29 Peri-insular Hemispherotomy
Anatomical hemispherectomy has proven to be an effective surgical treatment for epilepsy when the epileptic pathology involves a large part of the cerebral hemisphere. Dandy1 and L’hermitte2 originally conceived of the use of hemispherectomy in the late 1920s as a treatment for malignant brain tumors and the procedure was first applied to the treatment of epilepsy by McKenzie3 in Canada in 1938. In 1950, Krynauw et al4 demonstrated control of epileptic seizures in a series of 12 patients with infantile hemiplegia treated by hemispherectomy. In anatomical hemispherectomy, the entire affected cerebral hemisphere is resected, and a large cerebrospinal fluid cavity is created postoperatively. Superficial cerebral hemosiderosis (SCH) secondary to repeated bleeding into the large subarachnoid space may occur in up to 33% of the patients.5–7
Rasmussen8 introduced the functional hemispherectomy in the 1970s to achieve the goals of hemispheric disconnection while preventing SCH. In the functional hemispherectomy, the frontal and occipital poles are preserved but functionally disconnected from the rest of the brain. A temporal lobectomy with complete callosotomy and insular cortex resection is performed, along with the parasagittal frontal and parietal lobectomies to remove the central region. The benefits of the functional hemispherectomy are identical to those of the anatomical hemispherectomy, but by functionally disconnecting the hemisphere while leaving most of the brain intact, this technique avoids the complication of SCH. Other modifications of hemispherectomies have been developed to avoid late hemosiderosis, including reducing the subdural space by sewing the convexity dura to the falx, anterior and middle fossa dura, and tentorium,9 plugging the foramen of Monro with muscle,10 using postoperative subdural drainage and ventriculoperitoneal shunt placement,11 or undertaking hemidecortication.12,13
More recently, other forms of the functional hemispherotomy have been conceived to minimize cortical resection further while still functionally disconnecting the affected hemisphere by transection of the neuronal fiber tracts.14–16 These techniques have been described as hemispherotomy because the operative transection of fiber tracts predominantly occurs through a small peri-insular opening in the cerebral hemisphere. The advantages of these procedures include reduced operative time, reduced blood loss, better anatomical preservation of the surgically treated hemisphere, and decreased postoperative complications. Although other forms of functional hemispherectomy and the various hemi-spherotomy techniques are described in other chapters of this volume, our focus is the peri-insular hemispherotomy.
Preoperative Evaluation
The indications for peri-insular hemispherotomy are lesions that cause intractable epilepsy involving one cerebral hemisphere. The most common hemispheric syndromes include infantile hemiplegia, Sturge-Weber syndrome, Rasmussen encephalitis, atrophic cerebral hemisphere caused by vascular disorder or trauma, and cortical dysplasia involving a broad area of the cerebral hemisphere and hemimegalen-cephaly. These diseases, in most cases, lead to a hemispheric syndrome characterized by hemiplegia and hemianopsia. All patients undergo a detailed neurological evaluation, including multiple electroencephalographic evaluations, neu-ropsychological testing, and magnetic resonance imaging (MRI) studies, to confirm that the seizures originate from the diseased hemisphere.
Operative Procedure
The peri-insular hemispherotomy was first described by Vil-lemure and Mascott16 in 1995, and few modifications have been added since that time.17,18 The procedure is performed under general endotracheal anesthesia, with the patient positioned supine and the head turned so the intended surgical hemisphere is facing upward. The head can be secured by Mayfield pin head holder (Integra, Plainsboro, NJ) or Sugita clamp (Mizuho, Tokyo, Japan) or positioned laterally on a horseshoe rest. A modified question mark- or U-shaped scalp incision provides the necessary exposure for the operation. The incision should be planned to reach the zygoma inferiorly, and the superior aspect of the incision need not extend above the mid-convexity level. The bone flap is designed so that the anterior extent reaches the coronal suture, the superior aspect is at the superior temporal line, and the posterior extent provides access to the posterior insular and extendsinferiorly to the temporal floor. The preoperative MRI scan is useful to take into account the individual’s anatomy, atrophy, and brain shift to plan the craniotomy. After the craniotomy, the dura mater is opened to expose the underlying frontal, parietal, and temporal lobe opercular cortex.
The peri-insular hemispherotomy involves disconnecting the operative hemisphere from the rest of the central nervous system, and the majority of the procedure is performed principally within the ventricular system. Access to the ventricle is gained primarily by two stages via (1) the superior insular window (suprainsular window) and (2) the inferior insular window (infrainsular window) ( Fig. 29.1 ).
