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Parafalcine menigiomas originate from the falx, but unlike parasagittal meningiomas, they are completely covered by the cortex. They may grow through the falx and are often bilateral. Like their parasagittal counterparts, they are most commonly found around the middle third of the superior sagittal sinus, between the coronal and lambdoid sutures. They comprise 8.5% of intracranial meningiomas.
Similar to their parasagittal counterparts, the clinical presentation of a parafalcine meningioma depends on its location along the falx. Tumors near the central sulcus may cause sensory and motor seizures or contralateral hemiparesis/hemianesthesia. Speech dysfunction may also occur if the tumor is related to the dominant supplementary motor area.
Accordingly, the tumors along the anterior third of the falx can reach a large size and may cause cognitive dysfunction, headaches, and blurred vision/papilledema. Those at the posterior third may cause visual disturbances such as homonymous hemianopsia or hallucinations. Occasionally, a falcine meningioma may be the source of intracerebral, subdural, or subarachnoid hemorrhage.
There is also the rare circumstance when the tumor can only cause isolated contralateral lower extremity weakness. This presentation should not be confused with radiculopathy because of nerve root compression. Upper tract signs (can be subtle) and distribution of the weakness (not respecting muscle groups for individual roots) help the clinician differentiate between these scenarios.
Magnetic resonance imaging (MRI) reveals an extra-axial interhemispheric tumor that has a dural tail and is based on the falx. T2-weighted MR images estimate the location of the distal anterior cerebral arteries and their branches. Parenchymal edema signifies pial invasion and correlates with neurologic deterioration after surgery.
Magnetic resonance or computed tomography venogram assesses the degree of superior sagittal sinus invasion or occlusion and estimates the location of large midline parasagittal bridging veins or their collaterals. The locations of these veins guide dissection, ensuring an adequate interhemispheric corridor to reach the tumor.
Large tumors often encase the callosomarginal and pericallosal arteries. A preoperative CT angiogram identifies displacement or encasement of these vital vessels.
Other tumors that may mimic parafalcine meningiomas include metastatic tumors, osteochondromas and chondrosarcomas.
Indications for Surgery
Patients with progressive symptoms are reasonable candidates for surgery. Small asymptomatic tumors can undergo surveillance imaging. Tumors that partially obstruct critical bridging veins or the sinus may be subtotally removed and the growing tumor remnants treated with radiosurgery.
A lumbar drain affords brain relaxation that minimizes brain retraction (especially in the case of large tumors). This is especially important because cerebrospinal fluid (CSF) cavities are not entered during interhemispheric dissection to access the parafalcine tumor.
Elective cortical resection should not be necessary. I have not encountered any risk of transtentorial herniation with prudent intraoperative lumbar drainage for giant meningiomas causing significant mass effect and midline shift. CSF drainage is instituted upon opening of the dura and 10cc aliquots are gradually removed until adequate relaxation is achieved.
I do not advocate endovascular tumor embolization before surgery since the tumor can be readily devascularized from the falx early during dissection.
Advances in radiosurgery have affected my philosophy toward meningioma surgery. The goals of surgery, centered on patient safety, have evolved into favoring radical subtotal resection over aggressive total resection of tumors involving important arteries or veins. Radiosurgery is reserved for treating the growth of tumor remnants seen on follow-up surveillance imaging.
Falcotentorial meningiomas displace or encase the diencephalic (veins of Galen and Rosenthal) and straight sinus. Since the vasculature in this region is critical, a preoperative digital subtraction angiogram is beneficial to study these structures and their location in detail. Radical subtotal tumor resection ensures unharmed cerebrovascular architectures.
RESECTION OF PARASAGITTAL MENINGIOMAS
Please refer to the Interhemispheric Craniotomy or Parietal Craniotomy chapters for more details regarding accessing parafalcine meningiomas. There are certain considerations in craniotomy planning that are specific to parafalcine meningiomas.
Unlike parasagittal meningiomas, most parafalcine meningiomas can be readily resected through a linear incision. The incision is designed to cross the midline so that the superior sagittal sinus can be exposed by the craniotomy and mobilized through retraction sutures within the superior falx after dural opening. The lateral head position allows gravity retraction to mobilize the brain during operative maneuvers within the interhemispheric corridor.
Giant bilateral parafalcine tumors may be excised through a unilateral linear incision and transfalcine approach. The transfalcine corridor allows resection of the contralateral tumor without a need for a contralateral craniotomy and placing the contralateral parasagittal veins at risk.
The location of parasagittal veins has to be carefully studied on preoperative imaging. If the overlying ipsilateral parasagittal veins are numerous and obstructive, I have employed the contralateral transfalcine route to expose the tumor.
A thin layer of normal cortex is usually covering the parafalcine tumor. Due to increased cranial tension, the vibrations of the drill during craniotomy can injure this thin cortex unless CSF drainage is used to achieve decompression. In addition, uncontrolled tension can lead to cerebral herniation; this can be especially problematic and consequential during exposure of the sensorimotor cortex.
The following case demonstrates the intraoperative events related to resection of a large bilateral parafalcine meningioma.
The previously harvested pericranial graft may be sutured in place if a portion of the dura was resected. Tack-up stiches are used to prevent formation of epidural hematoma and fluid collections.
A postoperative MRI is obtained. Steroids may be slowly weaned as tolerated by the patient. Prophylactic anticonvulsants are administered perioperatively, but tapered off 1 week after surgery if the patient has not suffered from a seizure. The patient is kept well-hydrated during the immediate postoperative period to decrease the risk of venous thrombosis, especially if the venous sinus or parasagittal veins were manipulated or sacrificed during the operation.
Pearls and Pitfalls
- Preoperative studies must be reviewed (including T2-weighted MR images) to approximate the location of the distal anterior cerebral arteries.
- Sacrifice of the bridging veins should be kept to a minimum.
- The trigeminocardiac reflex may cause bradycardia or asystole when the falx is manipulated.
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