are of minor value as predictors of the recurrence of benign meningiomas.

Nakasu and coworkers⁹⁶ studied 101 patients who underwent macroscopically complete removal of meningiomas. The patients were followed postoperatively for at least 5 years (maximal duration, 18 years) or until tumor recurrence. Fifteen meningiomas recurred during the follow-up period. Multivariate analysis revealed that only the shape of the tumor was significant; “mushrooming” and lobulated meningiomas were more likely to recur than round ones.⁹⁶

Nonsurgical Therapy

Nonsurgical therapies are used for recurrent or incompletely resected meningiomas. Standard or stereotactic irradiation has been used.^97,98 A comprehensive discussion of radiation therapy, both conformal fractionated radiotherapy and radiosurgery, is beyond the scope of this chapter, although the past decade has seen an increasing application of these two techniques both as intitial therapy and as adjuvants to surgical resection.^99–106 In terms of long-term control, however, the clinical recurrence rate after 15 years with subtotal resection and radiation therapy is 75%, and the rate of complications is 56%.¹⁰⁶ Radiation-induced complications and the possibility of radiation-induced malignancy or tumor progression are concerning issues.^107–112 Also concerning is the pattern of aggressive growth after failed radiosurgery.¹¹³

Acknowledging that the potential benefits of radiotherapy should always be balanced against its known side effects and complications, Guthrie and associates⁹⁷ concluded that “while surgical excision is the treatment of choice, radiation therapy should be considered: (1) after surgery for a malignant meningioma, (2) following incomplete resection of a meningioma for which the risk of resection of an eventual recurrence is judged to be excessive, (3) for patients with multiple recurrent tumors for whom the surgeon judges repeat surgery to be too risky, and (4) as a sole therapy of a progressively symptomatic patient with a meningioma judged by the surgeon to be inoperable.” Although the adjuvant role of stereotactic radiotherapy is gaining ground among neurosurgeons, its use as primary treatment for poorly accessible meningiomas (e.g., cavernous sinus or clival) is the subject of ongoing debate.^114,115 In particular, data reported by Rowe and coworkers¹¹² indicate only a 53% actuarial 15-year survival rate for patients with meningiomas treated with the Gamma Knife, and 67% of these patients died from their meningiomas.

The presence of hormonal receptors in meningiomas has prompted research into hormonal manipulation as treatment. Tamoxifen, an estrogen antagonist, was shown to stimulate meningioma cells in culture, perhaps because of its partial estrogen-agonistic activity. Oura and colleagues¹¹⁶ reported an anecdotal case of a patient with gastric carcinoma treated with mepitiostane, an antiestrogen agent. The patient also suffered from a presumed falcine meningioma discovered incidentally during the metastatic work-up. The patient was treated with mepitiostane for 2 years, and the meningioma had decreased by 73% at the end of this period.¹¹⁶ Bromocriptine, a dopamine antagonist, inhibits meningioma cells significantly in vitro. Other studies are assessing the efficacy of mifepristone (RU 486), an antiprogesterone agent, in the treatment of meningiomas. Mifepristone has progesterone- and cortisol-blocking activities; most patients initially complain of nausea, vomiting, or tiredness. Although preliminary results were encouraging, it is still too early to assess the efficacy of RU 486 in the treatment of meningiomas. Other antiprogesterone agents such as gestrinone have been used in the medical treatment of meningiomas.¹¹⁷ Other antineoplastic drugs, including hydroxyurea^118,119 and interferon alpha-2B,¹²⁰ have also been used.

Trapidil, a drug with known antagonistic action against PDGF, showed a dose-dependent inhibition of meningioma cell proliferation.¹²¹ Trapidil significantly inhibited the epidermal growth factor–stimulated proliferation of meningiomas as well as nontumorous fibroblasts. When trapidil is administered in conjunction with bromocriptine, there is an additive inhibitory effect on meningioma cell growth that is more pronounced than when either drug is used alone.

Animal trials are being conducted to assess the possible use of a growth hormone receptor antagonist (pegvisomant) as a medical treatment for meningiomas.¹²² Although progress in treating meningiomas without surgery has been remarkably steady, much detailed and arduous work remains.

Meningioma Types and Their Surgical Management

Convexity Meningiomas

Convexity meningiomas are characterized by their dural attachment; they lie on the hemispheres without extension into the dura mater of the skull base and in general do not invade the dura of the venous sinus. Convexity meningiomas constitute approximately 15% of all meningiomas.^123–127

Cushing and Eisenhardt⁶ subclassified convexity meningiomas as precoronal, coronal, postcoronal, paracentral, parietal, occipital, and temporal. The clinical presentation is related to the specific location of the meningioma on the convexity. However, it is not uncommon for these lesions to attain a significant size before symptoms develop. Seizures and incidental findings on imaging remain the most common forms of presentation.

As noted earlier, Simpson graded the extent of resection of meningiomas in attempt to correlate the risk of recurrence.⁹¹ For convexity meningiomas, Al-Mefty advocated that an additional 2-cm margin of dura be removed—called grade 0 removal. Using this strategy, no recurrence was reported after the resection of 37 meningiomas (Fig. 131-12).⁹²

FIGURE 131-12 Grade 0 removal that includes the tumor, a margin of normal dura, and the involved bone. A, This coronal cut shows extension of the tumor through the bone under the galea. B, The axial image shows the extent of the dural tail. C, Postoperative computed tomography demonstrates grade 0 removal.

The diagnosis of convexity meningioma can be made superbly with CT or MRI, which shows the lesion, any dural extension or tail, and underlying involved bone. The underlying bone, which may be involved extensively, appears as a palpable mass or hyperostosis on the radiograph. Occasionally, this appearance is radiolucent. A bony window CT scan is complementary, particularly with regard to bone involvement. These tumors typically derive their blood supply centrally from the middle meningeal branches and peripherally from the intracranial vessels. Angiography is done only if embolization is contemplated. The external carotid injection shows the typical sunburst pattern; the internal carotid injection shows arterial displacement and the pial blood supply.

The technical aspects of removing convexity meningiomas are based on the following principles:

1 A series of bur holes circumscribes the tumor completely and is made far from the tumor margin to obtain healthy dura around the tumor.

