Brain Advance Access originally published online on April 27, 2005
Brain 2005 128(8):1818-1831; doi:10.1093/brain/awh512
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The temporopolar cortex plays a pivotal role in temporal lobe seizures
1 Department of Neurosurgery, 2 Department of Neurology and 3 Department of Neuroradiology, CHU Michallon, Grenoble, France and 4 Epilepsy Surgery Center ‘C. Munari’, Niguardia Hospital, Milan, Italy
Correspondence to: Dr Chabardès E-mail: SChabardes{at}chu-grenoble.fr
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Summary |
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We investigated the role of the temporal pole (TP) in 48 consecutive patients with drug-refractory temporal lobe epilepsy (TLE). Chronic depth recordings of TP cortex activity were used in association with video recording of ictal symptoms during 48 spontaneous seizures. In 23 cases (48%, group 1) the TP was involved at the onset of the seizure, before or concurrently with the hippocampus. In the remaining 25 patients (52%, group 2) the TP was involved 16.4 ± 13.8 s after the hippocampus. A past history of febrile seizures was found in both groups, with no statistical difference. Ictal symptoms did not differentiate TP seizures from seizures originating in the hippocampus but the first clinical sign occurred sooner in group 1 compared with group 2 (respectively 10.56 ± 9 and 25.7 ± 19 s, respectively, P = 0.005). Loss of awareness also occurred sooner in the case of TP seizures compared with mesiotemporal lobe (MTL) seizures (22.9 ± 22.6 versus 42.2 ± 18.6 s, P = 0.0002). MRI data analysis showed that hippocampal sclerosis was present in both groups of patients, although it was more frequent in patients with MTL onset. Anterior temporal white matter changes were found ipsilateral to the epileptogenic area and tended to be more frequent in patients with TP seizures. All the patients underwent tailored anterior temporal lobectomy that included the TP, the hippocampus, the parahippocampal gyrus and the anterior part of the lateral temporal cortex. A better postoperative outcome was achieved in group 1 compared with group 2 (Engel class 1, 95 and 72% respectively, P = 0.04). We conclude that the frequent TP involvement at the onset of seizures could be a supplementary explanation for some failures of selective amygdalohippocampectomy, which should be addressed preferentially to well-selected patients. Moreover, the involvement of the TP cortex at the onset of the seizures is a good predicting factor for postoperative seizure outcome.
Key Words: temporal lobe epilepsy; SEEG; anterior temporal lobectomy; temporal pole; mesial temporal sclerosis
Abbreviations: LVFA = low-voltage fast activity; MTL = mesiotemporal lobe; MTLE = mesiotemporal lobe epilepsy; MTPLE = mesiotemporopolar lobe epilepsy; MTS = mesiotemporal sclerosis; TL = temporal lobe; TLE = temporal lobe epilepsy; TP = temporal pole; SEEG = stereo electroencephalography
Received July 30, 2004. Revised January 30, 2005; Second revision March 7, 2005. Accepted March 9, 2005..
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Introduction |
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Temporal lobe epilepsy (TLE) is the most common form of medically intractable partial epilepsy in adults, and surgery has proved to be effective in the majority of patients. Mesial temporal sclerosis (MTS) is found in about 70% of these cases (Babb et al., 1987
; Wolf et al., 1993; Pasquier et al., 1996) and its presence, highly associated with a past history of febrile seizures and with EEG lateralization of the epileptogenic region, is predictive of an excellent postoperative outcome. These findings have led to the definition of the mesial-temporal lobe epilepsy (MTLE) syndrome (Wieser et al., 1993; Cendes et al., 1997; Engel et al., 1997a). This term should be restricted to patients with the typical clinical presentation, MRI evidence of MTS, anterior and mid-inferomedial temporal ictal and interictal discharges on scalp EEG, and additional evidence of temporal lobe dysfunction from functional imaging and neuropsychology consistent with pathology on the same side. In such well-selected cases, one can expect 70–80% of patients to become seizure-free after surgery (Garcia et al., 1994; Arruda et al., 1996). The choice of whether to perform an anterior temporal lobectomy or a selective amygdalohippocampectomy varies among surgical teams.
