EPIDEMIOLOGY AND CLASSIFICATION OF CONVEXITY MENINGIOMAS

Convexity meningiomas comprise 15% to 19% of all meningiomas.^1,2 Generally, excision of these tumors is considered to be easy because of their accessibility. However, some difficulties might arise if they are located adjacent to eloquent areas or if there is no clear demarcation from brain tissue. To differentiate the localization of the tumor, convexity meningiomas are divided into seven subgroups according to the brain area they overlie: precoronal, coronal, postcoronal, parietal, prerolandic, temporal, and occipital. Most of the tumors (about 70%) are located anterior to the rolandic fissure. Before the advent of magnetic resonance imaging (MRI) and functional mapping, those subgroups and the coronal suture were helpful to rule out a localization in the neighborhood or within the central area. Further, the vicinity of the tumor close to eloquent areas such as the speech area in the frontal or parietal lobe was indicated by that grouping.

SYMPTOMS

Increased intracranial pressure causes general symptoms due to direct compression of the adjacent surface of the brain or a mass effect caused by the accompanying edema, especially in meningiomas that are increasing in size. As a result, patients may suffer from headache, organic psychosyndrome, and seizures. Moreover, the detailed neurologic deficits correspond to the adjacent eloquent areas. A contralateral palsy and motor seizures are caused by tumors surrounding the precentral cortex, and sensory deficits and Jacksonian seizures by tumors adjacent to the postcentral cortex. Seizures may be preceded by a motor or sensory aura and followed by a Todd’s palsy. In general, seizures are reported in 40.7% of patients exhibiting a convexity meningioma.³ Imaging is therefore important during the evaluation of an initial epileptic seizure.

A motor or sensory aphasia can diagnosed if the frontal (Brocca’s area) or temporal (Wernicke’s area) lobe of the dominant hemisphere is affected. Tumors overlying the temporal lobe (especially the temporomesial area) might cause general seizures; larger meningiomas could also lead to compression of the contralateral peduncle to the edge of the tentorium and cause an ipsilateral spastic weakness of the leg or visual field defects.

As imaging techniques are evolving and the number of applications for screening purposes increasing, the diagnosis of incidental meningiomas is rising. Therefore, one should consider carefully the indication and time of surgery in this special case.⁴ The same is also true for elderly patients with regard to subtotal tumor removal and other treatment options such as pharmaco- or radiotherapy. In this group of patients, an accurate and detailed preoperative evaluation is essential.^5,6

PREOPERATIVE DIAGNOSIS

As discussed in Chapters 13–16, computed tomography (CT), MRI, and digital subtraction angiography (DSA) are used for preoperative diagnosis.

In most instances CT is adequate for a correct diagnosis and demonstrates bony alternations such as hyperostosis and invasion. Normally, the tumors are hypodense on plain imaging and show intense and rapid contrast enhancement, and in 25% of all patients a calcification ranging from diffuse adherence to dense sclerosis is found.⁷

MRI is the current gold standard for a diagnosis. Excellent imaging of these tumors is provided with the use of gadolinium, as they show rapid, mostly homogeneous enhancement (Fig. 23-1). On non-contrast enhanced images they are iso- or hypointense compared to the brain. A so-called dura tail, which is caused by contrast enhancement of the surrounding meninges, is helpful for diagnosis. A rim of subarachnoid space around the tumor is an indicator for easy surgical excision and possible total removal.^8,9 This well-defined boundary between tumor capsule and arachnoid membrane might be an indicator for a possible total resection of a tumor that is associated with a low risk of complications due to injury to the brain. Extrapial resection is essential to avoid any devascularization of the underlying cortex resulting in small cortical bleedings or a loss of function. If no cleavage plane is found in the tumor, vascularization derives from pial vessels. The latter can be shown via angiography and is also commonly associated with peritumoral edema and continuing tumor growth. In general, tumors exhibiting a diameter of less than 3 cm can be removed by an extrapial resection. In contrast, a larger tumor is associated with a higher risk pial vascularization, and a subpial resection therefore becomes necessary. The latter is also true if an irregular tumor border is found on T2-weighted images; however, there is no statistical significance in visible tumor–brain interface on T2-weighted images and a safe surgical plane for resection. In one third of patients no extrapial plane can be found and in 30% of them an adverse neurologic outcome occurs during subpial resection. Conversely, if an extrapial resection can be performed, an adverse outcome (Karnofsky < 80) will occur in only 25% of the patients. This is especially important if surgery is performed in eloquent areas, in which case it may be wise to leave some tumor remnants around the area to avoid neurologic deterioration.