2 The dura is opened circumferentially around the tumor 2 cm from the margin of the lesion.

3 With use of the operating microscope, the tumor capsule is dissected from the cerebral cortex. Maintaining dissection within the arachnoid plane is crucial to preserve the cortical layer.

4 Cortical vessels underneath the tumor should be preserved.

5 The tumor is then removed en bloc (Fig. 131-13).

6 In some larger, more adherent tumors, central enucleation under the microscope is needed, followed by the preceding steps.

FIGURE 131-13 After the arachnoid plane around the tumor is established, dissection should take place in this plane outside the pia. The arterial and cortical veins are separated, and surgical patties are gently introduced. This technique, which is used circumferentially around the lesion, delivers the tumor en bloc with little blood loss. The most important technical point is to preserve all cortical veins and arteries and avoid violating the pia. The tumor is then lifted from its bed.

Parasagittal Meningiomas

Cushing and Eisenhardt⁶ defined a parasagittal meningioma as one that fills the parasagittal angle, with no brain tissue between the tumor and the superior sagittal sinus. They defined falcine tumors separately, whereas other investigators, such as Olivecrona, Elsberg, and Merrem, grouped all parasagittal meningiomas with falcine meningiomas.^128–131

In 1922 Cushing³ reported that meningiomas accounted for 11.3% of his 751 intracranial tumors, and 32% of these tumors were parasagittal meningiomas. Subsequent large series have shown that parasagittal meningiomas account for 17% to 32% of all meningiomas.^{129,130,132–137}

In reviewing 154 patients with parasagittal meningiomas, Gauthier-Smith¹³⁴ found that at presentation 62% had seizures, 54% had headaches, 49% had unilateral weakness, and 43% had mental symptoms.

The technique of parasagittal meningioma resection is based on the following principles:

1 A bicoronal incision is preferred because it allows maximal vascularity to the skin, especially if subsequent craniotomies are performed.

2 A pericranial flap is reflected separately.

3 Multiple bur holes are made in close approximation to one another at the periphery of the tumor.

4 Bur holes straddling the superior sagittal sinus allow safe separation of the dura from the bone.

5 Microsurgical separation of the tumor capsule from surrounding cortex is performed while preserving the vessels overlying the normal cortex.

If the tumor has grown into the lateral aspect of the superior sagittal sinus, but the sinus is not occluded (Fig. 131-14), there are three choices. The first is to ligate the sinus. This option carries the risk of venous infarction¹³⁸ and can be performed only in the anterior third. The second is to leave the portion of the tumor attached to the sinus in place, with the understanding that this will most likely grow and possibly cause slow occlusion of the sinus with the formation of collateral venous channels, allowing easier resection in the future. The third option is to resect the portion of the sinus involved and then repair the sinus primarily or with a patch.

FIGURE 131-14 MRI of a parasagittal meningioma involving one wall of a patent sinus.

Choosing the appropriate method of handling the involved sinus constitutes the most important decision in the treatment of falcine and parasagittal meningiomas. One or two walls of the sinus can be excised, and reconstruction still provides a good patency percentage. Technically, total sinus replacement with a venous graft can also be successful; however, the long-term results of this maneuver show patency rates of only 50%. The potential for delayed occlusion of a totally replaced sinus graft makes us hesitant to excise and replace a patent sinus in the posterior third or in the posterior part of the middle third (Fig. 131-15).¹³⁹ We have been impaired by the presence of a cleavage plane between the venous channels and the invaded outer sinus wall, making tumor removal difficult while preserving the venous conduit. Decisions regarding the sinus, however, should be individualized for each case according to several factors: the patient’s age and symptoms, patency of the sinus, location of the tumor, and cortical venous collateral system. A truly occluded sinus can be totally excised at any point. The importance of preserving collateral venous channels cannot be overemphasized and is a vital part of the operation. The anterior third of the sinus can be excised with or without a graft or replacement. Tumor infiltration of one wall can be repaired primarily after the tumor is removed from the sinus.

FIGURE 131-15 Parasagittal meningioma with a patent posterior third of the sagittal sinus and hyperostosis invading the skull. The hyperostotic, invaded bone is divided into pieces before it is removed. The tumor is debulked with an ultrasonic aspirator, and the capsule is separated from the cerebral cortex. The pial plane is maintained, and utmost attention is paid to preserving the cortical veins. The tumor capsule is then amputated along the sinus, and a piece of tumor invading the sinus is left.

Olfactory Groove Meningiomas

The anatomy and pathology of olfactory groove meningiomas and the surgical prinicples realted to their management were eloquently described by Cushing⁶ in 1938. In this monograph, based on his management of 29 patients with olfactory groove meningiomas, Cushing emphasized the importance of tumor decompression before tumor capsule dissection and the importance of preserving the anterior cerebral arteries and their branches, which may be adherent to the tumor capsule. These tumors tend to grow slowly and gradually compress the frontal lobes. Because of a lack of focal functional areas in the inferomesial frontal lobes, these meningiomas may attain a large size before manifesting clinically. The diagnostic method of choice is MRI in multiple planes, which delineates the tumor and surrounding edema and shows tumor extension into the ethmoid, sphenoid, and nasal cavities. MRI also identifies the anterior cerebral artery and its relationship to the posterior pole of the tumor (Fig. 131-16). Magnetic resonance angiography further delineates the tumor’s relationship to the cerebral vasculature, including arterial displacement. CT, however, is superior in showing calcification and skull base invasion.

FIGURE 131-16 Sagittal MRI of an olfactory groove meningioma. A, Preoperative MRI demonstrates the giant size of the tumor and displacement of the anterior cerebral artery. B, MRI after total removal and reconstruction of the floor with a pericranial flap.

The tumor’s primary blood supply is from the anterior ethmoidal artery, and preoperative embolization is usually not performed. Angiography is no longer necessary in most cases.

These tumors may extend into the ethmoid and down into the nasal cavity. Although removal of the extracranial portion can be delayed and performed in a second operation, it is frequently preferable to remove intracranial and extracranial parts of the tumor in one operation.

