In 1614, Felix Plater first described meningiomas at an autopsy. In 1938, Cushing and Eisenhardt first used the term "meningioma" and introduced it as a separate category of extraparenchymal tumors. [1, 2] Meningiomas are believed to arise from the meningothelial cell (arachnoid cap cell) and are usually attached to the inner surface of the dura mater.  The parasagittal region, cerebral convexities, skull base, and falx are the most common locations for meningiomas, although they may arise at any location where meninges exist.  See the image below.
Meningiomas account for approximately 13%-20% of all brain tumors  and 34%-36.4% of all primary brain tumors, making meningiomas the most common primary brain tumors. [6, 7] Meningiomas represent only 4% of all orbital tumors.  Of lesions in the supratentorial compartment, sphenoid wing meningiomas represent about 15%-20% of all meningiomas. Sphenoid wing meningiomas are also known as “orbitosphenoid meningiomas,” “meningiomas en plaque of the sphenoid wing,” and “sphenoid wing meningiomas with osseous involvement.”  Sphenoid wing meningiomas may be associated with hyperostosis of the sphenoid ridge and may be very invasive, spreading to the dura of the frontal, temporal, and orbital regions. [9, 10, 11, 12] Two different growing patterns of sphenoid wing meningioma have been described: meningioma en masse, forming a nodular space-occupying lesion, and meningioma en plaque, which is flat.
Meningiomas of the anterior skull base are defined as arising anterior to the chiasmatic sulcus that separates the middle cranial fossa from the anterior cranial fossa.  Sphenoid wing meningiomas are the most common of the basal meningiomas. Medially, they may expand into the wall of the cavernous sinus, anteriorly into the orbit, and laterally into the temporal bone. Sphenoid wing meningiomas are categorized as lateral, middle, or medial (clinoidal), depending on the origin of the tumor along the sphenoid ridge.  Furthermore, medial (clinoidal) meningiomas are further differentiated into 3 subcategories based on their relation to the anterior clinoidal process.
Type I clinoidal meningiomas originate from the inferomedial surface of the clinoidal process proximal to the distal carotid ring. This type is very difficult to resect because of the absence of the arachnoid plane between the tumor and the internal carotid artery.
Type II clinoidal meningiomas originate from the superolateral surface, leading to widening of the sylvian fissure, and are relatively easy to remove.
Type III meningiomas originate at the optic foramen and extend into the optic canal.  Other frontal skull base meningiomas can arise from the olfactory groove or planum sphenoidale. Planum sphenoidale meningiomas arise 2 cm posterior to olfactory groove meningiomas, and they may be symmetrical around the midline or may extend to the side. Of olfactory groove meningiomas, about 15%-20% grow into the ethmoid sinuses. [16, 17]
The most widely known risk factor for meningiomas is ionized radiation exposure. [18, 19, 20, 21, 22] For example, children with tinea capitis who are treated with an average single dose or multiple doses of 1.5 Gy have a relative 9.5% risk of developing meningioma.  Dental radiography is a common source of radiation in the United States. The risk of meningioma has been found to double after full-mouth series.  In addition, radiation-induced tumors may develop after previous radiation treatment of another lesion. Cahan et al  were the first to propose criteria for identifying radiation-induced tumors, and these criteria have been applied to diagnose radiation-induced meningiomas. Radiation-induced tumors were defined as lesions arising after a latency period (4 years in the original article) within a previously irradiated field that have different histology than the radiated tumor.  These criteria have been augmented by others over the years with additional features, and the full set of criteria now includes the following: [26, 27, 28]
The tumor must arise in the irradiated field.
The histological features must differ from those of any previous neoplasm in the region.
The tumor must occur after an interval sufficient to demonstrate that the neoplasm did not exist prior to irradiation (usually years).
This type of tumor must occur frequently enough after irradiation to suggest a causal relationship.
This type of tumor must have a significantly higher incidence in irradiated patients than in an adequate control group.
There must be no family history of a phakomatosis
The tumor must not be recurrent or metastatic.
The most commonly reported radiation-induced tumor is meningioma. [26, 28] The Childhood Cancer Survivor Study in the United States reported a cumulative incidence of 3.1% and a relative risk of 2.7% for meningioma development at 30 years after the primary tumor diagnosis.  Radiation-induced meningiomas develop more frequently over the convexities than the skull base, at a ratio of 1.9:1 for both high-dose and low-dose radiation. 
Radiation-induced meningiomas more commonly occur as multiples and in a younger age groups than do spontaneous meningiomas.  Some authors reported that these tumors exhibit more malignant behavior, as indicated histologically by high cellularity, pleomorphism, multinucleation, and giant cell pseudoinclusions.  These findings have not yet been confirmed by others. 
There is a wide variation in the literature regarding the latency period for the development of radiation-induced meningioma. [26, 29, 30] It has been suggested that this variation is related to variation in the radiation dose, with a short latency for high doses and long latency for low-dose radiation.  The recognized latency period can be as short as 14 months and as long as 63 years, although the average latency period is 30-40 years in most cases. [28, 29, 30]
Aside from radiation, other factors that have been studied as potential causes of meningioma include genetic abnormalities, hormonal factors, and viral infections.
