Historical Background

The standard temporal lobectomy developed concurrently with the identification of the temporal lobe epilepsy syndrome and the emergence of electroencephalography (EEG) in the 1940s and 1950s. Much of the credit belongs to Penfield and Jasper at the Montreal Neurological Institute (MNI).¹ When Herbert Jasper joined the MNI in 1937, he brought the technique of EEG with him. By this time, Penfield and his partner William Cone had been performing surgery for epilepsy since Penfield arrived in 1928. As EEG became more established in the evaluation and diagnosis of focal epilepsy through the work of Jasper and Gibbs, the classification and study of temporal lobe epilepsies was under way. Through the close friendship of Jasper and Penfield, the MNI became a leader in the late 1930s in the surgical treatment of epilepsy. In 1941, Jasper and Kershman proposed a classification of epilepsy based on EEG waves.² In this report, they described the localization of psychomotor phenomena from within the deep regions of the temporal lobe. Despite increasing awareness of the role of the mesial temporal lobe in these seizures, the lack of understanding about the function of this tissue and the inability of the surgeon to “see” the lesion at the time of resection led to reluctance on Penfield’s part to remove these structures.¹ In 1950, Penfield reported his success in anterolateral temporal resections.³ In this series of 68 patients, 10 had partial removal of the uncus, and only 2 had hippocampal resections. EEG abnormalities were recorded from the temporal lobe in this group of patients, but an underlying substrate had not been identified. At the same time, Percival Bailey and Ernest Gibbs reported a series of 25 patients from the University of Illinois program who underwent temporal resection guided by EEG.⁴ Similar to Penfield’s series, these patients did not have hippocampal resections.

The second phase in the development of the modern surgical strategy to treat temporal lobe seizures took place in the 1950s as the role of the mesial temporal lobe structures in the pathogenesis of the epilepsy became better understood. This occurred as a number of scientists began to study the connections of the mesial temporal lobe to the rest of the brain and through the use of stimulation studies to reproduce seizure semiology in animals and humans.^5–8 In 1952, Penfield and Baldwin published a classic monograph describing their technique for anterolateral temporal lobectomy including the hippocampus and amygdala.⁹ They reported that the most frequent pathologic abnormality in two of three of their cases was an atrophic lesion termed incisural sclerosis. Falconer in a report in 1953 recognized a connection with febrile seizures and introduced a modification allowing for en bloc resection of the hippocampus, which allowed the pathologists to study the tissue.¹⁰ With the advent of neuroimaging, modifications to the technique reported by Penfield have been made to address specific pathology seen preoperatively. Despite these modifications, the operation developed and introduced by innovative neurosurgeons in the early 20th century remains one of the most successful operations for the treatment of epilepsy today.

Identification of Surgical Candidates: The Concept of Pharmacoresistance and Medical Intractability

The indications for epilepsy surgery generally include the presence of focal epilepsy resistant to treatment with an adequate trial of anticonvulsant therapy. The precise definition of an adequate anticonvulsant trial is open to interpretation, but a study by Kwan and Brodie produced useful information regarding the efficacy of anticonvulsant therapy in newly diagnosed epilepsy.¹¹ This study suggests that after three medications fail to control seizures, further success is unlikely, and other options should be considered. Consider also the paper by Wiebe and associates, comparing temporal lobectomy to optimal medical therapy in a group of patients with temporal lobe epilepsy.¹² In this prospective randomized trial, surgical therapy in combination with medical therapy was far superior to ongoing medical therapy alone. These papers lend credence to the idea that patients with ongoing epilepsy despite a trial with a few anticonvulsants should be expeditiously evaluated for possible epilepsy surgery.

Preoperative Evaluation

Surgical Decision Making

For the TLE patient, decisions revolve around localizing the epilepsy to the lateral or mesial temporal lobe, the presence and nature of the epileptic lesion, and the presence of language or important short-term memory deficits. For the patient undergoing a standard temporal lobectomy, the epilepsy should be localized to the anteromesial temporal lobe, and ideally a well-defined lesion should be present (mesial temporal sclerosis, malformation of cortical development, neoplasm, cavernoma). The planned posterior extent of resection should not encroach on possible neocortical temporal lobe language areas. If the dominant temporal lobe is involved, a baseline memory or naming deficit in the presence of mesial temporal sclerosis would support the conclusion that the correct brain site was targeted, and the risk for causing further neurological deficits would be acceptably low.^27,28

