Corpus Callosotomy

Multiple Subpial Transections

Corpus Callosotomy

The rationale for corpus callosotomy is to control generalized seizures by interrupting seizures as they evolve from the cerebral cortex and secondarily spread through commissural pathways. Interruption of this generalization or bilateral synchronization can reduce or eliminate the major seizure type exhibited by this spread, namely "drop attacks," or atonic seizures. The goal, though, of corpus callosotomy in children is palliative seizure control and improved quality of life. This is achieved by increased social function, less caution as to activity, and reduced constant vigil because atonic seizures often lead to extensive facial trauma. In patients with a defined, resectable epileptogenic focus, nearly all centers would agree that resection is the procedure of choice. Intractability of this seizure type in the setting of no identifiable focus for resection is the most favorable and common indication for callosotomy. Whether or not partial (anterior two-thirds) callosotomy is preferable to complete division as an initial procedure is less clear. In most patients, including children, a partial callosotomy is advocated initially, unless there is evidence of a predominantly posterior focus or if the patient is severely neurologically impaired and either poorly or non-verbal. In these instances, a complete callosotomy is recommended as the definitive procedure.

The surgery for corpus callosotomy is performed with the patient supine on the operating table, in a neutral and slightly flexed position. A linear incision is made 2 cm anterior to the coronal suture, extending from 3 cm to the left of midline to 7 cm to the right of midline. The bone flap must come exactly to midline or slightly to the left of midline for adequate visualization of the interhemispheric fissure and the corpus callosum and positioned anterior to the coronal suture. The dura is opened in a curvilinear fashion based on the sagittal sinus, and the initial dissection down the interhemispheric fissure is performed with the surgical microscope. Minimal retraction is aided by CSF drainage and, in some instances, the administration of mannitol during the opening. A self-retaining retractor gently retracts the right frontal lobe, and when needed, an additional retractor blade is used on the falx or contralateral cingulate gyrus to adequately visualize the corpus callosum.

The glistening white appearance of the corpus callosum distinguishes it from the more superficial cingulate gyrus, and exposure along the length of callosum to be divided is obtained prior to entering the commissure. Adhesions between the hemispheres, especially common in the setting of previous infection or trauma, may make exposure difficult. The authors have found approaching the callosum more posteriorly and using the deeper extension of the falx to be helpful in this situation. The callosal marginal and then the pericallosal arteries are identified as they pass over the callosum. The actual division of callosal fibers is carried out most often between the two pericallosal arteries in order to preserve the blood supply to each cortical surface. The initial division of callosal fibers is carried out using suction aspiration and bipolar coagulation through the genu and anterior callosum. Once the callosum has been entered and the midline raphe (the septum between the lateral ventricles) identified, a ball dissector can be used to divide the perpendicularly crossing fibers posteriorly, either partially through the anterior two-thirds, or completely through the splenium. For the partial callosotomy, a premeasured strip of cottonoid is used to gauge the extent of disconnection. Avoiding entry into the ventricle will decrease the likelihood of clinically significant subdural effusions postoperatively. Anticonvulsant medications are generally not altered until at least subsequent follow-up, depending on outcome, though again, the corpus callosotomy is a palliative procedure and the vast majority of children will not be able to be tapered completely off AED.

Drop attacks have the most marked response, with approximately 70% obtaining a seizure-free state (elimination of the atonic seizures) in the partial callosotomy group. A higher percentage of response in atonic seizures (up to 100%) is seen with the complete callosotomy. Despite the improvement, in the atonic seizure component of their seizure syndrome, these patients rarely are completely seizure free. An analysis of outcome for anterior corpus callosotomies from 14 centers (183 patients) found that 5.5% were seizure-free, 73.5% had improved seizure control, and 20.7% remained unimproved. A follow up survey in 1991 included 563 patients, of whom 7.6% were reported to be seizure free and 60.9% improved.

There are numerous residual neurologic effects of disconnection, but, in general, it has been unusual for these effects to represent significant deficits. With division of the anterior corpus callosum, decreased spontaneity of speech (ranging from subtle slowing to complete mutism) and decreased use of the nondominant hand and leg (often described as either a paresis or an apraxia) are sometimes seen though these findings usually resolve over several days. There remains discussion as to the relative contribution of surgical retraction versus acute disconnection to these changes. The great majority of patients exhibit no long-standing effects from anterior section, although exacerbation of previous lateralized deficits has been reported. In children, these complications rarely cause sustained long-term problems. Complete and posterior corpus callosotomies, in contrast, produce a now well-recognized disconnection syndrome characterized by interhemispheric sensory dissociation. This syndrome can be demonstrated with somatosensory, auditory and visual stimuli, with the language-dominant hemisphere not having direct access to information presented to the other hemisphere. With incomplete section, the language-dominant hemisphere may still gain access to contralateral hemisphere information, and the dissociation may not be demonstrable. With complete section, the sensory dissociation is complete and permanent. This is likely less of an issue in children because the majority of those coming for complete corpus callosotomy are fairly neurologically impaired, particularly as it relates to language.

