## Abstract

Focal cortical dysplasia (FCD) is an increasingly recognized cause of intractable epilepsy. Surgical data suggest that the dysplastic cortex should be removed to obtain freedom from seizures, but the prognosis remains poor as FCD is difficult to delineate by imaging. We retrospectively analysed a series of 28 patients (aged 5-41 years, median 16.5 years) with FCD who had been investigated by stereoelectroencephalography (SEEG) between 1964 and 1995. Neurophysiological data were correlated with histological findings and surgical outcome. MRI was available for only seven patients. Severe partial epilepsy of early onset, pre-existing neurological deficit (68%) and cognitive impairment were the main clinical features. FCD was distributed equally between all lobes except for the temporal lobe, and was found predominantly on the mesial aspect of the cerebral hemispheres. SEEG findings provided evidence of dysplastic tissue epileptogenicity, as demonstrated by intralesional rhythmic spike discharges, the onset of ictal discharges and a low epileptogenic threshold. The epileptogenic zone corresponded to histologically defined FCD in 82% of the cases. Despite the lack of adequate neuroimaging in most cases, 64% of the patients became seizure-free after surgery. The main predictors of a favourable outcome were complete removal of the epileptogenic zone (P< 0.01) and complete removal of the dysplastic cortex (P< 0.01). These results emphasize the usefulness of neurophysiological data in accurately assessing the extent of the FCD.

## Introduction

Focal cortical dysplasia (FCD) is increasingly recognized as a cause of intractable epilepsy of early onset (Palmini et al., 1991a; Hirabayashi et al., 1993; Polkey, 1996; Wyllie et al., 1998). Although the classification of cortical dysplasia remains under debate, FCD usually refers to the lesion described by Taylor and colleagues, which consists of congregations of large, bizarre neurons that are littered throughout the cortex and are associated in most instances with voluminous glial cells (balloon cells) in the affected cortex and the underlying white matter (Taylor et al., 1971). FCD accounts for 3–7.5% of all histological diagnoses in general epilepsy surgery series (Taylor et al., 1971; Robitaille et al., 1992; Hirabayashi et al., 1993; Brännström et al., 1996; Daumas-Duport, 1996) and for 18–26% in paediatric epilepsy surgery series (Polkey, 1996; Wyllie et al., 1998). FCD is detectable by contemporary imaging, particularly with high-resolution MRI (Palmini et al., 1991c; Kuzniecky et al., 1995, 1997; Barkovich and Kuzniecky, 1996; Chan et al., 1998). Epilepsy usually begins in early childhood, soon proves to be intractable (Chugani et al., 1990; Chugani and Conti, 1996; Wyllie et al., 1996) and may be associated with developmental delay and focal neurological deficits (Guerrini et al., 1996).

Focal surgical resection may relieve the patients of their seizures, but the prognosis remains poor when compared with that of patients undergoing resective surgery for other types of lesion (Palmini et al., 1991b; Hirabayashi et al., 1993; Raymond et al., 1995a; Wyllie et al., 1998). One early report suggested that the extent of dysplastic cortex removal was the determining factor for a favourable outcome (Palmini et al., 1991b). Further studies demonstrated that all of the dysplastic tissue needed to be resected because of intrinsic epileptogenicity (Kuzniecky et al., 1993; Palmini et al., 1995). However, small dysplastic lesions may be hard to detect, even with high-resolution MRI (Duncan, 1997), and lesional boundaries are often difficult to delineate by neuroimaging or on the basis of the macroscopic appearance of the cortex during the surgical procedure (Olivier et al., 1996; Polkey, 1996).

Neurophysiological investigations are usually considered unhelpful in determining the extent of lesions because the epileptogenic area appears larger than the dysplastic cortex (Palmini et al., 1991a; Hirabayashi et al., 1993; Kuzniecky et al., 1995). More recently, specific interictal and ictal patterns on electrocorticography have been reported as a means of identifying the dysplastic cortex (Palmini et al., 1995). However, the exact relationship between the extent of the lesion and the epileptogenic zone (EZ) remains unclear.

Stereoelectroencephalography (SEEG) provides a unique opportunity for obtaining direct intralesional recordings and correlating interictal and ictal activity in three-dimensionally orientated areas of the brain. Furthermore, the distribution of the neurophysiological abnormalities can be compared with that of histologically defined FCD in surgical brain samples. However, no reports focusing on cortical dysplasia investigated by SEEG have yet appeared. We analysed the neurophysiological and histological data in a series of 28 patients with FCD. The aim of the study was to assess the epileptogenicity of the dysplastic tissue and of the surrounding cortex, to analyse the interictal and ictal patterns, and to identify the factors influencing the postoperative seizure outcome. Only cases of FCD that conformed to the histological definition of Taylor and colleagues (Taylor et al., 1971) were considered in this study.

## Patients and methods

Twenty-eight of the 500 patients with intractable epilepsy undergoing corticectomy at Sainte-Anne Hospital, Paris, between 1964 and 1995 were identified as having FCD. The patient population consisted of 17 females and 11 males. Age at the time of operation ranged from 5 to 41 years (median 16.5 years); 14 patients were children <16 years old. Age at the onset of epilepsy ranged from the first days of life to 11.5 years (median 2.75 years). In eight cases, seizures occurred during the first year of life. The preoperative duration of epilepsy ranged from 3.5 to 38 years (median 12 years).

Presurgical evaluation consisted of an anatomical and electroclinical correlation study, based on the history of epilepsy, interictal and ictal EEG recordings, stereotaxic imaging and SEEG, as previously described by Bancaud (Bancaud, 1980) and Talairach and colleagues (Talairach et al., 1992). The stereotaxic mapping procedure included angiography and, until 1990, ventriculography, which was subsequently replaced by stereotaxic MRI. Most patients were investigated before the era of modern brain imaging. Therefore, only 10 patients had a CT scan and seven had MRI. SEEG was performed in all cases, twice in four patients; in two of these four patients the second SEEG was performed after failure of surgery; in the other two surgery was initially withheld because of functional risks, and a second evaluation was done 6 and 11 years later, respectively, because of catastrophic deterioration. Intracerebral activity was recorded for several hours after the depth electrodes had been positioned, either during a single session (28 cases) or over several days (four cases). Based on SEEG data, the lesional zone (LZ) was defined by the presence of slow waves or depression of activity, the irritative zone (IZ) by interictal spiking and the EZ as the site of subclinical ictal discharges or of the onset of spontaneous, chemically or electrically induced seizures.

