Table 1.
Revisiting the concepts of the epileptogenic zone (modified from Lüders and Awad [1]).
| Cortical zones | Descriptions | Modalities utilized for localization |
|---|---|---|
| Epileptogenic zone | Region of which resection is necessary and sufficient to achieve seizure-freedom. | Video; EEG; ECoG; MEG; MRI; PET; Ictal SPECT; Postoperative photograph of brain; Postoperative MRI; Postoperative seizure outcome. |
| Irritative zone | Region generating interictal epileptiform discharges. | EEG; ECoG; MEG. |
| Seizure onset zone | Region initially generating ictal discharges. | EEG; ECoG. |
| Symptomatogenic zone | Region responsible for generating ictal symptoms | Video; EEG; ECoG; Electrical stimulation; MRI. |
| Epileptogenic lesion | Structural lesion, which is causally associated with epilepsy. | MRI; Video; EEG; ECoG; MEG; Postoperative seizure outcome. |
| Functional deficit zone | Region functionally abnormal during interictal state. | PET; SPECT. |
| Eloquent cortex | Region essential for a given sensorimotor or cognitive function. | Wada test; Electrical stimulation; ECoG; MEG; functional MRI; Postoperative behavioral outcome. |
“Epileptogenic zone” is sufficiently included in the resection cavity seen on a postoperative photograph or MRI in a patient who has achieved long-term seizure-freedom following resective surgery. Still, the extent of the epileptogenic zone cannot be accurately measured since the region necessary to obtain seizure-freedom remains uncertain. In practice, investigators at each epilepsy center, often using their own combination of modalities and criteria, determine the “presumed epileptogenic zone” to be removed. In our previous study using electrocorticography (ECoG), for example, we defined the presumed epileptogenic zone as the summation of the seizure onset zone, the neighboring cortical lesion(s), and the neighboring region(s) showing frequent interictal epileptiform discharges [3]. It is unknown whether the presumed epileptogenic zone indeed needs to be completely removed to achieve seizure-freedom. This is further complicated by the fact that complete resection of the presumed epileptogenic zone does not always result in complete seizure freedom.
“Interictal epileptiform discharges” consist of spikes, polyspikes, and sharp waves on EEG, ECoG, or magnetoencephalography (MEG). Sharp transients or sharply-contoured waves explainable as physiological activities are not considered as interictal epileptiform discharges [4, 5].
“Ictal discharges” are defined here as sustained, rhythmic electrographic discharges accompanied by subsequent clinical habitual seizure symptoms; not simply explained by state changes and clearly distinguished from background and interictal electrographic activity [3, 6, 7]. Exceptions to this definition include myoclonic seizures and epileptic spasms, of which ictal discharges may be much shorter than 1 second in total duration [8, 9].
“Symptomatogenic zone” can be defined as the region of which electrographic involvement or electrical stimulation elicits a given ictal symptom. For example, the symptomatogenic zone for seizures characterized by left upper extremity jerking is estimated to be localized in the right pre- and post-central gyri [9]. Eloquent cortex, also known as functionally necessary cortex, is often a part of the symptomatogenic zone [2].
“Epileptogenic lesion” was originally defined as the MRI-visible lesion causally related to the epileptic pathophysiology [1, 2]. Proof of such a causal relationship would require further consideration of convergence of a given lesion with epileptiform discharges, semiology, or postoperative seizure outcome. In this review article, we have used the term: “cortical lesion” instead of “epileptogenic lesion”, in cases where the causal relationship between a given lesion and the epileptogenic process is not proven. Table 2 presents criteria to consider before concluding that a given cortical lesion is likely to be causal.
“Functional deficit zone” is commonly delineated as hypometabolic regions on 2-deoxy-2-[18F]fluoroglucose-positron emission tomography (FDG-PET) imaging. A pathologic specimen of a functional deficit zone may contain focal cortical dysplasia in some epileptic patients without lesions identifiable on MRI [10–11]. The interictal single photon emission computed tomography (SPECT) scan is less frequently utilized to delineate the functional deficit zone, partly due to poor spatial resolution.