Some corticectomies, which are at the anatomically peripheral limits of parts of the cerebrum, may be conducted without concern about the preservation of any tissue within the boundaries of the corticectomy. This, for example, would apply to most corticectomies involving the poles of the hemisphere, e.g., temporal, frontal, and occipital. (If sufficiently large, then they are better designated as polar lobectomies, as opposed to corticectomies.) However, it is rare that the surgeon might be faced with difficulties of preserving en passage arterial vessels in those instances, as these more peripheral corticectomies will rarely have overlying arterial vasculature that must be preserved, or underlying white matter that must be preserved. This derives from the fact that the vasculature is “end arterial vasculature”; that is to say, the vasculature in these cases ends in the cortex that is targeted for removal. Thus, there is no reason for its special preservation. The same may be said for the underlying white matter, in which all of the subcortical afferent and efferent axons are subservient to the cortex being removed.

In the conduct of a resection of a small portion of the cortex (corticectomy), the onus upon the surgeon is to distinguish between two groups of vessels traversing the cortex. One group consists of those vessels whose only areas of irrigations are confined to those within the cortex to be removed, the “end arterial vasculature” referred to in the foregoing paragraph. The other group consists of those vessels, which are traversing the area of the cortex to be removed but are involved in irrigating the cortex that is not to be removed, but rather to be preserved. Such vessels may also provide as well some irrigation of the parenchyma of the proposed corticectomy. Irrespective of whether they irrigate any part of the proposed corticectomy, any such traversing vessels, which provide significant irrigation of the cortex that is to be preserved, are termed en passage vessels. Those vessels on the surface of the gyri, whose areas of irrigation are solely within the cortex to be removed, can usually be visualized without difficulty and their resection is straightforward. The en passage vessels, whose end fields of irrigation are outside the corticectomy being conducted, must be preserved and can usually be recognized relatively accurately. The best rule of thumb is that any arteries, which may be en passage vessels, should be preserved, until, with further dissection, they can be clearly shown to be vessels irrigating only the cortex to be removed. The preservation of such vessels may be easily achieved, especially if they simply pass over a small corner of the corticectomy without contributing to the irrigation of the parenchyma of the corticectomy. However, en passage arteries that do contribute to the parenchyma of the proposed corticectomy must be what is called “skeletonized.”

Skeletonization literally means “reducing to a skeleton.” It has become used in vascular surgery to describe the isolation of a portion of a vessel, usually an artery, by the removal of the tissues adherently associated with it over a part of its course. In the conduct of a corticectomy over which an arterial vessel crosses it to irrigate cerebral parenchyma that is to be preserved, then the vessel, which will be called the “parent” vessel, must also be preserved. Its preservation is achieved by its skeletonization. The adherent tissues that must be removed from the artery over its course across the corticectomy site include leptomeninges and any branches of any size leaving the artery to irrigate only the corticectomy parenchyma. This requires the identification of the small branches of the en passage artery, which irrigate the parenchyma of the corticectomy. The skeletonization is most easily accomplished by initially carefully incising the leptomeninges throughout the course of the artery crossing over the corticectomy site. When this is completed the arteries confined to irrigating the corticectomy parenchyma are exposed over satisfactory lengths for their coagulations and incisions. For the very small branches, small narrow coagulation bipolar forceps blades and the use of very low coagulating current are most valuable in avoiding injury to the parent artery. Injury results from inadequate low current coagulation and an eventual tearing of the branch at its origin from the parent artery. This can nearly always be avoided by spending a small amount of extra time to isolate a good length of the small branches, e.g., 1–2 mm, in order to have sufficiently exposed vessels that their coagulation and incisions are somewhat remote from the parent vessel. Failure to gain such room in which to isolate them properly will, on occasion, lead to the branch vessel being ruptured at its exit from the parent vessel, which can then be a nuisance to stop, and may even end up with obstruction of the parent vessel. This unpleasant complication usually only requires a single occurrence before the described method of securing a satisfactory length of the vessel with which to work, before coagulating and incising it, is always followed! Once the small vessels irrigating the corticectomy parenchyma have been incised and the en passage vessel is separated from its leptomeningeal investment, it is truly “skeletonized” and the cortex beneath can be removed without injuring the vessel. When an en passage vessel is large, or delicate, or there is more than one, the cortex may have to be removed piece meal.