However, this concept of MTLE does not imply that the onset of seizures is always and exclusively confined to the sole sclerotic hippocampus. This point is illustrated by several studies using intracerebral electrodes (Munari et al., 1994; Spanedda et al., 1997; Isnard et al., 2000; Kahane et al., 2001), as well as by increasing evidence of extrahippocampal histological (Pitkanen et al., 1998) and morphological (Kuzniecky et al., 1987) abnormalities. These can involve other limbic structures, as well as paralimbic and temporal neocortical areas. Thus, the epileptogenic zone may extend beyond the atrophic mesial temporal structures, which may explain some failures or long-term relapses of selective mesial-temporal lobe (MTL) resections (Berkovic et al., 1995).
Among extrahippocampal areas possibly involved in the genesis of MTL seizures, several studies have focused on the temporal pole (TP), a paralimbic area strongly connected with the amygdala, the hippocampus, the parahippocampal gyrus, the cingulate gyrus, the orbitofrontal cortex and the insula. These studies have clearly highlighted the common occurrence of histological (Choi et al., 1999; Meiners et al., 1999; Mitchell et al., 1999), morphological (Jutila et al., 2001; Moran et al., 2001) and metabolic (Semah et al., 1995; Rubin et al., 1995; Ryvlin et al., 1998; Dupont et al., 2000) changes in the TP. However, data obtained from depth recordings are serendipitous (Munari et al., 1994), and whether this structure can be responsible for seizure onset remains debatable. In a small group of patients implanted with intracerebral electrodes we found that early TP involvement was quite common during temporal lobe (TL) seizures, even when clinical feature associated with MRI evidence of MTS indicated an MTL onset (Chabardès et al., 1999). Therefore, we conducted a retrospective study in a larger population of 48 consecutive patients suffering from TLE. Preoperative stereotactic intracerebral EEG recordings (SEEG) were performed to provide information on the EEG activity of the TP of these patients. The main aim of this work was to assess the involvement of TP cortex during TL seizures and to further delineate the spectrum of TLE with temporopolar onset. Patients were selected using the following inclusion criteria: (i) a final diagnosis of TL seizures; (ii) SEEG investigation including the TP, MTL structures (hippocampus or hippocampus plus amygdala) and the temporal neocortex; (iii) at least one representative spontaneous seizure recorded during video-SEEG monitoring; and (iv) postoperative follow-up of at least 48 months from the beginning of the study.
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Patients and methods |
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Patients
The 48 consecutive patients included in this retrospective study (see the clinical and neuroimaging findings in Table 1) all suffered from drug-resistant partial seizures suspected to be of TL origin and which were finally proved to arise from the TL after SEEG recordings. The epileptogenic onset zone within the TL could not be determined on the basis of non-invasive procedures alone. These latter included MRI (including at least coronal T1-weighted images perpendicular to the hippocampal axis and T2-weighted images parallel to the hippocampal plane), neuropsychological tests, and prolonged video-EEG monitoring in all patients. Interictal fluorodeoxyglucose PET (FDG-PET) and ictal ethyl cysteinate dimer SPECT (ECD-SPECT) were also performed in selected patients (data not presented).
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The population consisted of 24 males and 24 females, aged 15–47.5 years. The duration of the epilepsy ranged from 2.4 to 40.9 years and epilepsy onset age ranged from 1 to 32 years. Twenty-two patients (45.8%) had experienced febrile convulsions in childhood. Eight patients (17%) had normal MRI scans. In the remaining 40 patients (83%), MRI demonstrated a combination of various abnormalities, all ipsilateral to the epileptogenic region. In reviewing MRI findings, particular attention was paid to the presence of MTS (clear-cut unilateral hippocampal atrophy on coronal T1-weighted images with increased signal on horizontal T2-weighted images) and to the presence of TP abnormalities (loss of the grey–white matter differentiation with decreased T1- and increased T2-weighted signal in the anterior temporal region, without specific underlying histological changes). Thus, MTS was identified radiographically and later confirmed pathologically in 27 cases (56%). TP abnormalities were identified in 18 cases (37.5%), without any lesions revealed by neuropathology examination, with the exception of non-specific gliosis. Different types of anatomical lesions were found in 12 cases (25%), including four gangliogliomas, four hamartomas, two focal cortical dysplasias, two dysembryoplastic neuroepithelial tumours and one astroblastoma (one patient had two lesions). Eight of these 13 lesions were located in the basotemporal cortex, two in the lateral temporal cortex, two in the TP and one in the hippocampus.
