Meningiomas are benign and rather slow growing tumor arising from arachnoidal cap cells. Statistically speaking meningiomas constitute about 20% of all primary intracranial tumors. Out of these 20% olfactory groove meningiomas constitute 10%. It was the Italian surgeon Francesco Durante who first reported the first successful resection of olfactory groove meningioma in 1885. In 1938 Cushing reported the largest series of olfactory groove meningioma which were resected via frontal craniotomy / subfrontal approach. It is really worthwhile to differentiate olfactory groove meningioma from other intracranial menigiomas as they differ in their presentation, symptomatology and management.
The female : male ratio is 2:1. Exact explanation for this variation is not available.
Meningiomas arise from meningothelial cap cells that are largely distributed through the arachnoid trabeculations. The greatest concentration of meningothelial cells are seen in the arachnoid villi lining the dural sinuses, cranial nerve foramina, middle cranial fossa and cribriform plate area. This accounts for the common location of meningiomas i.e. Over the convexity, along the skull base and along the falx. Meningiomas are usually attached to the dura and are well encapsulated. Blood supply to these tumors arise usually from the dura and the anterior and posterior ethmoidal arteries.
Histologically these tumors show features of bening lesions. These lesions classically appear as whorls of arachnoid cells surrounding a central hyaline material that eventually calcifies. These calcified areas are known as Psammoma bodies. These cells are arranged in sheaths separated by connective tissue trabeculations.
Subtypes of meningiomas:
Transitional types – Psammamatous tumors
Secretory meningiomas – Secretes Vascular endothelial growth factor. These tumors are characterized by the presence of marked oedema. They may be papillary or rhabdoid variants. These tumors are usually considered to be malignant in nature.
WHO histological grading of meningiomas:
Grade I: This grade is usually benign and 90% of all meningiomas belong to this category. They also carry the best prognosis and a very low recurrence rate.
Grade II: Atypical meningiomas come under this category. About 5% of all meningiomas belong to this grade. Tumors belonging to this grade have a high recurrence rate (about 50%).
Grade III: This grade of meningioma is frankly malignant constituting about less than 3% of all meningiomas.
Majority of meningiomas are associated with one / more focal chromosomal deletions. Malignant versions of meningiomas involve multiple chromosomal aberrations. These multiple chromosomal abberations cause extreme instability to the genomic structure thereby increasing the risk of malignant transformation.
Deletion and inactivation of NF2 gene on chromosome 22 is the predominant feature in sporadic meningiomas.
Risk factors contributing to meningioma:
Exposure to ionizing radiation – Studies have demonstrated that survivors of atom bomb explosion showed increased incidence of meningioma
Role of Hormones – Histologically meningiomas present with oestrogen, progesterone and androgen receptors. This could explain the increased incidence of menigioma in females.
Head injuries have been shown to increase the incidence of meningiomas.
Location of olfactory groove meningioma:
These tumors are seen in the midline and arise over the cribriform plate and frontosphenoidal suture area. A majority of these tumors occupy the floor of anterior cranial fossa extending from crista galli up to the tuberculum sella. Extension to ethmoidal sinuses occur in about a third of these patients. There are obvious similarities existing between posteriorly extending olfactory groove meningiomas and tuberculum sellae meningiomas. These two masses can be differentiated by studying their relationship with that of the optic apparatus. Olfactory groove meningiomas have a tendency to push optic nerves and chiasma downwards and posteriorly as they grow, where as tuberculum sellae meningiomas push the optic nerves and chiasma upwards and superolaterally as they grow because of their subchiasmal position.
Blood supply of olfactory groove meningiomas:
These tumors are supplied by:
Anterior ethmoidal artery
Posterior ethmoidal artery
Anterior branches of middle meningeal artery
Meningeal branches of ophthalmic artery
These tumors are very slow growing ones and they are seen in the silent area. Hence to become symptomatic they need to enlarge their size to a great extent. Usually these lesions are incidental findings during routine imaging.