If the sphenoid sinus or nasal cavity is entered during surgery, the mucosa is exenterated, the sinus is packed with fat, the dura is grafted, and a pericranial flap is laid on the floor of the frontal fossa to prevent leakage of cerebrospinal fluid. Late recurrence of olfactory groove meningioma is significant and is believed to be due to conservative handling of the underlying invaded bone. The pattern of recurrence toward the ethomoid and nasal cavity reconstruction of the skull base requires a special technique.¹⁵²

Two surgical approaches to olfactory groove meningiomas are traditionally used: the unilateral pterional approach and the subfrontal approach. We prefer the supraorbital approach, which includes the orbital rim with the frontal flap. It provides superb low basal exposure and lessens the need for retraction of the frontal lobe. This approach is our preference for treating a tuberculum sellae meningioma. Olfactory groove meningiomas are removed through the supraorbital bifrontal approach. It allows a wider corridor to control tumor vascularity, enucleate the tumor, resect the basal dura, and repair the anterior cranial base (Fig. 131-17).^153,154

FIGURE 131-17 Exposure of an olfactory groove meningioma through the supraorbital bifrontal approach. After devascularization, the tumor is debulked to a thin capsule that is then dissected carefully from the anterior cerebral artery, optic apparatus, and cortex.

Sphenoid Wing and Clinoidal Meningiomas

Sphenoid wing and clinoidal meningiomas are classified based on their origin along the sphenoid wing as clinoidal, middle, and lateral sphenoid wing lesions. Meningiomas en plaque also occur in the area of the sphenoid wing. This is characterized by marked hyperostosis of the sphenoid bone and diffuse tumor invasion, leading to proptosis or cranial neuropathies caused by foraminal encroachment. The surgical approach to this type of meningioma is complete removal of the greater sphenoid wing, anterior clinoid, and superior and lateral orbital walls, followed by generous removal of the affected dura. Careful dural repair with autologous graft is undertaken, followed by aesthetic reconstruction by cranioplasty.^155,156

Lateral sphenoid wing meningiomas can be resected after extradural drilling of the sphenoid ridge, which also aids in devascularizing the tumor. Meningiomas that arise from middle third of the sphenoid wing are also resected after extradural drilling of the sphenoid wing. For tumors that involve the orbit and superior orbital fissure and grow toward the cavernous sinus, a cranio-orbital zygomatic craniotomy may be the optimal approach.

The more medially located clinoidal meningiomas represent a distinct clinicoanatomic entity, and historically, resection of these lesions has been associated with significant morbidity and mortality. Advances in cranial base surgery have enabled more complete resection with lower risks of morbidity and mortality. Al-Mefty¹⁵⁷ classified clinoidal meningiomas into three categories based on the anatomic site of origin and degree of surgical difficulty (Fig. 131-18). After emerging from the cavernous sinus inferior and medial to the anterior clinoid process, the carotid artery enters the subdural space, where it lacks an arachnoid covering over a distance of 1 to 2 mm. The carotid artery then enters the carotid cistern and is invested in arachnoid. If the meningioma’s origin is proximal to the carotid cistern (type I), as is the case in a meningioma originating at the inferior aspect of the anterior clinoid process, the tumor will enwrap the carotid artery, directly adhering to the adventitia without an interfacing arachnoid membrane. As the tumor grows, this direct attachment to the vessel wall advances to the carotid bifurcation and along the middle cerebral artery, pushing the arachnoid membrane ahead of it. This anatomic arrangement accounts for the surgeon’s inability to dissect the tumor from the carotid artery and the middle cerebral branches.

FIGURE 131-18 Anterior clinoid meningioma encases the carotid artery. A, Type I with severe adherence to the carotid. B, Type II with the presence of an interfacing arachnoid plane.

Type II clinoidal meningiomas originate from the superior or lateral aspect of the anterior clinoid process above the segment of the carotid artery that has already been invested in the arachnoid of the carotid cistern. As this tumor grows, the arachnoid membrane of the carotid cistern and, more distally, of the sylvian cistern separates the tumor from the arterial adventitia. This plane allows dissection of the tumor from the vessels, even though they may be entirely engulfed. In type I and II tumors, the optic chiasm and nerves are wrapped in the arachnoid membrane of the chiasmatic cistern, allowing microsurgical dissection of the tumor from these structures.

Type III clinoidal meningiomas originate at the optic foramen and extend into the optic canal and at the tip of the anterior clinoid process. These tumors are generally small, presenting early with optic nerve compression and decreased visual acuity. The arachnoid membrane investing the carotid artery is present. Because this tumor arises proximal to the chiasmatic cistern, however, there may be no arachnoid investment between the optic nerve and the tumor.¹⁵⁷

We recommend the cranio-orbital zygomatic approach for the resection of clinoidal meningiomas. This provides the surgeon with a low-based approach and multiple avenues of dissection, minimal brain retraction, and the ability to enter the cavernous sinus if necessary (Fig. 131-19).

FIGURE 131-19 Exposure of a clinoidal meningioma through the cranio-orbital zygomatic approach. After splitting the sylvian fissure, the distal branches of the middle cerebral artery are identified and followed toward the encased carotid bifurcation.

(From Al-Mefty O. Operative Atlas of Meningiomas. Philadelphia: Lippincott-Raven; 1998:160.)

Tuberculum Sellae Meningiomas

Tuberculum sellae meningiomas account for 5% to 10% of intracranial meningiomas.^158–162 The mean age at diagnosis is in the fourth decade, and the tumor affects three times as many women as men.^163–165 The chiasmal syndrome, a primary optic atrophy with bitemporal field defects in adult patients with essentially normal sellae, has been the classic presentation of this tumor since its recognition in 1927 by Holmes and Sargent.¹⁶⁶

In most patients visual loss has an insidious onset, and the course is progressive. It may, however, be acute or fluctuating.^161,163,164 Approximately two thirds of patients complain of failing vision in one eye as the first symptom, and monocular blindness may be present in half of patients before surgery.¹⁶⁵ Incongruity and asymmetry of the field defect are common findings.

