Functional hemispheric surgery: vertical versus lateral approach

Introduction

Hemispheric surgery has long been an established surgical treatment for pediatric patients with drug-resistant epilepsy stemming from diffuse, unilateral epileptogenic conditions. These include, but are not limited to, perinatal vascular insults, malformations of cortical development, Rasmussen’s encephalitis, and Sturge–Weber syndrome . Conceptually, this surgical approach involves separating a pathologic cerebral hemisphere from the healthy contralateral hemisphere, a concept first introduced by Dandy in 1928. Dandy initially applied this technique to remove the right cerebral hemispheres in patients suffering from diffuse right gliomas and resulting left hemiplegia . Although initially abandoned in tumor surgery due to significant perioperative morbidity and mortality without a corresponding survival benefit, hemispheric surgery has since undergone a resurgence for the treatment of epilepsy. Today, it encompasses a variety of techniques designed to safely achieve the separation of the cerebral hemispheres, either through the original method of resection or, more recently, through disconnection of the white matter tracts that facilitate neural signal propagation between the hemispheres.

The application of hemispheric surgery for unihemispheric, drug-resistant epilepsy was first refined by McKenzie in 1938. Building on Dandy’s initial approach, which involved removing the entire hemisphere including the ipsilateral basal ganglia, McKenzie introduced a crucial modification that preserved the basal ganglia. He achieved this by dividing the anterior and middle cerebral arteries distal to the origin of their deep perforators, rather than at the internal carotid artery bifurcation, thus maintaining their blood supply . The technique, now known as anatomic hemispherectomy, gained significant attention following Krynauw’s successful demonstration in 1950. In his series, 10 out of 12 children with infantile hemiplegia due to seizures achieved seizure freedom following the procedure . However, despite its initial success in controlling seizures, anatomic hemispherectomy’s popularity waned over the next decade due to increased reports of severe, sometimes fatal, delayed intracranial complications such as superficial cerebral hemosiderosis, hydrocephalus, and hematomas . These complications prompted a reassessment within the neurosurgical community, leading to substantial revisions of the procedure. This critical reevaluation aimed to retain the effectiveness of the surgery in controlling seizures while enhancing its safety, thereby shaping the modern practice of hemispheric surgery as it is known today.

The postoperative complications of anatomic hemispherectomy were thought to stem from the extensive tissue resection, intraoperative and postoperative bleeding, and the resultant large intracranial cavity created by the procedure . In response, advancements in hemispheric surgery have focused on developing techniques that reduce tissue resection and blood loss, thereby preserving more cerebral tissue to avoid the severe complications previously observed. A pivotal advancement was made in 1983 by Rasmussen, who introduced a functionally equivalent yet anatomically less extensive version of hemispherectomy. Termed “functional hemispherectomy,” Rasmussen’s technique involved removing the central portion of the hemisphere down to the cingulate gyrus and the entire temporal lobe but conserving the frontal and occipital lobes. He achieved separation from the contralateral brain by severing only the white matter tracts to the corpus callosum and brainstem . This innovative approach allowed Rasmussen to achieve seizure freedom in 10 of 14 patients without any instances of fatal superficial cerebral hemosiderosis, thereby confirming the efficacy and safety of reduced resection and cerebral disconnection over the more extensive traditional resections .

In the years following Rasmussen’s introduction of functional hemispherectomy, numerous modifications have emerged, collectively termed functional hemispherotomy. These variations are designed to achieve cerebral disconnection while minimizing tissue resection and are now favored for treating medically intractable epilepsy localized to one cerebral hemisphere . Notable among these are the vertical parasagittal hemispherotomy by Delalande , the lateral periinsular approach by Villemure , and the lateral transsylvian approach by Schramm . While these three are the most commonly practiced, there are additional techniques derived from these foundational methods. ( Fig. 12.1 ) . These primary approaches, along with their modifications, can broadly be categorized into two groups based on the anatomical plane of the operation—vertical or lateral ( Fig. 12.1 ) . Both categories follow essential surgical steps such as corticospinal tract interruption, corpus callosotomy, and basal-frontal disconnection. However, the differences between the vertical and lateral approaches are significant enough to hypothesize that they may influence the degree of seizure control achieved in clinical practice.