Cortical resections were individually tailored according to anatomical and neurophysiological data, including the EZ and ictal spread areas. The operation was guided by a diagram using the AC–PC stereotaxic landmarks defined by Talairach and Tournoux (Talairach and Tournoux, 1988, 1993). Cortical samples were fixed in formalin and paraffin sections were made. The whole corticectomy specimen was examined histologically in 22 cases; microscopic examination was limited to macroscopic cortical abnormalities in six cases.

The localization and assessment of the extent of FCD were based on histological findings in the cortical samples and compared with imaging data when available, as well as on the intra-operative findings. FCD was defined as limited if its volume was less than one-third of a lobe, as lobar when greater than one-third of but confined to one lobe, and as multilobar if it extended over more than one lobe. The extent of cortical resection was classified using the same criteria. The central area, defined as the paracentral lobule, the pre- and postcentral gyri and the corresponding part of the operculum, was considered as a separate lobe. The cortical specimens were labelled according to the anatomical location of intracranial electrodes; this allowed the establishment of a precise correlation between neurophysiological data (LZ, IZ, EZ) and the histological abnormalities in each cortical sample examined. Abnormal intracerebral activity was considered as co-localized with FCD when it was confined to lesional areas, as more extended when recorded both in lesional and perilesional or distant areas, as more restricted when recorded in an area smaller than the lesional areas, and as distinct when recorded outside the lesional areas. The EZ was considered as completely removed if the area of ictal discharge onset was totally included in the resection, and as incompletely removed in all other cases. FCD removal was considered complete when no dysplastic cortex was identified on resection borders, incomplete when the surgical limits clearly ran through the pathological zone or if all cortical samples were pathological, and possibly incomplete if the resection margins corresponded to the transitional area between the dysplastic and the non-dysplastic cortex.

Postoperative seizure outcome was evaluated according to the classification proposed by Engel and colleagues (Engel et al., 1996). Statistical analysis consisted of the calculation of non-parametric Spearman rank correlation coefficients, which were used to assess the degree of association between pairs of markers, and a logistical regression study to identify the main factors influencing seizure outcome. The threshold of significance was fixed at a probability of 0.05 for all tests.

## Results

### Clinical data

The early childhood onset of epilepsy stood out in our patient population. The earliest onset correlated with posterior localization (P < 0.01) and multilobar FCD (P < 0.01). The average age at onset was 0.9 years for posterior FCD (neonatal in three of five patients) and 3.27, 3.5, 4 and 7.5 years for frontal, temporal, parietal and central FCD, respectively.

Neurological deficits were noted in 19 cases (68%). Mild to moderate limb motor or sensory–motor deficits (10 cases), facial paresis (three cases) and bilateral incoordination of the hands (two cases) were noted in patients with central or parietal FCD. Hemianopsia (two cases) and nystagmus (two cases) were observed in patients with posterior FCD. The motor deficit became permanent after status epilepticus in two patients, was apparently progressive in two cases with continuous myoclonus, and worsened with increased seizure frequency in one case.

Mental impairment was observed in 11 of the 14 children studied (78.5%) and in five out of 14 adults (35%). The IQ (intelligence quotient; Wechsler Intelligence Scale) was within the average range (90–109) in only three children. Borderline scores (70–79) were obtained in two children, three were classified as mentally deficient (IQ < 69) and six children could not be formally assessed because of severe mental retardation. Nine of the 14 adults had a normal IQ, whereas five others had IQs ranging from borderline to mentally deficient. Ten patients had major psychiatric disturbances (35%). Severe cognitive disorders were correlated with early onset of epilepsy (P < 0.01). Normal functioning or mild impairment was correlated with limited FCD (P = 0.01). Psychiatric disturbances were correlated with early onset of epilepsy (P = 0.03) and with a posterior location of the FCD (P = 0.02).

No patient had dermatological or neuroimaging features typical of tuberous sclerosis. However, one patient presented both mild cutaneous lesions and two cerebral calcifications.

### Electroencephalographic data (Table 3)

The EEG background was abnormal in 19 cases, slow and/or asymmetrical in 15, and not identifiable in four patients, in whom the activity consisted of slow waves or continuous rhythmic spikes. It was normal in limited or lobar FCD affecting the anterior areas, but was severely abnormal in multilobar and posterior FCD.

Interictal spikes were focal (nine cases), regional (13 cases) or bilateral (six cases). In addition, spikes at a distance from the main focus were recorded in three cases: frontal spikes in two cases with occipitoparietal FCD and posterior spikes contralateral to a mesial premotor FCD in the third case. The site of maximum spike activity was concordant with the location of FCD in 17 cases (60%). Well-localized spikes were seen only in limited FCD. Regional spikes were seen in lobar and multilobar FCD, but were also recorded in limited FCD (five cases). Bilateral spikes were found only in limited mesial frontal FCD (six cases), with bilateral paroxysmal activity triggered by photic stimulation in one patient who had a family history of photosensitive epilepsy. The most characteristic interictal pattern consisted of rhythmic or pseudorhythmic spikes (16 patients), which was continuous in 14 patients, and was seen only during the postictal phase or after chemical activation in two patients. This pattern was observed mainly in limited FCD lying near the convexity of the brain, or in multilobar FCD.

