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Parietal and Occipital Avms The Neurosurgical Atlas by Aaron Cohen-Gadol, M.D. Parietal and Occipital AVMs Operative Anatomy The parietal and the occipital lobes are neighbors with arbitrary boundaries. At their medial aspect, they are separated by the parieto- occipital sulcus. At their lateral surface, there is no real fissure or sulcus to demarcate them, but they may be separated arbitrarily by an imaginary extended Sylvian fissure line. Because of such an intimate neighborhood, the vasculature of these two lobes is shared and interrelated. Therefore, arteriovenous malformations (AVMs) involve these lobes synchronously. As a result, I will consider these two lobes as a single unit during discussion of AVM excision. All three major cerebral arteries and their branches supply the parietal and occipital lobes, including the middle cerebral artery (MCA), anterior cerebral artery (ACA), and posterior cerebral artery (PCA). The distal and cortical branches of the MCA (both the superior and inferior trunks) supply the lateral parieto-occipital surface via: 1. Central artery 2. Anterior parietal artery 3. Posterior parietal artery 4. Angular artery 5. Temporo-occipital artery The distal ACA branches (A5) supply the medial parietal surface, including the cuneus and precuneus through the superior and inferior parietal arteries. The major blood supply to the medial and basal parieto-occipital lobe is by means of the PCA and its branches: 1. Posterior temporal artery 2. Calcarine artery 3. Parieto-occipital artery These branches originate from the P3 segment of the PCA that forms near the posterior border of the midbrain and courses within the quadrigeminal cistern. On the other hand, the calcarine artery (the P4 segment of the PCA) courses through the calcarine fissure and supplies the inferior cuneus and lingula (the visual cortex). The splenial artery separates proximally from the parieto-occipital artery at the posterior border of the splenium and anastomoses with the pericallosal artery, creating the “limbic loop.” Figure 1: Surface anatomy of the lateral parietal and occipital lobes is shown (images courtesy of AL Rhoton, Jr). Figure 2: Surface anatomy of the medial parietal and occipital lobes is shown (images courtesy of AL Rhoton, Jr). Venous drainage for the lateral surface primarily ascends to the superior sagittal sinus. The most dominant drainage system is the vein of Trolard. There are numerous anatomic variations, including double or triple Trolard veins. There is frequently no major draining vein along the distal 4- to 5-cm segment of the superior sagittal sinus except the lateral occipital vein. Careful evaluation of the major parasagittal draining veins is a practical detail for feasibility of the posterior interhemispheric approach. Venous drainage is mostly en route to the Sylvian and occipital veins at the inferolateral surface of these lobes; these veins generally descend to the vein of Labbé. Figure 3: Variations in the venous anatomy along the convexity are summarized. An absence of the vein of Labbé has led to an enlarged parasagittal vein along the posterior superior sagittal sinus (yellow arrow, right lower image) (images courtesy of AL Rhoton, Jr). PARIETO-OCCIPITAL AVM SUBTYPES Lateral Parieto-Occipital AVM Lateral parieto-occipital AVMs are typical cone-shape convexity AVMs. These lesions may sometimes lack cortical representation and only manifest as an arterialized vein on the surface. This AVM subtype is considered eloquent when located on the postcentral gyrus or on the visual cortex at the medial occipital pole. Feeding arteries are formed from the distal cortical MCA branches. However, ACA and PCA feeders may also participate at the depth of the AVM. Venous drainage is mostly superficial, ascends to the superior sagittal sinus and may also descend to the Sylvian vein or vein of Labbé. Figure: 4: A lateral parietal AVM is shown on lateral (upper image) and anteroposterior (lower image) left-sided ICA angiograms. The inset MRIs illustrate the location of the AVM. The deep ependymal/choroidal feeding vessels near the ventricle (arrows) can be a challenging source of intraoperative bleeding. I prefer to position the patient in the lateral position; head rotation depends on the anteroposterior location of the lesion. The cortical presentation of the AVM is the highest point on the operative field. I use a U-shaped incision, encompassing the AVM and the surrounding normal brain margins. I incise the dura to circumscribe the lesion; the base of the dural flap is next to the venous sinus. Dissection begins at the margins of the AVM by opening the thick arachnoid layers and sulci to identify, follow, and then divide the feeders near the nidus. The primary feeders are mostly recruited from the distal cortical branches of the MCA that are found at the anteroinferior margin of the AVM. Therefore, this area is disconnected first. Large AVMs reach into the occipital horn or atrium and engage feeders from the choroidal/ependymal arteries or deep MCA branches. Parenchymal dissection is conducted by “hugging” the nidus with great delicacy, especially