FIGURE 23-1 Preoperative (A) and postoperative (B) axial, contrast-enhanced, T1-weighted MRI examinations demonstrating gross total resection of an occipital convexity meningioma.

(Images courtesy of Professor M. N. Pamir.)

A displacement of major arteries, a dislocation of sulci and gyri, especially in eloquent areas, and a peritumoral edema can also be visualized via MRI. Functional imaging can be used in combination with intraoperative, frameless neuronavigation. In some rare cases, cystic tumors can be found and the differential diagnosis from a metastatic lesion may be complicated.¹⁰

DSA is used to investigate the degree of vascularity, to localize the main draining bridging veins toward the sinus and to evaluate for the possibility of preoperative embolization. Normally, the blood supply of the tumor arises from the external carotid artery (mainly the medial meningeal artery) but sometimes from pial vessels as well. In case of such pial parasitization, an internal carotid angiography becomes necessary. Information about the primary blood supply from these vessels and their location in relation to the operative approach are necessary for surgical planning. To avoid any harm to the patient it is important to know whether the tumor is extremely vascularized and if embolization of one or more major feeders is possible. Inadvertent embolization of the central retinal artery or necrosis of the skin needs to be avoided in any case.

Embolization, performed by using polyvinyl alcohol, should take place 1 to 3 days before operation, even though one author proposes waiting for 7 to 9 days after surgery.¹¹ Embolization is believed to be beneficial by reducing blood loss and leading to a shorter duration of the surgical procedure. However, in one study it was shown that intraoperative blood loss is reduced only if the tumor was embolized totally¹² and no significant reduction of the duration of the surgical procedure or the extent of resection was found. Thus the debate about effectiveness continues. In superficial meningiomas, embolization is believed not to show any benefit; rather, there is an increased risk of brain swelling and bleeding within the tumor bed after the procedure, creating a need for emergency surgery.

ADVANCED TECHNOLOGIES

Anatomic navigation is intended to enable the surgeon to keep trepanation small and well directed. After segmentation of the tumor and of important structures adjacent to the tumor such as the pericallosal artery or veins, a trajectory guides the surgeon in virtual and physical space to the tumor and helps to avoid damage to the healthy brain. Functional brain mapping for localization of the pre- and postcentral gyrus, as well as sensory and motor speech area and pyramidal tract, are very helpful as well. The functional mapping is performed via functional MRI or magnetic encephalography (MEG). Anatomic and functional data can be combined to perform functional neuronavigation, which is a useful tool in the preoperative surgical planning phase. According to the data obtained, in selected cases there is an indication to leave tumor remnants behind to avoid higher morbidity, especially if the tumor origin is adjacent to eloquent areas. Finally, intraoperative imaging is used to update the functional neuronavigation data with respect to the possible intraoperative brain shift.

In the past, localization of the central sulcus was achieved by using somatosensory evoked potentials (SEPs), which are still used at present if functional neuronavigation is not available. While the median nerve is electrically stimulated, an atypical “phase reversal” is observed in consecutive four or more cortical leads that are displaced above the suspected central sulcus. However, the use of this method is fading due to improved accuracy and the broad availability of functional imaging and neuronavigation.¹³

INDICATION FOR SURGERY AND AIMS OF THE OPERATION

General Indications

Even if convexity meningiomas exhibit the highest cure potential, especially if well capsulated, and total excision is possible in most cases because they can be easily accessed, decision making regarding surgery is extremely important. This is supported by the fact that complex structures are mostly absent except for eloquent regions of the underlying brain that can be easily spared.