A scalp ictal recording was obtained in 27 cases, with 155 spontaneous seizures (1–65 per patient) and 16 chemically induced seizures. The onset of the ictal discharge was characterized by low-voltage fast activity (22 cases) or rhythmic spikes (five cases); the latter cases corresponded to posterior FCD. Ictal activity onset coincided with FCD location in 14 cases (six limited, eight lobar or multilobar FCD). In contrast, among 13 cases with limited FCD, the onset of ictal discharge appeared to be multilobar in four and no localization was possible in nine. In these latter cases, the location of the FCD was mesial frontal (five cases), mesial central (four cases), lateral central (one case), parietal (two cases) or temporal (one case). Thus, the ictal discharge recorded on the surface EEG had a localizing value in half the cases, and failure to localize usually implied a mesial FCD location.

### SEEG data

The position of each intracranial electrode was decided on the basis of scalp electroclinical data, taking into account the lesional areas when they could be detected on MRI (six patients). A total of 257 intracranial electrodes were implanted (6–11 per patient, average 8). Their placement was unilateral in 13 cases and bilateral in 15. SEEG was carried out with electrodes located in multiple lobes (2–5) in 26 cases. Correlation with histological findings retrospectively demonstrated that 1–6 electrodes were clearly placed within the FCD in 25 patients. The electrodes were located at the periphery of the FCD in only three patients.

The interictal activity was recorded only in perilesional areas in three patients. For two of these patients, interictal activity remained rhythmic but consisted of slow spikes or sharp waves, similar to that observed in the perilesional areas in the other cases. The third patient was a 5-year-old child with continuous subclinical seizures due to limited mesial frontopolar FCD, in whom RSD were observed in the whole frontal lobe.

The LZ, defined by slow waves or depression of activity, was located in the same lobe or lobes containing FCD in 16 cases, also affecting a larger area than FCD in one-third of the cases. LZ was found to be multilobar in four limited FCDs and one lobar FCD, and hemispheric in one lobar FCD. It could not be identified in six cases, corresponding to a limited FCD in five out of six cases. Therefore, LZ did not appear to be an indicator of FCD.

The IZ was confined to the RSD areas in 15 cases, independently of the size of FCD. In the other cases, IZ included RSD and asynchronous spikes spreading to surrounding areas (11 cases including eight limited, one lobar and two multilobar FCD) or bursts of diffuse spikes (two cases with limited mesial frontal FCD). One of these last two patients had a personal and family history of photosensitivity. The extent of the IZ increased after seizures and diminished after injection of diazepam.

High-frequency electrical stimulation (monophasic rectangular pulses of 1–2.5 mA and 1 ms duration at 50 Hz for 5 s), performed in 27 cases, induced seizures similar to spontaneous electroclinical seizures in 20 patients. In each case, stimulation-induced seizures were obtained when the current was applied to the same electrodes as those recording RSD, within the dysplastic cortex.

The functional organization of the central area was studied by low-frequency stimulation (1 Hz, 1.5–3 mA) in nine patients who had FCDs in central, precentral or postcentral areas. Normal motor and/or sensory responses were obtained in all cases, suggesting no displacement of the motor strip. An unexpected finding was the triggering of 10 seizures in five patients using low-frequency stimulation. In these cases, the stimulation was performed within the FCD located in central or parietal areas.

No complications as result of the presurgical investigation occurred in this series, except for minor subdural haemorrhage with uneventful outcome in one patient.

### Neuroimaging findings

The CT scan was normal in four out 10 patients. In four cases, abnormalities consisted of hypodense areas apparently located in the white matter, and in two cases they consisted of cortical calcifications. The MRI was abnormal in six of seven cases and demonstrated a localized broad gyrus (one case), gyral abnormalities (four cases), a global increase in the volume of the occipital lobe (one case), increased signal on T2-weighted images with blurring of the demarcation between the grey and white matter (three cases), and an area of prolonged T2 relaxation in the underlying white matter (one case). MRI was normal in one of the seven cases. In all cases, FCD was histologically more widespread than the abnormal areas on CT scan, whereas on MRI this was true in three of six cases. In only three of the six patients who had abnormal MRI did the EZ correspond to the lesion as identified on MRI.

### Surgical data

The extent of corticectomy was limited to part of a lobe in 14 patients, lobar in nine and multilobar in five. When compared with the extent of FCD, the corticectomy was often larger than the extent of the dysplastic cortex, because both the EZ and the ictal discharge spread areas were included in the resection. The EZ was considered to be included in the resected tissue in 22 cases. In five cases, the resection was smaller than the EZ, essentially because of functional risks. One patient was operated on without definition of the EZ, but had a localized area of RSD in the parietal lobe.

Histological examination of the whole corticectomy specimen (22 cases) showed that all resection margins were beyond the limits of the dysplastic cortex FCD in nine cases, whereas three cases had margins in the transitional zone and six cases had one margin in the pathological zone. In four cases, all samples were pathological, but in one case the boundary consisted of calcifications without abnormal neurons. Based on histological findings, FCD removal was complete in nine cases, possibly incomplete in three and incomplete in 10.

### Seizure outcome

For each patient, seizure outcome was evaluated with a minimal follow-up of 2 years (up to 14 years, average 6 years). According to Engel's classification (Engel et al., 1996), 18 patients (64%) were found to be in class I (12 IA, two IB, one IC, three ID), two patients in class II (one IIA, one IIC), four patients in class IIIA and four patients in class IVB. Freedom from seizures was correlated both with the histologically confirmed complete removal of the dysplastic lesion (P < 0.01) and the complete removal of the EZ (P < 0.01) (Table 5).

When considering the status of FCD removal (22 patients), all patients in whom cortical resection included the totality of the FCD (nine cases) were seizure-free. Of the three patients whose resection boundaries were within the transitional zone, one was in class ID and two were in class II. Of the six patients with partial removal of FCD with one abnormal margin, three were in class I but none was in class IA; the other three had a less favourable outcome or no improvement (classes III and IV). Of the four patients in whom all tissue samples were pathological, three were in classes III or IV and only one was in class IA; in this patient, the edge of the resected tissue consisted of calcifications without abnormal neurons. In other words, the residual dysplastic cortex was found to be associated with postoperative seizures, even though some patients were greatly improved. However, a seizure-free outcome might still be observed, despite incomplete lesion removal, if the resection boundaries do not contain abnormal neurons.