In cytogenetic studies, the most commonly reported genetic abnormality is the loss of NF2 tumor suppressor gene on long arm of chromosome 22 (monosomy 22). This genetic alteration leads to loss of expression of NF2 protein product (neurofibromin) and has been reported in 40%-70% of meningiomas.  Other commonly reported genetic alterations in meningioma include deletion of short arm of chromosome 1; loss of chromosomes 6, 10, 14, 18, and 19; and gain of 1q, 9q, 12q, 15q, 17q, and 20q.  Abnormalities of chromosome 22 have been associated with type II neurofibromatosis, and 75% of patients with type II neurofibromatosis develop meningioma during their lifetime. Ten percent of these are multiple lesions. [32, 33]
Hormonal factors (eg, estrogen, progesterone, androgen, steroid) have been studied extensively as risk factors for meningiomas because of the striking predominance of meningiomas in women; the female-to-male ratio is 2:1 for intracranial tumors and 10:1 for spinal meningiomas.  Other evidence to substantiate the implication of sex-specific hormones comes from data showing increased growth of meningiomas during pregnancy and hormonal replacement therapy.  Estrogen receptor (ER) has been found in 30% of meningiomas in one series, predominantly the ER-beta receptor isoform. 
The progesterone receptor is the best candidate among the sex-specific factors as a cause for meningiomas. Progesterone receptors have been shown to be expressed in 81% of women and 40% of men with meningiomas. [36, 37] Other studies indicate that progesterone binds to meningiomas in 50%-100% of tested specimens.  Although progesterone receptor expression has been observed more frequently in benign meningioma (96%) than the malignant type (40%), no relation has been found between progesterone receptor status and age, sex, location of tumor, or menopausal state. [5, 37] These findings have prompted researchers to develop antiprogesterone medications, such as mifepristone (RU-486), which appears to inhibit tumor growth in vitro and in vivo. 
Androgen receptors have also been found in approximately 50% of meningiomas, but their receptor expression is variable, making them less likely candidates in the pathophysiology of meningiomas.  Similarly, meningiomas vary in expression of receptors for other hormones (eg, vascular endothelial growth factor receptor [VEGFR], epidermal growth factor [EGF], platelet-derived growth factor [PDGF], fibroblast growth factor, insulin-like growth factor-1 [IGF-1]), making them less likely candidates for oncogenesis of meningiomas. It has been suggested that the direct stimulatory effect of EGF on PDGF or PDGF itself may be partially responsible for angiogenesis and even oncogenesis in meningiomas. PDGF is a particularly attractive candidate because it has structural homology with the product of c-sis oncogene on chromosome 22. 
Some viruses have been found within meningiomas, including polyoma virus, simian vacuolating virus 40 (SV-40), and adenovirus. A suggested role for these viruses or parts of viruses is related to the proteins involved in the induction or maintenance of tumor growth and transformation.  However, this association has not proven.
Among the other potential factors for inducing meningiomas that have not been proven are head trauma and electromagnetic field exposure. Head trauma and skull fractures have been suggested as a risk factor for meningioma development by some authors.  However, a large population-based 2014 study from Taiwan found no association between head injury and meningioma development in two cohorts of patient with and without head injury. 
Similarly, electromagnetic field exposure, especially with the widespread use of cell phones, has generated interest in relation to the pathophysiology of brain tumors. Many studies suggest that little, if any, evidence supports the implications of cell phone use on meningioma development, although there is generally a lack of well-conducted studies to date and a significant amount of debate in this regard. 
According to the most recently published Central Brain Tumor Registry of the United States (CBTRUS) report, meningioma is the most common nonmalignant brain and central nervous system (CNS) tumor, making up 36.4% of reported tumors (7.86 per 100,000 population). The CBTRUS report also indicates that meningioma is the second most frequently reported brain and CNS tumor overall in adolescents and young adults (age 15-39 years), accounting for 15.9% of tumors. It is the most common tumor in patients aged 35-39 years, accounting for 25.1% of tumors, and is least common in patients aged 15-19 years, accounting for 4.9% of tumors. These numbers increase steadily with age. 
The annual incidence of meningiomas can be as low as 0.74 per 100,000 individuals younger than 34 years and as high as 18.86 per 100,000 individuals older than 85 years. It is 2.5 times more common in females than in males and 1.1% more common in blacks than in whites. [6, 43] In children, meningioma accounts for 4.6% of all primary brain tumors. [31, 44] Incidental meningioma is found in 0.52%-0.9% of brain images. [45, 46]
Meningiomas account for approximately 13%-20% of all brain tumors.
Worldwide, meningiomas account for approximately 13%-20% of all brain tumors.
In a 2016 study of 1549 operated meningiomas, the overall perioperative complication rate was 17.8%-18.8%; of these, the morbidity rate was 1.2%-2.2%. 