Debate over whether vertical or lateral hemispherotomy approaches are superior has only recently emerged within the last decade. Consequently, the evidence remains somewhat limited, preventing a definitive conclusion. However, two primary schools of thought have formed based on the research published so far. Initial studies suggested no significant differences in seizure outcomes between the two approaches . Yet, more recent investigations have proposed that vertical approaches might offer longer postoperative seizure freedom without an increased risk of complications compared to lateral approaches . This chapter will explore both vertical and lateral hemispherotomy techniques, providing a detailed comparison to help readers understand the distinctions and contextualize the current evidence and recommendations for choosing between these surgical options.

Vertical hemispherotomy

Introduced in 1992 by Delalande at the Rothschild Foundation in Paris, France, the vertical parasagittal hemispherotomy was the pioneering approach among vertical hemispherotomy techniques. This procedure ( Fig. 12.2 ) is typically performed with the patient in a supine position, their upper body and head flexed to 30–40 degrees to optimize surgical access. Intraoperative navigation is frequently employed to accommodate anatomical variations resulting from epilepsy-related pathology. The operation begins with a skin incision along the coronal suture, followed by a parasagittal precentral craniotomy. A C-shaped dural incision is then made and flapped toward the superior sagittal sinus.

Originally, the technique involved accessing the lateral ventricle via cortico-subcortical bloc resection for transependymal corpus callosotomy . However, a shift toward performing corpus callosotomy through an interhemispheric approach has gained popularity. This method, which involves corticotomy of the cingulate gyrus for direct access to the corpus callosum, minimizes tissue resection and provides more direct access . It is important to note that in cases of developmental diseases like hemimegalencephaly—where the septum pellucidum may be absent—this method may not be feasible, necessitating reliance on the original approach .

Following corpus callosotomy, the fornix is transected at the ventricular trigone, effectively disconnecting the mesial temporal structures. This is succeeded by a perithalamic corticotomy at the trigone level lateral to the thalamus, extending anteriorly toward the temporal horn. Surgeons must maintain a strictly vertical incision trajectory to avoid medial deviation into vital structures like the ipsilateral thalamus and basal ganglia. The initial cut is guided by the visibility of the thalamocaudate groove with the stria terminalis, while the entry of the choroid plexus into the temporal horn serves as a landmark for maintaining correct directionality and depth during disconnection. The perithalamic cut of the internal capsule completes the disconnection of projecting neocortical fibers from the occipito-parietal lobe, insular cortex, and most of the frontotemporal lobe, effectively isolating the pathological neocortex.

The procedure is finalized by resecting the most posterior part of the gyrus rectus, followed by an anterolateral cut through the caudate nucleus, which fully separates the anterior temporal lobe, amygdala, and remaining frontal lobe portions from the subcortical structures. This comprehensive approach ensures the complete disconnection of the affected cerebral hemisphere, aimed at maximizing seizure control while minimizing the risk of complications.

Having originated in Europe, vertical hemispherotomy approaches are predominantly performed in European countries, with recent gains in popularity noted in Asian countries as well. This trend was highlighted in a recent individual participant data metaanalysis (IPDMA) from our group, which pooled 193 cases of vertical hemispherotomy from published literature . The analysis reported impressive outcomes, with 81.2% of patients achieving seizure freedom at the last follow-up. Additionally, the likelihood of maintaining seizure freedom was 90.7% at 5 years and 85.5% at 10 years. These results are consistent with other independent patient cohorts subjected to vertical hemispherotomy .

