Secondary epileptogenesis in humans

Kindling in humans: Does secondary epileptogenesis occur?

The relevance of secondary epileptogenesis for human epilepsy remains a controversial subject decades after it was first described in animal models. Whether or not a previously normal brain region can become independently epileptogenic through a kindling-like process has not, and cannot, be definitely proven in humans. Rather than reliance on direct experimental evidence, attempts to answering this question must depend on observational data. In this review, observations based largely upon contemporary surgical series will advance the case for secondary epileptogenesis in humans. As will be argued, hypothalamic hamartoma-related epilepsy provides the strongest case for this process; all the stages of secondary epileptogenesis can be observed. Hippocampal sclerosis (HS) is another pathology where the question of secondary epileptogenesis frequently arises, and observations from bitemporal and dual pathology series are explored. The verdict here is far more difficult to reach, in large part because of the scarcity of longitudinal cohorts; moreover, recent experimental data have challenged the claim that HS is acquired consequent to recurrent seizures. Synaptic plasticity more than seizure-induced neuronal injury is the likely mechanism of secondary epileptogenesis. Postoperative running-down phenomenon provides the best evidence that a kindling-like process occurs in some patients, evidenced by its reversal. Finally, a network perspective of secondary epileptogenesis is considered, as well as the possible role for subcortical surgical interventions.

Gelastic seizures and the hypothalamic hamartoma syndrome: Epileptogenesis beyond the lesion?

The clinicoradiologic syndrome of hypothalamic hamartoma (HH) manifests with a variety of symptoms, including pharmacoresistant epilepsy with multiple seizure types, precocious puberty, behavioral disturbances, and cognitive impairment. Gelastic seizures are an early marker of epilepsy with HH in most of the cases. Despite a high variability, two major epilepsy phenotypes can be distinguished, based on electroclinical features: (i) focal seizures with epigastric or déjà-vu aura, loss of consciousness, and oroalimentary or gestural automatisms suggestive of temporal lobe involvement; and (ii) motor seizures with tonic, atonic, myoclonic, or versive phenomena, suggesting frontoparietal network involvement, with possible evolution toward an epileptic encephalopathy. The underlying physiopathologic mechanisms are not completely elucidated. The well-known intrinsic epileptogenicity of the HH represents the rationale for direct HH-aiming surgical procedures, with variable success in achieving seizure freedom. The concept of kindling-like secondary epileptogenesis has been suggested as a possible putative mechanism since the very beginnings of the hamartocentric era. Accordingly, a cortical area with enhanced epileptogenic properties due to an independent stage of secondary epileptogenesis would be responsible for seizures persisting after hamartoma ablation. However, recent intracerebral stereotactic EEG (SEEG) explorations demonstrated more complex, both reciprocal and hierarchical, relationships within the hypothalamo-cortical epileptogenic networks. Network formation may be due to either secondary epileptogenesis or widespread epileptogenicity present at the outset. A short time window from epilepsy onset to surgery seems to be crucial to cure epilepsy by direct surgery addressing a hamartoma. SEEG exploration may be reasonably proposed in cases where clinical data suggest an extension of the epileptogenic zone outside the limits of the HH, especially in focal seizures with impaired awareness and absence of gelastic seizures, or after a failure of the direct HH-aiming procedure.

Electroencephalographic abnormalities are correlated with cognitive deficits in children with benign childhood epilepsy with centrotemporal spikes: A clinical study of 61 cases

OBJECTIVE

The objective of this study was to explore the effect of spikes on cognition in patients with benign childhood epilepsy with centrotemporal spikes (BECTS) and to identify electroencephalography (EEG) markers enabling early detection of cognitive impairment.

METHODS

Sixty-one children with BECTS diagnoses and 60 age- and education-matched healthy controls were enrolled. Four-hour EEG recordings were analyzed for each patient to check for interictal spikes, high-frequency oscillations (HFOs), nondipole spikes, and other atypical EEG features and to examine the spike-wave index of nonrapid eye movement (NREM) sleep. All 121 children underwent a series of neuropsychological tests to assess cognitive function.

RESULTS

Patients with a high NREM sleep discharge index (≥55%) in the first sleep cycle exhibited significantly lower scores for arithmetic calculation, executive function, and attention and memory tests than patients with a low discharge index (<55%). Eight patients with HFOs exhibited even poorer performance than HFO-negative patients for arithmetic calculation, executive function, vocabulary comprehension, visual perception, vocal perception, spatial memory ability, and response ability. Children with bilateral discharge exhibited poorer ability in three-dimensional spatial imaging test, poorer memory, and slower response than did those with unilateral discharge (P < .05). Nondipole spikes, multiple asynchronous discharges, and generalized spike-wave discharges respectively had an impact on calculation ability, memory, and reaction ability respectively (P < .05).

