Corpus callosotomy (CCY) is not a resective epilepsy operation, but rather comes under the heading of “interruption of conducting pathways,” which is another group of operations that are utilized in special cases of epilepsy. I am covering it here very briefly, as it is a commonly utilized epilepsy operation; further, it does form part of the functional hemispherectomy (fHSPY), which will be discussed in the next chapter.

The development of the corpus callosum (CC) begins during the fifth week of fetal life (39 days) with the formation of the primitive lamina terminalis, which thickens to form the commissural plate. Failure of development of this plate results in a lack of the necessary guide for the CC crossing fibers and, hence, a probable agenesis. Glial cells coalesce to form a bridge-like structure, which serves as a guide for the callosal fibers crossing the longitudinal cerebral fissure to reach their target positions in the contralateral hemisphere. The interhemispheric projection of the first axons is preceded by microcystic degeneration in the commissural plate and physiological death of astrocytes. The lack of degeneration of the plate leaves a glial barrier that will interfere with the crossing corpus callosal fibers and, hence, an abnormal, or lack of, development of the CC. The earliest callosal axons appear at ~74 days in the human embryo, the genu and the splenium are recognized at 84 days, and the adult morphology is achieved by 115 days.

Neurons giving rise to commissural fibers from the neocortex are located in the supra-granular layers of the cortex, mainly layer III, and terminate in the vicinity of layer IV of the homotopic area in the opposite hemisphere. There is a paucity of fibers representing the distal portions of the limbs and most of visual area 17 (except near the border with area 18), as compared to most other neocortical areas.

The CC is the largest of the three hemispheric commissures. It primarily connects homotopic areas of the two hemispheres with one another. It develops in concert with the neocortex. In humans, it consists of ~200 million fibers. It consists of four relatively easily identifiable parts: rostrum, genu, body, and splenium, anteroposteriorly (see Fig. 11.1). The CC is something of the order of 10 cm in length, with its anterior end (genu) ~4 cm behind the frontal pole and the posterior end (splenium) ~6 cm anterior to the occipital pole.

Given that there is no neuronal cortex or neuronal networks in the CC, what is the rationale for a so-called corpus callosotomy? Experimental animal investigations have shown that (1) bilaterally synchronous discharges augment seizures with bilateral motor manifestations; (2) the CC can be shown to be a major pathway for seizure propagation, as noted in the well-described electroencephalographic “secondary bilateral synchrony” (with a transcallosal conduction time of ~5–15 ms); and (3) a CC section decreases or abolishes synchronous epileptiform discharges. Erickson showed in monkeys that electrically produced seizures in one hemisphere spread to the other, but that corpus callosotomy abolished the interhemispheric spread (1940). These types of observations have led to the view that the CC is heavily involved in the transmission of facilitation—particularly in the context of bihemispheric epileptogenicity. However, the experimental literature leaves little doubt but that there are alternate pathways, besides the CC, that provide the neuronal substrate for the appearance of bihemispheric synchrony.

Van Wagenen and Herren experimented with a corpus callosotomy (CCY) in a few human patients after a long practice history of observations in which the spontaneous destruction of transcallosal tissue by naturally occurring phenomena, e.g., stroke, tumors, etc., led to a significant reduction of seizures (1940). Their operations had a number of outcomes, but certainly the authors were convinced that there was a general reduction in seizures. Others followed who conducted also the incision of multiple interhemispheric commissures in addition to the CC, e.g., anterior commissure and hippocampal commissure (Bogen et al. 1965, 1969; Luessenhop 1970; Luessenhop et al. 1970; Wilson et al. 1975). However, surgery of multiple commissures caused significant concern about postoperative morbidity. Wilson and colleagues initially experimented with incising all three cerebral commissures (Wilson et al. 1975) but eventually felt that CCY alone was the most appropriate and safest, from the point of view of neurological and neuropsychological outcomes (Wilson et al. 1978, 1982). A review of the changes to the procedure over the middle of the twentieth century can be found in excellent reviews by Spencer et al. (1987) and Maxwell et al. (1987).

The indications for the conduct of a CCY are much less rigid than in most of the resective operations that have been discussed to this point. The group of indications might be looked upon by the uninitiated as a potpourri of intractable epileptic disorders. The indications are less generally accepted and vary considerably from one center to another. Indications include (1) seizures with absence of resectable epileptic foci; (2) cases of multiple seizure types, especially if the most troublesome consists of “drop attacks”; and (3) frontal epileptic foci with rapid secondary bilateral synchrony.

In the article by Maxwell et al. there is a very good account of the historical evolution of the CCY over the twentieth century after the pioneering work of Van Wagenen and Herren (1940). Their discussion is replete with the variety of supportive measures of the procedure used at the University of Minnesota. Certainly there is marked contrast between what I will describe below and what they utilized. For example, I have never used preoperative steroids or intraoperative antibiotics or mannitol nor found it necessary to consider dividing the sagittal sinus. I would agree that if the ventricular (ependymal) surface can be left intact, it is probably preferable, but if the surgeon feels that this might compromise the completeness of the incision in a particular part of the CC, then I have no concern about recommending being more radical, at the expense of placing rents in the integrity of the ventricular surface, which I have never felt resulted in any complications.

The common craniotomy, per se, is demonstrated in Fig. 11.2a; this is a narrow ~2–3 cm wide (mediolateral) parasagittal opening into the nondominant frontal lobe, approximately centered, in an anteroposterior direction, over the coronal suture. Figure 11.2b demonstrates the common scalp incisions that have been noted in the literature in order to gain access to the amount of bone requiring exposure for the craniotomy. Figure B1 depicts a typical parasagittal craniotomy incision, which is perhaps one of the commonest incisions used. Figure B2 discloses a transverse incision, near or just anterior and parallel to the coronal suture. I have included figure B3 simply because I used this incision for a number of years. It works perfectly well but it is unnecessarily more difficult in its production and certainly much more in its closure at the end of the CCY. Figure B4 is an anterior midline incision and one that was commonly used for the first phase of the CCY, by those carrying out the CCY in two phases (Wilson et al. 1982; Maxwell et al. 1987). The Dartmouth group chose to use two transverse incisions and 5 cm trephinations, one just anterior to the coronal suture and a second more posteriorly located (Roberts 1985; Roberts and Siegel 2001). My own preference is shown in figure B5, which is a simple generous length of a midline incision running from just behind the hairline of the forehead back to the lambda area. This incision is very easily extended anteriorly and/or posteriorly if more surgical “room” is required medially, laterally, or anteroposteriorly. Further, the clearly delineated tissue planes provide for a very easy surgical closure.