It is useful to modify Cushing's classification of these tumors and divide them into three groups because of the different clinical syndromes and operative approaches associated with each. These are the hyperostosing meningioma-en-plaque involving the lateral aspect and often a large portion of the sphenoid wing, usually associated with a plaque of tumor on the adjacent dura and in the orbit, and at times associated with a significant intracranial mass. The second group are the globular meningiomas arising from the middle third of the ridge. The final group are the medial, or clinoidal, meningiomas.
There is usually a history of a slowly progressive, painless, unilateral exophthalmos. In some patients there may be a palpable mass in the anterior temporal region. The exophthalmos may be present for many years before there is a change in visual acuity or extraocular movements. Some patients have headache and impairment of sensation in the first or second division of the trigeminal nerve, and in others significant intracranial extension of the tumor may cause a seizure disorder or evidence of impairment of neurological function.
These tumors are slow growing, and complete surgical removal is usually not possible by the time the diagnosis is made. If there is mild proptosis, it may be appropriate to follow the patient. Often little change will occur over a period of many years. If the proptosis is increasing or visual loss is developing, surgery is indicated. Radiation therapy has been used in the patient with visual loss due to soft tissue extension into the orbital apex.
The blood supply is from meningeal branches, but many of these tumors are relatively avascular. The tumor often involves the orbital capsule and the dura around the superior orbital fissure, orbital apex, and cavernous sinus.
The position and the skin incision are essentially the same as are used for a tuberculum sellae meningioma, but with more exposure of the anterior temporal bone and lateral wall of the orbit. If the exposed bone is not involved by tumor, a frontotemporal free bone flap is turned, If the tumor is growing into the bone, the flap is turned around the area.
Using the air drill and appropriate bone instruments, the lateral sphenoid wing is removed and the orbit entered, with care taken to keep the orbital capsule intact. Gradually bone is removed to expose both frontal and temporal dura and the orbital fascia. The lateral edge of the superior orbital fissure will be encountered and is marked by a small fold of dura that extends into the orbital fascia. There often is a small arterial vessel at that point. The entire lateral wall and roof of the orbit are removed. If hyperostotic bone extends over the optic canal, this is also removed, as is bone down to the foramen rotundum. The dura is then opened and, if indicated, the intracranial tumor is resected and the dura is replaced with a graft. In many patients, the tumor will be growing into the superior orbital fissure and medial dura and it will not be possible to remove this area. The last step is to open the orbital fascia; if this is done earlier, the orbital fat will be in the way of the intracranial procedure. A localized mass of tumor may involve the orbital capsule. Sometimes this can be removed. but in some patients the lateral rectus muscle and/or orbital apex are involved with tumor, and a judgment will have to be made about the extent of removal. It is not necessary to replace the orbital roof. Pulsations of the eye gradually diminish and usually will not be a bother to the patient.
It is hard to be sure of total removal in any patient. However. removal is usually extensive. Most make a good recovery. Exophthalmos improved in some patients. There are no major neurological complications except for worsened vision in 5%. 5% of patients require reoperation later. 15% of patients require radiation therapy for worsening vision due to tumor in the orbital apex. In some patients improvement of exophthalmos occurs, and in most it is at least stabilized. A more optimistic outlook has been reported following extensive extradural resection of bone and adjacent tumor. Preoperative exophthalmos regresses completely in 55% of cases and partially in 25%. Among patients with bone involvement of the lateral or middle ridge and orbit, complete removals with full recovery could be achieved in half of cases and 25% subtotal removals with improvement; 12% have recurrence of tumor or a permanent deficit due to postoperative complications.
Middle Third Meningiomas
These tumors arise from the edge of the ridge and grow in a globular fashion, compressing the frontal and temporal lobes. The patient usually presents with headaches or one or more seizures. The approach is the same as described for tuberculum sellae and medial sphenoid wing tumors. If the attachment of the tumor can be divided, most of the blood supply will be interrupted. Most of patients has complete removal of the tumor; with good prognosis.
Medial (Clinoidal) Meningiomas
These meningiomas involve the region of the anterior clinoid. adjacent medial sphenoid wing, superior orbital fissure. and cavernous sinus. They may grow into the orbit. The tumor often encases the internal carotid and proximal middle and anterior cerebral arteries as well as the optic nerve and may compress or provoke edema in the temporal or frontal lobes.
MRI outlines the extent of the tumor and shows the relationship to the arterial structures. It is often hard to define the optic nerves and chiasm. MRA is usually needed to determine the extent of encasement of the internal carotid artery and its branches. On rare occasion. especially with recurrence of an aggressive tumor, internal carotid occlusion is considered prior to surgery. Embolization of external carotid artery branches may occasionally be indicated.
The treatment decisions are often difficult. Prior to the development of CT and MRI and microsurgical techniques it was difficult to remove these tumors completely and there was significant morbidity. With the development of microsurgical techniques. some of these tumors now can be removed extensively. It remains to be established in the long term which patients benefit more from this extensive surgery with the increased risk of morbidity. from a partial removal of tumor followed by radiation therapy. or from radiation therapy alone.
Some general guidelines for the treatment of these tumors can be outlined. For patients with mild or nonprogressive symptoms it may be appropriate to follow the patient with periodic scans and examinations to determine if the lesion is growing and to see whether the symptoms are interfering significantly with the patient's life. The indications for surgery in younger patients are worsening symptoms and/or tumor growth seen on follow-up MRI, and in older patients a large tumor with worsening symptoms. Radiation therapy is used in older patients with small- and medium-size tumors with worsening symptoms and for regrowth after subtotal or radical subtotal removal.
Great care must be taken in the approach to these lesions. because the internal carotid and middle cerebral arteries may be embedded in the tumor. Usually only a subtotal removal is possible because of the growth of the tumor around these arteries and into the base of the skull.
The operative approach is similar to the frontotemporal exposure for tuberculum sellae meningiomas. with more temporal exposure and with the side being determined by the site of the tumor. A similar subfrontal approach is in use by others.
After the initial exposure, it may be necessary to open the medial aspect of the sylvian fissure. Internal decompression of the tumor is carried out. and the blood supply along the sphenoid wing, often a hypertrophied branch of the middle meningeal artery, is divided. As the dissection progresses on the lateral aspect of the tumor. it is important to look for the middle cerebral artery branches and to follow these medially to determine whether they or the internal carotid artery are involved with tumor.
When the tumor grows into the optic foramen or involves the bone around the foramen and superior orbital fissure. the dura over this area is excised and the bone is removed using an air drill with a diamond burr and then fine curettes. As the tumor grows. the arachnoid membrane remains intact. making microsurgical dissection from the internal carotid artery and optic nerve possible.
In most cases it is hard to be sure of a total removal because of involvement of important arteries and/or cranial nerves or the extension of the tumor into the cavernous sinus or orbit. In 90% of cases there is a good result. 5% develop permanent neurologic deficit. 35% of cases require radiation sooner or later. 20% of cases are radioresistant.