Figure 5.1a is an illustration of an en passage branch of the middle cerebral artery (“epMCA”), emerging from the Sylvian fissure, overlying a right posterior temporo-occipital corticectomy, supplying not only the parenchyma of the proposed corticectomy but in addition some occipital cortex near the pole, which is outside of the proposed periphery of the corticectomy. The pebbled linear outlines depict the periphery (A2) of the corticectomy on the occipital cortex and the subcortical depth (A1) of the corticectomy. Figure 5.1b illustrates the “skeletonization” of the en passage vessel. This includes incisions in the juxtaposed leptomeningeal investment (B2) of the corticectomy parenchyma and coagulation and incision of those arterial branches irrigating the cortex being removed (“*”). Figure 5.1c illustrates the resection bed of the corticectomy and the preserved en passage artery.

While the recognition of the groups of surface vessels, as noted in the foregoing, is usually relatively straightforward, there is also the possible presence of deep, sulcal, arteries, which may not be managed quite so easily. These intrasulcal vessels are smaller in general, more delicate, and less visible, and their fields of irrigation frequently cannot be easily determined. Further, they do not have a leptomeningeal investment, but rather only a pial meningeal investment. It is tantamount that the surgeon uses all the gentleness of suction and low current coagulation strength, which have been emphasized previously, in order to not penetrate the pial membrane. There is always some very minor bleeding from the penetration of the pia by these small branches of the intrasulcal vessels, during the subpial dissection, but some simple gentle pressure for a few minutes from an inserted cottonoid patty can nearly always easily stop this bleeding. Again, the potential fragility of the intrasulcal vasculature cannot be over emphasized. Many of the intrasulcal vessels are sufficiently delicate that minimally applied traction to them may cause the rupture of their branches, or even of them. This is where the technique of applying countertraction (see Sect. 2.​2.​3) on the pia is so important. That is to say, the countertraction must be enough such that the pull of the suction does not disturb the normal position of the pial membrane. If the latter is achieved, then the tributaries of these vessels will not be compromised. Having said the foregoing, I must confess that very few en passage vessels of any significance travel in the depths of the sulci.

There are also specific operative procedures in which relatively predictable preservations of blood supplies are integral parts of the operative procedures, e.g., the peri-callosal and calloso-marginal arteries in frontal lobe resections (see Sects. 7.​2.​4, 7.​2.​5, 7.​3, and 7.​4), the middle cerebral arterial (MCA) branches and posteriorly coursing arteries in an anterior temporal lobectomy (see Sects. 6.​2.​4 and 6.​2.​6), the MCA branches irrigating superior dorsolateral Rolandic cortex in an inferior Rolandic corticectomy (see Sect. 8.​4), and the arterial branches to the eloquent cortex and white matter in extensive posterior hemisphere corticectomies. These will be considered more completely in later chapters.

The biology of neurons teaches us that their metabolic centers are located in their bodies, or somata, not in their axonal and dendritic branches. Thus, the removal of these somata leads to the death of the neurons. This same consideration applies to the neurons constituting an epileptic focus. The focus resides in the somata of the neuronal population, or network, giving rise to the seizures, i.e., the “gray matter” of the focus. Thus, the surgical treatment of epilepsy is primarily concerned with the removal of the somata of the neurons, or the cortical gray matter. In conducting a corticectomy it is indeed only the gray matter that must be removed. Thus, the removal of any focus can be achieved by the removal of only the gray matter of the focus. Taken to its logical conclusion, therefore, a corticectomy could consist of simply removing only the gray matter of the gyri involved.

The purpose of the foregoing preamble was simply to emphasize that there is no necessity of removing subcortical white matter. In actual fact there is nearly always a significant removal of some white matter in a corticectomy for a number of reasons. First, usually the function of the immediately underlying white matter is entirely associated with the cortex being removed. Under these circumstances its removal has no secondary complications, e.g., such as the involvement of a postoperative neurological deficit. Its vascular counterpart is the removal of only the branches of an en passage vessel feeding the corticectomy parenchyma, leaving the parent artery that irrigates healthy parenchyma being left intact. Second, it precludes the possibility of leaving behind some gray matter (e.g., deeper layers of the cortex). It is not always easy to recognize exactly the junction of the deeper layer of gray matter with the superficial subcortical white matter. Thus, in these cases it is safer to remove some of the underlying white matter in order to ensure that all of the cortical gray matter, i.e., the neuronal somata, has been removed. Third, the inclusion of some white matter with the cortical gray mantle often facilitates the actual surgical removal of the cortex. Fourth, a better quality specimen of the cortex, harboring the epileptic focus, is achieved from which to obtain satisfactory morphology.