7.1 Introduction

It is useful to categorize temporal lobe epilepsy into one of two types based on the anatomical site of seizure onset: neocortical (lateral temporal) or mesial temporal lobe epilepsy (MTLE). Although they share many features that may make diagnosis difficult in a single patient, ¹ , ² there are sufficient distinguishing characteristics for MTLE to be considered a distinct syndromic entity. ³ In general, MTLE more commonly displays the typical temporal lobe seizure elements of an aura, staring, automatisms, and posturing. ⁴ MTLE is more related to childhood febrile convulsions, especially prolonged and complicated febrile convulsions. ⁵ The imaging hallmark of MTLE is mesiotemporal or hippocampal sclerosis, best seen on magnetic resonance imaging (MRI). ⁶ The interictal FDG-PET will identify more hypometabolic change in the mesial temporal and hippocampal region compared to the lateral neocortex. ⁷ Electroencephalography (EEG) of mesiobasal epilepsy shows interictal spiking prevailing in anterior temporal electrodes and ictal rhythmic theta activity in the same location. ⁸ Neocortical epilepsy may manifest signs related to perisylvian structures such as a simple auditory hallucination or, in the dominant hemisphere, postictal aphasia. ⁹ , ¹⁰

Localization of a seizure focus based on a noninvasive evaluation is often complex and may be unreliable. ¹¹ , ¹² , ¹³ On the other hand, when history, seizure semiology, EEG, and imaging findings point to MTLE, there can be a high degree of diagnostic certainty. ¹² , ¹⁴ Intracranial electrode monitoring may be required in a subset of patients to confirm the site of seizure onset. Our approach at Loma Linda University is to reserve invasive studies for patients in whom the diagnosis of temporal lobe epilepsy or lateralization of the seizure focus is in doubt.

In the 1950s, Paulo Niemeyer introduced the concept of selective amygdalohippocampectomy (SAH; ▶Fig. 7.1 ). ¹⁵ His description of a small corridor through T2 to the temporal horn for the selective disconnection of the mesial temporal structures represented a dramatic shift from the temporal lobectomy popular at that time. In fact, as a result of research performed by Scoville and Milner ¹⁶ and Penfield and Milner ¹⁷ that stressed the important role of the hippocampus in memory, the trend at that time was to preserve the hippocampus. In the first published report of surgery for temporal lobe epilepsy, Penfield and Flanigin largely preserved the mesial temporal structures in the patients on whom they operated. ¹⁸ Despite concerns of disrupting memory function, even in the early history of surgery for temporal lobe epilepsy, experimental evidence was mounting for the paramount role of the mesial structures in seizure genesis. ¹⁹ , ²⁰ , ²¹ , ²² , ²³ Much of this work was presented at the International Colloquium on Temporal Lobe in Marseille in 1954. ²⁴ Penfield and Jasper showed that further removal of the hippocampus could convert a failed surgery into a success. ²⁵ Removal of mesial temporal structures with temporal lobectomy was shown to be successful with long-term follow-up by Morris in 1956. ²⁶ Falconer identified that the best results from surgery were achieved with removal of the mesial temporal structures. ²⁷ Feindel and colleagues demonstrated the role of the amygdala in seizure generation and temporal lobe automatisms. ²⁸ , ²⁹

More recently, considerable interest was raised for SAH after Yasargil et al developed an approach using the transsylvian route. ³⁰ In the technique of Yasargil et al, the arachnoid over the sylvian fissure is divided and the bottom of the circular sulcus exposed. An incision between two opercular temporal arteries exposes the ventricular horn allowing the hippocampal formation to be resected by an extrapial approach and the amygdala removed by subpial aspiration. Later Hori et al described a novel subtemporal approach to access and remove the mesial temporal structures by traversing the parahippocampal gyrus. ³¹ Over many years now, a transcortical approach through the second temporal gyrus, similar to that demonstrated by Niemeyer, has been used and popularized at the Montreal Neurological Institute and by the author. ³² , ³³

The SAH approach is based on the concepts first developed from Hughlings Jackson’s description of a lesion in the uncus causing psychomotor seizures (Jackson called this a “dreamy state”) and the role played by the mesial temporal structures in human epilepsy. ³⁴ Subsequently, an abundance of experimental studies have pointed to a very important role of the mesial temporal structures in experimental and human epilepsies. ¹⁹ , ²⁰ , ²² , ²⁵ , ²⁶ , ³⁵ , ³⁶ Results from many patients operated for MTLE with SAH confirm the overwhelming participation of the mesial temporal and limbic structures in temporal lobe epilepsy. ³⁷ , ³⁸ , ³⁹ Furthermore, the International League Against Epilepsy determined that MTLE represents a sufficient cluster of signs and symptoms to comprise a specific epilepsy syndrome. ³

The goal of SAH is to achieve the best possible results on the seizure tendency in the most selective manner, and to spare from resection those cerebral structures not involved in the seizure focus. At centers with experience in SAH, the results on seizure tendency clearly rival the results of cortico amygdalohippocampectomy (CAH; see below). ³⁰ , ³¹ , ³² , ³⁶ , ³⁹ Although the neuropsychological advantages of SAH have yet to be entirely proven, ⁴⁰ , ⁴¹ , ⁴² the intuitive advantages are inescapable. Sparing brain tissue not involved with the seizure focus should be an important goal of epilepsy surgery.