Further supporting these data, a post hoc analysis of a large, multicenter, international cohort revealed that 86.1% of patients were seizure-free at the last follow-up, with the 5- and 10-year likelihood of seizure freedom holding steady at 85.5% . Similarly, a high-volume center, recognized as the birthplace of the vertical approach, reported a 10-year seizure freedom prevalence of 78.3%, consistent across various etiologies . An Italian multicenter study comparing vertical and lateral approaches also reflected favorable outcomes, with 84.2% seizure freedom in the vertical cohort at the last follow-up .

Notably, the lowest probability of seizure freedom reported for vertical hemispherotomy was 76.0%, from a recent European multiinstitutional retrospective study, which concluded no significant difference between vertical and lateral approaches . However, it is crucial to recognize that a significant portion of this cohort consisted of patients with hemimegalencephaly—a particularly challenging etiology for hemispherotomy—without comparing seizure freedom in a time-to-event analysis . Overall, the likelihood of postoperative seizure freedom following vertical hemispherotomy are remarkably high across various independent cohorts and prolonged follow-up periods, underscoring its effectiveness in managing intractable epilepsy caused by diffuse, unihemispheric pathology.

Vertical hemispherotomy approaches are specifically designed to minimize tissue resection and rely primarily on disconnection procedures. Advocates of these approaches often highlight their potential to reduce blood loss and cerebrospinal fluid (CSF) disturbances postoperatively. The benefit of reduced blood loss has been well established, particularly with the recent development of endoscopy-assisted vertical hemispherotomy by Chandra, which has resulted in cases with blood loss under 100 mL . However, claims that vertical approaches result in lower likelihood of hydrocephalus compared to lateral approaches have been harder to substantiate, mainly relying on retrospective, single-center studies. The first metaanalysis from 2021 to compare vertical and lateral approaches reported similar probability of hydrocephalus—9.9% and 10.6%, respectively . Similarly, our group’s IPDMA found no significant differences, with the probability of hydrocephalus at 10.9% for vertical and 11.4% for lateral approaches . While decreased risk of hydrocephalus is not a definitive advantage of vertical over lateral approaches, it is considered an improvement compared to the rates seen with anatomic and Rasmussen’s functional hemispherectomy .

Further advantages of vertical hemispherotomy are linked to the surgical plane. Delalande’s original vertical approach reduces the risk of inadvertently damaging the contralateral hemisphere because incisions are made vertically and parallel to it, unlike the lateral approaches which are perpendicular and directed toward the contralateral hemisphere. However, this may be less pertinent in modified interhemispheric approaches, which pose risks to the superior sagittal sinus, contralateral cingulate, and pericallosal arteries due to bilateral exposure . Additionally, vertical approaches do not manipulate the insular and opercular branches of the middle cerebral artery like lateral approaches do, minimizing risks of vasospasm and stroke due to their entry at the Sylvian fissure . Although vertical hemispherotomy provides excellent outcomes and several advantages, its use within a very narrow and deep surgical corridor can be suboptimal for cases with significant parenchymal distortion, potentially making lateral hemispherotomy approaches more viable in certain situations.

Lateral hemispherotomy

Lateral hemispherotomy, which disconnects the cerebral hemisphere through lateral access, can be executed through various techniques, reflecting numerous modifications since its inception . Despite the diversity of approaches, the core methodology remains consistent, broadly comprising a series of defined steps. The two foundational lateral hemispherotomy techniques were introduced in 1995: the periinsular hemispherotomy by Villemure at the Montreal Neurological Hospital, and the transsylvian hemispherotomy by Schramm at the University of Bonn.

The lateral approach ( Fig. 12.3 ) typically begins with the patient in a supine position, the head turned 90 degrees to the opposite side of the hemisphere being operated on. Initially, a question mark-shaped incision was made, followed by a large craniotomy designed for a temporal lobectomy. However, advancements have led to smaller incisions and craniotomies, with selective amygdalohippocampectomy increasingly replacing extensive resections. Upon opening the dura, the Sylvian fissure is dissected to expose the ascending M1 branch of the middle cerebral artery, the insular cortex, particularly the superior and inferior insular sulci, and the overlying M2 and M3 branches.