30 Corpus Callosotomy

10.1055/b-0034-84141

30 Corpus Callosotomy

Wong, Tai-Tong, Kwan, Shang-Yeong, Chang, Kai-Ping

Approximately 10% of children with newly diagnosed epilepsy develop intractable seizures.1,2 Surgical management of children with medically intractable seizures includes resective and palliative surgical interventions. For patients who are not good candidates for resective surgery, palliative procedures such as disconnection surgeries (corpus callosotomy) and neuromodulation procedures (vagus nerve stimulation) are considered. Corpus callosotomy consists of partial or total disconnection of the corpus callosum and aims to block interhemispheric spread of secondary generalized seizures. It is an effective palliative, although not curative, surgical intervention that is particularly useful for atonic, tonic—clonic, and tonic seizures from different types of medically refractory epilepsies and epilepsy syndromes.

Histological Refinement of Corpus Callosotomy

Corpus callosotomy in the surgical management for epilepsy patients was first reported by van Wagenen and Herron in the 1940s.3 They ligated the anterior one third of the superior sagittal sinus, divided the corpus callosum in various extents, sectioned the anterior commissure, and, in some cases, also divided the fornix unilaterally or bilaterally.3,4 This approach was complicated with significant morbidity and did not gain any popularity. Approximately 20 years later in 1960s, Bogen and his colleagues revived the operation again.5 They performed two different approaches: complete and partial disconnection. Complete disconnection was performed by dividing the entire corpus callosum, anterior commissure, unilateral fornix, hippocampal commissure, and even massa intermedia in some patients. The partial disconnection was performed by dividing the anterior one third of the corpus callosum, unilateral fornix, and anterior commissure in patients with bilateral independent seizure discharges with seizure foci restricted to frontal or temporal lobes.5,6 In 1970, Luessenhop et al reported their experience with corpus callosotomy in the treatment of intractable seizures in children.7 Then, again in 1970s, Wilson applied an operating microscope during this procedure, and he limited the operation to divide only the corpus callosum and hippocampal commissure. He did not enter the ventricles to avoid postoperative hydrocephalus.6,8,9 Thereafter, in the 1980s, the staged callosotomy concept was introduced in the field by Maxwell, and corpus callosotomy started to be used more frequently in epilepsy patients.6 The anterior two thirds to three quarters callosotomy was performed in the first stage, and the remaining posterior corpus callosum was divided in the second stage if the patient did not have much benefit from anterior two thirds callosotomy.6 In 1993, Wyler described his technique for anterior callosotomy.10 He split the corpus callosum from midline in between two peri-callosal arteries to enter the cavum septum pellucidum. He advocated sectioning most of the corpus callosum and left only the splenium intact.10 Later, although not commonly used, callosotomy with gamma knife radiosurgery was also described as a novel technique.11,12 After introduction of the vagal nerve stimulator, corpus callosotomy lost its popularity somewhat, but it is still an effective procedure in a well selected patient population.13–15

Anatomical and Physiological Basis and Rational

Several midline commissural structures connect two cerebral hemispheres. The corpus callosum is the largest commissure and principal anatomical and neurophysiological connection pathway between the two hemispheres. The others include the anterior commissure, posterior commissure, hippocampal commissure, and massa intermedia. The corpus callosum can be divided into four parts: rostrum, genu, body (anterior and posterior), and splenium. The anterior half of the corpus callosum starts from the rostrum and includes the genu and the anterior half of the body and carries fibers projecting from premotor, supplementary motor, motor, anterior insular, and anterior cingulated cortical areas. Thus, it is essential for the generalization of tonic and tonic—clonic convulsions and atonic drops. The posterior half of the corpus callosum includes the posterior half of the callosal body, isthmus, and splenium and connects cortical areas from the parietal, temporal, and occipital lobes. The posterior midbody and isthmus carries fibers from parietal, superior temporal, posterior insula, and posterior cingulate. The splenium interconnects occipital lobe, caudal portion of inferotemporal region, and lateral to caudal portions of the parahippocampal gyrus. The corpus callosum contains topographically organized fibers covering very large parts of the cerebral hemispheres, and these fiber tracts can be seen with current magnetic resonance imaging (MRI) techniques, such as diffuse tensor tractography (DT-MRI).16–18 ( Fig. 30.1 ). This topographical distribution is also significant from the surgical approach standpoint. A clinically sufficient number of fibers for interhemispheric transfer of some perceptual information can be preserved by sparing splenium during callosotomy and related complications occurring after complete callosotomy can be diminished.19,20 This is one of the major advantages of the anterior two thirds corpus callosotomy approach.

**Fig. 30.1** Using diffuse tensor tractography—magnetic resonance imaging for topography study of the corpus callosum, **(A)** the parcellation of the corpus callosum, **(B)** the cortical associated tracts, and **(C)** the related cerebral cortex can be defined. (Courtesy of Ching-Po Lin, Institute of Neuroscience, National Yang-Ming University, Taiwan)

Patient Selection Criteria, Surgical Indications, and Preoperative Assessment

Patient selection criteria for corpus callosotomy in children include several factors: clinically having documented resistance to major antiepileptic medications (AEDs), not being a candidate for curative surgical procedures, having certain types of seizures (described later) and epilepsy syndromes, having disabling seizures with high risk of injury, experiencing a decreased quality of life secondary to seizures, and, finally, parents’ preference among the available management options.

