Chapter 112 Corpus Callosotomy

Indications and Techniques

Edward F. Chang, Nathan C. Rowland, Nicholas M. Barbaro

In 1940, William P. van Wagenen and R. Yorke Herren described a small series of patients with various disruptive lesions of the corpus callosum.¹ Their report highlighted the phenomenologic distinction between generalized convulsive seizures in individuals with tumors near the intact callosum and only partial seizures following insidious invasion of the corpus callosum months to years later. The hypothesis that interruption of the corpus callosum could alter the bihemispheric spread of seizure activity in epileptic individuals led to the simple yet elegant conclusion that surgical division of the corpus callosum might confine epileptogenic foci to a single hemisphere and thus ameliorate secondary seizure generalization. At the time of their writing, the practice of excising regions of brain tissue to eliminate seizures had been firmly established, although with varying rates of success.² Nevertheless, the historical significance of van Wagenen and Herren’s careful observations cannot be overstated. They set the stage for the first corpus callosotomy lesions performed with the intent of stopping seizure propagation to the contralateral hemisphere in individuals with intractable epilepsy, marking a critical advancement in the field of modern epilepsy surgery.

In the 10 cases of corpus callosotomy that constituted van Wagenen and Herren’s landmark report, there was significant reduction in seizure frequency and with few permanent adverse effects.³ Curiously, however, the practice of corpus callosotomy for treatment of epilepsy was rarely used until over 20 years later, when the next case reports of corpus callosotomy in humans appeared and the procedure began to gain acceptance within the neurosurgical community.⁴^–⁶ The procedure itself has undergone various technical modifications, including total versus partial callosotomy, anterior versus posterior approaches, single versus multistaged operations and even radiosurgical callosotomy.^7,⁸ Furthermore, 21st-century technology for less invasive neurosurgical approaches, such as vagal nerve stimulation,⁹^–¹¹ and the advent of novel antiepileptics have led to reconsideration of the role of surgical corpus callosotomy for patients having intractable seizures without a focally resectable lesion. However, the safety and efficacy of this procedure have been substantiated in over 60 years of empiric and retrospective studies in humans and experimental studies in animals.^1,^4,^6,¹²^–¹⁴ Van Wagenen and Herren’s seminal work directly contributed to the first theoretical interpretation of “split-brain” syndromes by Andrew J. Akelaitis,¹⁵^–¹⁷ and eventually culminated in the awarding of a Nobel Prize to the neuropsychologist Roger Sperry for his innovative tachistoscopic experiments on disconnected patients.¹⁸ Sperry’s insights subsequently led to more detailed anatomical and physiological studies of callosal function, which have helped shape our contemporary understanding of the functional lateralization of the cerebral hemispheres.

Modern Indications

Corpus callosotomy was developed specifically for treatment of generalized seizures, which can include numerous abnormal movement phases (tonic, clonic, and atonic). Of these variations, drop attacks (both tonic and atonic) remain the primary indication for corpus callosotomy, as repeated studies have shown superior efficacy for this seizure type.^7,¹⁹^–²¹ Nevertheless, all forms of generalized seizures, including tonic, clonic, tonic-clonic, myoclonic and absence seizures, have been reported to respond favorably to corpus callosotomy.²² In some studies, corpus callosotomy has also been reported to reduce partial seizures as well, although at significantly lower rates. ²³ Additionally, callosotomy has been reported to reduce seizures in several childhood epilepsy disorders including Rasmussen’s encephalitis^24,²⁵ and Lennox-Gastaut syndrome,^26,²⁷ as well as infantile hemiplegia and frontal lobe epilepsy.¹⁹

In most modern epilepsy surgery centers, corpus callosotomy is not considered a primary surgical treatment in patients who might benefit from focal resection of epileptogenic regions.²⁸ Controversy remains regarding whether forms of mental retardation should preclude patients from callosotomy,^7,¹² although intelligence quotient has not been found to significantly predict outcome following callosotomy.²⁹ Patients with crossed dominance have been reported to have less favorable outcomes, presumably because these patients require cortical interconnections for routine function to a more significant extent than those without crossed dominance.¹⁹

Preoperative Evaluation

The preoperative assessment of patients considered for corpus callosotomy is comprehensive and should adhere to three general principles. First, seizure intractability ³⁰ with multiple anticonvulsants used in combination should be established. Second, a resectable epileptogenic focus should be excluded, since alternative procedures such as lesionectomy or lobectomy have higher seizure control rates. Third, it should be communicated to patients that callosotomy is considered a palliative procedure, and patient expectations and goals for the surgery should be carefully explored. Other specific components of the workup should include a complete physical examination, battery of neuropsychological testing, magnetic resonance imaging of the brain to evaluate structural lesions and corpus callosal anatomy, and EEG with video monitoring of ictal and interictal states.^7,¹⁹

Surgical Approaches

Rationale

The topographic organization of the corpus callosum follows a bilaterally connected arrangement in the anterior-posterior plane. The anterior portion of the corpus callosum carries fibers connecting frontal cortical regions, including anterior cingulate, premotor, motor and anterior insular areas.¹⁰ Secondary generalization is therefore dependent on anterior motor callosal fibers during tonic-clonic and atonic seizures. Posterior fibers transmit somatosensory and visual information from the parietal and occipital lobes as well as information from association cortices.³¹ Hence, severing these posterior connections can produce a perceptual disconnection syndrome, which is the rationale for preserving the splenium.⁷ Nevertheless, posterior callosotomy may still be reserved for seizures refractory to anterior section alone.³² Specific patients, such as those with severe, bilateral seizures, may even benefit from initial complete callosotomy, although this has not been proven.⁷ It should be pointed out that the corpus callosum is one of several midline commissures. Others include the anterior commissure, the dorsal hippocampal commissure, and the massa intermedia of the thalamus. The callosum is the primary pathway through which seizures become secondarily generalized. However, seizure propagation may also be transmitted through these alternative pathways. Presumably, in such cases, these alternate pathways may be the basis for residual seizure activity following callosotomy.

