22 Functional Hemispherectomy and Peri-insular Hemispherotomy
Abstract
Hemispherectomy techniques are used for intractable focal epilepsy when the seizure-causing pathology is confined to one hemisphere but is diffuse in that hemisphere such that focal resection or single lobectomy will not provide significant benefit. Of course, this usually comes at the cost of new or worsening neurological deficits on the contralateral side. Therefore, pathology such a perinatal middle cerebral artery infarct and Rasmussen encephalitis have been the primary indications for such aggressive procedures because these patients already have contralateral deficits. However, hemispheric disconnections/resections are also used in severe hemispheric cases without preexisting deficits when there are no other options; and in these cases, the surgery should be performed early in childhood when possible so that the young child can benefit from the maximum neuroplasticity during rehabilitation and so that the healthy hemisphere is allowed to develop normally unimpeded by the detrimental effects of ongoing contralateral seizures and/or antiepileptic medications. The history of hemispherectomy over the last 50 years has been one of reductionism going from complete anatomic removal of the diseased hemisphere, to functional hemispherectomy, to hemispherotomy techniques. More recently, advanced neuronavigation methods and intraoperative magnetic resonance imaging (iMRI) have become important adjuncts in the armamentarium. In addition to a brief review of the history of and indications for hemispherectomy techniques for focal epilepsy, here we describe the operative techniques of functional hemispherectomy and peri-insular hemispherotomy as originally introduced by Rasmussen and Villemure, respectively, along with some our surgical nuances including the use of iMRI. Finally, we describe seizure and functional outcomes, and potential pitfalls to be avoided.
22.1 Introduction
While Walter Dandy in the 1920s described hemispherectomy for the treatment of malignant tumors, 1 hemispherectomy techniques to control seizures were first introduced by McKenzie 2 and popularized by Krynauw. 3 After initial success, many patients were reported to have significant late complications, in the form of superficial cerebral hemosiderosis. 4 The best determinant of seizure outcome success is the extent of disconnection rather than the amount of tissue removed, and as such, disconnection techniques evolved. Rasmussen, in the early 1980s, introduced the functional hemispherectomy. 5 In 1993, Villemure introduced and later popularized the peri-insular hemispherotomy. 6 , 7 , 8 , 9 A vertical hemispherotomy was also described by Delalande 10 with the same intent of reducing perioperative morbidity while maintaining the same excellent seizure outcome as would otherwise be obtained with an anatomical hemispherectomy.
22.2 Indications
Nowadays, most hemispheric disconnections are proposed for children who show diffuse but unilateral epilepsy refractory to an adequate trial of at least two appropriately chosen antiepileptic medications. 11 Of course, these drugs can also have a negative influence on brain development. The damage to the involved hemisphere causing the epilepsy is often quite extensive, but should not affect the contralateral hemisphere, which the child will be depending on to further develop once the seizures are controlled. The damage to the hemisphere can be the result of perinatal middle cerebral artery (MCA) infarct, hemimegalencephaly, Sturge–Weber syndrome, or extensive cortical dysplasia, as well as acquired syndromes including Rasmussen encephalitis, hemispheric trauma, hemorrhages, or infection in the form of meningoencephalitis.
Electroencephalogram (EEG) studies should show widespread abnormalities involving the entire hemisphere contralateral to the clinical deficits. EEG abnormalities originating in the good hemisphere are relatively common, but are not an absolute contraindication to surgery, particularly if they are not occurring independently of the diseased hemisphere EEG abnormalities. 12
Children with such hemispheric insults often display hemiparesis or hemiplegia. These children are often too young to accurately determine their visual field status, but they usually show signs of a preexisting hemianopia. On occasion, the natural history of the condition causing the seizures will be that of progressive loss of function and the surgeon has to determine in the context of a multidisciplinary seizure conference whether the benefit of eliminating seizures to improve cognitive development of the good hemisphere outweighs the cost of precipitating or aggravating the loss of motor function of the contralateral hemibody. 13
Children who successfully undergo a hemispherectomy, if they are not already walking, will eventually walk with a limp. They will move the contralateral arm at least proximally and will usually have distal tone increase in the hand that prevents the use of this hand other than to support the opposite hand’s activities. This may be in part a result of weakness, but also in part a result of increased tone and dystonia.
Once the family understands the need to balance the motor deficits with cognitive gains from seizure (and medication) elimination, and is ready to face a prolonged rehabilitation period, the operation can be planned. Expectations have to be well delineated.