At Taipei Veterans General Hospital (Taipei VGH), the main indication for callosotomy is having medically refractory seizures without an identifiable or resectable epileptogenic focus. Sometimes, children with unilateral hemispheric lesions or bihemispheric malformation of cortical development are also considered as candidates for corpus callosotomy. Our criteria for clinically documented medical intractability of seizures are as follows: (1) presence of medically intractable seizures in a patient who has been treated by a pediatric epileptologist for at least 1 year, (2) poor response to currently available major AEDs with sufficient treatment dosages and serum levels, and (3) having more than two disabling seizures within a month. We do not accept mental retardation as a contraindication.21 Currently, we perform corpus callosotomy in the amelioration of medically refractory seizures of some generalized epileptic syndromes such as Lennox-Gastaut syndrome (LGS), infantile spasms, severe epilepsy with multiple independent spike foci, and hemiconvulsion-hemiplegia-epilepsy syndrome. LGS patients constitute the largest group in our series, and they usually present with multiple seizure types including drop attacks, generalized tonic—clonic seizures (GTCS), partial seizures, and atypical absence. Drop attacks are the most responsive seizure type to callosotomy.22–24

At Taipei VGH, the preoperative evaluation of the patients for callosotomy includes the following:

Detailed history and documentation of seizure semi-ology,
Physical and neurological examinations,
electroencephalography (EEG) and long-term video/ EEG monitoring,
MRI of brain, and
Neuropsychological tests for intelligence scale and psychosocial behavior evaluation.

MR-venography for presurgical planning may also be obtained; however, it is not an absolute necessity.23

Surgical Technique

The operation is performed under general anesthesia. The patient’s head is placed on a headrest with the sagittal plane perpendicular to the floor. A curvilinear, right-sided scalp incision is made along the coronal sutures by crossing midline approximately 2 cm. A right frontal craniotomy is performed with a midline cut on the left side of the sagittal sinus and the posterior edge of the skull flap stays just behind the coronal suture on the right and extends 6 to 7 cm anteriorly. Then a free skull flap is removed by exposing the right frontal region, and the dura is opened by coming as close to the sagittal sinus as much as possible. The dural flap is reflected over the sagittal sinus. Although the location of cortical bridging veins varies patient to patient, it rarely constitutes an obstruction to the surgeon in front of the coronal suture. If any dural venous lake or large drainage vein is encountered, the dural incision is extended accordingly along these bridging veins to avoid any injury and to reach the edge of the sagittal sinus. If it is unexpectedly difficulty to make a right interhemispheric approach because of large bridging veins or venous lakes, in our experience, it is always possible to do the procedure through the other side ( Fig. 30.2 ). We cover the exposed cortical veins with Gelfoam strips (Pfizer, New York, NY), and Cottonoids to avoid any accidental injury during the procedure. Before starting interhemispheric dissection, we obtain precallosotomy electrocorticography (ECoG). Then, the interhemispheric microdissection assisted by microscope or binocular loupes is performed. First, the arachnoid attachments are divided and the interhemispheric fissure is gradually exposed by draining cerebrospinal fluid (CSF). While dissection continues, the brain can be further relaxed by suctioning CSF from interhemispheric space. Gradually, the brain becomes relaxed enough to retract it sufficiently to be able to perform the procedure without any need for lumbar drainage or application of osmotic agents. The next step is dividing the interhemispheric adhesions adequately to avoid injury to the mesial frontal cortex before stepwise hemispheric retraction. This also provides a good exposure to pericallosal arteries and their branches. As the dissection is advanced toward the callosum, the retractor blade is further replaced to expose the cingulum. The cingular gyri can be quite adherent and difficult to separate in some cases because of arachnoid adhesions; in these cases, the trajectory is redirected to a portion where dissection can easily be performed and then extended further using fine-tipped bipolar forceps and appropriate dissectors. The supracallosal cistern is opened, and the underlying glistening white corpus callosum and the pericallosal arteries are exposed. Although rare, it can be seen in some cases that a single pericallosal artery supplies both hemispheres. Pericallosal arteries are gently separated with bipolar tips and dissectors and Cottonoid micropatties were placed on both ends to keep them separated. The corpus callosum is then divided between the pericallosal arteries at the midline of the rostral midbody of corpus callosum by a microspatula. The grayish-blue color of the ependymal lining of the ventricle can be easily appreciated while deepening the callosal dissection, and it is preferable not to enter the ventricle by opening the ependyma to avoid risk of chemical meningitis. If the goal is anterior two-thirds callosotomy, then the challenge at this stage is translating the planned length of the callosotomy on the surgical field. As the midline cleft between the roofs of the lateral ventricles is exposed, we follow this cleft to split the genu anteriorly down to the anterior commissure and then to divide the caudal part of body of the corpus callosum posteriorly to reach the splenium as much as possible ( Fig. 30.3 ). In selected patients with severe mental retardation, single-stage total callosotomy may be performed. Sometimes, we tilt the table up or down to elevate or lower the head a bit to gain space to see the genu or posterior part of the corpus callosum better. After the callosotomy, a postdivision ECoG is performed ( Fig. 30.4 ). Then the surgical area is irrigated with saline, and meticulous hemostasis is obtained. Then the entry points of cortical drainage veins are rechecked, and Gelfoam pieces are packed around them, if needed. Then the dura is closed primarily and the skull flap is replaced. Postoperatively, the extent of corpus callosum division is assessed by MRI. We recently started to use a special software package for this purpose.