MRI is the most preferred imaging modality as this would clearly show the origin of the tumor from dura. These lesions appear isointense / hypointense to gray matter of brain in T1 weighted images and isointense to hyperintense in T2 weighted images. When gadolinum is used as contrast these lesions demonstrate homogenous enhancement.
Majority of meningiomas show marginal dural thickening that tapers peripherally. This tapering is classically known as the dural tail which is the characteristic feature which is revealed in the images.
This entirely depends on the age and physical fitness of the patient. If the tumors are small and seen in elderly and ill patients then serial imaging and observations would do. In symptomatic cases irradiation can be resorted to,
Surgical resection is the best option. Removal of these lesions is similar to that of any other skull base tumor.
Cushing was the first to describe surgical resection of the tumor via unilateral frontal craniotomy. Other approaches available include:
Bifrontal craniotomy combined with subfrontal approach:
This approach provides wide exposure for complete removal of tumor. In this approach it is easy to drill out the hyperostotic area in the cribriform plate area. In this approach optic nerves also can be deroofed if need be. Major disadvantage of this approach is the amount of brain retraction that is needed.
Unilateral frontal craniotomy with subfrontal approach:
This approach has the advantage of sparing the opposite frontal lobe and superior saggital sinus. The disadvantages include:
Excessive brain retraction
This is a rather new approach. It is less invasive than frontal craniotomy approaches. It avoids CSF leaks because the frontal sinus is not damaged. The optic nerve can be localised and exposed before tumor manipulation. Major disadvantage of this approach is the lack of working space. The wole dissection process needs to be carried out within a narrow angle.
This procedure is performed binaurally with the endoscope introduced through one nose and the surgical instruments via the other.
This procedure involves:
Bilateral maxillary antrostomies
Unilateral / bilateral nasal septal flaps are harvested first. This helps in covering the dural defect.These flaps are tucked into the nasopharynx well out of the way of surgical field.
Modified Lothrop procedureis performed. The frontal intersinus septum should be completely removed.
Controlling the bleeding to the mass is the top most priority. The anterior ethmoidal artery should be identified and ligated. Anterior ethmoidal arteries should be sought and ligated on both sides. Posterior ethmoidal arteries also should be drilled out and ligated. Image guidance is used to identigy the anterior and posterior extent of the mass. The anterior cut is usually made at the level of posterior wall of frontal sinus and continued along the fovea ethmoidalis using drills and kerrison punch. The posterior resection is made as posterior as possible. This is usually governed by the posterior extent of the mass. It can be as posterior as the planum of sphenoid.
Dura is incised exposing the entire olfactory groove meningioma. Dissection is completed using a combination of blunt dissection, debrider and CUSA. Tumors involving the medial wall of the orbit may be considered to be rather suboptimal for endoscopic resection. While performing the resection of the tumor care should be exercised to dissect it between the tumor and arachnoid plane. The defect in the skull base is repaired using abdominal fat, reinforced with fascia lata and tissue glue is used to fix them in place.
Advantages of endoscopic approach:
The main site of recurrence i.e. The floor of the anterior cranial fossa is completely resected / drilled out.
Allows two surgeons to operate simultaneously there by ensuring clear surgical field
Very useful from the cosmetic point of view.
Role of irradiation in the management of meningiomas:
RT is indicated only in patients with recurrent tumors following surgical resection
In patients with atypical / malignant meningiomas after surgical extirpation of the tumor
Role of stereotactic radio surgery:
The major advantage of this procedure is that the irradiation dose at the edge of the neoplasm is greatly reduced thereby sparing the normal adjacent tissues. This procedure can be safely used to treat even large volume tumors close to critical intracranial structures.
Targetted molecular therapy:
This type of therapy aims at blocking the various signals leading to unbridled proliferation of cells. These include:
Inhibitor of PDGFR – This is infact a key driver of cell proliferation in meningiomas. Drugs that block this can help in arresting the growth of meningiomas. Classic example of these drugs is Imatinib.
Inhibitors of angiogenesis factor – Sorafenib and Sunitinib are examples of drugs belonging to this group