CONCLUSIONS

Spike frequencies in stage 3 and 4 sleep varied from those observed in stage 1 and 2 sleep; the highest spike frequency was in stage 2 sleep. High NREM sleep discharge index (i.e., ≥55%) and HFOs were linked to the highest risk for cognitive deficit, while bilateral discharges, nondipole spikes, multiple asynchronous discharges, and generalized spike-wave discharges were less indicative of cognitive impairment.

Analysis of serial electroencephalographic predictors of seizure recurrence in Rolandic epilepsy

PURPOSE

We aimed to assess the relationship between electroencephalography (EEG) markers and seizure recurrence in cases with benign epilepsy with centrotemporal spikes (BECT) in a long-term follow-up study.

METHODS

We analyzed the data of 52 children with BECT who were divided into 2 groups: the isolated group and recurrence group. The clinical profiles and initial/serial visual EEG recordings of both groups were evaluated. The entire follow-up period ranged from 12 to 65 months.

RESULTS

None of the clinical characteristics differed between the 2 groups. Serial EEGs showed that the appearance of Rolandic spikes in the frontal region was more prevalent in the recurrence group. Moreover, a significant correlation was found between bilateral asynchronous discharges and seizure recurrence. However, on initial EEG of these patients, neither of the EEG features exhibited statistical significance.

CONCLUSION

The presence of frontal focus and bilateral asynchrony appeared to be hallmarks of BECT patients with higher risk for seizure recurrence.

Hypothalamic hamartoma: Epileptogenesis beyond the lesion?

The discovery of intrinsic epileptogenicity of the hypothalamic hamartoma (HH) marked a new area in understanding the associated clinical syndrome, often manifesting as progressive epileptic encephalopathy. However, therapeutic procedures targeting the HH proved to be inefficient to cure seizures in up to 50% of cases, whereas in cases with partial improvement, the electroclinical patterns of persisting seizures suggest an involvement of distant cortical regions. The concept of kindling-like secondary epileptogenesis has been suggested as a possible underlying mechanism. Yet the role of the hypothalamic lesion in the pathophysiology of the syndrome remains debatable. In the Strasbourg-Kork series, the best outcomes were obtained when the duration of epilepsy before endoscopic HH surgery did not exceed 10 years. In two patients with HH ablation followed at a later time by a temporal lobectomy, only this second surgical step allowed complete seizure freedom. These findings suggest the existence of an independent, third stage of secondary epileptogenesis in human. In the Grenoble series, stereotactic intracerebral recordings (stereo electroencephalography [SEEG]) of five HH cases demonstrated that gelastic/dacrystic seizures were correlated with discharges within the HH, whereas other seizure types were related to discharges affecting cortical regions, which sometimes seemed to be triggered by HH. In the Marseille series, two cases explored by SEEG provided evidence of extended epileptogenicity outside the limits of the HH, forming complex epileptogenic networks, with HH still triggering clusters of neocortical seizures in the first, but not obligatory involved in spontaneous seizures in the second case. Taken together, our data argue for the existence of dynamic ictal network organization, with possible "kindling-like" relationships between the HH and the neocortex or widespread epileptogenesis. Despite the existence of secondary epileptogenesis, the epileptogenic zone could still be limited to the hamartoma, for which early surgical treatment should be pragmatically considered as a first surgical step.

Low-frequency stimulation of the primary focus retards positive transfer of secondary focus

Positive transfer of secondary focus (PTS) refers to new epileptogenesis outside the primary focus and is minimally controlled by existing treatments. Low-frequency stimulation (LFS) has benefits on the onset of epilepsy and epileptogenesis. However, it's unclear whether LFS can retard the PTS in epilepsy. Here we found that PTS at both contralateral amygdala and ipsilateral hippocampus were promoted after the primary focus was fully kindled in rat kindling model. The promotion of PTS at the mirror focus started when the primary kindling acquisition reached focal seizures. LFS retarded the promotion of PTS when it was applied at the primary focus during its kindling acquisition, while it only slightly retarded the promotion of PTS when applied after generalized seizures. Meanwhile, we found the expression of potassium chloride cotransporter 2 (KCC2) decreased during PTS, and LFS reversed this. Further, the decreased expression of KCC2 was verified in patients with PTS. These findings suggest that LFS may be a potential therapeutic approach for PTS in epilepsy.

