The corticectomies need very little individual description, outside of the general principles outlined in Chap. 5. That is to say, they include the general technique of subpial dissection/resection, the adherence to sulcal boundaries where possible, the preservation of any blood supply that is en passage over the corticectomy designated for resection, the preservation of as much subcortical white matter as possible, and the minimization of the scar left behind.

The commonest frontal corticectomies are those of the dorsolateral and orbital surfaces. The former are usually large. If it is remote from the precentral sulcus and Broca’s area, then it is not a complex resection and requires only the reiterated summary of the surgical principles, pertaining to corticectomies in general, as outlined previously (Chap. 5).

In those corticectomies encroaching upon the precentral sulcus and/or the sulcus immediately in front of Broca’s area, there is emphasis on making certain that there is no associated injury to the gyral surfaces across these sulci that are being left behind and the preservation of their function is mandatory in order to preclude a postoperative neurological deficit. In my view all such resections in which the eloquent cortex is involved should be undertaken in the awake state and with continual clinical monitoring of the function of the eloquent cortex that must not be compromised.

As was outlined under the periphery outline of a corticectomy (Sect. 5.​5.​3) and in the chapter devoted to a description of Rolandic cortical resection (see Chap. 8), the dissection at the bottom of the sulci around the speech and motor areas should be kept as shallow as possible. Thus, when the depth of the sulcus has been reached, then the immediate subcortical white matter should be left as completely intact as possible. This is outlined in Sect. 5.​5.​3 and is illustrated again here in Fig. 7.2. Figure 7.2a is very similar to previously illustrated examples (see Figs. 2.​8 and 5.​3a), in which the depths of sulci are junctions between epileptic and eloquent cortices and in which the former is to be removed, while the latter is to be preserved with respect to its integrity and connectivity. In this particular case the sulcus is the precentral sulcus. Figure 7.2b illustrates the very restricted removal of the cortex on the side of the sulcus while leaving the eloquent cortex intact on the other side of the sulcus. Finally, Fig. 7.2c illustrates that once the inferior extent of the gray matter of the cortex in the depth of the sulcus has been reached, then the ensuing incision in the subcortical white matter should not be taken more deeply, but rather should be brought out superficially. This will guarantee optimizing the preservation of the normal neuronal connectivity of those afferent and efferent fibers of the eloquent cortex. In this case the incision will be directed anteriorly, e.g., “away from” the precentral gyrus, as also previously illustrated in Fig. 5.​3a2. Further, and most importantly, it should be done immediately after reaching the bottom of the sulcus. Any resection of the abnormal cortex juxtaposed to the eloquent cortex usually will be associated with at least some transient clinical deficit, which reflects the function of the eloquent cortex, as discussed in Sect. 5.​5.​3. If there is a significant delay between the removal of the abnormal cortex that initiates a clinical deficit and the termination of the resection of that area of the cortex, there is always a risk that the surgeon may lose the clinical marker by which to judge whether additional surgery in the area is encroaching on parenchyma, the destruction of which may produce a permanent postoperative neurological deficit. Taken to the extreme, if the delay allows a completed deficit of the function of the eloquent cortex, then the safety of any further resection is completely unknown (again this was strongly emphasized in Sect. 5.​5.​3). Thus, in this circumstance the length of the time to complete this part of the surgery should be minimized as much as possible.

“Pure” medial corticectomies are the least common of the frontal lobe corticectomies. However, all of the resections of the frontal lobe that involve the medial surface, e.g., within the interhemispheric fissure (IHF), may encounter adherence between the leptomeningeal investment of the two hemispheres over those areas in which there is not an intervening dural membrane, e.g., the falx cerebri or crista galli. The latter, if present, precludes any such adherence. If this bihemispheric leptomeningeal adherence is either very patchy or very weak, then it may be relatively easily separated and it is usually an advantage to separate it. However, if the adherence is very strong, e.g., to one another, in which the eventuality of its separation is difficult and thought to possibly be associated with parenchymal damage in the contralateral hemisphere, then it is better to cease trying to separate it and use an alternative strategy.