A temporal lobectomy was performed in all patients, in 28 cases on the right side and in 20 cases on the left side. It consisted of a tailored temporal resection which included in all cases the entire hippocampus, amygdala, parahippocampal gyrus and TP. The posterior limits of the neocortical resection varied according to SEEG results. Postoperative outcome was assessed according to Engel's classification (Engel et al., 1996). One patient was operated on twice since seizures recurred after the patient had been initially seizure-free. MRI scanning showed incomplete resection of the posterior hippocampus, so a second operation was performed; this rendered the patient seizure-free.
Intracerebral electrode implantation and SEEG recordings
In order to further delineate the extent of TL resection, to evaluate possible early extratemporal involvement, and to verify surface EEG information, all patients were evaluated with chronic stereotactically implanted intracerebral electrodes according to the SEEG method developed by Talairach and Bancaud (Talairach and Bancaud, 1973).
Seven to 13 semi-rigid electrodes were stereotactically implanted in cortical areas which varied depending on the suspected origin and region of early spreading of seizures (Munari et al., 1994). For all patients, the following regions were explored: the TP, located in front of the rhinal sulcus, and including Brodmann areas 38 and 21; the hippocampus (head and body); and the superior temporal gyrus (STG). The amygdala was explored in 36 patients. The lateral temporal neocortex was explored in all patients by use of the superficial contacts of the electrodes inserted into the hippocampus and/or the amygdala. When the involvement of a particular area was suspected based on video-EEG analysis, additional electrodes were implanted within the frontal lobe (mainly the orbitofrontal and opercular cortex), the temporoparietal cortex (with the mesial contacts exploring the posterior cingulate gyrus), and the temporobasal cortex (with the mesial contact exploring the parahippocampal gyrus).
Each of the 392 electrodes had a diameter of 0.8 mm and had five, 10 or 15 leads of 2 mm length, 1.5 mm apart (Dixi, Besançon, France), depending on the target region. The TP was investigated using a 10-lead electrode implanted perpendicular to the midsagittal plane and anterior to the vertical plane, passing through the anterior commissure (Figs 1 and 2). In each patient, the position of each electrode contact was anatomically plotted onto the corresponding slice of Talairach's stereotaxic atlas (Talairach and Tournoux, 1988). The deepest and most superficial contacts were used to investigate the mesial and lateral aspects of the TP, respectively.
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Depth EEG activity was recorded between contiguous contacts at different levels along the axis of each electrode, so that a large sample of mesial and lateral temporal cortices was assessed (Fig. 4). Intracerebral recordings were conducted extraoperatively in chronic conditions with reduced medication using an audio–video–EEG monitoring system (Biomedical Monitoring System, Campbell, USA; since 1996, Micromed, Treviso, Italy). This allowed precise correlation between clinical events and depth EEG activity.
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Clinical seizure analysis
A member of the epilepsy team observed the patients continuously during monitoring in order to obtain a precise description of subjective patient experience at seizure onset, as well as to test awareness, language, muscle tone and sensory–motor functions. A total of 98 spontaneous seizures (mean, 2.6 ± 2.4 per patient) were recorded during video-SEEG monitoring in the 48 patients. In order to avoid biasing our quantitative analysis, and because the number of seizures per patient ranged from one to 17, we analysed only one seizure per patient for the purpose of the study. We decided to analyse the first seizure clearly recognized by the patient or the patient's family as typical of the patient's epilepsy and in whom we could identify the commonest clinical signs previously described by the patient or his family. For these reasons, we did not take into account seizures in which auras only occurred, or seizures which ended with unusual secondary generalization.
Seizure semiology was analysed according to a working definition of ictal symptoms (Table 3). The clinical onset of the seizures was defined as the time of the first visible change in the patient's behaviour, or when the patient indicated that he was experiencing an aura.