BOLD responses related to focal spikes and widespread bilateral synchronous discharges generated in the frontal lobe

OBJECTIVE

To investigate whether specific frontal regions have a tendency to generate widespread bilateral synchronous discharges (WBSDs) and others focal spikes and to determine the regions most involved when WBSDs occur; to assess the relationships between the extent of electroencephalography (EEG) discharges and the extent of metabolic changes measured by EEG/functional magnetic resonance imaging (fMRI).

METHODS

Thirty-seven patients with interictal epileptic discharges (IEDs) with frontocentral predominance underwent EEG/fMRI. Patients were divided into a Focal (20 patients) group with focal frontal spikes and a WBSD group (17 patients). Maps of hemodynamic responses related to IEDs were compared between the two groups.

RESULTS

The mean number ± SD of IEDs in the Focal group was 137.5 ± 38.1 and in the WBSD group, 73.5 ± 16.6 (p = 0.07). The volume of hemodynamic responses in the WBSD group was significantly larger than in the Focal group (mean, 243.3 ± 41.1 versus 114.8 ± 27.4 cm(3), p = 0.01). Maximum hemodynamic responses occurred in both groups in the following regions: dorsolateral prefrontal, mesial prefrontal, cingulate, and supplementary motor cortices. Maxima in premotor and motor cortex, frontal operculum, frontopolar, and orbitofrontal regions were found only in the Focal group, and maxima in thalamus and caudate only occurred in the WBSD group. Thalamic responses were significantly more common in the WBSD group (14/17) than in the Focal group (7/20), p = 0.004. Deactivation in the default mode network was significantly more common in the WBSD group (14/17) than in the Focal group (10/20), p = 0.04.

SIGNIFICANCE

The spatial distribution and extent of blood oxygen level-dependent (BOLD) responses correlate well with electrophysiologic changes. Focal frontal spikes and WBSDs are not region specific in the frontal lobe, and the same frontal region can generate focal and generalized discharges. This suggests that widespread discharges reflect widespread epileptogenicity rather than a focal discharge located in a region favorable to spreading. The thalamus plays an important role in bilateral synchronization.

Involvement of GABA in mirror focus: a case report

Mirror focus (MF) is a cortical epileptogenic lesion that is posited to develop in the contralateral site to a cortical primary focus (PF) by secondary epileptogenic mechanisms. Previous animal evidence supports the implication of gamma-aminobutyric acid (GABA) in this phenomenon, but this contention has not yet been substantiated by clinical findings. Here we report for the first time clinical evidence suggesting the involvement of GABAergic cortical transmission in MF pathogenesis, in a 37-year-old man affected by a lesional PF in the right frontal lobe and a homotopic MF in the contralateral hemisphere, triggered by hyperventilation. One year after surgical excision of the PF, the electric activity of the MF remained unchanged, but was accompanied by a significant increase in the density of GABA(A)/benzodiazepine receptor binding in the left frontal lobe, as measured by (123)I-Iomazenil SPECT. These results extend previous evidence on the involvement of GABAergic signaling in MF pathophysiology.

Electroencephalographic generalized features in idiopathic childhood focal epilepsies

PURPOSE

Idiopathic focal epilepsies in childhood including benign childhood epilepsy with occipital paroxysms (BEOP) or benign childhood epilepsy with centro-temporal spikes (BCECTS) are characterized by specific focal electrographic patterns as the name indicates. Generalized spike-wave discharges in children with idiopathic focal epilepsy can suggest a neurobiological continuum with the idiopathic generalized epilepsies. We assessed the prevalence of generalized epileptiform discharges and generalized seizures in BEOP/BCECTS patients.

METHODS

Between August 2005 and November 2008, we identified 220 cases with electroclinical features typical of idiopathic focal epilepsies, 172 patients with BCECTS and 48 patients with BEOP, excluding patients whose neurological examinations or brain MRI were abnormal. We analyzed gender, age at onset, manifestation of generalized seizures, and serial EEG records to detect generalized abnormalities.

RESULTS

Of our population, 42 patients (19.1%, 22 boys), 30 (17.4%) of 172 BCECTS patients and 12 (25.0%) of 48 BEOP patients, showed generalized spike-wave discharges once or more during follow-up. The typical 3-Hz generalized spike wave discharge was noticed in 7 patients and concurrence with clinical generalized seizure was observed in 11.