There is no doubt that surgery is necessary if any neurologic deficits due to the size, extension, and/or increase of intracranial pressure occur. A perifocal edema that is mainly present in the special subgroup of secretory meningiomas shows high epileptogenic potential even if the tumors are small (Fig. 23-2). An antiedematous pretreatment is essential in those cases. From a cosmetic aspect and to avoid infections, surgery is also indicated for tumors that are mainly osteophytic and perforating the skin. Fortunately, such tumors are rare.

FIGURE 23-2 Axial, coronal, and sagittal preoperative (A, B, C) and postoperative (D, E, F) contrast-enhanced, T1-weighted MRI examinations demonstrating gross total resection of a frontal convexity meningioma.

(Images courtesy of Professor M. N. Pamir.)

OPERATIVE PROCEDURE

Dural Opening

Before the incision of the dura, tumor feeders are coagulated or sutured, especially in highly vascularized tumors. Further, the surgeon has to pay attention to avoid a tense dura or brain swelling which will lead to herniation of the cortex after incision of the dura. If not treated immediately a brain infarction may commonly occur. Adequate treatment consists of an elevation of the head, hyperventilation, and continuous administration of steroids and mannitol. In uncontrolled high intracranial pressure a ventriculostomy and cerebrospinal fluid (CSF) diversion is performed.

Generally, the dural opening is performed 0.5 cm distant from the tumor margin, the dura is elevated with a sharp hook or a dura stitch, and the incision is carried out using a knife. Ultrasonography or neuronavigation might be helpful to define the tumor border. Afterwards, the dura incision is carried out circumferentially using blunt-tipped scissors and care has to be taken to avoid any damage of the draining veins that run around the tumor, especially in eloquent areas (Fig. 23-3). After resection of the tumor, the dura should be excised at least 5 to 10 mm distant from the tumor margin as tumor remnants could be shown to be present on histologic examinations, even in normal appearing dura²⁰ in up to 40% of all cases. As a dura tail apparent in imaging may be caused by tumor invasion or hypervascularization, removal needs to be performed according to angiographic or radiographic contrast enhancement results or to the extent that the tumor tissue is visually judged within the dura. If there is any doubt about total resection, multiple histologic samples should be drawn. As sometimes an en plaque tumor or a spread of tumor cells within the arachnoid can occur, the dura has to be opened in an adequate size.^21,22 To avoid recurrences, removal of the tumor remnants is superior to any cauterization of the dura edges.

FIGURE 23-3 Resection of a convexity meningioma. On dural circumferential opening the tumor is visualized (A), followed by gentle elevation of the tumor and the involved dura with sharp dissection from the arachnoid (B). The intact cortical surface is visualized after tumor removal (C).

(Images courtesy of Professor M. N. Pamir.)

After initial opening, it is helpful to put stitches through the edges of the resected dura to handle itself and the tumor by gentle retraction and manipulation. Accordingly, the need for retraction directly applied to the tumor or brain will be minimized.

