Roles of fMRI and Wada tests in the presurgical evaluation of language functions in temporal lobe epilepsy

Abstract

Surgical treatment of pharmacoresistant temporal lobe epilepsy (TLE) carries risks for language function that can significantly affect the quality of life. Predicting the risks of decline in language functions before surgery is, consequently, just as important as predicting the chances of becoming seizure-free. The intracarotid amobarbital test, generally known as the Wada test (WT), has been traditionally used to determine language lateralization and to estimate their potential decline after surgery. However, the test is invasive and it does not localize the language functions. Therefore, other noninvasive methods have been proposed, of which functional magnetic resonance (fMRI) has the greatest potential. Functional MRI allows localization of language areas. It has good concordance with the WT for language lateralization, and it is of predictive value for postsurgical naming outcomes. Consequently, fMRI has progressively replaced WT for presurgical language evaluation. The objective of this manuscript is to review the most relevant aspects of language functions in TLE and the current role of fMRI and WT in the presurgical evaluation of language. First, we will provide context by revising the language network distribution and the effects of TLE on them. Then, we will assess the functional outcomes following various forms of TLE surgery and measures to reduce postoperative language decline. Finally, we will discuss the current indications for WT and fMRI and the potential usefulness of the resting-state fMRI technique.

Keywords: epilepsy surgery; functional MRI; intracarotid amobarbital test; naming; outcome; resting-state.

Figure 1

Diffusion tensor tractography of the fascicles involved in language function (by Dr. Loxlan W. Kasa).

Figure 2

Forest plot indicating concordance between the Wada test and fMRI for language lateralization (as mean estimates and confidence bars) in each of the 31 studies included in our meta-analysis, both separately and as the sum of all patients (as "Overall"). For further details, see Massot-Tarrús et al., (5). Adopted with minor modifications from the original figure published in Massot-Tarrús et al. (5).

Figure 3

Flowchart to guide clinical decision-making when using fMRI and WT for presurgical evaluation of language in TLE. ATL, anterior temporal lobectomy; DFT, dichotic fused word listening test; fMRI, functional magnetic resonance imaging; fNIS, functional near-infrared spectroscopy; fTCD, functional transcranial Doppler sonography; Intra/extra-op ESM, intra/extra-operative electrocortical stimulation mapping; MEG, magnetoencephalography; MST, multiple subpial transections; RNS, responsive neurostimulation; TLE, temporal lobe epilepsy; VNS, vagus nerve stimulation; WT, Wada test.