Preoperative evaluation for temporal lobe surgery

doi:10.1016/S0967-5868(03)00080-8

Journal of Clinical Neuroscience

Volume 10, Issue 5, September 2003, Pages 535-539

https://doi.org/10.1016/S0967-5868(03)00080-8 Get rights and content

Abstract

Temporal lobe epilepsy (TLE), the most common form of partial epilepsy in adults is often refractory to medical treatment and in these patients epilepsy surgery is considered. Successful surgery is dependent on accurate localisation and lateralisation of the epileptogenic zone. The preoperative evaluation involves a series of assessments and investigations including detailed clinical history, interictal EEG, video-EEG monitoring, MRI, PET, SPECT, and neuropsychology and neuropsychiatric assessment. The role of each of these investigations and assessments in the preoperative evaluation is discussed. Advanced MR techniques including magnetic resonance spectroscopy, MR diffusion and MR perfusion have recently been assessed and are likely to enhance the pre-surgical evaluation of patients with TLE.

The surgical outcome and preoperative investigations performed of 80 consecutive patients who underwent temporal lobe surgery between 1993 and 2002 at Royal Melbourne Hospital were reviewed. All patients had MRI, video-EEG monitoring and neuropsychology assessment and 56% a PET scan. During a mean follow-up of 5.9 years 75% had Class 1 outcome, 22% non-Class 1 outcome and 3% were lost to follow-up. The results of preoperative investigations were correlated with outcome. For interictal EEG, seizure semiology, ictal EEG, PET and neuropsychology assessment the surgical outcome of patients in whom results were concordant to side of surgery was compared with those discordant or non-lateralising. There was no significant difference. In 78 of 80 patients MRI revealed mesial temporal sclerosis or a foreign tissue lesion. The outcome was no different between these two groups. Results suggest that in patients with unilateral temporal lobe lesion on MRI and where ictal EEG is either concordant or non-lateralising, other investigations including PET, provide little additional prognostic information.

Introduction

Temporal lobe epilepsy (TLE), the most common form of partial epilepsy in adults, is often refractory to medical treatment and in these patients epilepsy surgery is considered. Although surgery has become a common form of treatment for patients with intractable TLE, it is only in recent times that a randomised control trial of surgery for TLE has been undertaken. It has demonstrated that surgery at one year follow-up is superior to medical therapy.¹ Most would now agree that patients with TLE who are inadequately controlled by antiepileptic drugs should be considered for epilepsy surgery.

There are two main subtypes of surgically remediable TLE: mesial temporal lobe epilepsy (MTLE) and lateral or neocortical temporal lobe epilepsy (NCTLE). MTLE due to mesial temporal sclerosis, accounts for at least 60% of patients proceeding to temporal lobe surgery.² This condition is often associated with a history of febrile seizures in infancy with onset of habitual complex partial seizures usually late in the first decade of life, with initial response to antiepileptic drugs but later becoming resistant. Secondarily generalised seizures are infrequent. The MRI features of mesial temporal sclerosis include atrophy, increased signal and loss of internal architecture of the hippocampus.

NCTLE is more difficult to diagnose. Studies assessing clinical and neurophysiological features, including interictal and ictal EEG patterns, have detected few distinguishing features.³ Patients may have a lesion on MRI in the lateral temporal lobe but more often the MRI is normal making differentiation from MTLE without hippocampal atrophy difficult. Patients with MTLE due to mesial temporal sclerosis usually undergo an anterior temporal lobectomy, whereas patients with NCTLE in whom the hippocampus and amygdala are assumed to be normal, may have a surgical procedure sparing the mesial temporal structures. Hence the presurgical assessment involves not only confirmation of temporal lobe seizures but also localisation of the seizure onset within the temporal lobe.

The proportion of patients seizure free one to two years following temporal lobe surgery varies from one study to another.[1], [2], [4], [5], [6], [7] This variation in outcome in part relates to selection of patients for surgery, preoperative evaluation and methodology of follow-up. It is well recognised that successful surgery is dependent on accurate localisation and lateralisation of the epileptogenic zone with appropriate preoperative evaluation, and ensuring that surgery is unlikely to result in significant morbidity. The preoperative evaluation involves a series of assessments and investigations including detailed clinical history, interictal electroencephalography (EEG), video-EEG monitoring, magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), neuropsychology and neuropsychiatric assessment and in some cases the Wada test. Each test provides valuable information about the seizure focus and suitability for surgery. The value of these tests individually and in combination remains unclear but probably depends on the patient population assessed.