The classic and most common funduscopic finding is primary optic atrophy, which is asymmetric in both eyes and different in degree.¹⁶⁷ Anosmia rarely manifests in suprasellar meningioma cases.¹⁶⁷ Poppen¹⁶⁸ observed that olfactory groove meningiomas manifest with anosmia early on and that visual symptoms are a late finding; in contrast, patients with suprasellar meningiomas present with a history of visual deficits, and anosmia is a late finding. The tumor frequently extends in one or two optic canals (Fig. 131-20). Mental changes (e.g., memory impairment, personality changes, depression, anxiety) are present in 10% of cases.^159,169

FIGURE 131-20 Tuberculum sellae meningiomas frequently extend to both optic canals.

Diaphragma sellae meningioma is a separate clinicoanatomic entity, although in the literature, it is usually grouped with the tuberculum sellae tumor. This tumor usually grows retrochiasmatically and manifests with hypothalamic dysfunction. Kinjo and colleagues¹⁷⁰ classified these lesions into three categories: type A originates from the upper leaf of the diaphragma sellae, anterior to the pituitary stalk; type B originates from the upper leaf of the diaphragma sellae, posterior to the pituitary stalk; and type C originates from the inferior leaf of the diaphragma sellae. Careful evaluation of radiologic studies differentiates these tumors from pituitary tumors, assisting in selection of the appropriate cranial approach. Diaphragma sellae meningiomas are technically more difficult to remove than tuberculum sellae meningiomas because of the deep location and the difficulty of dissecting them from the pituitary stalk. Surgical resection of tuberculum sellae meningiomas is based on a supraorbital approach that allows minimal brain retraction and early control of arterial feeders. Tuberculum sellae meningiomas typically displace both optic nerves outward and backward, often to the extent that the optic nerve lies above and lateral to the internal carotid artery, with the chiasm stretched far back from the tuberculum sellae (Fig. 131-21).

FIGURE 131-21 Exposure of a tuberculum sellae meningioma through the supraorbital approach, depicting preservation of the olfactory nerve, lateral displacement of the optic nerves, and posterior displacement of the chiasm.

(From Al-Mefty O. Operative Atlas of Meningiomas. Philadelphia: Lippincott-Raven; 1998:40.)

Identifying the optic nerve can be quite difficult if it has been completely engulfed by the tumor or distorted to an almost unrecognizable, thin band in the tumor capsule. After debulking, the tumor is slowly stripped from the flattened and engulfed nerve. Despite apparent encasement or severe adherence, a plane of dissection can be obtained under high magnification.¹⁷¹ The A1 segments in particular are usually severely stretched, adherent, or engulfed and can be the source of morbid injury. Arterial supply to the optic nerves and chiasm should be preserved by the same method of tumor dissection.¹⁷¹ When the tumor extends into the optic canal, the anterior clinoid process, optic canal, and roof of the superior orbital fissure are drilled away with the diamond bit of a high-speed air drill. The dura propria is then opened. Tumor tissue around the optic nerve is removed by microdissection, and the surgeon must pay particular attention to preservation of the ophthalmic and central retinal arteries.¹⁷¹ Recent reports detail the use of a transnasal endoscopic approach to these lesions,^172,173 but the safety and efficacy of this approach have yet to be proved.

Cavernous Sinus Meningiomas

In 1965 Parkinson¹⁷⁴ was the first to systematically address the cavernous sinus and its anatomy specifically for vascular and neoplastic lesions. Advances in skull base and microvascular surgery over the past two decades have made it possible to safely resect lesions within the cavernous sinus.

Meningiomas involving the cavernous sinus may start primarily within it, or the cavernous sinus may be involved by extension from a clinoidal, sphenoid wing, tuberculum sellae, or sphenopetroclival meningioma. Detailed study of the internal carotid artery and collateral system is important before surgical excision of a cavernous sinus meningioma. Magnetic resonance angiography, conventional angiography, and balloon test occlusion in association with cerebral blood flow studies such as xenon CT or radioisotope spectroscopy are among the tools available to study the cerebral blood flow dynamics and physiology.

The surgical approach to the cavernous sinus is based on the cranio-orbital zygomatic approach. Proximal and distal control of the internal carotid artery is required before dissection within the cavernous sinus. Proximal control can be achieved by exposing the internal carotid artery within the petrous bone in the middle fossa or exposing the cervical internal carotid artery.

Entry into the cavernous sinus can be achieved through the medial or lateral triangles (Fig. 131-22). Dissection of the tumor progresses in a stepwise fashion, beginning by opening the dura propria of the optic nerve sheath longitudinally along the length of the optic canal. The distal dural ring is opened next, with the opening extending posteriorly to the oculomotor trigone, thereby also opening the proximal dural ring and allowing a wide entry into the anterior and superior cavernous sinus space. The carotid artery can be mobilized laterally by releasing it from its proximal and distal dural rings, which allows dissection in the medial cavernous sinus space. Lateral entry into the cavernous sinus is achieved by elevating the outer dural layer of the lateral wall of the cavernous sinus, which is peeled away. The internal carotid artery can be located by dissection between cranial nerves III and IV and the first division of the trigeminal nerve (Parkinson’s triangle). The course of cranial nerve VI, which runs lateral to the internal carotid artery and is generally directly apposed to it, is usually parallel but deep to cranial nerve V₁. The tumor is removed from within the cavernous sinus space by suction, bipolar coagulation, and microdissection. A plane of cleavage along the carotid artery can be developed in half these cases. Venous bleeding, typically not a problem when the tumor fills the sinus, may occur as the venous plexus is decompressed by tumor removal. In that event, homeostasis can be obtained by packing the cavernous sinus space with oxidized cellulose or a similar hemostatic agent.

FIGURE 131-22 Exposure of a sphenocavernous meningioma through the cranio-orbital zygomatic approach. The anterior clinoid and superior orbital fissure are opened. The cavernous sinus is entered from the superior and lateral walls.

(From Al-Mefty O. Operative Atlas of Meningiomas. Philadelphia: Lippincott-Raven; 1998:180.)