CONCLUSION

A relatively high incidence of generalized spike-wave discharge and concurrence with generalized seizure were observed in patients with BEOP/BCECTS, with the incidence being higher in BEOP patients than in those with BCECTS. It may be inferred that idiopathic focal epilepsy is not a fixed syndrome but is a part of a broad, age-related, benign, seizure susceptibility syndrome.

Nonconvulsive status epilepticus with an unusual EEG: a fresh look at lateralities of motor control and awareness

This article is an account of a patient with nonconvulsive status epilepticus associated with an unusual EEG. The importance of recording speed in lateralizing the hemisphere of onset of epilepsy is emphasized, on the basis of one-way callosal traffic theory. From this vantage point, the following were deemed responsible for the currently divergent views on the lateralizing significance of various signs and symptoms in epilepsy: (1) the dichotomous nature of laterality of motor control, which is the same as that of seizure onset; (2) the probabilistic nature of the availability of the callosal channel for transfer of epileptiform discharges from the major to the minor hemisphere (i.e., the random variability of synaptic transfer); (3) the dynamic (varying) expanse of the epileptic region within the major hemisphere. Other data reviewed indicated that measuring the reaction time of two symmetrically located effectors is the most robust way of determining the laterality of the major hemisphere, with the side of shorter reaction time being opposite to the major hemisphere. Clinical presentations of seizures reflect the probabilistic involvement of different regions of the major hemisphere by the epileptic process and the spread of the epilepsy to the minor hemisphere via the callosum. Termination of seizure activity with diazepam was associated with simultaneous recovery of awareness and speech in this case.

Value of non-invasive testing when there are independent bitemporal seizures in the scalp EEG

We investigated the value of non-invasive data for predicting the outcome of intracranial EEG and anterior temporal lobectomy (ATL) (follow-up>1 year) in patients who have bitemporal independent seizures in the scalp EEG. No previous report has dealt with this patient group. Independent variables were duration of epilepsy, febrile seizures, interictal and ictal scalp EEG, ictal behavior, MRI, [18F]-fluorodeoxyglucose-PET (PET) and Wada test and dependent variables were surgical outcome (seizure free or not) and localized on intracranial EEG (finding all symptomatic seizures from one temporal lobe). Non-parametric statistics were used. Of 24 patients, 20 patients had IEEG, of which 12 were localized and 8 were not. Sixteen patients had ATL and, of these, 13 (81%) became seizure free and the remaining three improved. Lateralized findings on MRI and PET, a history of febrile convulsions and shorter duration of epilepsy were all associated with a focal onset on intracranial EEG, while there was a non-significant trend with ictal behavior. The non-invasive data did not predict surgical outcome. We conclude that some of these patients can do well with surgery. In most cases, intracranial EEG is necessary for localization of seizure focus, but if PET and MRI show focal abnormalities and there is a history of febrile convulsions no further evaluation could be needed. These findings need confirmation.

Ictal Brain Hyperperfusion Contralateral to Seizure Onset: The SPECT Mirror Image

PURPOSE

Ictal single-photon emission computed tomography (SPECT) may help localize the seizure-onset zone (SOZ) by detecting changes in regional cerebral blood flow induced by epileptic discharges. This imaging method also reveals hyperperfusions in areas of seizure propagation, including the hemisphere contralateral to the SOZ. We have studied the occurrence, the topography, and the clinical value of such contralateral ictal hyperperfusion areas (HPAs).

METHODS

We examined data from presurgical evaluations of 36 consecutive patients with pharmacoresistant partial epilepsy of various localizations. Ictal and interictal SPECT examinations were made with 99mTc-ECD, and the scans were processed for coregistration, normalization, subtraction, and merging with MRI images.

RESULTS

Contralateral HPAs were observed in 72% of the patients: 50% of mesiotemporal epilepsy cases with hippocampal sclerosis, 85.7% of the other mesiotemporal epilepsies, 85.7% of neocortical lateral temporal epilepsies, and 87.5% of extratemporal epilepsies. Contralateral HPAs were usually symmetrical to the SOZ, forming a mirror image, observed in 57.1% of the patients. They could be slightly asymmetrical in mesiotemporal epilepsies, perhaps because of the particular anatomic pathways linking temporal lobes. In neocortical epilepsies, they were located in the cortex homotopic to the SOZ.

