Multidisciplinary Presurgical Evaluation for Epilepsy Surgery

Introduction

When we decide that surgery is an option for treating epilepsy, there are a number of considerations we have to make. First and most important, the decision of operating on a patient with seizures does not reside in one person, it is the result of a multidisciplinary team: epileptologist, neurophysiologist, neuropsychologist, neuroradiologist, and neurosurgeons play an important role in selecting, studying, and, above all, tailoring the best surgery for the patient.

Despite the advances in medical treatment, 30% of epileptic patients do not respond to adequate antiepileptic treatment. We consider that a patient has medically uncontrollable (or refractory) epilepsy when satisfactory seizure control cannot be achieved with any of the potentially available effective antiepileptic drugs (AEDs), alone or in combination, at doses or levels not associated with unacceptable side effects. On the other hand, surgery has a success rate of about 75%, depending on the site of surgery, the type of surgery performed, and the underlying etiology. In spite of this, surgical treatment of medically uncontrollable epilepsy is often delayed or withheld. Referral for epilepsy surgery takes an average of 20 to 25 years, resulting in a number of avoidable seizure-related deaths, including drowning, motor vehicle accident, fatal status epilepticus, and sudden unexpected death in epilepsy. In children, appropriate timing for surgical procedures is critical, as seizure control may interfere with consolidation of cognitive and motor functions in a developing brain. On the other hand, brain maturation may decrease epileptogenesis, and a nonprogressive brain lesion like a scar or dysplasia may be associated to transient epileptogenesis.

Other than lack of efficacy, causes of poor response to medical treatment are noncompliance, pseudo-epileptic seizures or a combination of epileptic and pseudo-epileptic seizures, incorrect classification of seizures, and incorrect pharmacological treatment (either drug or dosage). So, the first action in evaluating surgical candidates is to confirm that a proper diagnosis and an appropriate medical treatment have been offered.

A patient’s satisfaction with the results achieved with medical treatment often depends on their professional and social circumstances, as well as the type of seizures and the time of day in which they occur. Some patients seek surgical treatment for reasons other than medical intractability; intolerable side effects of AEDs, avoidance of congenital malformations in women who desire to get pregnant, and avoidance of the social stigmata of the disease are some of the reasons patients have expressed when coming to our clinic. Self-assessment of quality of life is an important step in evaluating the patient’s perception of their disease. At our clinic we use the questionnaires QOLIE-31 for adults ^, and QOLIE-48-AD for adolescents. ^,

When epilepsy surgery is planned, we have three goals:

• 1.

  Eliminate or decrease epileptic seizures,

• 2.

  Prevent neurologic deficit due to surgery,

• 3.

  Improve the quality of life.

In order to be able to reach the best surgical choice for the epileptic patient, a multidisciplinary team has to work to solve the surgical questions: Where do the patient’s seizures start? Are there functional or eloquent areas involved? What is the prognosis after surgery? Answering these questions will permit us to know what type of surgery is indicated and to be able to customize the procedure according to our patient’s needs. Nowadays, we have a great number of studies that can be performed, but it is important to remember that there is not a single one that can be considered the golden standard for diagnosis by itself. It is the clinician’s ability to interpret and find the concordance between studies that will lead us to a correct diagnosis and a successful surgery.

The diagnostic workup can be divided into noninvasive and invasive phases:

Phase I Testing: Noninvasive Studies

Neuropsychological Assessment in Surgical Candidates

The overall objective of neuropsychological assessment in candidates to surgery for epilepsy is to seek evidence of cognitive deficits and their relationship with other studies (clinical and electrophysiological). ^,

Neuropsychological testing answers specific questions that other specialists of the epilepsy clinic have made and whose response has a significant impact on surgical decision. (1) What is the patient’s overall neuropsychological status? (2) Which is the hemispheric dominance for language? (3) If cognitive deficit exists, is it lateralized? (4) Is there concordance between neuropsychological findings and structural and electrophysiological results? (5) Should the patient have any sequelae from surgery, can we predict the degree of recovery?

There are specific tests for each of the above questions. We will focus on questions two and three of the above, with particular attention to those adult patients with temporal lobe epilepsy (TLE).

Hemispheric Dominance for Language

Determining the lateralization of language can have a significant impact in surgical planning. It has been established ^, that 95% to 99% of right-handed subjects and 15% to 19% of left-handed have a left cerebral hemispheric dominance for language. Due to the small percentage of subjects who may have a right hemisphere dominance, it is very important to clarify each case. ^,

This statement brings up another important fact: a right-handed patient who presents with difficulties of language might be mistakenly classified as having a left hemisphere affection when in fact he could have dysphasia of the dominant hemisphere, which is not necessarily the left.

For decades, the Wada test has been the gold standard for determining hemispheric language dominance (HLL). ^{, , , , , ,} Recently, a growing number of publications report that other techniques such as functional magnetic resonance imaging (fMRI) may have the same degree of reliability ^{, , ,} without the disadvantages of cost, risk, and complexity of other techniques. ^, found a 91.3% concordance between the Wada test and fMRI. Other authors have also found correlation between these two methods, even in cases with bilateral representation of language. New noninvasive techniques are evolving to substitute Wada test, however, they still present limitations.

To determine the HLL using fMRI, most authors use the task of evoking words silently ^, based on a semantic category (e.g., names of animals) to activate temporal regions, or a phonological category (words beginning with same letter) for activating frontal regions of the dominant hemisphere. ^, Fig. 99.1 illustrates a fMRI study using a phonological category.

Dichotic listening technique (DLT) has also been proved useful in determining the HLL. ^{, , ,} It consists of simultaneous presentation of two words (one in each ear) with the same characteristics in terms of sound, number of syllables, and common use, with the intention to present some competition for processing stimuli between the two hemispheres. With this methodology it has been shown that the majority of right- and left-handed subjects have right ear advantages as a reflection of a left HLL. Fig. 99.2 shows the variability in the index of HLL according to the dichotic listening test in right-handed healthy subjects. The lateralization index language (LI) is a predictor of postoperative verbal memory deficit as shown by some studies.

Lateralization of Memory Deficits

Lateralization of memory deficits (LMDs) is another objective of preoperative neuropsychological assessment. Table 99.1 presents some of the available memory tests. At our Clinic we use the Battery of Learning and Memory and the Visuospatial Learning and Memory tests. When the LMD is found in the same hemisphere where the EZ has been localized through clinical, EEG, and imaging data, the decision for surgery is strengthened, otherwise the complete testing described in Table 99.1 is applied. ^{, ,}

Evaluation of memory is performed by well-established tests ( Fig. 99.3 ) that may vary slightly from one center to another. In approximately 50% of patients it is not possible to determine the LMD through neuropsychological studies alone, but that percentage decreases information provided by neuropsychological testing is concordance with other studies.

A major problem in LDM is that there is a high error rate due to language dominance, as discussed above: that is, verbal memory deficit does not always indicate affection of the left cerebral hemisphere.