In our series, gross resection of cavernous sinus meningioma was possible in 76% of patients.¹⁷⁵ The surgical major morbidity and mortality rates were 4.8% and 2.4%, respectively. Preoperative cranial nerve deficits improved in 14%, remained unchanged in 80%, and permanently worsened in 6%. Because of the oculomotor function outcome after operating in the cavernous sinus, some authors shy away from the surgical removal of cavernous sinus meningiomas and resort to radiosurgery.¹⁷⁶

Posterior Fossa Meningiomas

Ten percent of all intracranial meningiomas arise in the posterior fossa. Almost half of these meningiomas are located in the cerebellopontine angle (CPA), 40% are tentorial or in the cerebellar convexity, 9% are clival, and 6% are at the foramen magnum.^194–196 Meningiomas arising medial to the trigeminal nerve (petroclival meningiomas) must be differentiated from those arising lateral to it (CPA or posterior petrous pyramid meningomas), because petroclival meningiomas carry a significantly higher rate of surgical morbidity.^194,197,198

Cerebellopontine Angle Meningiomas

Cranial nerve findings are quite common with meningiomas in the CPA. Hearing loss, facial pain or numbness, and facial weakness or spasm are common, as are headaches and cerebellar hemispheric signs.¹⁹⁹

The cranial nerves of the posterior fossa have relatively constant relationships to CPA meningiomas. The trochlear nerve is usually superior and lateral to the tumor, the trigeminal nerve is superior and lateral to it, and the trigeminal nerve is superior and anterior to the tumor; the abducens nerve is found anteriorly, whereas cranial nerves VII and VIII (contrary to petroclival meningiomas) are anterior and cranial nerves IX through XI are inferior.²⁰⁰

A retrosigmoid approach usually allows sufficient exposure for the removal of these tumors^197,201; however, exposure of the presigmoid dura is generally performed to allow retraction of the sigmoid sinus laterally and decrease its obstruction of the surgeon’s view. The meningioma’s dural attachment is to the posterior pyramid (Fig. 131-23), which is progressively coagulated and divided to devascularize the tumor; this must be done with care to avoid injury to the exiting cranial nerves. If the tumor’s size precludes its safe removal, the tumor capsule should be opened and the tumor centrally debulked and devascularized (Fig. 131-24). The capsule is carefully dissected from the surrounding cranial nerves, brainstem, superior cerebellar artery (superior and medial), anterior inferior cerebellar artery (medial), and posterior inferior cerebellar artery (inferior and medial). After the tumor is removed, the dural attachment should be removed or coagulated (with the bipolar coagulation or laser), and any hyperostotic bone is drilled away, keeping in mind the location of the nearby inner ear structures.

FIGURE 131-23 MRI of a cerebellopontine angle meningioma with insertion on the posterior pyramid.

FIGURE 131-24 Exposure of a cerebellopontine angle meningioma through a retrosigmoid approach with a skeletonized sigmoid sinus from a lateral exposure.

(From Al-Mefty O. Operative Atlas of Meningiomas. Philadelphia: Lippincott-Raven; 1998:322.)

Petroclival Meningiomas

Cushing and Eisenhardt⁶ recognized that petroclival meningiomas are a formidable entity because they involve the supratentorial and infratentorial compartments. Differentiation among clival, petroclival, and sphenopetroclival meningiomas is based on the surgical anatomy of the lesion. Tumors that arise from the superior two thirds of the clivus and displace the brainstem posteriorly are considered clival meningiomas. Petroclival meningiomas also involve the superior two thirds of the clivus and are located medial to cranial nerve V (Fig. 131-25), with the bulk of the tumor being more lateral along the spheno-occipital synchondrosis. In the latter group, the brainstem and basilar artery are typically displaced to the contralateral side. Sphenopetroclival meningiomas have characteristics similar to those of petroclival meningiomas, but the sphenoid forms also invade the lateral wall of the cavernous sinus along the medial sphenoid wing.

FIGURE 131-25 MRI of a sphenopetroclival meningioma originating medial to cranial nerve V.

Headache and ataxia from cerebellar compression are the most frequently identified clinical findings. Long-track signs, spastic paresis, and cranial neuropathies are also common findings. The natural history of these tumors is such that, if left untreated, the larger ones almost uniformly cause death. However, surgical treatment of these lesions before the 1980s was associated with very poor outcomes, with the operative mortality rate exceeding 50%.^202–204

With advancements in microsurgical technique, neuroanesthesia, and intensive care unit management and improved knowledge of skull base anatomy, surgical resection of these lesions is now feasible with acceptable morbidity and mortality risks.^{198,205–211} However, the demanding surgical undertaking, the burden of treating potential complications, and the risk of cranial nerve deficits that may negatively influence the patient’s quality of life have enticed some authors to adapt a philosophy of subtotal resection frequently augmented with radiotherapy or radiosurgery.^212–215 The past decade has seen an increase in the radiosurgical treatment of petroclival meningiomas, both as initial therapy and as an adjuvant to surgical resection.^100,103 However, the concern is that radiation is associated with significant complications, long-term failure, and the potential induction of malignancy. The option of partial removal followed by radiation, if used deliberately and routinely, bypasses the best opportunity to achieve curative surgical removal.^216,217

The principles of petroclival meningioma resection are based on using a lateral skull base approach while avoiding brain retraction; avoiding venous injury, especially the vein of Labbé; and connecting the middle and posterior fossa by drilling the petrous ridge and splitting the tentorium. The main approaches considered for these lesions are the posterior petrosal approach (presigmoid transtentorial; Fig. 131-26),¹⁹⁸ the anterior petrosal approach to the middle fossa through petrosectomy,²¹⁸ or a combination of both (Fig. 131-27).²¹⁹ In the case of hearing loss, a total petrosectomy is performed.

FIGURE 131-26 Exposure of a petroclival meningioma through the petrosal (presigmoid transtentorial) approach.

(From Al-Mefty O. Operative Atlas of Meningiomas. Philadelphia: Lippincott-Raven; 1998:304.)

FIGURE 131-27 Exposure of a clival and petroclival meningioma through a double petrosal (posterior presigmoid) approach, with anterior drilling of the petrous apex.

(From Cho CW, Al-Mefty O. Combined petrosal approach to petroclival meningioma. Neurosurgery. 2002;51:713.)