When considering the status of EZ removal (27 patients), of the 22 patients for whom EZ resection was considered to be complete, 19 had an excellent or good outcome (17 in class I and two in class II). However, three patients had a less favourable or poor outcome (one in class III, two in class IV). For these latter patients, it was retrospectively concluded that SEEG had underestimated the EZ. The patient who was operated on without definition of the EZ but with evidence of a limited RSD area was in class IA. Among the five patients with incomplete EZ resection, three were in class IIIA and two were in class IVB. Thus, complete EZ removal was clearly related to favourable outcome, although SEEG failed to demonstrate the real extent of EZ in three cases. Conversely, resections that did not include the EZ were associated with an unfavourable outcome in all patients. FCD removal status could not be determined with precision in four patients with a favourable outcome (class I) with complete EZ removal, as well as in two other patients with less favourable outcome (class III) and incomplete EZ removal.

Insufficient information was available for six patients, but analysis of the remaining 22 patients demonstrated that outcome results according to EZ removal or histologically defined dysplastic cortex removal were similar, suggesting that they tend to co-localize.

## Discussion

This series of 28 cases constitutes a histologically homogeneous group of FCDs, as described by Taylor and colleagues (Taylor et al., 1971), for which SEEG findings have not been reported previously. Moreover, precise orientation of the cortical specimen according to the location of the intracerebral electrodes demonstrated that the dysplastic tissue generated RSDs, and was the site of onset of spontaneous ictal discharges, and that intralesional electrical stimulation elicited seizures similar to spontaneous seizures. We found that the EZ corresponded to the histologically defined FCD in 82% of the cases. The co-localization of the EZ and FCD is supported by analysis of the surgical results. Seizure-free outcome was correlated with complete removal of the EZ (P < 0.01) and with complete removal of the histologically defined FCD (P < 0.01). The high rate of freedom from seizures after surgery guided by SEEG (64%) confirms the value of neurophysiological investigations, and particularly of SEEG in determining the extent of corticectomy, even in the absence of adequate neuroimaging. Failure of surgery can be explained by constraints on resection due to eloquent areas of the brain that have dysplastic involvement.

Except for the temporal lobe, which was affected in a single case, FCD was represented equally in all lobes. Crucial areas, such as the posterior frontal lobe and the central and postcentral areas, were affected in 39% of the cases. The smallest FCDs were seen in the central and parietal areas and in the anterior cingular gyrus. The largest FCDs were located primarily in the posterior areas. In most series, FCDs are found predominantly in frontal and central locations (Perot et al., 1996; Palmini et al., 1991a; Hirabayashi et al., 1993; Kuzniecky et al., 1996; Polkey, 1996). However, paediatric series show a mainly posterior or hemispheric location (Chugani and Conti, 1996; Wyllie et al., 1996). Such differences can be explained by an earlier onset of epilepsy in larger and more posteriorly situated FCDs, as noted in our population. Temporal involvement is rare in our series, but not uncommon in other studies (Taylor et al., 1971; Palmini et al., 1991b; Raymond et al., 1995a; Brännström et al., 1996). This discrepancy may be the result of a bias in selection for surgery, or of the inclusion of different types of cortical dysplasia. On the other hand, in most reported cases involving the temporal lobe, including our single temporal case, seizures persisted after temporal lobectomy, suggesting the involvement of extratemporal areas associated with temporal FCD. We found FCD located predominantly on the mesial aspect of the brain (64%). This high frequency of mesially situated FCD is not reported by other authors, except by Kuzniecky (Kuzniecky, 1996) for the frontal lobe. This mesial location can explain some of the peculiarities of EEG patterns and the difficulties encountered during intracranial recording.

The major clinical features in this series are the early onset of epilepsy, a high seizure frequency with occurrence of clusters and status epilepticus, the severity of the cognitive impairment, especially in children, and the high frequency of neurological deficits (68%), which were concordant with the location of the FCD. The motor deficit was clearly acquired secondarily with central FCD in five patients (26% of neurological deficits), after bouts of high seizure frequency, including epilepsia partialis continua and status epilepticus. Partial seizure semiology, as previously described (Bancaud et al., 1991; Bancaud and Talairach, 1992; Chauvel et al., 1992), clearly corresponded to the site of the FCD except for the only temporal case in which there were perisylvian signs (Munari et al., 1980). Secondarily generalized seizures were rare (14%). This series presents most of the characteristics already mentioned in earlier series of histologically demonstrated FCD. Early onset and severity of epilepsy (Palmini et al., 1991a; Hirabayashi et al., 1993; Polkey, 1996; Wyllie et al., 1996) with the occurrence of status epilepticus (Perot et al., 1966; Palmini et al., 1991a; Kuzniecky et al., 1995; Wyllie et al., 1996) and of epilepsia partialis continua (Palmini et al., 1991a; Kuzniecky and Powers, 1993; Aicardi, 1994) appear to be characteristic of FCD. A relationship between the location of FCD and the neurological deficit has been noted in central (Barkovich and Kjos, 1992; Raymond et al., 1995a) and posterior FCD (Taylor et al., 1971; Kuzniecky et al., 1997). Progressive motor deficits related to epilepsy have been described (Perot et al., 1966; Kuzniecky and Powers, 1993; Guerrini et al., 1996; Polkey, 1996; Wyllie et al., 1996). Autistic features combined with severe mental retardation in children with posterior or anterior involvement have also been reported (Jambaque et al., 1993; Rossi et al., 1996). Few data concerning ictal semiology are available, but reported partial seizure manifestations consist of focal motor features or drop attacks in frontocentral FCD (Palmini et al., 1991b; Barkovich and Kjos, 1992; Kuzniecky et al., 1995) and of visual symptoms in occipital FCD (Kuzniecky et al., 1997). A distinct feature of our series is the low incidence of secondary generalization (14%) instead of the 35–73% reported in other studies (Palmini et al., 1991b; Hirabayashi et al., 1993; Kuzniecky et al., 1995). Such differences in secondarily generalized seizure frequency may be explained by heterogeneity in the histological diagnosis of focal dysplasias. It may also be difficult to distinguish generalized seizures from seizures with somatomotor manifestations due to rapid bilateral involvement of the mesial premotor areas (Chauvel et al., 1992).

