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Parasagittal meningiomas originate from the parasagittal angle without brain tissue between the tumor and the superior sagittal sinus. Resection of parasagittal and convexity meningiomas is one of the more satisfying procedures in neurosurgery because these benign tumor types at these locations provide an ample opportunity for cure and relief of preoperative neurologic deficits.
A parasagittal location for meningiomas is fairly common in the middle third of the superior sagittal sinus (45-70%), followed by the anterior (15-34%) and posterior thirds (9-30%). In this categorization, the anterior third of the sagittal sinus is defined as between the crista galli and coronal suture; the middle third is located between the coronal and lambdoid sutures; and the posterior third is between the lambdoid suture and torcula.
Parasagittal meningiomas occur more frequently in areas with a higher density of arachnoid granulations; 15% of these tumors invade the dural sinus. In addition, a meningioma in this location may be associated with a higher incidence of atypical and malignant histologic subtypes.
The clinical presentation for parasagittal meningiomas depends on the location of these tumors along the sinus. Tumors near the central sulcus may cause sensory or motor seizures and contralateral hemiparesis/hemianesthesia. Accordingly, tumors at the anterior third may cause frontal lobe dysfunction and nonspecific signs such as dementia and papilledema. Those at the posterior third may cause visual disturbances such as homonymous hemianopsia. Additionally, large tumors can often present with headaches, vomiting and changes in sensorium.
Magnetic resonance imaging (MRI) allows diagnosis of an extra-axial parasagittal tumor with a dural tail. Computed tomography arteriogram/venogram (CTA) or magnetic resonance venogram (MRV) assesses the degree of dural sinus invasion or occlusion and estimates the location of vital large parasagittal bridging veins or their collaterals relative to the tumor. Digital subtraction angiography may be indicated for confirmation of sinus patency if CTA or MRV is nondeterminate, high-grade sinus obstruction is evident, and the blood flow in these vessels is slow.
Parenchymal edema often signifies pial invasion and correlates with neurologic deterioration after surgery. Parasagittal meningiomas may also cause noticeable hyperostosis; a computed tomography (CT) scan is important to delineate the extent of tumor-infiltrated calvarium as part of presurgical planning for extent of resection.
Occasionally, large intradiploic veins around the tumor can become very prominent and highly functional for cerebral venous drainage; their indiscriminate sacrifice can lead to unexpected postoperative venous infarction. Hyperostotic meningiomas may cause torrential epidural bleeding during excision because of their large dural tumor feeders. Adequate preparation for managing large-volume blood loss and venous air embolism is mandatory.
Indications for Surgery
Patients with progressive symptoms or growing tumors are reasonable candidates for surgery. Small asymptomatic tumors can be observed. Tumors that affect critical bridging veins or the sinus and lead to partial obstruction may undergo subtotal removal and the tumor remnants treated with radiosurgery. If recurrence is evident, a conservative approach leads to slow tumor growth and development of proper collateral venous circulation during progressive sinus obstruction by the tumor. These changes subsequently allow safe gross total resection of the tumor and involved segment of the venous sinus.
Dural sinus stenosis or occlusion by the tumor may occasionally lead to venous hypertension and papilledema. Intrasinusal tumor resection and sinus reconstruction is associated with significant risks and not advised. Ventriculoperitoneal shunting may be considered in medically refractory cases.
A lumbar drain affords brain relaxation that minimizes brain retraction. Elective cortical resection should not be necessary. I have not encountered transtentorial herniation with judicious use of intraoperative lumbar drainage (gradual CSF drainage during and after dural opening; total 40-60cc in 10cc aliquots,) especially for giant meningiomas.
An occluded sinus is amenable to resection as the venous drainage channels of the hemispheres adapt to gradual closure of the sinus through development of venous collaterals. Preoperative CT can determine the need for synthetic bone flap construction (cranioplasty) following resection of the tumor-infiltrated calvarium. I do not advocate endovascular tumor embolization before surgery since the tumor can be readily devascularized early during dural opening and dissection.
Advances in radiosurgery have affected my philosophy regarding meningioma surgery. The goals of surgery, centered on patient safety, have evolved into favoring “radical” subtotal resection over aggressive “total” resection with venous sinus reconstruction in the case of partially occluded sinuses or involvement of important draining veins. Radiosurgery is also reserved for growth of tumor remnants on follow-up surveillance imaging.
RESECTION OF PARASAGITTAL MENINGIOMAS
Please refer to the Parietal Craniotomy chapter for more details regarding access to parasagittal meningiomas. There are certain considerations in craniotomy planning that are specific to parasagittal meningiomas.
The skin incision must be generous (horseshoe) and designed to cross the midline so that the infiltrated dura can be safely and liberally excised. The pericranium must be spared for dura reconstruction since the tumor-infiltrated dura is resected. Large collateralized diploic and scalp veins need to be protected, if possible, to avoid postoperative cerebral edema or venous infarction.
If significant hyperostosis is encountered and the inner table is adherent to the dura, aggressive elevation of a single large bone flap may lead to torrential bleeding and injury to the bridging veins or the sinus. It is safer to perform a step-wise craniectomy while securing hemostasis during gradual and controlled bone removal. The sinus should be exposed early during and protected.
Coagulation of the epidural space leads to early partial tumor devascularization. The peripheral dura is tacked up to the edges of the craniotomy.
The previously harvested pericranial graft may be sutured in place where the dura was resected. In the case of calvarial hyperostosis, the tumor-infiltrated bone flap is not replaced and a cranioplasty may be performed using titanium plates, methylmethacrylate or a prefabricated synthetic plate. Tack-up stiches are used to prevent formation of epidural hematoma or fluid collections.
The patient is observed in the intensive care unit overnight for frequent neurologic evaluations and control of pain and blood pressure. A postoperative MRI is obtained. Steroids may be slowly weaned as tolerated. Prophylactic anticonvulsants are administered perioperatively, but tapered off one week after surgery if the patient has not suffered from a seizure.
Meningiomas causing significant hyperostosis of the bone may cause torrential bleeding during elevation of the bone flap. Moreover, adherence of the bone to the tumor can lead to avulsion of the tumor away from the cortex during manipulation of the bone flap.
This patient presented with mild altered mental status and a growing mass in the frontal region for the past few years.
Growing tumors within the dural venous sinus (intrasinusal tumors) may undergo resection of the affected/occluded segment of the sinus.
Pearls and Pitfalls
- Preoperative studies should assess the patency of the superior sagittal sinus.
- Elevation of the bone flap can be associated with torrential bleeding. Gentle pressure on a large sponge placed over the dura can tamponade the bleeding while portions of the dura are being coagulated in a stepwise fashion.
- Sacrifice of the bridging veins should be kept to a minimum, even around the anterior third of the sagittal sinus. Patent venous sinuses should be preserved at the expense of small tumor remnant.
- Early devascularization of the tumor is a key first step and must not be overlooked. It will minimize blood loss and facilitate an efficient operation.
- Central debulking of the tumor will allow aggressive manipulation of the tumor capsule and prevent unnecessary brain retraction.
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