When preparing for epilepsy surgery, neurologists may consider using functional MRI (fMRI) instead of the intracarotid amobarbital procedure (IAP) to map language and memory functions in the brain, according to a practice guideline developed by the American Academy of Neurology (AAN). The current evidence is weak, however, and clinicians should advise patients carefully about the benefits and risks of IAP (also known as the Wada test) and fMRI, according to the guideline, which was published online ahead of print January 11 in Neurology.
IAP, an invasive technique, is the current standard for presurgical evaluation in epilepsy. fMRI is noninvasive and also considered safe. Neither of the two methods has been standardized. “Because fMRI is becoming more widely available, we wanted to see how it compares to the Wada test,” said lead author Jerzy Szaflarski, MD, PhD, Professor of Neurology at the University of Alabama at Birmingham. “While the risks associated with the Wada test are rare, they can be serious, including stroke and injury to the carotid artery.”
Jerzy P. Szaflarski, MD, PhD
The AAN formed an 11-member panel to draft the guidelines. Two panelists independently selected 37 possibly relevant articles. Studies with fewer than 15 cases, case reports, meta-analyses, and editorials were excluded. Two panelists rated each article according to the AAN’s diagnostic and prognostic classification of evidence, and the panel ultimately developed consensus recommendations.
Data Support fMRI for Certain Situations
Class II data indicated that fMRI possibly provides language lateralization information concordant with that of IAP in 87% of people with medial temporal lobe epilepsy and in 81% of patients with extratemporal epilepsy. Current comparative data for temporal tumors or lateral temporal cases are insufficient to draw conclusions. Thus, fMRI may be considered as an option for lateralizing language functions in place of IAP in patients with medial temporal lobe epilepsy (Level C), temporal epilepsy in general (Level C), or extratemporal epilepsy (Level C), said the authors. The evidence is unclear for patients with temporal neocortical epilepsy or temporal tumors (Level U).
One Class II study and one Class III study suggested that fMRI is possibly effective in aiding the prediction of postsurgical language deficits in patients undergoing presurgical evaluation for possible temporal lobectomy. The authors recommended that fMRI may be considered for predicting postsurgical language outcomes after anterior temporal lobe resection for the control of temporal lobe epilepsy (Level C).
Class II evidence suggests that in patients with medial temporal lobe epilepsy, fMRI is comparable with IAP in its ability to lateralize memory functions and may be used for this purpose. The authors recommended that fMRI may be considered as an option to lateralize memory functions in place of IAP in patients with medial temporal lobe epilepsy (Level C).
Nine Class II studies with different methods together suggested that fMRI leftward activation asymmetry during encoding of verbal material, regardless of whether measured in the medial temporal lobe or in the language network, probably predicts verbal memory decline after left medial temporal lobe surgery. The authors therefore recommended that presurgical fMRI of verbal memory or of language encoding should be considered as an option to predict verbal memory outcome in patients with epilepsy who are undergoing evaluation for left medial temporal lobe surgery (Level B).
A Class II study indicated that fMRI activation asymmetry during nonverbal (ie, scene and face recognition) memory tasks possibly predicts nonverbal memory decline after medial temporal lobe surgery. The authors recommended that presurgical fMRI using nonverbal memory encoding may be considered as a means to predict visuospatial memory outcomes in patients with epilepsy who are undergoing evaluation for temporal lobe surgery (Level C).
Based on data from one Class II study and one Class III study, the authors recommended that presurgical fMRI may be used instead of the IAP for language lateralization in patients with epilepsy who are undergoing evaluation for brain surgery (Level C). “However, when fMRI is used for this purpose, task design, data analysis methods, and epilepsy type should be considered,” they added.
In addition, based on nine Class II studies, the authors recommended that fMRI of language and verbal memory lateralization may be an alternative to IAP memory testing for prediction of verbal memory outcome in medial temporal lobe epilepsy (Level C). The authors note that fMRI is not yet established as an alternative to the IAP for prediction of global amnesia in patients who have undergone anterior temporal lobe surgery.
More and Larger Studies Are Needed
“The imperfect concordance between fMRI and IAP language lateralization leaves open the question of which test is more accurate in discordant cases,” said the authors. Although the IAP is the reference standard, it is subject to limitations resulting from individual variation in arterial anatomy, variable effects of anesthesia, the rate of amobarbital injection, variability in patient cooperation, and variation in testing methods.
Like the IAP, cognitive fMRI “is a complex diagnostic procedure that requires both advanced technical expertise in imaging and expert interaction with patients to elicit adequate levels of task performance, select a set of activation tasks appropriate to the patient’s ability and the clinical aims of the study, instruct the patient on the tasks, administer the tasks during scanning, and evaluate and provide corrective feedback on task performance during the scanning session,” said the authors.
Global amnesia may result from bilateral medial temporal lobe damage, and some neurologists depend on the IAP to evaluate a patient’s risk for this outcome. “Global amnesia is rare after unilateral temporal lobe surgery, however, and occurs mainly when there is preexisting contralateral medial temporal lobe dysfunction,” said the authors. “One possible approach, therefore, is to reserve use of the IAP memory test for those patients at greatest risk for global amnesia, that is, patients undergoing unilateral anterior temporal lobe resection who have structural or functional evidence of damage to the contralateral medial temporal lobe.”
“Larger studies need to be conducted to increase the quality of available evidence,” Dr. Szaflarski concluded.
—Erik Greb
Suggested Reading
Szaflarski JP, Gloss D, Binder JR, et al. Practice guideline summary: Use of fMRI in the presurgical evaluation of patients with epilepsy—Report of the Guideline Development, Dissemination, and Implementation Subcommittee of the American Academy of Neurology. Neurology. 2017 Jan 11 [Epub ahead of print].