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• Parasagittal approaches are used to treat lesions located near the falx, corpus callosum, or other deep midline structures. These lesions include parasagittal and falcine meningiomas; midline gliomas; cavernomas and arteriovenous malformations (AVMs) located in the thalamus; paraventricular masses; and lesions located in the medial frontal gyrus, cingulate gyrus, or corpus callosum.
• Surgical intervention should be considered for tumors that exhibit growth, cause neurologic deficit, or cause uncontrolled seizures.
• Surgical treatment of AVMs in this area is a challenge because the risk of hemorrhage is greater than in other cortical locations; however, surgery itself can carry a high morbidity.Contraindications
• Conservative management should be considered in patients who are older, have multiple medical comorbidities, have poor baseline neurologic function, or have a poor Karnofsky score.
• AVMs involving the posterior limb of the internal capsules should be treated nonsurgically because of the extremely high risk of permanent neurologic deficit.
• Pathologies that affect eloquent structures should be approached with caution.
• The vasculature needs to be carefully studied preoperatively; complete obliteration of the sinuses can increase the risk of venous infarcts.Planning and positioning
• Preoperative magnetic resonance imaging (MRI) with intraoperative, frameless neuronavigation has become widely used in approaching parasagittal lesions. For lesions located posterior to1 the coronal suture, incorporating data from functional MRI into the neuronavigation system can help the surgeon avoid eloquent areas of the brain. Evaluation of functional MRI preoperatively may decrease morbidity.
• Preoperative four-vessel catheter angiography is imperative for the diagnosis and planning of treatment of AVMs and can provide crucial information regarding the vascularity of tumors. Before treatment of an AVM, it is important to understand the arterial supply and venous drainage of the lesion. Angiography can dramatically change the approach in complex midline tumor resections by visualizing arterial feeders and draining veins, identifying whether the superior sagittal sinus (SSS) is patent and whether significant collateralization has occurred in areas where the SSS is occluded, and determining whether there is a role for preoperative embolization.
• Preoperative staged embolization in the appropriate setting may decrease the risk of hemorrhage and decrease lesion volume before surgery.
• Magnetic resonance venography can also be useful for imaging meningiomas to better understand the involvement of the sagittal sinus and anticipate potential bridging or draining cortical veins. When the sinus is occluded, particular attention should be paid to surrounding cortical veins because there is an increased risk of venous infarcts from surgery.
• Intraoperative monitoring is an important tool in resection of midline lesions. Typically, somatosensory evoked potentials of the upper and lower extremities bilaterally, motor evoked potentials, and cortical mapping can direct the surgical approach and resection and decrease morbidity.
• The operative position chosen depends on the location of the lesion. For lesions involving the SSS or deep to the anterior third of the SSS, the patient is positioned supine. Lesions located in the area of the middle third of the SSS can be approached with the patient supine and the neck and head of bed flexed or with the patient in the semisitting position. Lesions located in the area of the posterior third of the SSS can be approached with the patient in the park bench or prone position. If prolonged retraction is planned, sometimes placing the patient’s head with one side down allows gravity to retract the brain naturally.
• Intraoperative medications need to be discussed with the anesthesiologist before initiation of the operation. Antiepileptic medication (e.g., levetiracetam, phenytoin) is useful for seizure prophylaxis if the patient is not already receiving an antiepileptic. Administration of mannitol or furosemide before skin incision and mild hyperventilation produce brain relaxation and minimize the need for excessive brain retraction. We prefer to give a dose of 10 mg of intravenous dexamethasone before skin incision with redosing every 4 hours during the case, especially in cases in which there is significant edema surrounding the lesion. A single dose of intravenous antibiotics (e.g., cefazolin, clindamycin, vancomycin) should be administered within 60 minutes of skin incision with redosing as appropriate during the operation.
• Monitoring for development of air embolism is especially important in cases performed with the patient in the semisitting position. Precordial Doppler ultrasound equipment i1s often placed to screen for development of air embolism. Monitoring and maintaining adequate central venous pressure via a central venous catheter is also imperative during cases performed with the patient in the semisitting position.
• Placement of the Mayfield head clamp should ensure that the three pins are not in the operative field but are appropriately engaged to prevent pin slippage during the case. Delineation and marking of the tumor location and borders using intraoperative navigation is recommended as a guide to ensure the ideal position and trajectory and to mark out the skin flap. A bicoronal incision is preferable in patients with a receding hairline or for anterior frontal lesions. A trapdoor incision that crosses the midline or linear incisions for smaller lesions can also be considered. Clipping of the hair is done according to the surgeon’s preference. Infiltration of the incision line with bupivacaine and epinephrine before patient preparation maximizes hemostasis.
Parasagittal Approach 13-2: A, For the frontal interhemispheric approach, the patient is positioned supine with slight translation and flexion of the neck so that it lies above the heart. B, For the middle parietal interhemispheric approach, the patient can be placed in the supine position with the head flexed or in the semisitting position. C, For the posterior parietooccipital approach, the patient can be placed in the supine position with the head tilted 90 degrees using a shoulder roll for additional elevation. Alternatively, the patient can be placed in the park bench position with the head turned laterally toward the floor.
Parasagittal Approach 13-3: Using intraoperative navigation, the tumor borders and SSS are marked out on the skull. The craniotomy limits are marked out to ensure adequate exposure. The lesion size and depth dictate the number of burr holes needed. Typically, two burr holes are placed 1 cm lateral to the contralateral side of the SSS, and two to three burr holes are placed on the ipsilateral side. A No. 3 Penfield dissector is used to strip the dura off of the inner table.
Parasagittal Approach 13-7: Placement of Bicol collagen sponges (Codeman, Raynham, MA) or Telfa strips on the exposed brain protects the brain from desiccation. Veins draining into the SSS are identified, and dissection around them is performed to avoid venous infarction. The lobe is retracted laterally taking great care to apply pressure slowly. The surgeon also needs to be cognizant of the deeper structures (i.e., anterior cerebral arteries, corpus callosum). If the lesion is superficial, a plane is dissected around it and the surrounding brain. Parasagittal meningiomas may strip off of the SSS or be adherent, depending on the characteristics of the tumor. Reconstruction of the SSS may be required. The operating microscope is draped and brought into the field. Care must be taken to not retract on the anterior cerebral arteries or on draining veins.
• After the lesion is resected, meticulous hemostasis is obtained. A tight dural closure should be attempted but is not required in these locations. The bone flap is replaced with burr hole covers placed over each burr hole. For lesions that involve the skull, the inner table should be drilled off. Alternatively, if a larg1e portion of the skull was involved, the defect is covered with a cranioplasty, and the bone flap is left off.Tips from the masters
• Make every effort to understand the venous anatomy, particularly the draining veins, when planning the dural opening.
• Be patient when initially retracting the brain and look for important vessels such as the anterior cerebral arteries.
• If you need to remove tumors, such as a meningioma, that may have invaded the lateral aspect of the sagittal sinus, consider reconstructing the sinus as you take the tumor out.