Continuous or subcontinuous spikes or rhythmic sharp waves were observed on the surface EEG in 57% of our cases. This striking pattern of rhythmic epileptiform discharges is considered characteristic of surface EEG recordings in FCD and is found in about half the cases in some studies (Palmini et al., 1995; Raymond et al., 1995b; Gambardella et al., 1996). Rhythmic epileptiform discharges are spatially more restricted than intermittent sharp waves and spikes, correspond to maximally epileptogenic areas, as demonstrated by electrocorticography, and are concordant with FCD location in 80% of the cases (Gambardella et al., 1996). Comparison with SEEG recordings allowed us to show that rhythmic epileptiform discharges observed on the surface EEG clearly correspond to RSDs recorded within the dysplastic cortex, although the pattern appears to be attenuated and distorted on surface recordings: intracerebral rapid spikes may correspond to slow and low-amplitude spikes or sharp waves on the surface EEG. However, intracerebral and surface recordings may be similar in some cases. Rhythmic epileptiform discharges are recorded on EEG in larger FCDs, but also when small dysplastic lesions are located near the convexity or the cerebral poles. Conversely, RSDs generated by FCD located on the mesial aspect of the hemispheres may not be recorded on the surface EEG, which will show only propagated or bilateral interictal spikes. Scalp ictal EEG recordings are often considered unhelpful in locating FCD (Palmini et al., 1991a; Hirabayashi et al., 1993; Kuzniecky et al., 1995). In this series, ictal EEG proved to be useful for localization in half of the cases. Comparison with intracerebral recordings demonstrates that clearly lateralized discharges on SEEG can appear bilateral on the surface EEG, especially in central mesial FCD. Thus, interictal and ictal EEG patterns appear to be determined by the location, orientation and extent of FCD.

Surface EEG evaluation was of considerable help when planning an adequate SEEG procedure. Since most patients were investigated without contemporary imaging, it is noteworthy that the intracranial implantation plan, based on electroclinical correlation, led to the placement of at least one electrode in the dysplastic cortex in all cases but three. Direct intralesional recordings demonstrated unequivocally the epileptogenicity of the dysplastic cortex, which was the site of interictal RSD, spontaneous ictal discharge onset, and induced seizures after electrical stimulation. In most of our cases, a clear correlation between the extent of FCD and the EZ could be demonstrated. The few discrepancies observed may have been due to methodological reasons (i.e. the lack of an intralesional electrode, an inadequate number of electrodes or the absence of a recorded seizure). Overall, FCD was delineated by SEEG in 82% of the cases. The main difficulties in accurately assessing the extent of FCD occurred in the central area, where the EZ may be larger than the dysplastic lesion, probably because of a low epileptogenic threshold. In this area, the RSD interictal pattern appeared to be of particular interest for delineating the dysplastic cortex. In the only previously reported case of FCD investigated using SEEG (Munari et al., 1996), the authors found that EZs were more widespread than the lesion on MRI, and postulated that ictal discharges may start in the surrounding cortex. However, the histologically defined extent of the dysplastic cortex found at surgery was not given, and therefore their hypothesis does not necessarily contradict our data.

Epileptogenic areas defined by electrocorticography are often more widespread than the structural lesion on MRI (Palmini et al., 1991a; Hirabayashi et al., 1993; Kuzniecky et al., 1995). They can also be larger than the hypometabolic area identified by PET (Olson et al., 1990). The usefulness of peculiar patterns of ictal or continuous epileptogenic discharges recorded on electrocorticography from electrodes overlying the dysplastic cortex, even in cases with normal imaging or visual inspection, has been stressed recently by Palmini and colleagues (Palmini et al., 1995). These authors concluded that the described electrocorticographic pattern allowed the identification of the dysplastic cortex. They emphasized the intrinsic epileptogenicity of FCD, which they found to be more epileptogenic than other structural lesions. However, other authors found that this electrocorticography pattern was correlated with high seizure frequency and a subtype of balloon cell cortical dysplasia, but could be also observed in other types of lesions (Rosenow et al., 1998). We reach conclusions similar to Palmini and colleagues, as we demonstrated that a striking pattern (RSD) was highly valuable in identifying and delineating the FCD. However, as mentioned (Rosenow et al., 1998), this pattern cannot be considered to be specific for dysplastic tissue. We indeed observed it in other conditions, mainly in Rasmussen encephalitis and in some cases of dysembryoplastic neuroepithelial tumours (personal observation). Therefore, this pattern appears to be a marker of high lesional epileptogenicity rather than a histological marker.

Both electrocorticography and SEEG allowed detection of dysplastic cortex. However, some differences related to methodology should be noted. First, we observed RSDs on intracerebral recordings in all but two patients (93%), whereas a similar pattern was found in 67% of the cases by electrocorticography (Palmini et al., 1995). Secondly, we demonstrated that both subclinical ictal discharges and electroclinical seizure onset clearly arose from the dysplastic cortex that also generated RSD. Thus, keeping in mind that the definition of the EZ is based on ictal discharges in our method, we showed that only part of the interictal spiking (RSDs) could be considered to have a similar value to ictal discharges for the identification of the EZ. Thirdly, SEEG permits a satisfactory evaluation of mesial FCD. Furthermore, it provides a three-dimensional approach to the delineation of the FCD, despite the fact that depth electrode recording implies limited sampling of lesional and perilesional areas.

