Indications for surgical consideration in patients with drug-resistant epilepsy include a defined epileptogenic focus and a low likelihood of new neurologic deficit after surgery. In adults, the most common etiology is temporal lobe epilepsy, that is, mesial temporal sclerosis. The most common epileptogenic etiologies in pediatric surgical candidates are low-grade tumors and malformations of cortical development.

Surgical options for pediatric patients who have marked dysfunction of a single epileptogenic hemisphere have evolved over time. Complications resulting from highly resective operations such as anatomic hemispherectomy, including superficial siderosis and secondary hydrocephalus, have led to the development of less resective and more disconnective functional hemispherectomy. Functional hemispherectomy has recently given rise to hemispherotomy, the least resective operation primarily aimed at disconnecting the abnormal hemisphere. Hemispherotomy is effective in reducing or eliminating seizure frequency and likely decreases the risk of postoperative complications when compared with its predecessors.

Hemispherotomy is a technically challenging operation that requires a thorough understanding of three-dimensional cerebral anatomy to ensure adequate hemispheric disconnection without placing the deep structures at risk or leaving certain hemispheric connections intact that could lead to residual postoperative seizures.

In this chapter, I will discuss the relevant operative nuances for a modified form of peri-insular hemispherotomy. Through hemispherotomy, experienced surgeons can effectively treat patients with unilateral epileptogenic hemisphere dysfunction while limiting potential complications. First I will briefly review the historical evolution of hemispherectomy and hemispherotomy techniques.

Hemispherotomy evolved from hemispherectomy. Walter Dandy first performed the latter operation for treatment of malignant gliomas in 1928, and hemispherectomy for treatment of epilepsy was first reported in 1938. Superficial siderosis was later recognized as a potential long-term complication. Up to 33% of patients developed this condition at a median time of 8 years after the procedure. Superficial siderosis occurs as a consequence of chronic granular ependymitis associated with multiple bleeding areas on the membrane that replaces the resected hemisphere in continuity with the ventricular system, leading to neurologic decline, hydrocephalus, and sometimes death.

To avoid this complication, less resective modifications of the operation were implemented, such as functional hemispherectomy. In this procedure, the frontal and occipital poles are disconnected but not removed, with the goal of achieving similar seizure control as with hemispherectomy while preventing the delayed complication of superficial siderosis. This operation has continued to be modified in an attempt to minimize cerebral resection and also to decrease intraoperative blood loss while maintaining its effectiveness, leading to hemispherotomy techniques.

Severe unihemispheric dysfunction causing medically refractory epilepsy in the pediatric population is frequently selected for surgical treatment through hemispherotomy. Some of the most common indications are multilobar cortical dysplasia, hemimegalencephaly, polymicrogyria/polypachygyria, posttraumatic epilepsy, Rasmussen encephalitis, perinatal stroke, and Sturge-Weber syndrome. The goal of surgery in these patients is to interrupt the spreading pathways of the epileptic discharge in order to isolate the epileptogenic zone.

Children suffering from refractory seizures who have received two unsuccessful but appropriate trials of anticonvulsant medications should be referred for surgical evaluation. Pediatric patients suffering from epilepsy may develop developmental regression or arrest. An early comprehensive surgical evaluation may avoid these adverse effects which can be a direct result of epileptic syndrome.

The preoperative workup begins with a thorough history and physical examination. Almost all patients have significant preoperative hemiparesis related to the abnormal hemisphere. Preoperative hemianopsia is assessed. Neuropsychological evaluation of older pediatric patients provides a baseline for postoperative comparison and can provide insight regarding the functional impairment that may result from surgical intervention.

Evaluation of speech dominance is another important consideration. Language lateralization most likely occurs by age 6. Careful determination of language dominance is especially important after this age because older children are less likely to achieve contralateral function transfer after surgery, resulting in higher risk of a permanent language deficit.

Magnetic resonance (MR) imaging localizes the epileptogenic hemisphere and excludes structural abnormalities in the intact hemisphere. All patients should undergo a video electroencephalography (EEG) recording in an attempt to confirm the epileptogenic hemisphere and exclude any potential seizure activity originating from the intact hemisphere. Any evidence of contralateral abnormality on imaging or electrical monitoring is associated with significant functional decline after surgery.

The length of surgery and blood loss should be minimized because pediatric patients have a limited reserve of blood volume. A central venous catheter is strongly recommended as it provides rapid repletion of intravascular volume. The length of surgery should also be minimized to decrease the surgical stress on the patient; the technical challenges of this procedure can lead to lengthy operative sessions. Intraoperative image guidance based on MR imaging is beneficial.