Section snippets

Clinical history

The clinical history and description of seizures including auras can help in localising and lateralising the epileptogenic focus.⁸ Complex partial seizures of temporal lobe origin typically begin with arrest of motor activity followed by oral and manual automatisms. Patients have impaired responsiveness during the seizure and are usually amnestic for the event. A history of febrile seizures in infancy is common in patients with MTLE due to mesial temporal sclerosis. Auras such as epigastric

Interictal EEG

Interictal EEG may provide valuable information in lateralising the seizure focus in patients with TLE. Although a single outpatient awake scalp EEG recording has a relatively low yield,⁹ prolonged interictal scalp EEG recording during inpatient EEG monitoring significantly increases the yield of epileptiform activity. One study reported 71% showing unilateral temporal epileptiform discharges ipsilateral to the seizure focus and 25% bilateral discharges.¹⁰ The presence of interictal unilateral

Video-EEG monitoring

Despite advances in neuroimagining, video-EEG monitoring remains an important investigation in the preoperative evaluation for temporal lobe surgery. It has been suggested ictal EEG recordings are not necessary if the interictal EEG demonstrates consistent unilateral anterior mid-temporal epileptiform discharges and other data is not discordant.¹² A good surgical outcome in patients with MTLE proceeding to temporal lobe surgery without video-EEG monitoring has been reported.[12], [13] Most

Magnetic resonance imaging (MRI)

High resolution MRI plays a crucial role in the presurgical evaluation of patients with refractory partial epilepsy and in many epilepsy centres forms the basis for determining the seizure origin. MR imaging should include T₁ weighted volumetric acquisition allowing quantitative assessment of the hippocampal volume, T₂ weighted coronal sequences with or without T₂ mapping, and fluid attenuated inversion recovery (FLAIR) to detect increased signal in the hippocampus, as well as inversion

Positron emission tomography (PET)

In the majority of patients with hippocampal atrophy on MRI, PET reveals hypometabolism in the ipsilateral temporal lobe.[2], [21] In this situation PET usually provides little additional information in localising and lateralising the seizure focus. Contralateral or bilateral temporal hypometabolism suggests bilateral temporal pathology and possibly a poorer prognosis following temporal lobe surgery. A recent study suggests the pattern of temporal hypometabolism using PET is predictive of

Single photon emission computed tomography (SPECT)

Ictal SPECT is extremely helpful in the presurgical evaluation of TLE providing valuable information of the location of the seizure focus. In patients with an MRI lesion and well lateralised ictal EEG studies the test provides little useful additional information. Ictal SPECT can, however, be useful in patients in whom ictal EEG data is inconclusive. A number of reports have highlighted the value of ictal SPECT in localising the seizure focus, reporting hyperperfusion at the site of seizure

Neuropsychology assessment

A neuropsychology assessment is an essential component of the presurgical evaluation. The test provides information regarding the functional integrity of the tissue comprising the epileptogenic focus and the remaining brain structures. This information can be used to help localise and lateralize the seizure focus and assess the post-operative risk for memory loss. A presurgical neuropsychological assessment showing verbal memory deficits suggests pathology in the left (dominant) hippocampus,

Psychiatric assessment

Psychopathology is common in patients with TLE.³⁰ Although there are few psychiatric conditions that are absolute contraindications to temporal lobe surgery, psychiatric assessment is essential in the preoperative evaluation to help minimise psychiatric morbidity following surgery and to ensure expectations from surgery are appropriate. Episodic psychosis, a relatively common psychiatric complication of TLE, may diminish when patients become seizure free following surgery. Chronic psychosis

Magnetic resonance spectroscopy (MRS)

MRS provides a non-invasive means of detecting and quantifying chemicals of interest in selected regions of the brain. N-acetylaspartate (NAA), a compound exclusively found in neurons and neuronal processes, creatine (Cr) and choline (Cho), markers of both glial and neuronal tissue, can be detected using proton MRS. Given mesial temporal sclerosis represents neuronal loss with varying degrees of gliosis, it is possible MRS may detect changes of mesial temporal sclerosis not evident on MRI.