Historical Considerations

Over the course of the past century, two main factors have driven the surgical approach to epilepsy. First, an appreciation of the central role of the mesial basal temporal structures in many cases of human epilepsy has been gained from clinical observation and neuropathologic, electrophysiologic, and radiologic studies.¹ Second and in parallel has been accumulating evidence from numerous surgical series demonstrating that temporal lobe resection, in particular, resection of mesial temporal structures, leads to an 80% to 90% rate of freedom from seizures in patients with medically intractable temporal lobe epilepsy.¹^–⁷

A number of surgical techniques have evolved in the process. The standard temporal resection procedure has been en bloc anterior temporal lobectomy (ATL) involving anterior neocortical resection of up to 4.5 to 6.5 cm, depending on whether the dominant temporal lobe is being operated on, and mesial temporal resection encompassing the amygdala and at least 3 cm of hippocampus. This approach was originally advocated by Falconer and colleagues in 1955 ⁸ and is still used today.

Selective surgical approaches to the amygdala and hippocampus evolved as evidence increasingly indicated a critical role for these structures in epileptogenesis, and methods were sought to minimize collateral surgical injury to important temporal neocortical structures. Niemeyer was the first to describe selective transcortical transventricular amygdalohippocampectomy (STTAH) in 1958.⁹ A direct approach to the mesial temporal structures via the middle temporal gyrus was used, an approach that Olivier would later refine with the assistance of frameless stereotaxy.¹

Other surgeons have approached the mesial temporal structures via less direct paths. Weiser, Yasargil, and their coworkers reported a transsylvian approach to the medial temporal structures that had the theoretical advantage of complete avoidance of neocortical injury; however, the approach was technically more demanding and placed critical vascular structures at risk.¹⁰^,¹¹ Hori and associates described a subtemporal selective approach and reported fewer neuropsychological sequelae as an advantage.¹²^,¹³

With the recent confirmation that temporal lobectomy is superior to the best medical therapy for patients with intractable temporal lobe epilepsy,⁷ focus has now shifted toward understanding what is the best surgical approach to achieve this end. Selective amygdalohippocampectomy (SAH) represents an approach to the mesiobasal temporal structures that is as effective as ATL in selected patients, has the potential to minimize neocortical injury and its neurological sequelae, and is an approach that may be used by most neurosurgeons.

Surgical Rationale for Selective Amygdalohippocampectomy

The surgical rationale for performing SAH is to limit surgical resection to brain structures that must be removed to eliminate seizures while minimizing the risk for injury to important collateral structures. For example, in the case of well-defined mesial temporal sclerosis, these objectives can be met with a more selective approach than the traditional ATL.

Seizure outcome after epilepsy surgery has been noted to depend largely on diagnostic and clinical variables that have an impact on patient selection.³^,⁶ The efficacy of selective approaches versus traditional en bloc ATL with mesial resection for seizure control has been examined in a number of noncontrolled series and found to be comparable in most,³^,⁵ but not all reports,² and this variability may reflect differences in the types of patients selected for surgery. Indeed, even in studies that have claimed the highest rates of freedom from seizures with the selective approach,¹ it is evident that strict attention to preoperative selection is important.

In the absence of surgical complications such as stroke or infection, neuropsychological complications, particularly memory loss, are the major potential morbidity after all temporal resections. It is argued that these complications must arise from resection of functioning normal tissue and are therefore less likely to occur in patients undergoing more selective resection. Although confounding variables such as the dominance of the resected side, the absence of ipsilateral hippocampal sclerosis, the focality of the epileptic source, the presence of intact memory, and methodologic differences in neuropsychological testing make direct comparisons difficult, there is accumulating evidence that individually tailored or selective approaches may have a more favorable cognitive outcome than standard resections ^3,^5,^14,¹⁵; however, not all studies have shown such results.

Although SAH has been performed via the transsylvian and subtemporal routes, the rationale for the frameless stereotaxy transcortical approach described in this chapter is that it is the most direct and simplest approach that results in minimal disruption of nontargeted tissue and avoids the arachnoid dissection and vessel manipulation or excessive retraction inherent with other selective approaches.