Olfactory Groove Meningioma
Last Updated: March 12, 2020
Olfactory groove meningiomas (OGMs) constitute 9–18% of all intracranial meningiomas. These tumors arise in the midline of the anterior cranial fossa at the junction of the cribriform plate and planum sphenoidale.
Tumors arising from this area are in close proximity of tuberculum sellae meningiomas, but these two tumor types behave very differently, primarily with respect to their presentation and mass effect on the optic apparatus. Olfactory groove meningiomas mobilize the chiasm inferiorly and posteriorly, whereas tuberculum sellae meningiomas elevate and symptomatically compress the chiasm.
Olfactory groove meningiomas commonly cause hyperostosis of the anterior cranial fossa floor, and in about 15–20% of cases, erode inferiorly into the ethmoid sinuses. The olfactory nerves are usually displaced laterally by these tumors, whereas the A2 segments of the anterior cerebral arteries are found draped over the tumor capsule superiorly and posteriorly. The orbitofrontal and frontopolar branches are displaced laterally and superiorly, respectively.
Olfactory groove meningiomas have an insidious clinical course and often grow very large before they are diagnosed. Patients commonly present with headaches, mental status changes including personality changes, short-term memory loss, and lack of motivation. Visual impairment, seizures and anosmia are also likely presenting symptoms.
Visual impairments are varied and include acuity loss, visual field deficits, scotomata, or papilledema. Anosmia is less often a presenting complaint because the patient is usually unaware of this deficit. The triad of anosmia, unilateral optic atrophy, and contralateral papilledema is highly suggestive of an olfactory groove meningioma (Foster-Kennedy syndrome). With the availability of advanced imaging studies, this syndrome is rarely diagnosed today.
After a thorough history and physical examination with particular attention to olfactory, visual, and frontal lobe functions, magnetic resonance imaging (MRI) is performed. The characteristics of the tumor are assessed, including its size, associated cerebral edema, extent of calcification, vascular encasement, and anterior cranial floor hyperostosis.
I also review the tumor’s relationship to the optic apparatus and other parasellar structures. Dedicated imaging of the vasculature is considered helpful to assess displacement or encasement of the anterior cerebral vessels and their branches and potential involvement of the superior sagittal sinus.
I rarely pursue any specific vascular imaging study because the MRI is adequate for providing the necessary information. Embolization of the feeding vessels to these tumors is usually not attempted because of the risk of ophthalmic artery occlusion. Preoperative visual field testing is performed to document visual field deficits. Endocrinological assessment is appropriate for the rare tumors extending into the sella.
If a large portion of the tumor grows into the nasal sinuses, surgical planning should consider a bifrontal craniotomy or a staged approach to maximize resection of all the tumor compartments. I usually do not pursue an aggressive resection of small intrasinus tumor components.
Indications for Surgery
Management strategies for OGMs include surgical resection, stereotactic radiosurgery, and observation with serial MRIs. Indications for treatment include neurologic deficit due to the tumor’s mass effect or edema and documented tumor growth on serial imaging studies.
The best chance for cure of a meningioma is gross total resection with removal of the involved dura and bone. Moreover, the first surgery provides the best chance for achieving a cure. However, it may be necessary to leave small amounts of adherent tumor tissue on vulnerable neurovascular structures in order to maintain preoperative neurologic function. For smaller asymptomatic or minimally symptomatic tumors in high surgical-risk patients, stereotactic radiosurgery can be used as the primary treatment modality.
I have completely abandoned the use of bifrontal approach for OGMs of any size, and believe that the unilateral pterional or lateral frontal corridor is more than adequate to remove the large or giant OGMs. Many of my colleagues, however, prefer the bifrontal route as it provides more generous exposure of the tumor.
Small and medium-size OGMs may be excised through endoscopic endonasal surgery or an eyebrow supraorbital osteotomy. For these tumors, transcranial surgery offers an opportunity for preservation of olfaction, but the endoscopic route does not. Therefore, if the patient owns intact olfaction before surgery, the transcranial route offers the only opportunity for preservation of the senses of taste and smell. These details must be discussed with the patient before surgery.
Through its lateral operative trajectory, the pterional corridor allows for early identification of the optic nerve and carotid artery, avoidance of the frontal sinuses/bilateral frontal lobe retraction, and preservation of the superior sagittal sinus. An indication for bilateral frontal craniotomy in this tumor type is an attempt to reach the extensive erosion of the tumor into the ethmoid sinuses.
I place a lumbar drain before the incision and drain cerebrospinal fluid (CSF) at the time of the dural opening. Gradual low-volume CSF drainage (10 cc aliquots) provides the frontal lobe relaxation needed to safely reach the opticocarotid cisterns to release additional CSF.
If the cisterns are filled with tumor, lumbar drainage is even more important for brain relaxation to allow mobilization of the tumor and the frontal lobe early to complete the much needed tumor devascularization along the anterior skull base as the first stage of intradural dissection.
The use of prophylactic anticonvulsant medications is highly recommended due to significant cortical manipulation associated with resection of these large tumors.
The anterior and posterior ethmoidal arteries arising from the ophthalmic artery and the sphenoidal branches originating from the middle meningeal artery supply the meningiomas in this region. Pial feeders often parasitize vessels from the anterior cerebral arteries.
RESECTION OF OLFACTORY GROOVE MENINGIOMAS
Following is a review of the nuances of the technique for resection of large olfactory groove meningiomas through the pterional route. The details of the extended pterional craniotomy are reviewed in the volume dedicated to cranial approaches.
The patient’s head is turned approximately 45 degrees to allow me to work on the contralateral side of the crista galli during the later intradural phase of resection. For giant tumors, the craniotomy is enlarged more medially, but should remain just lateral to the frontal sinuses. A very flat operative trajectory, parallel to the roof of the orbit, is necessary to reach the base of the tumor. An orbitozygomatic osteotomy is usually not necessary.
Early identification of the skull base neurovascular structures is imperative for their protection in the midst of handling of a highly vascular tumor.
All meningiomas are resected using the following algorithm of 3Ds:
- Devascularize the base of the tumor,
- Debulk the tumor, and finally
- Dissect the tumor along the preserved arachnoid planes to protect the adjacent cerebrovascular structures.
The correct order of executing these maneuvers is imperative.
The optic canals are inspected to exclude intracanalicular extension of tumor.
To further elaborate upon the above operative nuances, I am including the following case example.
After surgery, patients are typically observed in the intensive care unit overnight. The steroid dosage is tapered off within one week, depending on the extent of cerebral edema and the patient’s neurologic status. Seizure prophylaxis is continued for about three months after surgery; if the patient has experienced a seizure, then antiepileptic medication is continued for six months to a year thereafter.
Pearls and Pitfalls
- Regardless of their size, most OGMs may be resected though the pterional route.
- These tumors typically reach a very large size and can be highly vascular. Early aggressive and thorough tumor devascularization along the skull base significantly facilitates safe and efficient tumor resection.
- Preservation of perforators along the posterior aspect of the tumor capsule is paramount for minimizing postoperative ischemic complications.
- Indiscriminate pulling of tumor fragments along the operative “blind spots” should be avoided as this often leads to vascular and pial injury.
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