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Electrophysiological seizure analysis
The SEEG seizure onset was defined as the first clear electrical change that occurred prior to the clinical onset. These changes were classified in one of the following three categories: (i) low-voltage fast activity over 20 Hz (LVFA); (ii) recruiting fast discharge (around 10 Hz) of spikes or polyspikes (SpFD); (iii) rhythmic activity (around 5 Hz) of spikes or spike-and-wave complexes, defined as a ‘hypersynchronous seizure onset pattern’ (Wieser et al., 2004
). Attention was focused on the initial location of the SEEG discharge and on the rapidity of TP involvement. Particular attention was paid to the time of clinical onset and to the time that consciousness was altered with respect to the SEEG seizure onset, as well as to the delay before TP and MTL involvement. Later seizure propagation within the TL and through extratemporal areas was also assessed.
Case grouping and statistical analysis
We divided the patients into two groups: those with early TP involvment (group 1) and those with delayed TP involvement (group 2). The TP was considered to be involved early when one of the ictal depth EEG patterns mentioned above occurred at the TP recording electrode at the onset of the seizure (group 1, Figs 5 and 6). Conversely, when the TP involvement occurred 1 s or more after the electrical onset of the seizure, patients were classified into group 2 (Fig. 5). The two groups were compared with respect to ictal SEEG and clinical data, general clinical features, MRI findings and outcome.
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The mesial temporal lobe epilepsy syndrome is well defined and commonly accepted. For this reason, we also performed an additional analysis of the specific subgroup of patients with MTS, excluding the cases with a gross lesion (12 patients with 13 lesions) (Table 5).
Due to the small number of patients, all descriptive statistics are given parametrically as the mean and standard deviation. Statistical comparisons were made using Fisher's exact test for quantitative data, and the Mann–Whitney U-test for qualitative data. Significance was accepted at the 0.05 level, two-tailed.
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Results |
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Ictal SEEG findings
At SEEG onset, all 48 seizures involved MTL structures and/or the TP (Table 1). Early TP involvement was found in 23 of the 48 seizures (group 1, 48%), of which eight started in the TP with late (7.12 ± 6.01 s) MTL spreading. Fifteen seizures involved both the TP and the MTL structures at seizure onset. Of the latter, eight also initially involved the temporal neocortex (the STG in seven cases and the basotemporal cortex in one). Late TP involvement (16.4 ± 13.8 s from seizure onset) was found in 25 of the 48 seizures (group 2, 52%), all of which originated in the hippocampus without any initial temporal neocortical involvement. The amygdala, explored in 36 patients, was involved at the onset of seizure in 13 cases, either concurrently with the TP (in nine out of 18 cases in group 1) or concurrently with the hippocampus (in four out of 18 cases in group 2).
Temporopolar SEEG activity (Table 1)
The electrical pattern recorded at the TP electrode in group 1 consisted most often of LVFA (n = 17/23), and less frequently of the recruitment of fast discharges of spikes or polyspikes (n = 6/23). The hypersynchronous seizure onset pattern was never observed in group 1. In group 2, the TP SEEG pattern consisted of LVFA (n = 12/25), recruiting fast discharges of spikes or polyspikes (n = 9/25), and the hypersynchronous seizure onset pattern (n = 4/25). Spike activity was therefore observed more often in group 2 (48%) than in group 1 (26%), but this difference did not reach a significant level. LVFA was predominant in group 1 (73%) and the difference between the two groups almost reached significance (P = 0.08).
Seizure spread patterns
As shown in Table 4, both group 1 and group 2 seizures could spread mainly over the perisylvian cortex (STG and the precentral operculum), the midtemporal neocortex, the basotemporal cortex, the orbitofrontal cortex, the anterior and posterior cingulate gyrus and the parietotemporal cortex. However, group 1 seizures propagated more frequently over the perisylvian cortex compared with group 2 seizures (87 and 44%, respectively, P = 0.002). Conversely, group 2 seizures propagated more frequently to the orbitofrontal cortex (68.8 versus 22.2%, P = 0.01) and to the midtemporal neocortex (32 vs 4.3%, P = 0.02). We did not observe any propagation of either group 1 or group 2 seizures over the frontopolar cortex, the dorsolateral frontal convexity or the intermediate mesial frontal cortex.