The epileptogenicity of dysplastic cortex may be accounted for by several mechanisms. Seizures elicited by low-frequency stimulation of the dysplastic cortex in this study and one other study (Munari et al., 1996) suggest a low epileptogenic threshold, since such stimulation does not usually elicit seizures outside the hippocampal structures (Kahane et al., 1993). Persisting RSDs in the dysplastic cortex after intravenous injection of diazepam when surrounding spikes have disappeared and [11C]flumazenil-PET findings (Chassoux et al., 1995; Richardson et al., 1996) both suggest that abnormal benzodiazepine receptors are involved in FCD. Moreover, abnormal synaptic interconnectivity (Mattia et al., 1995), changes in the pattern of catecholaminergic innervation (Trottier et al., 1994) and serotonergic hyperinnervation (Trottier et al., 1996) and abnormalities in GABAergic neurons (Ferrer et al., 1992) have been found in dysplastic cortical specimens.

Because EZ and FCD areas tend to co-localize, it can be postulated that lesion removal is both necessary and sufficient for the achievement of freedom from seizures. However, in this series corticectomies were often larger than the dysplastic cortex as they included areas of ictal spread, and it is therefore not possible to arrive at a conclusion concerning the necessity of removing perilesional areas. When removal of the EZ was considered to be complete, the outcome was favourable in the majority of cases. Surgical failure in three patients was interpreted as being due to an insufficient definition of the EZ. Incomplete EZ removal was performed because of functional risks in the eloquent cortex and was associated with poor outcome in all patients. Complete removal of the FCD led to results in class IA. The residual dysplastic cortex was clearly associated with an unfavourable outcome, except in patients who presented no abnormal neurons at the margins of the resection. Although Taylor and colleagues reported half the cases to be seizure-free (Taylor et al., 1971), in later series freedom from seizures was <20% (Palmini et al., 1991b; Hirabayashi et al., 1993; Raymond et al., 1995a; Brännström et al., 1996). Poor surgical outcome was explained by difficulty in identifying the margins of the dysplastic cortex on imaging and at surgery (Kuzniecky et al., 1995; Palmini et al., 1995; Olivier et al., 1996; Polkey, 1996). A more favourable outcome has been reported in children, the proportion of seizure-free patients ranging from 52% (Wyllie et al., 1996) to 65% (Chugani et al., 1993), but wider resections, including hemispherectomy, may account for this discrepancy. With 64% seizure-free patients, our results figure among the best reported, despite the lack of adequate brain imaging and limited cortical resections in extratemporal areas in the vast majority of cases; these results therefore validate SEEG methodology. It must be recalled that, until the advent of modern imaging techniques, neurophysiological investigation was the only method available for planning surgical resection in epileptic patients. The value of the information provided by SEEG data is more than just historical as it has led to a better understanding of the epileptogenicity of FCD and the organization of the EZ. It also aids in the delineation of the dysplastic cortex for surgical resection. We conclude that, despite the risks and costs entailed, the combination of neurophysiological criteria with high-resolution MRI will retain its usefulness in planning adequate resection for focal cortical dysplasia until invasive procedures can be replaced by newer, more advanced imaging techniques.

Table 1

Location and spread of FCD in 28 cases

SMA = supplementary motor area.
Frontal (n = 9) Mesial (8) Limited (8) Frontopolar (1)
Mesial + lateral (1) Lobar (1) Orbitocingular (1)
Anterior cingular gyrus (3)
Prefrontal–premotor (3)
SMA (1)
Frontocentral (n = 1) Mesial + lateral (1) Multilobar (1) All frontal and central areas
Central (n = 6) Mesial 5 Limited (6) Paracentral lobule (1)
Lateral 1  Paracentral lobule + SMA (4)
Precentral (1)
Parietal (n = 6) Mesial (1) Limited (5) Postcentral (3)
Lateral (3) Lobar (1) Area 7 (3)
Mesial + lateral (2)
Occipital (n = 1) Mesial (1) Lobar (1)
Occipito-parietal (n = 2) Mesial (1) Multilobar (2)
Occipito-parieto-temporal (n = 2) Mesial + lateral (2) Multilobar (2) All areas examined
Temporal (n = 1) Mesial (1) Limited (1) Ammon's horn, parahippocampal gyrus, basal temporal cortex
SMA = supplementary motor area.
Frontal (n = 9) Mesial (8) Limited (8) Frontopolar (1)
Mesial + lateral (1) Lobar (1) Orbitocingular (1)
Anterior cingular gyrus (3)
Prefrontal–premotor (3)
SMA (1)
Frontocentral (n = 1) Mesial + lateral (1) Multilobar (1) All frontal and central areas
Central (n = 6) Mesial 5 Limited (6) Paracentral lobule (1)
Lateral 1  Paracentral lobule + SMA (4)
Precentral (1)
Parietal (n = 6) Mesial (1) Limited (5) Postcentral (3)
Lateral (3) Lobar (1) Area 7 (3)
Mesial + lateral (2)
Occipital (n = 1) Mesial (1) Lobar (1)
Occipito-parietal (n = 2) Mesial (1) Multilobar (2)
Occipito-parieto-temporal (n = 2) Mesial + lateral (2) Multilobar (2) All areas examined
Temporal (n = 1) Mesial (1) Limited (1) Ammon's horn, parahippocampal gyrus, basal temporal cortex
Table 2