Multiple variations of hemispherotomy have been described, and the most common variations are peri-insular hemispherotomy, modified peri-insular hemispherotomy, and vertical parasagittal hemispherotomy. All hemispherotomy techniques have four commonalities: resection of medial temporal structures, interruption of the fibers forming the internal capsule and corona radiata, transventricular corpus callosotomy, and disruption of the frontal horizontal fibers.

The surgical technique described below is a modification of the procedure first developed by Schramm and colleagues in an attempt to decrease operative time and blood loss, remove less brain tissue, perform a smaller craniotomy, and achieve similar seizure relief when compared with functional hemispherectomies. I advocate this technique over other hemispherotomy techniques because access to the lateral ventricle is gained through the temporal horn, a technique that is common and familiar to epilepsy surgeons.

Hemispherotomy is a technically challenging operation that requires a thorough understanding of three-dimensional cerebral anatomy to ensure a safe and thorough hemispheric disconnection.

The patient is positioned supine on the operating table. The single pediatric pin of the skull clamp is placed behind the ipsilateral ear above the superior nuchal line, while the double pin is placed on the contralateral superior temporal line.

The dissection begins with a standard temporal lobectomy with removal of the medial structures. The details of this procedure are included in the Anteromedial Temporal Lobectomy chapter. The temporalis muscle is reflected forward in a single layer with the skin flap, and a craniotomy is performed to expose the peri-insular region and temporal lobe. Following the craniotomy, the dura is opened and reflected in the direction of the skin flap.

Care is taken to preserve large cortical branches of the MCA crossing the areas of cortical resection during the entire procedure to preserve distal cortices and minimize their resultant ischemia.

These illustrations idealize the operative view, but in reality, the operator’s working depth is long.

Identification of the pericallosal segment ensures that the dissection has not crossed the midline, placing the contents of the contralateral intact hemisphere at risk. Following identification of the proximal A2 branches, I extend the genu callosotomy anteriorly while pursuing the A2 branches through their encasing arachnoid membranes. Due to significant thickness of the genu, this arterial landmark and neuronavigation guidance are important to facilitate surgical orientation. Genu callosotomy is stopped at a coronal plane about 5 mm short of the foramen of Monro to avoid injury to the diencephalic structures.

It is also important to confirm visualization of the pia on the medial side of all of the disconnection routes to ensure a complete hemispheric deafferentation before closure.

I do not routinely place a catheter within the ventricle for temporary postoperative drainage unless immaculate hemostasis is impossible. The ventricles are copiously irrigated to remove debris.

Residual seizures most often occur when deafferentation is incomplete. Most commonly, this is the result of an incomplete disconnection of the corpus callosum and, more specifically, the genu and splenium. The possibility of this complication may be minimized by exposure of the pericallosal arteries and falx cerebri as described above. The use of an intraoperative EEG on the ipsilateral occipital lobe and the contralateral hemisphere may help with confirmation of the callosotomy.

Inadequate basal frontal lobe disconnection and insular resection have also been implicated in seizure recurrence. I define the extent of the basal subcortical dissection along the clinoid to ensure complete disconnection. I have also modified the previous techniques to pursue the horizontal and ascending edges of the tentorium to ensure complete disconnection in the region. Partial genu callosotomy is another potential reason for inadequate hemispheric disconnection; neuronavigation along with the route of A2 branches provides an adequate surgical roadmap.

Patients suffering from recurrent seizures should undergo high-resolution MR imaging and diffusion tensor imaging to assess the completeness of hemispheric deafferentation. I do not routinely obtain a postoperative MRI. Due to the working space limitations within the ventricles, patients with hemimegalencephaly are likely to have a higher risk of inadequate disconnection.

Although hydrocephalus occurs less commonly after hemispherotomy than hemispherectomy, it still complicates approximately 2% to 15% of operations. Hydrocephalus necessitating permanent shunt placement is more likely to occur when the underlying pathology is hemimegalencephaly or another multilobar cortical dysplasia.

A number of studies have confirmed the effectiveness of hemispherotomy in appropriately selected patient populations. Functional hemispherotomy or peri-insular hemispherotomy is associated with an 80% chance of the cessation of disabling seizures. The remainder of the patients who do not attain complete cessation of their seizures benefit at least from a worthwhile improvement compared with their preoperative status.

• Hemispherotomy is an effective treatment for pediatric epilepsy resulting from marked dysfunction of a single cerebral hemisphere. Although technically more challenging than similar operations, the procedure has the potential to provide adequate relief from seizures while limiting postoperative complications.

DOI: https://doi.org/10.18791/nsatlas.v7.ch04

This chapter was previously presented in a similar format as part of the following publication:

Kovanda TJ, Rey-Dios R, Travnicek J, Cohen-Gadol AA. Modified peri-insular hemispherotomy: operative anatomy and technical nuances. J Neurosurg Pediatr. 2014;13:332-338. PMID: 24410122. Permission for the use of the text was granted.