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Ictal clinical data (Table 3)
Auras of varying types were experienced in all seizures but one. These were mainly characterized by a rising epigastric feeling (52%), autonomic sensations (e.g. tachycardia, flushing) (48%), psychic symptoms (e.g. fear, anxiety) or dysmnesic phenomena (e.g. déjà vu, memory flashback, dreamy state) (37.5%). Consciousness was impaired at different times relative to electrical seizure onset in all but one case and was incomplete in about 44% of the cases. Both simple motor signs and complex behaviours were seen in a majority of cases (Table 3). The former most often consisted of contralateral dystonic posturing (54%), whereas the latter consisted of gestural (65%), oroalimentary (69%) or verbal (44%) automatisms. However, tonic or clonic manifestations and hypermotor behaviour, which are less common in temporal lobe seizures, were also encountered in a significant number of cases (42, 23 and 29%, respectively). These almost always occurred late in the course of the seizure. Other signs, such as sialorrhea, urination and aphasia, were also noted in some cases. Secondary tonic–clonic generalization was rare.
Overall, the only statistically significant differences in clinical behaviour between the two groups were as follows. First, the clinical onset of the seizure was earlier in group 1 (10.56 ± 9.01 s after electrical onset) than in group 2 (25.72 ± 19 s, P 
0.005). Secondly, the impairment of consciousness was earlier in group 1 (22.9 ± 22.6 s from electrical onset) than in group 2 (42.2 ± 18.6 s, P 0.0002). Thirdly, the delay between the first clinical sign and the loss of consciousness was shorter in group 1 (9.8 ± 14.9 s) than in group 2 (16.7 ± 14.4 s, P = 0.04). Interestingly, the analysis of the time interval between electrical TP involvement and the occurrence of impairment of consciousness did not show any difference between group 1 (26.50 ± 22.96 s) and group 2 (20.94 ± 15.49 s, P = 0.58).
General clinical features (Tables 1 and 2)
There was no statistical difference between group 1 and group 2 patients when comparing sex ratio, age at SEEG, age at onset of epilepsy, duration of epilepsy, follow-up and laterality of epileptogenic regions.
A history of complex febrile seizures was more frequent in group 2 (70%) than in group 1 (30%), but the difference was not significant.
Neuroimaging findings (Tables 1, 2 and 5)
The only statistically significant difference between the two groups was the presence of MTS in 72% of patients in group 2 and in 43% of patients in group 1 (P = 0.03). Nine patients in group 1 and 17 in group 2 had MTS without any additional gross lesion. TP abnormalities were more frequently observed in group 1 (47.8%) than in group 2 (28%), but this difference was not significant.
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Postoperative outcome (Tables 1 and 2)
Overall, the average follow-up period was 65 ± 19 months (range 48–100 months). Postoperative seizure status was 85.4% in class I (IA, 58.3%), 12.5% in class II, and 2% in class III. Results were better in group 1 (class I, 95.6%; IA, 69.5%) than in group 2 (Class I, 72%; IA, 48%), and this difference was statistically significant (P = 0.04) for Engel class I.
TP involvment in patients with MTS, with cryptogenic epilepsy and with anatomical lesion (Table 5)
Among the 26 patients in this study who had MTS without structural lesion, nine (34.6%) had a seizure onset in the TP before or concurrently with the hippocampus. Among the 12 patients with a structural lesion, eight patients (66%) had a seizure onset that involved the TP before or concurrently with the mesial structures, in addition to the cortex surrounding the lesion. In the remaining four cases (33%), the onset of seizures originated in the hippocampus before involving the TP.
Eight out of 48 patients did not have either a lesion or MTS in the presurgical MRI evaluation. Among these cryptogenic cases of TLE, the onset of seizures originated in the TP in six cases (75%) and in the hippocampus in two cases.
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Discussion |
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The main finding of the present study is that, in seizures affecting the temporal lobe, the TP cortex is involved at the onset of seizures in about 50% of cases with TLE and in about 35% of cases of TLE with MTS. Indeed, two groups of patients can be clearly delineated on the basis of SEEG recordings according to the early (group 1, 48%) or late (group 2, 52%) involvement of the temporal pole. However, these two groups of patients are difficult to distinguish on the basis of general clinical features, MRI findings, or ictal clinical semiology. Therefore, particular attention must be paid to anatomoclinical characteristics that might help to further delineate MTPLE (group 1) and MTLE (group 2). In this respect, the three main findings of this study are: (i) MTS on MRI is present in MTPLE and therefore cannot definitely differentiate MTLE and MTPLE; (ii) early clinical onset and early impairment of consciousness are the only ictal clinical signs that differentiate MTPL and MTL seizures; and (iii) initial involvement of the TP cortex is a good predicting factor for postoperative outcome.