Jugular Foramen Meningiomas

Although basal posterior fossa meningiomas often extend into the jugular fossa, primary jugular foramen meningiomas are rare, with fewer than 40 cases reported in the literature as of 2002.²²⁰ They are presumed to arise from arachnoid cells lining the jugular bulb. They may compress the lower cranial nerves; invade the temporal bone; invade, compress, narrow, or obstruct the jugular bulb; or extend into the posterior cranial fossa or extracranially. They manifest with lower cranial nerve deficits.

The preoperative radiologic diagnosis and the differential diagnosis are important for jugular fossa lesions because their preoperative management and operative planning differ considerably. Two common differential diagnoses for lesions in the jugular fossa are glomus jugulare tumors and neuromas of the lower cranial nerves. Meningiomas are differentiated by their characteristics on MRI and associated hyperostosis on CT (Fig. 131-28). By virtue of the tumor’s involvement of the jugular fossa and beyond, removal of the lesion requires the skull base approach. The patency and dominance of the involved jugular bulb, however, dictate the surgical approach. The procedure can be performed by opening the jugular bulb itself (transjugular approach, commonly used in glomus tumor resections) or by maintaining the integrity of venous flow through the jugular bulb (suprajugular or retrojugular approach), depending on the extension of the tumor (Fig. 131-29).²²⁰

FIGURE 131-28 A, MRI of a jugular foramen meningioma. B, Computed tomography shows hyperostosis.

FIGURE 131-29 Various avenues to removing a jugular foramen meningioma.

(From Arnautovic KI, Al-Mefty O. Primary meningiomas of the jugular fossa. J Neurosurg. 2002;97:13.)

Forman Magnum Meningiomas

Foramen magnum meningiomas may originate at any location on the foramen perimeter. They are divided into two main types: craniospinal (originating intracranially and extending downward) and spinocranial (originating in the upper spinal canal and extending intracranially).^6,221 Ventral foramen magnum meningiomas originate from the basal groove at the lower third of the clivus, anterior to the medulla, and project inferiorly toward the foramen magnum (Fig. 131-30). Spinocranial meningiomas, which originate from the upper cervical area, are usually posterior or posterolateral to the spinal cord and project superiorly into the cerebellomedullary cistern.

FIGURE 131-30 Sagittal MRI of a ventral foramen magnum meningioma.

Patients with lesions in this location may present with unusual symptoms and are often misdiagnosed. Cervical pain, usually unilateral; motor and sensory deficits, especially involving the upper extremities and, in later stages, progressing to spastic quadriplegia; and cold, clumsy hands with intrinsic hand atrophy constitue the well-documented clinical triad that identifies foramen magnum meningiomas.

Lateral or posterior foramen magnum meningiomas can be resected using a standard inferior suboccipital approach. Ventral foramen magnum meningiomas, however, can pose a formidable challenge because of the involvement of the lower cranial nerves and vertebral-basilar artery complex and significant brainstem compression. We think these tumors are best resected using the transcondylar approach (Fig. 131-31).^221–226

FIGURE 131-31 Exposure of a ventral foramen magnum meningioma through the transcondylar approach, with mobilization of the vertebral artery. (From Arnautovic KI, Al-Mefty O, Husain M. Ventral foramen magnum meningiomas. J Neurosurg. 2000;92[Suppl]:72.) JB, jugular bulb; JV, jugular vein; PICA, posterior inferior cerebellar artery; SCS, subocciptial cavernous sinus; SS, sigmoid sinus; VA, vertebral artery.

In up to half of patients, a meningioma at the foramen magnum encases the intracranial segment of the vertebral artery. The tumor can also displace this intracranial segment of the vertebral artery backward and laterally. In many instances, the tumor around the vertebral artery can be separated from the vessel because of an intervening arachnoid membrane. If the tumor is located laterally, the vertebral artery is hidden beneath it. In patients with ventrally located meningiomas, the vertebral artery is at the lateral aspect of the tumor. The posterior inferior cerebellar artery is usually displaced dorsally or medially; it may also be embedded in the tumor. The anterior and posterior spinal arteries usually adhere to the tumor. Intra-arachnoidal dissection allows theses arteries to be preserved.

The brainstem and cervical spinal cord are displaced posteriorly and to the contralateral side by an anteriorly or anterolaterally located meningioma. This displacement facilitates the exposure and approach to the most anterior segment and the tumor’s dural attachment. Early debulking of the tumor decreases the pressure on and displacement of the surrounding structures and makes subsequent surgery easier and safer. The tumor is debulked with an ultrasonic aspirator or suction and bipolar coagulation. The upper pole of the tumor should be separated from the medulla and the spinal cord at its lower extension into the spinal canal. Dividing the first dentate ligament, which is attached to the dura behind and above the dural ring, increases the available space for maneuvering.

Historically, the overall surgical results of foramen magnum meningioma resection have been poor, especially for ventral magnum meningiomas. Advances in microsurgical techniques, neurological anesthetic management, and skull base approaches have led to improved results. Arnautovic and coworkers²²¹ documented statistically significant improvement in Karnofsky performance scores in 18 patients with ventral foramen magnum meningiomas treated surgically via the transcondylar approach. Deficits of cranial nerves IX and X were the most common complication, and there was no perioperative mortality.

Basal Meningiomas

The goal of achieving total surgical removal of basal meningiomas while alleviating surgical morbidity led to the development of innovative skull base approaches aimed to provide access, widen exposure, eliminate brain retraction, and facilitate dissection. These approaches are imperative for the successful curative removal of these lesions.²²⁷

Cranio-Orbital Zygomatic Approach

Because it allows excellent exposure of the cranial base with minimal brain retraction, the cranio-orbital zygomatic approach has replaced the traditional frontotemporal approach in our practice. Deep lesions can be handled through subfrontal, transsylvian, or subtemporal routes during the same operation. This approach is most suitable for large lesions in the suprasellar, parasellar, and retrosellar areas and for those extending into the cavernous sinus or orbit and along the tentorial notch. Little or no functional, anatomic, or cosmetic defect is evident after the single bone flap is removed.

Patient Position

The patient is placed supine. The head is rotated 30 to 40 degrees to the opposite side and tilted slightly toward the floor. The head is fixed in position using the Mayfield head holder.