Neuronavigational techniques have become an important part of the surgical planning for such procedures. In addition, for almost a decade, we have been using intraoperative magnetic resonance imaging (iMRI) at our center for this purpose. In particular, as part of our preoperative planning for a peri-insular hemispherotomy (our preferred procedure—described below), we delineate the corpus callosum, pericallosal arteries, gyrus rectus, amygdala, hippocampus, and fimbria and fornix as well as important veins (▶Fig. 22.1), and then in the operative suite with the anesthetized patient’s head fixed in the operative position, we perform a new predissection iMRI and fuse these images with our preoperative plan (▶Fig. 22.2, ▶Fig. 22.3). Of course, the fusion of this preoperative plan and iMRI and subsequent neuronavigation also helps in designing an appropriate craniotomy that will allow us access to temporal horn, atrium, and frontal horn of the lateral ventricle through perisylvian corticectomies, the frontobasal area to the sphenoid ridge, and sufficient exposure of the middle fossa for the temporal resection. Because most of the targeted structures are in the relatively “fixed” midline, even after “unroofing” the ventricle it has been our experience that the navigation remains quite accurate (▶Fig. 22.4). Furthermore, the navigational accuracy can always be recalibrated by obtaining another interdissection iMRI and fusing the preoperative plan to the new set of images.
22.3 Operative Techniques
In both functional hemispherectomy and peri-insular hemispherotomy, the procedure is performed under general anesthesia and the patient’s head is immobilized in a specialized iMRI (Noras) head holder, with a cushion under the ipsilateral shoulder placing the zygomatic arch as the most elevated point of the surgical area and the falx cerebri relatively parallel to the floor. A three-dimensional (usually T1) sequence is obtained in the operative position and this navigational image is fused to the preoperative plan. After registration, with the preoperative plan at hand, the craniotomy is tailored to be only as large as is needed to perform the different stages of the surgery.
22.3.1 Functional Hemispherectomy
In some cases, such as in diffuse cortical dysplasia or hemimegalencephaly, the hemisphere is not atrophic and does not display encephaloclastic porencephalic cysts, and the ventricles are of normal size. In such situations, a larger amount of tissue may need to be resected to ensure complete disconnection through good visualization.
A question mark skin flap is raised from the zygomatic arch posteriorly toward the mastoid and transverse sinus to reach the level of the lambdoid suture and then turned superiorly toward an area 1 to 2 cm from the midline as determined by identifying the sagittal sinus on neuronavigation based on the preoperative plan. The incision is then extended more anteriorly toward the hairline. The skin flap is reflected forward, and the temporalis muscle is cut above the zygoma. The temporalis muscle is lifted, leaving a small cuff of fascia and muscle on the bone of the superior temporal line for later reattachment. The bone flap is raised using burr holes placed at the squamosal temporal bone, pterional keyhole, low parietal bone above the mastoid area, and at the coronal suture, again using neuronavigation to determine the optimal position of the burr holes. The dura is opened in a semicircular fashion and reflected on top of the temporal-cutaneous flap. The dura is fixed with sutures in such a way that it is stretched and kept moist so that it will not dry out and retract during the case. This facilitates primary dural closure so that an allograft patch will not be needed.
Navigation is then used to confirm that the craniotomy is wide enough and whether all key anatomical sites to be disconnected are readily accessible.
The aim of the functional hemispherectomy is to completely disconnect the hemisphere without removing it completely. The key steps are: (1) the excision of the temporal lobe to include the mesiotemporal structures, (2) excision of the central cortex including the parasagittal tissue down to the corpus callosum, which simultaneously disconnects the central part of frontal lobe but also the parietal cortex and the frontopolar/frontobasal cortex from their respective areas of the corona radiata, (3) interrupting the corpus callosum further anteriorly and posteriorly to disconnect completely the hemisphere at the genu and rostrum, as well as at the splenium of the corpus callosum, and (4) resecting the insular cortex.
Temporal Lobectomy
A large anterior temporal resection is performed to expose the temporal horn at the level of the ventricular atrium to facilitate the posterior temporal disconnection, and to allow resection of amygdala and hippocampus.
A subpial technique is used to resect from under the pia of the sylvian fissure, the first temporal gyrus to the anatomical plane of the superior aspect of the circular sulcus of the insula. This superior temporal gyrus, or “T1,” resection is then extended inferiorly toward the temporal base taking care not to damage the vein of Labbé or other important vasculature that should be preserved to avoid postoperative swelling of the remaining but disconnected tissue. The temporal horn is identified and unroofed using an ultrasonic aspirator and bipolar forceps toward its anterior blind-ended tip where the amygdala will be identified. A subpial aspiration anteromedially is then carried out preserving the pial bank, which is essential to identify the free edge of the tentorium and the third nerve through the pia, ultimately to empty the uncus. The hippocampus is resected using the ultrasonic aspirator at low power still with the aim of preserving the medial pia, which will protect the cerebral peduncle located deep to it.
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