SEEG evidence of temporal pole involvement
We found that in 48% of cases the TP was involved at seizure onset either before or at the same time as involvement of MTL structures (mean delay of hippocampal involvement after TP, 2.4 ± 4.8 s). In this group, the TP was involved concurrently (nine cases) or before (nine cases) the amygdala involvement (mean delay of amygdala involvement after TP, 7.16 ± 9.62 s). In the remaining 52% of cases, seizures arose from the hippocampus and subsequently spread much later (16.4 ± 13.8 s) over the TP. In this group, the amygdala was involved four times concurrently with the hippocampus, and 14 times after the hippocampus, seven times concurrently with the TP, four times before and seven times after the TP. This is in accordance with a quantitative study using H2O15-PET, which emphasized that ictal cerebral blood flow changes during TL seizures were rarely limited to the MTS, but frequently involved both the amygdalohippocampal complex and the inner portion of the TP (Kahane et al., 1999). This is also in agreement with the recent study of Chassoux and colleagues (Chassoux et al., 2003), who reported, in a subgroup of patients all suffering from MTLE and explored with FDG-PET, that interictal hypometabolism was not limited to the hippocampus but was also observed in the insula and the TP. This pointed out the role of neuronal networks preferentially involved during seizures.
The existence of several subgroups of neuronal network involved in TL seizures has been discussed previously but evidence from electrophysiological studies was rare (Wieser, 1983). TL seizures are classically subdivided into MTL seizures and lateral TL seizures; the former are often associated with MTS while the latter are associated with neocortical lesions (Ebner, 1994). However, Rasmussen showed in 100 patients, all of whom were seizure-free after temporal resection, that the EZ was mesial in 28%, lateral in 46% and mesial plus lateral in 27% (Rasmussen, 1982). More recently, Bartolomei and colleagues, using SEEG recordings, have confirmed that other types of TL seizures exist, including the classical medial and lateral subtypes as well as a medial-lateral or lateral-medial subtype (Bartolomei et al., 1999).
Interestingly, our study showed that the SEEG pattern recorded in the TP consisted most frequently of LVFA in MTPL seizures, while this pattern consisted preferentially of spike activity in MTL seizures. Although the difference was not statistically significant, this finding is in accordance with recent studies which found that fast activities are most often found in extrahippocampal seizures (Wennberg et al., 2002), whereas MTL seizures begin more frequently with low-frequency, high-amplitude spikes.
We also found that MTPL seizures propagated preferentially to the STG and to the periopercular cortex whereas MTL seizures propagated most often to the basotemporal and lateral temporal cortex. This preferential spread pattern of MTPL seizures is consistent with anatomical studies in monkeys using retrograde tracers, in which robust projections from the TP to the STG were described (Amaral et al., 1983; Insausti et al., 1995). Moreover, it might be explained by strong connections between the TP cortex and the perisylvian cortex (Moran et al., 1987), the parahippocampal gyrus and the hippocampus, which surround the insula and together constitute the paralimbic cortex described by Mesulam and Mufson (Mesulam and Mufson, 1982). The spread to the orbitofrontal cortex, previously described by Wieser (Wieser et al., 1993), was predominant in MTLE, possibly due to the shorter and direct connections, via the fasciculus uncinatus, between the amygdala, the uncus, the head of the hippocampus and the orbitofrontal region.