Under these circumstances the retraction of the leptomeningeal investment along with the underlying parenchyma, in the presence of bilateral adherence of the leptomeninges, is best discontinued in favor of simply utilizing pure subpial dissection over the area of adherence. The adherent leptomeningeal investment is then simply left attached to its contralateral counterpart and is incised around its periphery (of the adherence). In this way the adherence is circumvented and the ipsilateral meningeal investment is simply left intact with its contralateral counterpart. This is illustrated in Fig. 7.3. Figure 7.3a is a diagrammatic representation of the interhemispheric fissure (IHF) between the frontal lobes anterior to the corpus callosum in which there is an area of adherence of the bilateral medial leptomeninges, i.e., bihemispheric leptomeningeal adherence (“bh.l.ad”), above and below which there is lack of adherence. Figure 7.3b discloses the initiation of the right frontal corticectomy with the retraction of the right frontal superomedial cortex inferiorly until the upper part of the area of bilateral leptomeningeal adherence is reached. Prior to reaching this adherence, the isolation of the medial cortex, preparatory to its removal, has been achieved by simply retracting the superior frontal gyrus. However, upon encountering the very adherent middle part the simple retraction is supplanted by an incision in the leptomeninges superior to the adherence (“l.i.1”) and then progressing with subpial resection (Fig. 7.3c). The latter is then continued until the inferior aspect of the leptomeningeal adherence is reached after which a further leptomeningeal incision (“l.i.2”) is made and carried around the remainder of the periphery of the adherence. Following this, as illustrated in Fig. 7.3d, the adherence has been taken care of and is now followed by the same technique that was used initially at the superior frontal gyrus, i.e., the simple separation of the remaining inferior medial cortex of the corticectomy. The ipsilateral adhering membrane is thus simply left intact with its contralateral counterpart.

The least common of these medial frontal corticectomies are those that are anterior to the genu of the corpus callosum. Many of these involve also one of the juxtaposed surfaces, i.e., inferior (orbital) or the parasagittal part of the dorsolateral surface. The latter usually involves the parasagittal surface of part of or the entire superior frontal gyrus. Since this parasagittal cortex represents the watershed between the anterior (ACA) and middle cerebral (MCA) arteries, there is less concern about compromising the blood supply of the dorsolateral cortex being left behind. This is true for the anterior boundary of this corticectomy that is nearly always taken to the pole, and it is similarly true of the inferior surface, which, again, is nearly always taken to the floor of the anterior fossa. Thus, these three peripheries of the corticectomy are usually functional watershed areas, forming the equivalent of end arterial irrigations.

The posterior periphery of the anteromedial corticectomy is also usually straightforward. However, if the posterior boundary is at the genu of the corpus callosum (CC) and the callosomarginal branch of the pericallosal artery leaves the parent artery anterior to the genu, then it becomes an en passage artery overlying the posterior surface of the corticectomy, as it is usually a major branch irrigating the functional cortex (e.g., paracentral lobule) more posteriorly, and thus it must be preserved. The problem confronting the surgeon at this point is whether indeed it is the important callosomarginal artery or whether it is the frontopolar artery. Usually the distinction between the callosomarginal and frontopolar arteries can be achieved by simply following the course of the artery in question until its primary direction, e.g., anterior or posterior, is clarified. The frontopolar artery proceeds anteriorly and irrigates the medial and parasagittal cortex that is being removed and thus can be sacrificed, while the former proceeds posteriorly, as noted in the foregoing, and requires preservation. Under these circumstances the easiest manner of achieving its preservation is simply by “skeletonizing” it (see Sect. 5.​2). Thus, incisions through the covering leptomeningeal investment of the curvilinear portion of the artery allow the removal of underlying parenchyma of the corticectomy and coagulation and incision of any associated branches that are irrigating it.

It should be recorded that it is not uncommon to find the bilateral leptomeningeal adherence between the two frontal lobes, requiring the procedure noted in the last section (Sect. 7.2.3 and Fig. 7.3).

The posteromedial corticectomy is that which is behind the genu of the corpus callosum. It is more complex by virtue of the fact that it usually involves some part of the supplementary motor cortex (SMA), may involve a posterior resection line (PRL) in the depth of the precentral sulcus, and will nearly always involve the necessity of preserving at least one major branch of the pericallosal artery, i.e., the callosomarginal artery. Often the latter artery or additional arteries will course throughout the anteroposterior extent of the corticectomy and irrigate parenchyma behind the PRL; that is to say, they are en passage vessels. These then must be skeletonized and preserved (see Sects. 5.​2, 7.2.3, 7.2.4, 7.3, 7.4, and 8.​4).