The suprainsular window requires resection of the frontal and parietal opercular cortex to expose the upper insula and circular sulcus. The white matter is entered above the insula through the inferior frontal gyrus or superior circular sulcus until the lateral ventricle is entered. The approach to the ventricle along this trajectory disrupts the corona radi-ata and the ascending and descending fibers of the internal capsule.
Development of the infrainsular window requires removal of the temporal opercular cortex to expose the inferior insula and inferior circular sulcus whereby the temporal horn of the lateral ventricle is entered. Once the temporal horn is entered and opened, the hippocampal formation and amygdala are visualized. The order of which window is to be opened first is at the discretion of the operating surgeon. Generally, it is easier to extend the opercular resection from the suprainsular window posterior and inferiorly around the insula to the temporal operculum with maintained visualization of the underlying lateral ventricle.
Suprainsular Window
After the dura mater is opened, the pia mater of the frontal and parietal opercular cortex is coagulated and incised approximately 5 mm above the sylvian fissure on the inferior frontal gyrus. The opercular cortex is removed by subpial aspiration using the operating microscope until the insular cortex and the superior circular sulcus that surrounds it are completely exposed ( Fig. 29.1A ). The body of the lateral ventricle is entered through the white matter of the superior circular sulcus if the ventricles are large or entered with extension into the white matter of the corona radiata beneath the inferior frontal gyrus if the ventricles are small ( Fig. 29.1B ). Once the ventricle is entered, the superior sylvian window is extended anteriorly to the frontal horn of the ventricle to open the superior circular sulcus completely and posteriorly to the ventricular trigone. It is important to limit the entry of blood into the remainder of the ventricular system by placing a Cottonoid patty over the foramen of Monro. After the lateral ventricle is opened along its anterior-to-posterior axis, the septum pellucidum and corpus callosum are visualized.
Corpus Callosotomy
Clear visualization of the septum pellucidum within the lateral ventricle is necessary to initiate the corpus callosotomy. Intraoperative ultrasonography or frameless stereotactic guidance may be used to identify the anterior cerebral arteries above the corpus callosum or the falx to clarify the trajectory for the corpus callosotomy. A small, 2-mm wide incision at the junction of the septum pellucidum with the roof of the ventricle in the coronal plane allows for identification of the corpus callosum. Occasionally, in patients with small ventricles, subpial resection of the most inferior aspect of the cingulate gyrus may be necessary to achieve exposure of the pericallosal arteries anteriorly along the corpus callo-sum. Once the corpus callosum has been identified and confirmed with subpial visualization of the pericallosal artery, the ependyma of the ventricular roof is resected in an anterior-to-posterior direction while disconnecting the callosal fibers. The pericallosal arteries are used as a guide, and they are followed anteriorly by subpial aspiration of the corpus callosum from the rostrum to the splenium of the corpus callosum from within the lateral ventricle. This allows disruption of the fibers of the corpus callosum in a just slightly parasagittal plane ( Fig. 29.1C ). It is important not to actually enter the corpus callosum perpendicular to the midsagittal plane, because this could result in inadvertent entry into the contralateral hemisphere. Posteriorly, the pericallosal arteries course superiorly into the interhemispheric fissure so the dissection relies on visualization of the pia/arachnoid of the interhemispheric fissure and superior surface of the corpus callosum as landmarks.
Disruption of the frontal horizontal fibers is initiated after completion of the complete callosotomy. Attention is directed away from the inside of the lateral ventricle to the inferior and basal frontal lobe in the coronal plane at the anterior insula at the level of the sphenoid ridge. The pia over the frontal basal cortex is interrupted, and a corticectomy of the frontal lobe in the coronal plane is initiated at the level of the lesser wing of the sphenoid. The frontal basal corti-cectomy is extended into the white matter medially in the coronal plane and allows for visualization of the olfactory tract. The olfactory tract is preserved, and the subpial resection of the inferior frontal cortex continues until the pia begins to curve superiorly at the medial frontal lobe. Once this has been achieved, subpial aspiration of the white matter from the rostral callosotomy can be extended inferiorly just anterior to the rostrum of the corpus callosum and head of the caudate in the lateral wall of the ventricle in the coronal plane to connect the corticectomy across the frontobasal white matter of the frontal lobe. The corticectomy of the frontal basal cortex across the subcallosal, orbitofrontal, and gyrus rectus disrupts the commissural connections between the caudal orbitofrontal cortices, the ascending and descending projections of the frontobasal cortex via anterior sublen-ticular fibers, the connections between the orbitofrontal and insular cortex, and connections to the temporal lobe via the uncinate fasciculus.