MRI findings and MTPL epilepsy
Mesiotemporal sclerosis
Since the late 1960s, the high incidence of MTS in refractory temporal lobe seizures and its frequent association with a history of complex febrile seizures has been highlighted by many reports (Cendes et al., 1993; Engel et al., 1997). In our study, 58% of the patients had ipsilateral MTS and 45% had a history of febrile seizures. These rates are similar to those usually reported in TLE surgery series, in which MTS has been found in 58–75% of cases (Wolf et al., 1996) and a past history of febrile seizures in 40–60% (Engel et al., 1997). Interestingly, we found that the incidence of MTS was significantly higher in the MTLE group compared with the MTPLE group. This suggests that the classical presentation is likely to produce a mesial temporal onset of the seizures, and therefore is amenable to treatment by selective mesial temporal resection. However, in our series, among the 26 patients who had isolated MTS (i.e. without dual pathology) and who could have been classified prior to depth EEG evaluation as MTLE based on the MRI findings and clinical data, 35% of them had MTPLE based on SEEG exploration. Therefore, this data demonstrates that a subgroup of patients, despite a history of febrile seizures and MRI evidence of MTS, may present with seizures which arise from the TP alone or concurrently with MTL structures. Thus, we can assume that MRI evidence of MTS is correlated with the side of epileptogenesis but that it is not always predictive of the site of epileptogenesis within the temporal lobe, as already emphasized by a few depth EEG (Munari et al., 1994) and imaging (Spencer, 1995) studies. Relationships between MTS and seizure onset are still under debate. Spanedda and colleagues observed, in bitemporal epilepsy with MTS, that only 20% of seizures originated in the hippocampus, while 67% of the seizures originated in both the amygdala and hippocampus (Spanedda et al., 1997). King and colleagues reported a series of 119 patients suffering from MTLE with unilateral or bilateral MTS in which approximately 20% had discordant or unlocalized ictal depth EEG (King et al., 1997a, b). They concluded that MTS was not predictive of the site of epileptogenesis. However, the TP was not investigated in these two studies.
Temporal pole abnormalities
In addition, our data provide information on the relationships between MRI detection of TP abnormalities and the origin of seizures in the TL. First, we found TP abnormalities in 37.5% of our patients, all ipsilateral to the side of epilepsy. Among these patients, TP abnormalities were more frequent in group 1 (62%) compared with group 2 (38%). Our study confirms that TP abnormalities are helpful in lateralizing seizure onset. However, when we looked at the relationships between TP abnormalities and onset of seizures, we failed to find a clear correlation that differentiates MTPLE from MTLE, probably due to the small number of patients in our study. However, our data suggest that 50% of patients (Table 2) in whom TP abnormalities was associated with MTS exhibited MTPL seizures, while the remaining 50% exhibited seizures arising from hippocampus. In the case of TP abnormalities without MTS, seizures arising from TP tended to be more frequent (61%) compared with those arising from the hippocampus but the difference was not significant. Whether we could differentiate the two groups by studying a larger sample of patients or by using more sensitive MRI techniques, such as FLAIR sequences (which were not used routinely at the time of our study), remains debatable.
Structural lesions
We also found that TP abnormalities were associated in five cases with an ipsilateral anatomical lesion located remotely from the TP but still within the temporal lobe (Table 1). This association was more common in our series than in the more traditionally defined dual pathology involving the hippocampus plus a secondary lesion. It was more frequent in the MTPLE group (80%). Therefore, we can cautiously hypothesize that the presence of TP abnormalities might suggest the existence of a more diffusely distributed epileptogenic network within the TL, including the TP.
This latter conclusion might have surgical implications, especially when combined limited neocortical resections plus amygdalohippocampectomy are considered. Recently, Clusmann and colleagues found a lower success rate when temporal lesions plus MTL structures were removed and they debated whether ‘some of these patients might have profited from anterior temporal lobectomy’ (Clusmann et al., 2002). Based on our data, we can argue that, in such cases, lesionectomy alone or combined only with amygdalohippocampectomy could have left epileptogenic cortex at the TP.
Clinical findings
Despite extensive descriptive studies of clinical semiology in TLE, there are still no clear clinicoanatomical correlations between individual signs and regions within the TL. Even the analysis of symptom clusters and sequences, though helpful for differentiating TL from extra-TL seizures, does not allow the differentiation of distinct subgroups of TLE, as recently demonstrated by Henkel and colleagues (Henkel et al., 2002). Our study documents an ictal semiological similarity between MTL seizures and MTPL seizures, with the exception of two ictal clinical features, i.e. early occurrence of the first clinical sign with respect to the first depth EEG changes, and early loss of consciousness. This could be helpful in differentiating the two types of seizures.