Among patients with skull base meningiomas, a 2016 study reported the overall mortality rate was 5%, with transient cranial nerve deficits occurring in 32% of cases, definite cranial nerve lesions in 18%, and cerebrospinal fluid (CSF) leak in 14%.  Another study reported an overall mortality rate of 5.8%, transient cranial nerve deficit rate of 11.7%, definitive morbidity of 5.8%, and second recurrence rate of 5.8%. 
Potential complications include bleeding, deep venous thrombosis and embolism, air embolism, venous infract, wound-healing deficits, paresis, sensory deficits, cranial nerve palsy, aphasia, seizures, brain edema, hygroma, CSF fistula hydrocephalus, ischemia, and pituitary insufficiency. These complications vary depending on preoperative morbid conditions, age, and tumor size and location.
Previous studies reported variability in the prevalence of meningiomas among whites, Africans, African-Americans, and Asians and greater incidence among blacks than whites. [7, 50] However, the 2015 CBTRUS report showed that the rates among whites, blacks, and Hispanics were similar. 
The incidence rate of meningioma is higher in females (10.87 per 100,000 population) than in males (4.98 per 100,000 population). 
No sexual predilection was found among Africans. 
The average age at onset is 63 years. The incidence of meningiomas increases steadily thereafter. 
A 1983 study showed that recurrence-free survival rates after complete surgical removal in 114 patients (60% of which were sphenoid wing meningioma) at 5, 10, and 20 years were 80%, 70%, 50%, respectively.  The extent of surgical resection and histological grade are very important prognostic factors.
In 1957, Simpson classified meningiomas based on the extent of resection as follows: 
Grade I - Macroscopically complete removal of the tumor with excision of its dural attachment and of any abnormal bone
Grade II - Macroscopically complete removal of the tumor and of its visible extensions with endothermy coagulation of its dural attachment
Grade III - Macroscopically complete removal of the intradural tumor without removal or coagulation of its dural attachment or extradural extensions
Grade IV - Partial removal of the tumor
Grade V - Simple decompression with or without biopsy
The 10-year risk of recurrence for grades I through IV were 9%, 19%, 29%, and 44%, respectively. 
The relevance of this system in the current era of microsurgical procedures advancement has been questioned by some authors;  however, a 2016 study of 458 patients with World Health Organization (WHO) grade I meningioma indicated that the extent of surgical resection based on Simpson grading is an important predictor of tumor recurrence.  The overall tumor recurrence rates for Simpson resection grades I, II, III, and IV were 5%, 22%, 31%, and 35%, respectively.  When WHO histological grading of meningiomas is considered regarding survival, a 2016 study of 905 patients demonstrated that the 5-year overall survival rate was 85%-90% for WHO grade I, 75%-78% for WHO grade II, and 30%-35% for WHO grade III tumors.  Recurrence rates of tumors graded according to the 2007 WHO classification of tumors of the central nervous system were 7%-25% for WHO grade I, 29%-52% for WHO grade II, and 50%-94% for WHO grade III. 
Studies on the prognosis of sphenoid wing meningioma and spheno-orbital meningioma have generally showed good outcomes.
The results of a retrospective study of 25 patients who underwent surgical resection and orbital reconstruction with an average of 5 years of follow-up showed that a gross-total resection was achieved in 70% of patients, with surgery limited by the superior orbital fissure and the cavernous sinus. Proptosis improved in 96% of patients, with 87% improvement in visual function. Ocular paresis improved in 68%, although 20% of patients experienced temporary ocular paresis postoperatively. Overall, 95% of patients reported an improved functional orbit. There were no perioperative deaths or morbidity related to the surgical approach or reconstruction. Tumor recurrence occurred in 8% of patients. 
In another series of 67 patients with sphenoorbital meningioma who underwent surgical resection with orbital wall reconstruction, a total removal was achieved in 14 cases (82.3%), with only one recurrence (7.1%) over a mean follow-up period of 36 months. Proptosis was corrected in all cases, and visual acuity improved in 7 (70%) of 10 cases. Radical resection was followed by cranio-orbital reconstruction to prevent enophthalmos and to obtain good cosmetic results. No deaths or serious complications occurred in association with surgery. Revision of the orbital reconstruction was required because of postoperative enophthalmos (two cases) or restricted postoperative ocular movement (one case). 
A 2016 series of 33 patients with spheno-orbital meningioma who underwent resection without formal orbital wall reconstruction (mean follow-up, 4.5 years) showed improved proptosis in all patients, and only 2 patients had tumor recurrence at the orbit that required surgery. 
The authors of these studies emphasize aggressive removal of the hyperostotic bone to achieve satisfactory results and to decrease the risk of recurrence. For example, a study of 47 patients with spheno-orbital meningioma who underwent surgery via the frontotemporal approach without orbital wall reconstruction showed that complete resection was achieved in 51% of cases. At a mean follow-up of 52 months, proptosis normalized in 90.9% and improved in the remaining patients, visual acuity normalized in 20.8% and improved in 45.8% patients, and cranial nerve deficit subsided in all but two cases. The recurrence rate was 29.7%.  According to the authors, the high recurrence rate in this study was likely related to the incomplete removal of the invaded bone to minimize perioperative morbidity.
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