Seizure semiology according to location of FCD

Aura First signs Loss of consciousness Postictal deficit Duration of seizures
Frontal (n = 9) Fear (3) Frightened face (4)  None (6) 20–40 s; 30–90 s if involving anterior cingular gyrus
Autonomous (1) Screaming (4) 8/9 Brief confusion (2)
Indefinable (3) Sudden agitation (2)  Motor deficit (1)
Not reported (2) Tonic posturing (2) Incomplete/inconstant (5)
Deviation of head and eyes (2)
Central (n = 6) Somatomotor (3) Somatomotor:  Motor deficit (6) 15–60 s
Sensory (5)  tonic (5) 0/6
Fronto-central (n = 1) Headache (1)  clonic (1)
Not reported (1) Vocalization (1) 1/1
Parietal (n = 6) Somatomotor (2) Fixed gaze (4)  Motor deficit (2) 30–60 s
Sensory (1) Somatomotor (4) 3/6 Spatial agnosia (1)
Pain (2) Autonomous (2) Incomplete/inconstant (2)
Asomatognosia (2) Version (2)
Vertigo (1) Vocalization (2)
Indefinable (1) Deviation of head and eyes (1)
Not reported (1)
Occipital Visual Deviation of eyes  Not observed ≥30–60 s
Parieto-occipital Hallucination (1) jerking (1) tonic (1) 4/5  (90–150 s/> 4 mn in 1 case)
Parieto-occipito-temporal (n = 5) Ocular ocular jerks (4) Incomplete/inconstant (4)
Not reported (3) Somatomotor (3)
Groaning 1
Hiding eyes (1)
Temporal n = 1 Anxiety Somatomotor  Temporospatial disorientation 60 s
Aura First signs Loss of consciousness Postictal deficit Duration of seizures
Frontal (n = 9) Fear (3) Frightened face (4)  None (6) 20–40 s; 30–90 s if involving anterior cingular gyrus
Autonomous (1) Screaming (4) 8/9 Brief confusion (2)
Indefinable (3) Sudden agitation (2)  Motor deficit (1)
Not reported (2) Tonic posturing (2) Incomplete/inconstant (5)
Deviation of head and eyes (2)
Central (n = 6) Somatomotor (3) Somatomotor:  Motor deficit (6) 15–60 s
Sensory (5)  tonic (5) 0/6
Fronto-central (n = 1) Headache (1)  clonic (1)
Not reported (1) Vocalization (1) 1/1
Parietal (n = 6) Somatomotor (2) Fixed gaze (4)  Motor deficit (2) 30–60 s
Sensory (1) Somatomotor (4) 3/6 Spatial agnosia (1)
Pain (2) Autonomous (2) Incomplete/inconstant (2)
Asomatognosia (2) Version (2)
Vertigo (1) Vocalization (2)
Indefinable (1) Deviation of head and eyes (1)
Not reported (1)
Occipital Visual Deviation of eyes  Not observed ≥30–60 s
Parieto-occipital Hallucination (1) jerking (1) tonic (1) 4/5  (90–150 s/> 4 mn in 1 case)
Parieto-occipito-temporal (n = 5) Ocular ocular jerks (4) Incomplete/inconstant (4)
Not reported (3) Somatomotor (3)
Groaning 1
Hiding eyes (1)
Temporal n = 1 Anxiety Somatomotor  Temporospatial disorientation 60 s
Table 3

Interictal and ictal EEG scalp recordings according to FCD spread

FCD spread Limited (20) Lobar (3) Multilobar (5)
EEG background
Normal
Slow/asymmetrical 12
Not identified
Slow waves
Absent
Focal
Regional
Bilateral
Spiking activity
Focal spikes
Regional spikes
Bilateral spikes
Rhythmic or pseudorhythmic spikes 13
Ictal discharges (n = 27)
Focal/lobar
Regional/multilobar
Bilateral/not identified
FCD spread Limited (20) Lobar (3) Multilobar (5)
EEG background
Normal
Slow/asymmetrical 12
Not identified
Slow waves
Absent
Focal
Regional
Bilateral
Spiking activity
Focal spikes
Regional spikes
Bilateral spikes
Rhythmic or pseudorhythmic spikes 13
Ictal discharges (n = 27)
Focal/lobar
Regional/multilobar
Bilateral/not identified
Table 4

Relationship between histologically defined FCD extent and SEEG data

SEEG data (neuropatholology) n = 22 LZ/FCD IZ/FCD RSD/FCD EZ/FCD
>, more extended; <, more restricted; =, co-localized.
Limited FCD n = 14 = 2 = 5 = 11 = 10
Not identified (5)
>4 >9 >1 >4
Distinct (2)  Uncertain (2)
Lobar FCD n = 3 = 1 = 2 = 3 = 3
>1 >1
<1
Multilobar FCD n = 5 = 1 = 2 = 5 = 5
Not identified (1) >3
>3
Total n = 22 LZ = FCD IZ = FCD RSD = FCD EZ = FCD
19 18
18% 41% 86% 82%
SEEG data (neuropatholology) n = 22 LZ/FCD IZ/FCD RSD/FCD EZ/FCD
>, more extended; <, more restricted; =, co-localized.
Limited FCD n = 14 = 2 = 5 = 11 = 10
Not identified (5)
>4 >9 >1 >4
Distinct (2)  Uncertain (2)
Lobar FCD n = 3 = 1 = 2 = 3 = 3
>1 >1
<1
Multilobar FCD n = 5 = 1 = 2 = 5 = 5
Not identified (1) >3
>3
Total n = 22 LZ = FCD IZ = FCD RSD = FCD EZ = FCD
19 18
18% 41% 86% 82%
Table 5

Seizure outcome according to histologically defined FCD removal and epileptogenic zone (EZ) removal

Class I Class II Class III Class IV
FCD removal/seizure outcome (n = 22)
Complete FCD removal n = 9 9 (8 IA, 1 ID)
Margins in transitional area n = 3 1 (ID) 2 (1 IIA, 1 IIC)
Incomplete FCD removal (single pathological boundary) n = 6 3 (2 IB, 1 IC) 1 (IIIA) 2 (IVB)
Incomplete FCD removal (all pathological boundaries) n = 4 1 (IA) 1 (IIIA) 2 (IV B)
EZ removal/seizure outcome (n = 27)
Complete EZ removal n = 22 17 (11 IA, 2 IB, 1 IC 3 ID) 2 (1 IIA, 1 IIC) 1 (IIIA) 2 (IVB)
Incomplete EZ removal n = 5 3 (IIIA) 2 (IV B)
Class I Class II Class III Class IV
FCD removal/seizure outcome (n = 22)
Complete FCD removal n = 9 9 (8 IA, 1 ID)
Margins in transitional area n = 3 1 (ID) 2 (1 IIA, 1 IIC)
Incomplete FCD removal (single pathological boundary) n = 6 3 (2 IB, 1 IC) 1 (IIIA) 2 (IVB)
Incomplete FCD removal (all pathological boundaries) n = 4 1 (IA) 1 (IIIA) 2 (IV B)
EZ removal/seizure outcome (n = 27)
Complete EZ removal n = 22 17 (11 IA, 2 IB, 1 IC 3 ID) 2 (1 IIA, 1 IIC) 1 (IIIA) 2 (IVB)
Incomplete EZ removal n = 5 3 (IIIA) 2 (IV B)
Fig. 1

SEEG recordings. Intralesional interictal activity in FCD is characterized by continuous or subcontinuous RSDs with frequency varying from 1 to 10 Hz. Various patterns were observed in areas that were close together in a given patient (Case 10). Case 16, with frontocentral FCD, illustrates one of the most severe patterns of this series, with pseudoperiodic bursts of spikes interrupted by suppression of activity. For this later case, the upper line shows central area activity and the lower line shows frontal lobe activity (recorded after failure of central resection).