The occurrence of the first ictal clinical sign after the seizure onset was earlier in MTPL seizures than in MTL seizures, although the clinical characteristics were similar in the two groups. One explanation of this difference is that it might be due to the larger number of synapses involved in the hippocampal network compared with the small number of synaptic connections between the TP cortex and surrounding paralimbic areas. In this respect, it is likely that TP ictal activities will involve the surrounding cortex, such as the insula (Mesulam et al., 1982), much sooner compared with ictal activities occurring in the hippocampus. The latter will likely stay confined in the hippocampal loops and will give rise, in the same way, to the first clinical sign after a longer delay compared with MPTL seizures. Several authors (Isnard et al., 2000; Bouilleret et al., 2002) have recently suggested that the insular cortex is involved in about 60% of cases in MTL seizures and this could explain the occurrence of the most frequent auras observed in this form of seizure. Therefore, one may cautiously postulate that the cortical network that gives rise to the first ictal symptoms is the same in the two groups, but it is involved sooner when discharges start in the TP. This hypothesis is in accordance with the work of Ostrowsky and colleagues, who found that symptoms observed following stimulation of the anteromesial temporopolar cortex were reminiscent of those seen during stimulation of mesial temporal lobe structures (Ostrowsky et al., 2002). In addition, these symptoms could be elicited by insular stimulation (Penfield and Faulk, 1955; Isnard et al., 2000; Ostrowsky et al., 2000).
The idea that TP discharges interact with and spread more rapidly to surrounding areas also supports the finding that time to impairment of consciousness, measured from the electrical onset of the seizure, occurred earlier in MTPL seizures than in MTL seizures. As a matter of fact, the first clinical sign was followed by loss of consciousness sooner in MTPL than in MTL seizures. However, the interval between the TP SEEG involvement and the occurrence of consciousness impairement was the same in both groups. These findings suggest that impairment of consciousness is the consequence of the extent of the discharge, as previously reported (Munari et al., 1980). They also suggest that the TP appears to play a pivotal role in seizure propagation from local to regional cortex.
Postoperative outcome
Surprisingly, we found that patients suffering from MTPL seizures, proven by SEEG recordings, had better postoperative outcome than patients with MTL seizure onset, despite a surgical resection which included the TP in both groups. This difference was significant (Tables 2 and 5).
This finding is in agreement with PET studies (Manno et al., 1994; Wong et al., 1996) which showed that good postoperative outcome was correlated with the presence of interictal TP hypometabolism. In this respect, Dupont and colleagues argued that, in TLE, TP hypometabolism could reflect a preferential pattern of seizure which switches anteriorly to the TP, and could explain the good postoperative outcome after anterior temporal lobectomy (Dupont et al., 2000). Conversely, MTLE could preferentially spread to the insula, as illustrated by metabolic studies (Bouilleret et al., 2002), and could explain a poorer outcome compared with MTPLE. Our patients did not have insular recordings at the time of the study, so that our study cannot deal with insular issues, but it could be interesting in a future study to compare the specific role of insular involvement in MTPL and MTL seizures.
We also found some differences in spread pattern of seizures that might interfere with postoperative outcome. In MTLE, seizures spread preferentially to the orbitofrontal cortex and lateral temporal cortex, whereas MTPL seizures spread to the perisylvian cortex, especially the STG. As the anterior part of STG is usually removed during anterior temporal lobectomy whereas the orbitofrontal cortex is usually spared, we can cautiously hypothesize that it might be an additional reason to explain the discrepancy of the postoperative outcome.
Conclusion
Thus, when contemplating the diagnosis of MTLE, one must be cognizant that the presence of a short delay between the electrical onset of seizure, the first clinical sign, and the occurrence of alteration of consciousness may indicate a mesiotemporopolar onset of the seizure rather than an exclusively amygdalohippocampal onset. In our series, the presence of MTS on presurgical MRI scanning was associated with a mesiotemporopolar onset of seizure in about 35% of cases.
Therefore, our study provides new arguments for the belief that selective amygdalohippocampectomy should be carefully considered in well-selected patients suffering from exclusive amygdalohippocampal seizures. Recently, the Istanbul Workshop commission report on MTLE with hippocampal sclerosis concluded that ‘MTLE with HS appeared to involve areas of structural and functional disturbances that are much more extensive in area than the hippocampus, and maybe even the mesial temporal area’ (Wieser et al., 2004). Our study provides new insights into this issue.
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Acknowledgements |
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The authors wish to thank Dr A. Golbi for critical reading and helpful comments for the English translation. This work is dedicated to Claudio Munari.
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