Infrainsular Window
After the corpus callosotomy and disconnection of frontal horizontal fibers via the suprainsular window, attention is directed to the infrainsular window. The pia overlying the superior temporal gyrus is coagulated approximately 5 mm below the sylvian fissure. This is usually accomplished by starting in the superior temporal gyrus at the posterior margin of the insula. After creation of the suprainsular window, the lateral ventricle is opened posteriorly to the splenium of the corpus callosum. The opening into the lateral ventricle can be followed inferiorly by aspirating the cortex around the posterior margin of the insular and posterior circular sulcus to the superior temporal gyrus ( Fig. 29.1D ). Extra care should be given to preserving middle cerebral artery (MCA) branches and large venous draining vessels when feasible to prevent postoperative infarcts and cerebral edema. This opening allows for continuous visualization of the ep-endyma and choroid plexus of the lateral ventricle from the frontal horn to the posterior temporal horn. The removal of the superior temporal gyrus proceeds from posterior to anterior from the level of the trigone of the lateral ventricle and by removing the cortex and underlying white matter along the inferior margin of the insula and inferior circular sulcus extending the opening into the lateral ventricle into the temporal horn. Once the temporal horn has been completely exposed, the hippocampus and amygdala can be easily visualized.
The choroid fissure extending from the temporal horn of the lateral ventricle is opened to the posterior extent of the complete callosotomy at the level of the splenium ( Fig. 29.2 ). This incision disrupts parietooccipital connections via the short and long arcuate fibers of the superior longitudinal fasciculus, the cingulum, and fimbria fibers from the hippocampus. This resection of the medial white matter of the lateral ventricle inferiorly from the level of the splenium proceeds to the choroid fissure, and the basal vein and vein of Galen are often visualized through the arachnoid when this incision is completed.
After complete disconnection of parietooccipital fibers, the choroid fissure of the temporal horn is followed anteriorly to identify the anterior head of the hippocampus and amygdala ( Fig. 29.1E ). The amygdala is removed using a subpial aspiration. The superior extent of the amygdala resection proceeds to the level of the roof of the temporal horn. The resection may extend posteriorly into the hippocampus but is not necessary because the fimbria has been sectioned at the hippocampal tail. On completion of the inferior insular window and opening of the lateral ventricle from the atrium to the tip of the temporal horn, the retrolentiform and sublentiform internal capsule have been sectioned. Removal of the amygdala and anterior hippocampus disrupts the fibers of the posterior limb of the anterior commissure coursing between the superior amygdala and inferior lenti-form body, the stria terminalis, and connections between the amygdala and basal ganglia. The hippocampal projections to the parahippocampal gyrus and then on to other cortical areas are disrupted by the opening of the infrainsular window and sectioning of the fimbria.
Fibers projecting from the insular cortex to the basal ganglia, thalamus, hypothalamus, and brainstem are disconnected by aspiration of the insular cortex ( Fig. 29.3 ). At the conclusion of the procedure, the pia should be identified extending from along the floor of the anterior fossa and lesser sphenoid wing at the base of the frontal lobe over the olfactory tract, superiorly anterior to the insula and basal ganglia extending anterior to the genu and superior to the rostrum of the corpus callosum on the medial frontal lobe, posteriorly along the corpus callosum in a parasagittal plane to the splenium, and then inferiorly from the splenium to the choroid fissure of the temporal horn. It is important to identify this pial margin completely along its course as it is often difficult to visualize the disconnection on the postoperative MRI. At the conclusion of the procedure, a drain is left within the lateral ventricle and tunneled through the scalp to evacuate debris and bloody cerebrospinal fluid.
Related posts:
15 Implantation of Strip, Grid, and Depth Electrodes for Invasive Electrophysiological Monitoring
18 Transsylvian Selective Amygdalohippocampectomy
22 Surgical Approaches in Cortical Dysplasia
34 Cortical and Deep Brain Stimulation
35 Radiosurgical Treatment for Epilepsy
28 Transsylvian Hemispheric Deafferentation
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