Once again, if the periphery is composed of the precentral sulcus, then the resection in an awake patient is much more satisfactory than one under general anesthesia. There is no doubt that a potential postoperative weakness of the contralateral leg is less alarming than one in the arm, which would be the risk of a similar corticectomy involving the dorsolateral cortex. Nevertheless, I would consider that the ability to clinically monitor the patient for any threat to the motor function of the leg during such surgery makes it mandatory to carry out such operations under local anesthesia. Further, once the resection down to the depth of the precentral sulcus is reached, then the immediately ensuing subcortical incision should be carried superficially in the subcortical white matter, for at least a centimeter or more (see previous Sects. 7.2.2 and 5.​5.​3).

The surgical preservation of the pericallosal branches in a large posteromedial corticectomy is similar to other resections that require the preservations of significantly sized arteries, e.g., the radical aFLY (vide infra), the radical TLY, the inferior Rolandic corticectomy, etc. In both this corticectomy and the radical aFLY, the approach to the posteromedial cortex area will be from above, e.g., superior to the major vascular branching of the pericallosal artery. If the corticectomy involves the dorsolateral aspect of the superior frontal gyrus (SFG), then the lateral resection line will be initiated over that surface, as well as the posterior resection line (PRL) in the SFG, prior to engaging the medial surface. The posterior incision line, including the parenchyma with its overlying investment, will then be continued over the junction of the dorsolateral and medial surfaces down the medial side of the SFG. Assuming that the inferior margin of the corticectomy includes the entire cingulate gyrus, then, as this incision is advanced inferiorly, there must be an ongoing continual visual examination of the incision line for arterial vessels. Those on the surface will be seen easily, but it is the surgeon’s responsibility to examine very carefully each (medial) sulcus during the advancing subpial dissection, before it is coagulated and incised, in order to be certain that it does not contain a vessel that must be retained. In the absence of a significant vessel in the depth, then the sulcal leptomeninges can be coagulated along with some more inferior medial surface leptomeninges and incised. This forms the first of such steps in the gradual establishment of a medial surface PRL, e.g., coagulation and incision of gyral surface leptomeninges followed by subpial dissection of the next sulcus and in the absence of a significant vessel its coagulation and incision, and so on! This is illustrated diagrammatically in Fig. 7.4. Figure 7.4a shows the callosomarginal artery located in the depth of a medial sulcus in both the coronal section (a1) and in the medial midline sagittal section (a2). Once a vessel is encountered, the onus on the surgeon is to determine its importance, vis-à-vis whether it needs to be saved. In nearly all instances in this very posterior location, any reasonably sized vessel should be saved, as almost certainly its size would primarily be a reflection of a significant field of irrigation posterior to the posterior resection line.

Once a significant vessel is encountered, the incision, which up to this point has included both parenchyma and leptomeningeal investment, will now be one which involves only cortical parenchyma, as the surgical strategy will now involve only subpial resection, per se. It may be conducted by blunt dissection or suction and is similar to the leaving behind of the area of bilateral leptomeningeal adherence outlined in the preceding section (Sect. 7.2.3) and illustrated in Fig. 7.3. That is to say, the remaining part of the incision, down to the corpus callosum, must be conducted subpially, involving therefore only removal of parenchyma and leaving the leptomeningeal investment intact. Figure 7.4b illustrates the gradual deepening of the PRL involving leptomeningeal incision and resection, during which the callosomarginal artery is identified within the cingulate sulcus. In this figure the callosomarginal artery which was initially in the depth of the sulcus (Fig. 7.4a) has now been freed from any associated parenchyma as a result of subpial resection and, along with the leptomeninges overlying the sulcus and the pial meninges of the sulcus, which in fact remains covering it, is retracted medially (Fig. 7.4b1), and the ongoing subpial resection is carried inferiorly onto the cingulate gyrus. Figure 7.4b2 illustrates the advancement of the dissection/resection anteriorly through the initiation of a posteroanterior leptomeningeal incision (“p-a.l.i.”) along the leptomeninges superior to the callosomarginal artery, i.e., by either separation by blunt dissection or piece meal subpial resection of the cortex down to the corpus callosum; at this juncture the ipsilateral pericallosal artery will be evident through the leptomeningeal barrier when the bottom of the cingulate gyrus is reached. Once the commitment to subpial resection has been made, the surgeon is not dependent upon identifying any further large arteries more inferiorly, in order to preserve them, as all these arteries will be preserved by virtue of their being outside the meninges, which are being used as a protective barrier in the subpial dissection. The subpial dissection will be completed inferiorly with the removal of gray matter down to and including that in the sulcus of the corpus callosum (Fig. 7.4c1). Figure 7.4b2, c2 also depicts the important aid of the superior fronto-lateral retraction of the freed portion of the corticectomy.