Tumor Excision

Depending on the size and localization of the tumor, the decision is made whether an en bloc resection is possible or an intracapsular debulking has to be performed. In the latter case, the blood supply has to be diminished first. Generally, this decision is to be made before starting microsurgical resection, which is considered standard by now. Further, the use of brain retractors and cotton pads must be avoided whenever possible and if necessary should only be transitory. Preparation is started with the intention to spare eloquent areas and localization of those areas is provided by SEPs or neuronavigation. As a result, in precentral meningiomas the resection is started from the frontal and in postcentral meningiomas from the posterior border. During preparation, the surgeon gently pulls the dura to visualize the border between arachnoidea and tumor. This cleavable plane is found in one half to two thirds of all patients, especially in those exhibiting no pial vessel parasitization.⁸ In case a tumor capsule is absent and the tumor is growing along vessels without clear demarcation, any preparation will be very difficult. Conversely, if a clearly defined capsule exists, it is very easy. Cautery forceps and microscissors are used to perform a blunt dissection following the cleavable plane extrapial. Those instruments are also used to visualize, coagulate, and cut adhesive bands of the arachnoidal layer and tiny tumor vessels. More aggressive resection becomes necessary in recurrent and malignant meningiomas that exhibit atypical features, in general. Especially in the latter type of meningiomas, the extent of surgery is to determine the course of the disease as the tumor might show frank invasion of the brain.

According to the surgeon’s experience, magnification provided by the microscope can be used from the beginning of preparation of the tumor borderline or during separation of the supplying vessels in the depth, especially if conducted in eloquent areas (Fig. 23-4). The use of the microscope has to be considered carefully, as an overestimation can be very time consuming and sometimes confusing while underestimation can lead to damage of the brain or important vessels.

FIGURE 23-4 A frontal convexity is visualized on axial (A), coronal (B), and sagittal (C) T1-weighted, contrast-enhanced MR images. Note the involvement of the overlying bone on MR images and bone destruction on conventional radiography (D). The dura is opened circumferentially around the tumor (E) and the tumor is seen to involve both the dura and the overlying bone (E and F). The tumor is separated from the underlying parenchyma by sharp arachnoidal dissection (G) and the intact cortical surface is visualized after tumor removal (H).

(Figures courtesy of Professor M. N. Pamir.)

Care has to be taken not to injure the branches of the middle cerebral artery that might be adherent to the tumor capsule overlying the sylvian fissure. Meticulous dissection and preparation of the capsule should begin in uninvolved areas. The same is true for large draining veins in eloquent areas such as the rolandic vein. Brain resection in case of larger tumor masses and overlying brain tissue is today avoidable in most cases.

If an en bloc resection is not an option, it is helpful to exenterate the corner of the tumor piecemeal by the use of diathermy cutting loops or an ultrasonic aspirator. The former is used in firm tumors. Generally, this intracapsular debulking is carried out after one has started to develop the space between the tumor capsule and the arachnoidal plane. Care always needs to be taken not to sacrifice the tumor capsule and the adjacent brain. After debulking the tumor, the thinned capsule can be retracted into the created space and one always has to switch between both, excavating the tumor and developing its border, especially in large tumors. If the tumor is soft, not vascularized, and necrotic it might be possible to debulk it by suction and curettage.

Especially in highly vascularized and angioblastic tumors, bleeding might become a problem and will be reduced by preoperative embolization. Generally, in meningiomas fibrin products are broken down more rapidly because of an increased fibrinolytic activity causing intracapsular bleeding that is controlled by transitory application of pressure using cotton balls. If it cannot be controlled, thrombin-soaked absorbable gelatin sponge and oxidized cellulose or microfibrillar collagen hemostat are applied instead. If ever possible, experienced surgeons will tolerate milder bleeding to achieve a rapid tumor enucleation.

If there is any risk of provoking neurologic deficits and a complete tumor removal cannot be achieved, remnants will be left in situ. In older patients, observation of the tumor growth and in younger patients adjuvant treatment strategies such as discussed above were applied, instead.²³ The extent of tumor removal is graded according to Simpson:²⁴

Grade 1: Complete removal with excision of the dural attachment

Grade 2: Complete removal with endothermic coagulation of the dural attachment

Grade 3: Complete resection without resection or coagulation of dural attachments or extradural extensions

Grade 4: Partial removal, leaving tumor in situ

Grade 5: Simple decompression or biopsy.