Techniques

Historically, in addition to sectioning the corpus callosum, section of other major commissures have been attempted, including fornicotomy, anterior commissurotomy, and division of the massa intermedia of the thalamus.^1,¹⁹ Presently, an anterior two thirds callosotomy is the most commonly performed initial procedure.

Patients preparing for surgical callosotomy should be maintained on antiepileptic therapy at preoperative doses. Corticosteroids and appropriate prophylactic antibiotics are given at the beginning of the case.¹⁹

Modern corpus callosotomy techniques are aided by the use of a frameless stereotactic guidance system. This technology assists in precisely mapping the margins of the corpus callosum, especially the rostral-caudal extent. The patient is positioned in the supine position and draped for an incision which will ideally allow for visualization of both genu and splenium without significant brain retraction.⁷ Various incision shapes have been used, including an S-shape, bicoronal and midline straight incision with a curve toward the side of the craniotomy (preferred by these authors) (Figure 112-1). Lateral decubitus positioning has also been described with gravity assisting in retraction of the dominant hemisphere.¹⁹ Most reports use a right-sided approach presuming that this is the language non-dominant hemisphere. Others, including these authors, have used pre-operative Wada tests to determine which hemisphere is non-dominant. These patients have been shown to have a higher than normal incidence of right-sided language function, thereby making a routine right-sided approach somewhat more at risk for postoperative language dysfunction. Up to four burr holes are created, with two parasagittal holes 1 cm from midline, separated by 12 to 15 cm in the anteroposterior plane, and two lateral holes approximately 10 cm from midline.⁷ This creates a bone flap that gives a direct approach to the interhemispheric fissure with minimal risk of entry into the superior sagittal sinus. The dimensions of the bone flap can be modified with regard to subsequent planned posterior procedures, and can be directed by the frameless stereotactic navigation images. Dural incision is made in relation to the arrangement of draining bridging veins, which should be preserved to reduce the possibility of venous infarction. An acceptable exposure should maintain a 1.5- to 2-cm window between bridging veins.⁷ The superior sagittal sinus should serve as the dural hinge. Brain relaxation can be achieved using standard techniques such as positioning, CSF drainage during arachnoid dissection, and ventilatory control.¹⁹

FIGURE 112-1 Positioning, craniotomy, and exposure during corpus callosotomy. Supine positioning with a C-shaped scalp incision is made for interhemispheric exposure. Craniotomy should be carried as close as possible to the superior sagittal sinus or slightly distal to it to reduce retraction needed for the interhemispheric approach. Meticulous microdissection should be carried out to identify the callosomarginal and pericallosal arteries, as well as the cingulate gyrus.

For a partial anterior callosotomy, dissection should begin at the posterior border of the planned resection and proceed anteriorly. The posterior border of resection typically lies within 5 cm from the genu. Some authors also utilize electrical stimulation to map the interhemispheric lower limb motor cortex in estimating the posterior edge of resection, although use of frameless stereotactic guidance makes this less necessary and allows the anesthesiologist to maintain paralysis throughout the microsurgical portion of the procedure.⁷ In complete callosotomy, visualization of arachnoid covering the vein of Galen and internal cerebral veins establishes the posterior edge of resection. Interhemispheric dissection can be accomplished with self-retaining retractors (Figs. 112-1 and 112-2). When an incomplete falx is encountered, distinct pia and arachnoid layers may not be discernible. In such cases, it is necessary to develop a plane between the cingulate gyri. Complete preservation of one cingulate gyrus with mild retraction damage to the other is preferable to bilateral damage with resultant neurologic deficits. Care should be taken to complete the dissection below the cingulate, as one can mistake a fused cingulum for the callosum.⁷ This can be avoided with staged use of the frameless stereotactic navigation system which should be very accurate with respect to depth of the callosum even when there is mild sagging and retraction of the superficial brain structures. In addition, experienced neurosurgeons can identify the glistening, white, and relatively avascular corpus callosum. While the dissection is typically accomplished between the paired pericallosal arteries (Fig. 112-3), anatomic variations such as fused or densely adherent arteries can make dissection lateral to both arteries the preferred approach. However, one must be aware of delicate perforators extending from the cingulate gyrus.⁷ The callosal transection is typically carried out under the microscope using a combination of microaspiration and bipolar electrocautery.¹⁹ The inferior edge of dissection should ideally extend to the ependyma but remain extraependymal as to avoid entering the ventricular system. However, complete preservation of the ependymal layer is rarely possible. Complete transection of all commissural fibers should take precedence over preservation of the ependymal layer. The anterior cerebral arteries and anterior commissure typically define the anterior and inferior borders of resection, respectively. Here one encounters anterior cerebral artery perforators, which should be preserved to prevent septal infarcts.⁷