The Box

Arteriovenous malformations (AVMs) come in many shapes: spheres, ellipses, cylinders, and, classically, cones. The cone is based on the cortical surface and tapers to a tip at the ependyma. Although AVMs are never cube shaped, it is useful to conceptualize them as a box with six sides: one top (or superficial side), four sides, and a bottom (or deep side). This orthogonal conceptualization applies structure to an otherwise amorphous tangle, giving the AVM definable sides that are medial and lateral, anterior and posterior, superior and inferior.

These distinct sides can then be characterized by relationships to dural structures, cortical landmarks, feeding arteries, draining veins, fissures, and cranial anatomy. When preparing for surgery, I study computed tomography (CT) scans, magnetic resonance (MR) images, and angiograms scan by scan, sequence by sequence, and run by run, to familiarize myself with the AVM in ways that are not possible in surgery. Angiograms can replay forward and backward to see relationships between arteries and veins, to analyze bifurcations or tortuosities that may be buried in a sulcus, and to identify embolic agents that might serve as landmarks intraoperatively. Magnetic resonance images can be scrolled up and down to localize the motor strip or Broca’s area, to identify clean pial planes in a fissure, or to map out the adjacent clot as a route of access. The box enables the neurosurgeon to organize AVM anatomy as this information is gathered. A side can be “hot” or “cold” depending on its arterial input; a side can be eloquent or non-eloquent depending on its proximity to critical neurologic structures; a side can be pial, parenchymal, or ependymal, depending on its depth in the brain; and a side can be ruptured or unruptured depending on the presence or absence of an adjacent hematoma.

The box concept also organizes microsurgical dissection. Hot sides are confronted early to de-arterialize the AVM, whereas eloquent sides are confronted late when the nidus is quiet. Ruptured sides are also dissected early when hematoma evacuation accesses the nidus or relaxes the brain. The box localizes specific arterial feeders. For example, paramedian parietal AVMs receive anterior cerebral artery (ACA) input on its anterior side, middle cerebral artery (MCA) input on its lateral side, and posterior cerebral artery (PCA) input on its inferior side. Similarly, the box localizes draining veins, deep tracts, and ventricles, and the box paces progress as dissection advances from top to the four sides to bottom.

Surfaces