The anatomy of the temporal lobe deserves a brief discussion at this point. The temporal lobe has well-defined anterior, lateral, basal, and mesial surfaces. The posterior boundary is arbitrary, having no obvious anatomic demarcation separating it from the parietal area. The temporal lobe is made up of five gyri and their corresponding sulci. The lateral surface lies below the sylvian fissure and extends to the floor of the middle cranial fossa. The gyri from top to bottom include the superior temporal gyrus (T1), the middle temporal gyrus (T2), and the inferior temporal gyrus (T3), which often extends onto the basal surface. The basal surface includes the inferior temporal gyrus (T3), the fusiform gyrus, and the parahippocampal gyrus. The mesial surface includes the amygdala and the parahippocampal gyrus, including the uncus. The collateral sulcus separates the fusiform and parahippocampal gyri and serves as an important reference to locate the temporal horn of the ventricle. Within the temporal horn, important anatomic structures include the inferior choroidal point (anterior choroidal artery enters the choroid plexus here), the hippocampus occupying the mesiobasal portion of the ventricle, the fornix, the choroid plexus, the choroidal fissure, and the amygdala in the anterior-superior-medial portion of the ventricle. The reader is referred to an excellent series of articles describing in detail the temporal lobe anatomy.²⁹

Functional anatomy in the temporal lobe includes comprehensive language cortex in the dominant temporal lobe, visual field fibers (Meyer’s loop) subserving the contralateral upper quadrantic visual field information, and potential important memory and naming centers. The anatomy of language cortex in the temporal lobe can be quite variable.^30,31 Cortical stimulation testing can be performed to further identify and protect lateral language cortex when posterior temporal lobe resections are anticipated on the dominant side. This can be done either intraoperatively with the patient awake or extraoperatively with implanted electrodes. After the language cortex has been identified, the temporal resection can be tailored to the patient’s individual anatomy. The standard temporal lobectomy is designed to avoid temporal lobe cortical language sites by limiting the resection of the superior temporal gyrus to 3 to 4.5 cm from the anterior temporal pole. There is some controversy as to whether even this practice is safe, and some centers advocate leaving the entire superior temporal gyrus in place. This is based on language-stimulation data suggesting the presence of language sites in the anterior 3 cm of the superior temporal gyrus in a small percentage of patients.³⁰ Of course, these sites were not resected, and thus it is difficult to know whether they were essential language sites. Other “nonessential” language sites in the temporal lobe have been demonstrated through cortical stimulation followed by resection.³² These sites were located in the basal temporal lobe by direct cortical stimulation but left no permanent language deficit after they were resected. Insufficient data, however, are available from the limited numbers of patients reported to conclude with certainty that no patient will develop a permanent language deficit after resection of a basal temporal language site.

Visual field fibers are also located in the temporal lobe as they extend forward from the lateral geniculate body before turning posterior on their way to calcarine cortex. These fibers are located unpredictably in the roof of the temporal horn, and standard temporal lobe resections cause injury to this fiber tract in as many as 50% of cases.³³ This leads to the “pie in the sky” visual field deficit with loss of peripheral vision in the opposite upper quadrant. In most cases, the visual field defect noted with careful perimetry testing is not clinically significant.

Finally, other important anatomic structures the surgeon should be familiar with include the sylvian fissure and associated structures (sylvian vein, middle cerebral arterial cascade, and underlying insula), the vein of Labbé, and the region of the tentorial incisura, including the brainstem, posterior cerebral artery, basal vein of Rosenthal, and third and fourth cranial nerves. Familiarity with these anatomic structures is critical to avoid a potentially devastating injury during resection of the mesial temporal lobe structures.

Surgical Technique

When the decision has been made to perform a standard temporal lobectomy, the technique varies little from patient to patient. The word standard implies a reproducible operation from patient to patient and even surgeon to surgeon. It is best used to treat the “standard” syndrome of mesial TLE consisting of reproducible EEG, MRI, and seizure semiology elements. The targets of this operation are the mesial temporal lobe structures that are “sclerotic”: the parahippocampus, hippocampus, and amygdala. The exact posterior extent of resection of the hippocampus to improve outcomes is unknown, but at least one prospective trial correlated improved outcome with more aggressive resection of the hippocampus.³⁴

The procedure does vary slightly for the language-nondominant and language-dominant sides. On the dominant side, the surgeon limits resection of the superior temporal gyrus to avoid a possible postoperative language deficit. This usually translates into a resection of 3 to 4.5 cm measured along the superior temporal gyrus from the anterior aspect of the middle fossa. On the nondominant side, the posterior extent of resection can be farther from the temporal pole, although it is usually limited by the vein of Labbé about 4.5 to 6 cm posterior to the temporal pole. One must be careful with resections extending posteriorly beyond these measurements because there is increased risk for injuring the geniculocalcarine tract with resultant homonymous hemianopsia.