7.2 Surgical Anatomy of the Temporal Lobe with Emphasis on the Mesial Structures

Nineteenth-century neuroanatomists developed a numbering system for the gyri and sulci of the brain that today is the preferred method of nomenclature. ⁴³ , ⁴⁴ It is a simple but useful system that emphasizes the concept of gyral ribbons interconnecting the convolutions of the brain. ⁴⁴ , ⁴⁵ , ⁴⁶ There are five gyri running the length of the temporal lobe. The superior temporal gyrus is T1, middle temporal gyrus is T2, inferior temporal gyrus is T3, fusiform gyrus is T4, parahippocampal gyrus is T5, and the most medially located gyrus in the temporal lobe is the hippocampal complex (composed of the hippocampus proper and dentate gyrus; ▶Fig. 7.2a, b). Similarly, the temporal gyri are separated by four longitudinal sulci: S1, S2, S3, and S4. The superior temporal gyrus (T1) is bounded above by the sylvian fissure. Below T1, the superior temporal sulcus (S1) is the deepest sulcus in the temporal lobe extending toward the temporal horn, and it is an important anatomical guide to localizing the ventricle. T4 is well demarcated from the parahippocampus (T5) by the constant and strong collateral fissure (S4) that produces an obvious bulge into the ventricle of the temporal horn called the “collateral eminence” (▶Fig. 7.2b, ▶Fig. 7.3, and ▶Fig. 7.4).

The fifth temporal gyrus is better known as the parahippocampal gyrus because of its close relation to the hippocampus proper (▶Fig. 7.2b). The parahippocampus lies adjacent and inferior to the hippocampus. The parahippocampus is clearly delineated laterally by the collateral fissure (S4) and superomesially by the hippocampal sulcus. Its anterior extent does not reach the temporal pole but ends approximately 2 cm behind it. Peter Gloor defined the entorhinal cortex in man to be the region of parahippocampus anterior to the most posterior limit of the uncus. ⁴⁷ Posteriorly, the parahippocampal gyrus is divided into two branches by the anterior calcarine sulcus. The superior branch merges into the isthmus of the cingulate gyrus, and the inferior one becomes the lingual gyrus of the occipital lobe (▶Fig. 7.2b).

The uncus has a conical shape (▶Fig. 7.5). Its medial convex surface can be further subdivided into several small gyri including the semilunar gyrus superiorly, ambient gyrus, uncinate gyrus, band of Giacomini, and intralimbic gyrus that form its posterior apex. In fact, the rostral end of the hippocampus and dentate bends medialward to form most of the uncus. The semilunar gyrus in the anterosuperior part of the uncus is created by the amygdala.

The hippocampus proper envelops the hippocampal sulcus (▶Fig. 7.4). Its two pial walls closely adhere to each other and contain the vascular supply of the hippocampus. The superomesial wall of the sulcus is formed by the dentate gyrus, whereas the subiculum and hippocampus proper form the inferolateral wall (▶Fig. 7.4). With the ventricle opened, the hippocampus appears as a distinct bulge on its inner surface covered by a white matter tract, the alveus, and a glistening ependymal layer (▶Fig. 7.3). The hippocampus lies just lateral to the cerebral peduncle. Its anterior portion, or head, is short and occupies the rostral extent of the temporal horn and curves mesially to form the bulk of the uncus. Its middle part (body) extends posteriorly into a narrower part called the “tail” that curves backward and upward to the trigone of the ventricle.

The fimbria is a white matter band that runs horizontally along the medial border of the hippocampus, just above the dentate gyrus, separated by the fimbriodentate sulcus. Its free aspect forms the inferior edge of the choroidal fissure. It ends anteriorly at the posterior gyrus of the uncus, the intralimbic gyrus. Posteriorly, under the splenium, the fimbria becomes the fornix.

The choroid plexus partially covers the hippocampus, as they both bulge into the temporal horn and is an important surgical landmark for orientation while working in the ventricle (▶Fig. 7.3). The choroid plexus arises from the tela choroidea, an ependymal and pial layer that fills the choroidal fissure. The tela choroidea spans the space between the fimbria and the stria terminalis, attached to the two white matter bundles (▶Fig. 7.4). The anterior choroidal artery enters the temporal horn at the inferior choroidal point to form the choroid plexus. Therefore, the choroid plexus can be reflected posteriorly to see the intralimbic gyrus. Tilting the choroid plexus inferiorly exposes the stria terminalis, and superomesial retraction uncovers the fimbria and hippocampus.

The mesial temporal structures are defined as including the uncus, amygdala, hippocampal complex, fimbria, parahippocampus, and entorhinal cortex. The collateral sulcus and choroid plexus are important landmarks that bound the mesial temporal area. The anatomical region of the mesial temporal area lies between the choroid plexus and collateral sulcus, also including the more anterior structures of the amygdala and uncus. Therefore, the lateral temporal region (often called iso- or neocortex to contrast with the archicortex named for the phylogenetically more primitive three-cortical-layered hippocampus) comprises the temporal lobe lateral to the collateral fissure. The parahippocampus (and especially the subiculum) is the transition between the six-layered neocortex and three-layered archicortex of the hippocampus.