21 Epilepsy: Temporal Lobectomy with Invasive Monitoring
Abstract
Over the past half century, temporal lobectomy has emerged as a well-established surgical therapy for temporal lobe epilepsy. In this chapter, we review basic concepts involving temporal lobectomy surgery, including recent developments in patient selection, preoperative evaluation, surgical technique, and postoperative outcomes. We describe in detail the surgical technique we use at our institution and briefly discuss emerging future developments.
21.1 Patient Selection
Temporal lobectomy emerged in the 1950s as a surgical procedure used to treat seizures arising from the temporal lobe. 1 , 2 The primary indication is medically-refractory temporal lobe epilepsy, however, it can also be useful to treat lesions in the anterior temporal lobe such as cavernomas or low-grade gliomas. Classically, patients were identified as candidates for temporal lobectomy if they had partial seizures that were medically refractory, and they did not have diffuse or progressive brain disease. 3 However, these requirements have been substantially revised over the past several decades. 4 , 5 , 6 When selecting patients for this procedure, three basic questions must be answered.
21.1.1 Have the Patients Failed Medical Treatment?
The patient must have failed medical treatment with two antiepileptic medications either in terms of seizure control or tolerance of these medications.
21.1.2 Are Seizures Arising from the Temporal Lobe?
Temporal lobectomy is only successful if there is a seizure focus in the temporal lobe. To assess whether this is the case, it is important to integrate data from several diagnostic modalities. These include clinical semiology, neuropsychological testing, neuroimaging, non-invasive electrocroticography (EEG), and in select cases, invasive electrocorticography (EEG). The ideal candidate for temporal lobectomy will have converging evidence to suggest that seizures arise from the temporal lobe. However, in many cases the data is mixed and requires collaborative decision-making from a multi-disciplinary team to make a surgical decision.
21.1.3 Is There a High Risk of Postoperative Functional Deficits?
Temporal lobectomy can result in damage to structures that are important to language (lateral dominant temporal lobe), vision (optic radiations, Meyer’s loop), and memory (hippocampus). Preoperative studies to assess the baseline status of these cognitive functions and their anatomical localization relative to the temporal lobe are important to assess the risk of further injury during surgery. Additionally, as with all major surgical procedures, the patient’s cardiovascular status, nutritional status and overall health should be assessed to as to estimate a perioperative risk of cardiovascular and infectious complications.
21.2 Preoperative Evaluation
We briefly discuss several diagnostic modalities that are useful for pre-operative assessment of patients (see Chapter 19 for a further discussion of these modalities, including specific indications for surgery).
21.2.1 Semiology
Seizures arising from the temporal lobe are classically complex partial seizures with hand and face automatisms such as lip smacking. With involvement of the amygdala, they are often preceded by aura involving fear (“feeling of impending doom”), or a difficult to describe epigastric sensation. Involvement of the ipsilateral basal ganglia can lead to contralateral hemiplegia or dystonia. When these signs are observed in the setting of persistent ipsilateral automatisms, they may be falsely localized to the contralateral motor cortex.
21.2.2 Non-invasive Scalp Electroencephalograpy (EEG)
Scalp EEG can be used to assess for interictal spikes, markers of pathological neural activity arising from epileptogenic tissue in between seizures. The observation of unilateral interictal spikes arising from the temporal lobe provides a strong indication for temporal lobectomy. 7 Additionally, scalp EEG recordings can be used in conjunction with video recordings to capture clinical and subclinical seizures. Here, one can assess intra ictal electrophysiological patterns of activity to anatomically localize a seizure focus.
21.2.3 Neuropsychological Testing
Specific patterns of cognitive deficits as identified by neuropsychological testing may provide clues to the site of the epileptogenic focus. For example, a selective reduction in verbal memory performance may suggest that the epileptogenic focus involves the patient’s dominant medial temporal lobe.
21.2.4 Language Localization: The WADA Test and Functional Magnetic Imaging (fMRI)
The main goal of these tests is to determine which cerebral hemisphere is dominant for language function, which of course has major implications for surgical planning. The WADA test is an invasive procedure and involves injecting a barbiturate into each of the carotid arteries and testing the subsequent effects on language. Functional MRI (fMRI) is a non-invasive neuroimaging method to study patterns of blood oxygenation during various cognitive tasks. Recent data suggests that fMRI provides concordant results to the WADA test in most cases of temporal lobe epilepsy. 8
21.2.5 MRI Evaluation
MRI scans can reveal evidence of sclerosis in the medial temporal lobe, suggesting ischemic injury and gliosis, and a potential substrate for seizure generation. Unilateral mesial temporal sclerosis (MTS) on MRI is a strong indication that the epileptogenic focus is within the temporal lobe; as such, these cases are associated with favorable postsurgical outcomes (n = 161, 71% seizure freedom at 2 years). 9 However, favorable post-surgical outcomes have been observed in “MRI-normal” patients with other indicators of temporal lobe epilepsy, albeit in a smaller study (n = 40; 60% seizure freedom with at least 1 year of follow-up). 10 See the outcomes section below for further discussion.
21.2.6 Other Imaging Modalities (PET, SPECT, MEG)
Other imaging modalities can provide important evidence in preoperative evaluation but vary in their utility across institutions. 11 Positron emission tomography (PET) can be used to study metabolic activity in the medial temporal lobe. Relative hypometabolism in one hemisphere is an indicator of epileptic activity. Subtraction ictal single photon emission computed tomography (SPECT) imaging can be used in with EEG activity to assess whether the mesial temporal lobe shows increased activity during seizure generation. Magnetoencephalography (MEG) can be used with source-localization algorithms to spatially localize the onset of ictal activity with greater detail than scalp EEG.
21.2.7 Intracranial Invasive EEG Monitoring with Depth and Subdural Electrodes
When the above studies fail to identity a seizure focus, patients can undergo invasive monitoring with intracranial EEG subdural and depth electrodes. Intracranial EEG offers the temporal resolution of scalp EEG recordings with greater spatial resolution. However, because of the limited coverage offered by these electrodes, they are most useful when preoperative studies yield specific hypotheses about particular locations where the seizure focus may be located. Subdural electrodes can be “grids” or “strip” electrodes that can be used to cover cortical surface, and can also be used to map motor and language function. Depth electrodes are inserted into the parenchyma and can sample from deeper subcortical tissue, such as the mesial temporal lobe.
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