Craniotomy Technique

A curvilinear incision is made behind the hairline, extending from the zygomatic arch on the ipsilateral side, past the midline, and toward the superior temporal line on the opposite side. The superficial and deep fasciae of the temporalis muscle are cut parallel to the zygomatic arch, preserving the frontal branches of the facial nerve. A large, vascularized pericranial flap is reflected. The zygomatic arch is incised obliquely anteriorly and posteriorly. The zygoma is then reflected inferiorly. The temporalis muscle is detached from its insertions and retracted inferiorly. A bur hole is place in the anatomic keyhole. This allows access to the anterior cranial fossa and the periorbita. Bur holes are then placed on the floor of the middle fossa. Using a high-speed drill, the bur holes are connected. The orbital roof is cut using a V-shaped osteotome. The bone flap is reflected as one piece. The lateral wall and roof of the orbit are removed in a separate osteotomy (Fig. 131-32).

FIGURE 131-32 Cranio-orbital zygomatic soft tissue and bone flap.

(Adapted from Arnautovic KI, Al-Mefty O, Angtuaco E. A combined microsurgical skull-base and endovascular approach to giant and large paraclinoid aneurysms. Surg Neurol. 1998;50:506.)

Further steps depend on the size and extent of the lesion. Extradural removal of the anterior clinoid process, exposure of the subclinoid internal carotid artery, and exposure of the petrous internal carotid artery are key steps to unlocking the cavernous sinus. The route and site of entering the cavernous sinus depend on the anatomy of the tumor within it.²²⁸

Zygomatic Extended Middle Fossa Approach

We prefer a transzygomatic approach to the standard subtemporal approach because it allows a more inferior window and less temporal lobe retraction. This approach can be combined with an anterior petrosectomy for lesions extending posteriorly into the upper petroclival area.

Patient Position

The patient’s head is rotated approximately 30 to 40 degrees to the contralateral side and tilted slightly contralaterally. The head is fixed in this position using a Mayfield head holder. A spinal drain is placed.

Craniotomy Technique

A preauricular, curvilinear incision is made starting at the inferior margin of the root of the zygoma, anterior to the tragus, encircling posteriorly just above the external auditory meatus and then curved anteriorly and medially toward the midline just behind the hairline. The skin flap is separated from the underlying temporalis fascia. The superficial and deep temporalis fascia layers are cut sharply anteriorly, preserving the frontal branches of the facial nerve. The zygoma root and arch are dissected in the subperiosteal plane and cut obliquely anteriorly and posteriorly. The zygoma with its masseter muscle attachment is reflected inferiorly. The temporalis muscle is sharply separated from the underlying bone and retracted inferiorly. Two bur holes are placed low in the middle fossa. One or two additional bur holes can be placed at the superficial temporal line. The bur holes are connected using a high-speed drill (Fig. 131-33). An additional craniectomy is done to ensure maximal access to the inferior middle fossa.

FIGURE 131-33 Zygomatic middle fossa flap of the anterior petrosal approach after drilling the petrous apex.

For sphenopetroclival meningiomas with extension along the tentorium, the anterior petrosal approach may be needed. The dura of the middle fossa is separated under the operative microscope. Spinal drainage is useful during this portion of the procedure. The second and third divisions of cranial nerve V are identified. The middle meningeal artery is identified at its entrance from the foramen spinosum and is coagulated and cut. The greater petrosal nerve is identified posteriorly. Inferior and medial to the greater petrosal nerve lies the petrous internal carotid artery. In most cases, the petrous internal carotid artery is separated from the middle fossa only by a thin, fibrous layer of tissue. Exposure of the petrous internal carotid artery is done when lateral entry into the cavernous sinus is anticipated. Detailed understanding of the anatomy of the temporal bone is required to perform an anterior petrosectomy (Fig. 131-34). The dural opening for the standard extended middle fossa approach or the anterior petrosal approach can be performed more medially to avoid excessive direct manipulation of the temporal lobe and its underlying veins.²¹⁸

FIGURE 131-34 Zygomatic anterior petrosal approach with drilling of the petrous apex.

Petrosal Approach and Extended Petrosal Approach

Tumors involving the petroclival area and extension into perimesencephalic or peripontine structures can be accessed using the petrosal approach. For more extensive lesions involving the anterior clinoid area and carotid cistern area, the extended petrosal approach should be considered. To access these regions, the petrosal approach offers a number of advantages. The surgeon’s operative distance to these regions is shorter than with the retrosigmoid approaches; there is minimal retraction of the cerebellum and temporal lobe; the neural structures (cranial nerves VII and VIII) are preserved; the otologic structures (cochlea, labyrinth, and semicircular canals) are preserved; and the major venous sinuses (transverse and sigmoid), along with the vein of Labbé and other temporal and basal veins, are preserved.

Patient Position

The patient is placed in the supine position on the operating table. The table is flexed approximately 20 degrees to allow head and trunk elevation. The patient’s ipsilateral shoulder is slightly elevated using a shoulder roll. The head is rotated away from the side of the tumor (approximately 50 degrees) and flexed slightly toward the floor. The head is fixed in a three-point Mayfield headrest.

Craniotomy Technique

The incision starts at the zygoma, anterior to the tragus, and is carried to approximately 2 to 3 cm above and encircling the ear, where it descends behind the mastoid process. The skin flap is sharply dissected from the underlying pericranium and fascia. The temporalis fascia is reflected sharply and is kept in continuity with the sternocleidomastoid muscle; the temporalis muscle itself is subsequently sharply dissected off the bone and reflected anteriorly and inferiorly.

Four bur holes, two on each side of the transverse sinus, are made. The first bur hole is placed just medial and inferior to the asterion, which is located at the inferior junction of the transverse and sigmoid sinuses. The second bur hole is placed at the squamous and mastoid junction of the temporal bone, along the projection of the superior temporal line, which opens into the supratentorial compartment. The final two holes are placed approximately 2 to 3 cm more medially and closer together on either side of the transverse sinus. A temporoparietal craniotomy and lateral occipital craniotomy are performed without connecting the bur holes across the sinus. The holes across the sinus are then connected using the B-1 attachment (without a footplate) of the Midas Rex drill (Midas-Rex, Fort Worth, TX). After meticulous separation of the wall of the sinus from the flap, the bone plate is elevated.