Depending on the grade of resection, follow-up or further treatment are performed. Generally speaking, in convexity meningiomas the prognosis is very good as there is an easy access to the tumor and a high opportunity to achieve a total resection. Complete resection, even if there is en plaque growth, will exclude recurrences in most of the cases. Regrowth is possible if microscopic residuals remain in situ or, as described in one study, within the arachnoid membrane.²⁵ This might be due to a secretion of vascular endothelial growth factor (VEGF) which induces neovascularization and tumor regrowth.²⁶ The higher pathologic grading of the tumor is not as much associated to incidence of recurrence than is a tumor remnant.^27,28

POSTOPERATIVE CARE AND HANDLING OF COMPLICATIONS

After surgery of small meningiomas, an extubation is striven for while high blood and intracranial pressure are avoided. In larger tumors, however, especially if brain swelling occurred, ventilation for several days becomes necessary. Even if some authors state that in those cases CT scans should be performed, we do not support this approach in uneventful cases. Neurologic conditions such as the level of consciousness, focal neurologic deficits, seizures, as well as vital signs, pulmonary function, fluid intake, and electrolytes need to be monitored during the patient’s stay at the ICU. As a rule, the head has to be elevated 20 to 30 degrees, a corticosteroid therapy is applied up to two weeks supported by H₂-receptor antagonists to cover the whole period of maximal brain swelling and is tapered afterwards. In case this treatment is not sufficient, mannitol or furosemide might become necessary as well. From day 4 after surgery coagulation is inhibited by administering heparin to avoid thromboembolism. In case of preexisting seizures, the regular anticonvulsant drugs are continued and discontinued later according to the results of follow-up EEG examination. Care should be taken to avoid any increased intracranial pressure caused by brain swelling, hematoma, or thrombosis. If there is any new neurologic deficit or a seizure, immediate CT examination and adequate treatment of the findings consisting of a reoperation in case of any mass effect or treatment of the brain swelling by the use of medical therapy needs to be carried out.

As hemorrhage and resulting blood loss are the most important single causes of death after meningioma surgery, attention needs to be paid to bleeding from the scalp, the bone, the dura and intracranial veins. For this reason a meticulous coagulation should be performed in every case and some cases even might require embolization of the tumor feeders to minimize the risk of bleeding. As compromising the arachnoidea might lead to seizures or an indirect injury of the brain resulting in contralateral deficits, it should be avoided, like accidental closure of the major bridging veins.

If the bone flap becomes infected, it has to be removed and antibiotics are administered for at least 14 days. Three months after extraction of the bone a cranioplasty can be performed.

In patients suffering from all types of meningiomas, the mortality and morbidity are somewhat higher in elderly patients (30-day mortality: 16%, complication: 39%). Especially in the former patients, a postoperative confusion is commonly found but a severe adverse event has to be ruled out. It must be emphasized that a fatal pulmonary embolism progresses in about 1.2 % of all patients after meningioma surgery.²⁹

Prediction on epilepsy is more difficult, but patients suffering from convexity meningiomas are affected more often than patients exhibiting a tumor at any other localization within the cranium. It is more difficult to deal with them irrespective of whether they were preexisting or arose after surgery. This might be due to in their involvement of highly functional areas and in their relationship to the major draining veins. The time from onset and frequency of epileptic seizures may predict outcome in preexisting seizures; the causes of new seizures might be recurrence, intraoperative complications such as excessive retraction of the brain, sacrificing larger draining veins, or subpial dissection. In those cases anticonvulsive treatment is started or continued and stopping it should not be attempted before 1 year after surgery.

Even now, Gamma-Knife radiosurgery is supported as optional therapy in meningioma surgery, due to the higher risk of peritumorous changes, a higher rate of complication (43%) compared to surgery,³⁰ and the risk of malignant transformation it should be performed only in older patients with a higher risk for anesthesia. This is important, as convexity meningiomas especially can be treated surgically with a low risk of mortality or morbidity.