2 Subarachnoid Dissection

The subarachnoid approach can be short with superficial AVMs on a cortical surface or long with deep AVMs in central or ependymal locations (Fig. 2.1). Superficial approaches to convexity AVMs require minimal subarachnoid dissection because the AVM is based right on the cortical surface (transfrontal, transtemporal, and transparieto-occipital approaches, Table 2.1). Intermediate approaches require more subarachnoid dissection down a fissure such as the sylvian and interhemispheric fissures or along a basal or dural plane (transsylvian, medial interhemispheric, and basal approaches, Table 2.1). Deep approaches are forays to the bottom of these fissures and beyond, through the corpus callosum, ventricles, and choroidal fissures, and along the skull base or tentorial planes (transcallosal, transchoroidal, and posterior fossa approaches, Table 2.1).

Although superficial AVMs are easily localized on their free surface, they are often concealed in arachnoid thickened by microhemorrhages, chronic inflammation, radiosurgery, or prior embolization. An opaque white veil must first be opened to reveal the AVM’s anatomy. Distended veins are freed from their arachnoidal investments and peeled back to expose AVM margins. Hemorrhage, both new and old, stains adjacent surfaces with hemosiderin and presents a trail to fresh clot, liquefied hematoma, or encephalomalacia. Hemorrhage can create extra-anatomic cavities that often connect with subarachnoid spaces. Some AVM margins separate from adjacent brain as a subarachnoidal plane, often following gyral contours into a sulcus. An example is the pure sylvian AVM, which sits entirely in the sylvian fissure without invading frontal, temporal, or insular parenchyma, and can be circumdissected completely with subarachnoid dissection and without subpial invasion.

Other than these cerebral and cerebellar convexity AVMs that are superficial and immediately apparent on brain surface, the rest require more dissection to expose the AVM. Transsylvian approaches expose eight lateral and deep AVM subtypes; interhemispheric approaches expose eight medial and deep subtypes; skull base approaches expose eight basal, brainstem, and cerebellar subtypes; and transtorcular approaches expose six posterior, cerebellar, and brainstem subtypes (Table 2.2). All of these AVMs have their base on a surface buried in a fissure that must be opened, or along a basal plane that does not face the axis of the approach. The initial survey of the brain after dural opening may reveal only an arterialized vein or nothing abnormal at all. Consequently, these AVMs require bilateral craniotomies, gravity retraction, or transventricular access. These buried AVMs are localized by following arterialized veins around corners and into fissures, maneuvering around and deep to these dilating veins, and separating adhesions to the falx, tentorium, or skull base. For the deepest approaches, subarachnoid dissection extends further into ventricles or deep cisterns around the brainstem to reach the AVM base and optimize the angle of attack. At such depths, the AVM axis and angle of attack are often perpendicular, and expanding the surgical corridor with extensive dissection can improve visualization and maneuverability in the working channel.

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