Fig. 1

SEEG recordings. Intralesional interictal activity in FCD is characterized by continuous or subcontinuous RSDs with frequency varying from 1 to 10 Hz. Various patterns were observed in areas that were close together in a given patient (Case 10). Case 16, with frontocentral FCD, illustrates one of the most severe patterns of this series, with pseudoperiodic bursts of spikes interrupted by suppression of activity. For this later case, the upper line shows central area activity and the lower line shows frontal lobe activity (recorded after failure of central resection).

Fig. 2

Interictal activity in a small mesial central FCD. The mesial part of each electrode corresponds to code 1 (i.e. N'1, M'1,…) and the lateral part to code 2, the contacts being denoted 1-2, 2-3, etc. The RSDs are recorded in a limited area (R'1-T'1), that corresponded to the FCD. The rhythmic spiking activity was recorded simultaneously on surface EEC (Cz).

Fig. 2

Interictal activity in a small mesial central FCD. The mesial part of each electrode corresponds to code 1 (i.e. N'1, M'1,…) and the lateral part to code 2, the contacts being denoted 1-2, 2-3, etc. The RSDs are recorded in a limited area (R'1-T'1), that corresponded to the FCD. The rhythmic spiking activity was recorded simultaneously on surface EEC (Cz).

Fig. 3

Interictal activity in a multilobar occipitoparietotemporal FCD. All right posterior areas are involved, although RSD frequency varies in different parts of the FCD. Histologically, all areas displaying RSDs corresponded to the dysplastic cortex. The surface EEG shows the rhythmic activity in the right posterior areas. EOG = electrooculography.

Fig. 3

Interictal activity in a multilobar occipitoparietotemporal FCD. All right posterior areas are involved, although RSD frequency varies in different parts of the FCD. Histologically, all areas displaying RSDs corresponded to the dysplastic cortex. The surface EEG shows the rhythmic activity in the right posterior areas. EOG = electrooculography.

Fig. 4

Effect of intravenous diazepam on RSD activity (limited left central FCD). Interictal recording demonstrates focal RSDs (G'1-G'2), apparent on the surface EEG on the left central area (Fz-Cz, C3-P3). A large frontocentroparietal spiking activity occurs in the postictal state, whereas after intravenous injection of diazepam the RSDs persist only in a restricted area (G'1-G'2), corresponding to the FCD.

Fig. 4

Effect of intravenous diazepam on RSD activity (limited left central FCD). Interictal recording demonstrates focal RSDs (G'1-G'2), apparent on the surface EEG on the left central area (Fz-Cz, C3-P3). A large frontocentroparietal spiking activity occurs in the postictal state, whereas after intravenous injection of diazepam the RSDs persist only in a restricted area (G'1-G'2), corresponding to the FCD.

Fig. 5

SEEG and histological correlations in a limited right precentral FCD. (A) The site of maximal RSDs corresponds to the FCD (M2–X2–X3), which presented as a localized broad gyrus on MRI (T2-weighted image and 3D reconstruction). A less continuous spiking activity was recorded in perilesional areas (especially Q3) and areas distant from them (P2). The patient became seizure-free after precentral resection that did not affect the parietal area (electrode P). Note the normality of the activity in the mesial white matter and cortex (M1). (B) Histological findings (haemalum–phloxin–safranin staining). The cortical specimens corresponding to M2 and X2 activity demonstrated characteristic features of FCD, with abnormal large neurons and balloon cells. Balloon cells were also seen in the adjacent white matter [M1 (w)]. However, there were no cortical abnormalities (except neuronal damage) in the mesial cortex [(M1 (c)] or in the posterior and inferior boundaries of the resection (Q3–R3). Magnification: M2, X2 and M1 (w), ×300; M1 (c) and R3, ×150; Q3 ×70.

Fig. 5

SEEG and histological correlations in a limited right precentral FCD. (A) The site of maximal RSDs corresponds to the FCD (M2–X2–X3), which presented as a localized broad gyrus on MRI (T2-weighted image and 3D reconstruction). A less continuous spiking activity was recorded in perilesional areas (especially Q3) and areas distant from them (P2). The patient became seizure-free after precentral resection that did not affect the parietal area (electrode P). Note the normality of the activity in the mesial white matter and cortex (M1). (B) Histological findings (haemalum–phloxin–safranin staining). The cortical specimens corresponding to M2 and X2 activity demonstrated characteristic features of FCD, with abnormal large neurons and balloon cells. Balloon cells were also seen in the adjacent white matter [M1 (w)]. However, there were no cortical abnormalities (except neuronal damage) in the mesial cortex [(M1 (c)] or in the posterior and inferior boundaries of the resection (Q3–R3). Magnification: M2, X2 and M1 (w), ×300; M1 (c) and R3, ×150; Q3 ×70.

We wish to thank Suzanne Trottier, Catherine Chiron and Olivier Dulac for reviewing the manuscript and P. Chauvel, C. Munari, D. Broglin, L. Merienne, D. Toussaint, J. P. Gagnepain, S. Baudouin-Chial, P. Buser, M. Lamarche, S. Liminana and F. Semah for their constructive advice and assistance. This manuscript is dedicated to Jean Bancaud and Alain Bonis, in memoriam, and to Jean Talairach, in respectful homage.

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