This stage of the operation requires familiarity with the anatomy of the temporal bone and its surrounding structures. A complete mastoidectomy is performed using a high-speed air drill. The diamond bit should be used when drilling near vital neural and otologic structures. The sigmoid sinus is skeletonized down to the jugular bulb. The sinodural angle, or Citelli’s angle, which identifies the location of the superior petrosal sinus, is exposed. The surgeon next drills the superficial mastoid air cells and the deep (retrofacial) air cells. The facial canal and the lateral and posterior semicircular canals are identified. The petrous bone is thinned by drilling along the pyramid toward the apex (Fig. 131-35).

FIGURE 131-35 Posterior petrosal approach bone flap and mastoidectomy (inset).

The posterior fossa dura just anterior to the sigmoid sinus is opened. The dura at the floor of temporal fossa is also opened to the drainage point of the superior petrosal sinus. Depending on the specific anatomy of the vein of Labbé, it may need to be dissected along its course to avoid injury during temporal lobe exposure. The superior petrosal sinus is coagulated or ligated and then transected. The tentorium incision is extended parallel to the pyramid toward the incisura. Care must be taken to avoid injury to cranial nerve IV by cutting the tentorial edge posterior to the insertion of the nerve. For larger tumors with extensive extension into the posterior fossa and CPA, the dura posterior to the sigmoid sinus can be opened, allowing wider and more inferiorly directed access. Access to the transverse sinus is achieved above and below the sinus. We advocate the avoidance of sinus sacrifice in this and all other approaches.^229,230

Transcondylar Approach

Patient Position

Although called by different names—far lateral, posterolateral, extreme lateral, or transcondylar—this is, in essence, one approach with variations in the patient’s position, skin incision, muscle reflection, and craniotomy. We describe our use of the transcondylar approach.

The head and neck are kept in a neutral position to maintain the anatomic course of the vertebral artery and for easier stabilization, if necessary. The patient is then log-rolled and rotated 45 degrees. The patient’s head is fixed in position with a Mayfield headrest. The ipsilateral shoulder is gradually pulled downward and taped to keep it from obstructing the field. The ipsilateral thigh is also prepared and draped for the removal of fat and fascia lata if needed.

Craniotomy Technique

The skin is incised behind the ear in a curvilinear fashion two fingerbreadths behind the mastoid. The curved incision begins at the level of the external auditory canal and turns downward to the level of the C4 vertebra, where it gradually curves anteriorly into the horizontal neck crease. The skin flap is then retracted laterally and secured with fish-hook retractors. This skin flap is well vascularized and can easily be tailored to accommodate other approaches if necessary.

The muscles are presented in three layers. First, the sternocleidomastoid muscle is detached from its origin at the occipital bone. Its innervation by the accessory nerve should be preserved and freed as it enters the medial aspect of the muscle at the level of the C3 vertebra. The splenius capitis, longissimus capitis, and semispinalis muscles—the muscles of the second muscular layer—are also detached along with the sternocleidomastoid muscle and retracted inferiorly and medially.

The muscles of the third, deep layer create two triangles: the superior and inferior suboccipital triangles. The superior suboccipital triangle is delineated by the rectus capitis posterior major, obliquus capitis superior, and obliquus capitis inferior muscles. In the depth of this triangle is the venous compartment, which cushions the horizontal part of the suboccipital vertebral artery, its branches, and the C1 nerve. The inferior suboccipital triangle is delineated by the obliquus capitis inferior, semispinalis cervicis, and longissimus cervicis muscles. In the depth of this triangle is the vertical part of the suboccipital vertebral artery, its branches, its surrounding venous plexus, and the C2 nerve with its anterior and posterior rami.²³¹ The lateral corners of both triangles meet at the transverse process of the atlas, which is located 1 cm below the mastoid tip.

The muscles of this third layer are detached and reflected medially or resected. To improve exposure, the posterior belly of the digastric muscle can also be detached and reflected laterally.

The vertebral artery is usually larger on the left side. During surgery on foramen magnum meningiomas, two segments of the vertebral artery are encountered: the suboccipital (V₃) and the intracranial (V₄). Exposing and mobilizing the V₃ complex allows full proximal control of the artery. Transposing the V₃ complex allows drilling of the condyle, ventral exposure of the tumor, and safe dissection. The V₃ segment is subdivided into two parts: vertical (V_3v) and horizontal (V_3h).²³¹

Opening of the posterior atlanto-occipital membrane is followed by drilling of the posterior wall of the transverse foramen of the atlas and careful subperiosteal dissection. This dissection spares the lateral (periosteal) ring, which is used to manipulate the V₃ complex.

The posterior ramus of the C2 nerve (suboccipital nerve) must be sacrificed. The fibrous membrane around the sinus along with the areolar tissue on it must be kept intact to prevent bleeding and the possibility of an air embolus.^224,225,231

For bony resection, a lateral, posterior fossa craniotomy is done, and the mastoid tip is drilled to expose the occipital condyle. The sigmoid sinus and jugular bulb are fully exposed, and the atlantal and occipital condyles are drilled. Drilling of the condyle must be tailored to suit the needs of each case. Ventrally located tumors require more extensive drilling of the condyle (Fig. 131-36). Laterally placed tumors need only partial drilling of the condyle. Laminectomy of the atlas and sometimes of the axis must often be done.

FIGURE 131-36 Transcondylar flap and mobilization of the vertebral artery.

The dural incision is centered on the dural ring surrounding the vertebral artery. Opening the dural ring leaves a dural cuff of sufficient size around the artery. An incision along the sigmoid sinus is extended inferiorly to the dural ring. This incision extends further inferiorly and laterally to the level of the atlas or lower if necessary. The dura is tented laterally.

Closure and Reconstruction

Skull base approaches require especially meticulous closure. Cerebrospinal fluid leaks must be avoided by achieving watertight dural closure. The dura may be expanded using autologous fascia lata grafts. Vascularized pericranial graft provides the principal protective layer for skull base reconstruction. A vascularized temporalis muscle graft can also provide an additional strong reconstructive element for the larger, temporally based approaches. Microplating systems have enhanced the cosmetic results, especially in the zygomatic and maxillary areas.