at the eloquent sides of the AVM, protecting the central lobule, Wernicke’s area, and the visual cortex as well as their radiating fiber tracts. The deep white matter feeders in this area can be unsettling to the surgeon; their aggressive pursuit can lead to injury to the descending fibers of the central lobule. All venous drainage to the vein of Labbé, superficial temporal veins, and superior sagittal sinus should be saved intact until the completion of circumdissection of the nidus. The lobules of the AVM projecting into the white matter should not be inadvertently transected and left behind. This inadvertent error can especially occur at the level of the periventricle. The residual nidus is the source of intraoperative bleeding and unexpected brain swelling, notably if the bleeding fills the ventricles. AVMs located on the inferior edge of the central sulcus should be treated like Sylvian frontal AVMs and approached with the same surgical considerations, including careful Sylvian fissure dissection and preservation of en passage or bystander arteries. Medial Parieto-Occipital AVMs Medial parieto-occipital AVMs are located on the medial aspect of the hemisphere without any extension or presentation on the lateral convexity. They usually do not involve the splenium of the corpus callosum. The location of the lesion with respect to the parieto-occipital sulcus defines a medial parietal or occipital AVM; these AVMs share a similar surgical approach and technical nuances. Their main feeders arise from the PCA branches, including the parieto-occipital or Calcarine artery and distal ACA branches, including the paracentral artery, as well as the superior and inferior parietal arteries. Complex nidi also procure feeders from the distal MCA branches. The dominant drainage system is cortical, ascending to the superior sagittal sinus and/or descending to the inferior sagittal sinus and then leading to the straight sinus and finally into the vein of Galen. Figure 5: This hemorrhagic medial parietal AVM is primarily fed by the branches of the distal PCA. Note the lateral (upper image) and anteroposterior (lower image) vertebral angiograms and corresponding MRIs (insets). The nidus reaches the atrium. The ipsilateral interhemispheric approach may not safely reach the periventricular extent of the lesion. The small and superficial medial parietal lesions can be reached via the ipsilateral interhemispheric corridor; I use the contralateral interhemispheric transfalcine approach for lesions with predominant subcortical extension to avoid aggressive lobar retraction. Regardless of the operative route, the surgeon’s view is tangential to the surface of the AVM without any obvious convexity cortical access. Since the operative trajectory is oblique, I use the lateral head position with the AVM in the dependent hemisphere for the ipsilateral route and the unaffected hemisphere in the dependent position for the contralateral transfalcine approach. For medial parietal AVMs, the position of the patient’s head is midline horizontal (the axis of the sagittal suture is parallel to the floor), but for medial occipital AVMs, I turn the head 45 degrees toward the floor (“nose down”) to allow the occipital lobe to fall away from the falx and open the operative working corridor. I install a lumbar drain to release cerebrospinal fluid (CSF) early and facilitate interhemispheric dissection. The incision is semicircular or U-shaped with a wide vascular pedicle. The craniotomy should be generous and for occipital AVMs should expose the posterior superior sagittal sinus, a portion of the torcula, and the ipsilateral transverse sinus. The base of the dural opening is divided on the superior sagittal sinus medially and the transverse sinus inferiorly. I dissect along the falx within the interhemispheric fissure to reach the falcotentorial junction, and open the quadrigeminal cistern to drain additional CSF and slack the brain. Parasagittal draining veins can impede arachnoid dissection by tethering the medial hemisphere to the midline and counteracting gravity retraction. The veins can be carefully untethered along their subdural length over the interhemispheric space so that the hemisphere can be mobilized. However, they should be saved until the end of nidal dissection. This anatomic obstacle should be noted preoperatively and a plan formulated for a wider and adjusted craniotomy so that the appropriate working angles around the veins can be attained. The steps for AVM devascularization are as follows: First, I identify the interhemispheric ACA feeders, isolate them to the level of the nidus, and disconnect them. Distal ACA feeders are typically found at the anterior and inferior margins of the AVM. These feeders are mostly terminal ACA branches that can be safely sacrificed. Next, I isolate the PCA feeders at the posterior border of the AVM; these vessels can be en passage arteries and therefore require skeletonization. I continue pial and sulcal dissection along the superior and posterior margins and progress in a spiral-like fashion circumferentially around the deeper sections of the lesion.
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