Multiple Subpial Transections

Multiple subpial transections (MST) are utilized to treat epileptogenic foci located within unresectable, "eloquent" areas of the cerebral cortex. The goal of MST is to interrupt the horizontal coursing intracortical fibers, thereby isolating localized interactions between the regions of cortex and decreasing the likelihood of synchronized neuronal discharge over a wide expanse of cortex. Although first designed as an adjunct to focal resection of epileptogenic area or structural lesions, further application of MST as a solitary procedure has expanded its utility.

MST was designed based on the anatomic and functional structure of the columnar organization of the higher mammalian neocortex. When applied primarily or additionally to seizure focus resection, this procedure reduces the side-to-side spread of epileptogenic activity along cortical neurons without causing major functional impairment of the centrifugal and centripetal cortical neuronal connections. By reducing the horizontal spread of electrical activity along the cortex and preserving vertically oriented neuronal circuitry, surgeons performing MST can attempt treatment of patients who were not previously considered for possible surgical interventions.

The indications for performing MST must include (1) medically refractory seizures, and (2) epileptic foci arising from within unresectable eloquent cortex. Eloquent cortex includes precentral motor cortex, postcentral sensory cortex, visual cortex, and language cortices (motor and sensory areas, including Broca’s and Wernicke’s areas). MST has typically not been used in patients who demonstrate an electrically active lesion outside of unresectable cortical areas without resection of such a focus. Exceptions to this include diffuse disease processes such as Rasmussen’s encephalitis. In addition, MST may be used primarily or in conjunction with focal resections of a lesion that is adjacent to a focus that involves eloquent areas. Some surgeons are advocating MST as a primary therapy in certain cases. Although each patient is treated individually, MST must be considered solely or as an adjunct to resection in treating epileptogenic lesions in or near functional cortex.

The original surgical technique has been only subtly revised . Multiple subpial transections are planned by careful ECoG and, in some cases, stimulation trials during an extraoperative monitoring period using surface electrodes. With MST, the epileptic neurons are not removed, but the spread is eliminated, and may result in elimination of the sharp waves. To enter a sulcus prior to subpial transection, a pinhole incision is performed with the tip of an 18-gauge needle or a #9 injection syringe. Alternatively, complete dissection of the sulcus on either side of the transection can be performed. Whether the entry is a hole or a complete sulcal dissection, care must be taken to avoid the traversing vessels. The transecting instrument is fashioned from a heavy steel wire, with a 4- 5 mm right-angle hook, with either a blunt tip or ball at the end. Morrell describes the end to be smooth and rounded to 0.3mm, but others have described a 0.5mm ball tip. To begin a transection, the instrument is placed into the sulcus, again avoiding the nearby vessels, and the tip oriented across the gyrus to be transected. The instrument is swept tangentially under the gyrus while keeping the blade vertically oriented. Once the opposite sulcus is reached, the instrument is drawn back across the gyrus, keeping the tip in view below the pia. Care is taken to avoid vessels in the distal sulcus as well as on the cortical surface while pulling the instrument back. Transections are made 2-5 mm apart and are kept parallel . Multiple transection stripes are typically seen along the dissections and arise from capillary damage. These strips can help guide the measurement for the next transection. Electrical coagulation should be avoided to limit damage. The authors use topically placed thrombin-soaked sponges to abate excessive capillary hemorrhage. Transections are made until no further sharp wave activity is noted from the area planned for transection. If a resection is also planned, MST should be performed afterwards, because seizure foci in resectable cortex may directly modulate the epileptogenicity of the eloquent cortex where the MST is planned. Each patient should have the lesioning planned individually because no two cases are identical.

MST has been very successful in providing seizure-free states in selected patients, however, no surgical procedure is without the risk of complications. Besides, the myriad of complications associated with an intradural craniotomy, certain problems are specific to MST. Because eloquent cortex is being violated, motor, speech, and sensory dysfunction have been reported, likely secondary micro hemorrhage in eloquent areas. Additionally, an extensive hemorrhage from the MST has been observed with can lead to a greater order deficit particularly since the MST are site specific for eloquent areas. Most of the data to date have been in adults, the efficacy in childhood has not yet been fully determined though the principles and indications for its use remain the same.