CONCLUSIONS

We show that the symmetrical nature of the mirror image usually does not disturb SPECT interpretation. It can confirm the location of the SOZ (11 patients) and even occasionally improve the precision of its definition (nine patients) by restraining several potential SOZ-related HPAs to a single one or by permitting a restricted localization of the SOZ in a large HPA.

The hypothalamic hamartoma: a model of subcortical epileptogenesis and encephalopathy

Although uncommon, the hypothalamic hamartoma (HH) is often associated with a devastating clinical syndrome, which may include refractory epilepsy, progressive cognitive decline, and deterioration in behavioral and psychiatric functioning. Contrary to conventional thinking which attributed seizure origin to cortical structures, the hamartoma itself has now been firmly established as the site of intrinsic epileptogenesis for the gelastic seizures (i.e., characterized by unusual mirth) peculiar to this disorder. It also appears that the HH contributes to a process of secondary epileptogenesis, with eventual cortical seizure onset of multiple types in some patients. Anticonvulsant medications are known to be poorly effective in this disorder. Treatment, including some innovative approaches to surgical resection, is now targeted directly at the HH itself, with impressive results. Younger patients, in particular, may avoid the deteriorating course described earlier. Access to tissue from larger numbers of patients at single or collaborating centers specializing in HH surgery will allow for research into the fundamental mechanisms producing this little understood disorder. Refractory epilepsy associated with HH is the premier human model for subcortical epilepsy and an excellent model for secondary epileptogenesis and epileptic encephalopathy.

Long-term outcome of nonsurgical candidates with medically refractory localization-related epilepsy

PURPOSE

Epilepsy surgery can result in complete seizure remission rates of upto 80% in patients with mesial temporal sclerosis and unilateral seizures. The seizure-free rate after surgery for patients with extratemporal nonlesional epilepsy has ranged between 30% and 40%. Some patients with medically refractory localization-related epilepsy cannot be offered surgical resection because of inadequate localization of the epileptogenic zone, documentation of bilateral ictal onsets, or functionally important areas of cortex that prohibit resection. The short-term rate of complete remission with medications in temporal lobe epilepsy is poor. Less is known about remission rates in patients who are not surgical candidates. In this study, we evaluated the outcome of medical treatment in patients with medically refractory partial epilepsy who were evaluated for possible epilepsy surgery but deemed to be inadequate surgical candidates.

METHODS

A retrospective chart review and telephone survey with a self-rating questionnaire were completed for all patients who underwent epilepsy surgery evaluation but were not ultimately offered surgical treatment at the University of Michigan from 1990 through 1998. We assessed changes in seizure frequency and type, imaging characteristics, ictal recordings, interim medication history, and subjective changes in quality of life.

RESULTS

Thirty-four subjects were available for follow-up study, at an average of >4 years after surgical evaluation. A significant reduction in seizure frequency was noted at the time of follow-up compared with that at the time of surgical evaluation. Of patients, 21% achieved seizure remission and remained seizure free for an average of 2.5 years. Four of the seven seizure-free patients attributed their remission to new antiepileptic drugs (AEDs). On a global self-rating item, 15 of 34, or 44%, felt more or much more satisfied with their lives, and 41% felt their quality of life was stable.

CONCLUSIONS

A surprisingly large number of patients we surveyed, with refractory partial epilepsy not eligible for surgical management, reported reduced seizure frequency at follow-up, and 21% were seizure free. Our findings suggest that the long-term prognosis in patients with refractory partial epilepsy who are not surgical candidates may be more positive than might be generally expected.

Extratemporal hypometabolism on FDG PET in temporal lobe epilepsy as a predictor of seizure outcome after temporal lobectomy

We investigated the relationship between the presence of extratemporal hypometabolism on fluorine-18 fluorodeoxyglucose positron emission tomography (FDG PET) and seizure outcome after temporal lobectomy in patients with medically intractable temporal lobe epilepsy (TLE). In 47 patients with intractable unilateral mesial TLE, regional metabolic changes on FDG PET images obtained during the 2 months preceding anterior temporal lobectomy were compared with postoperative seizure outcome. Postoperative seizure outcome was evaluated with a mean follow-up period of 6.1+/-0.6 years (range 5.2-7.2 years). Forty-two (89%) of the 47 patients achieved a good postoperative seizure outcome (Engel class I or II). All patients had hypometabolism in the temporal cortex ipsilateral to the epileptogenic region on FDG PET scans. Fourteen (78%) of the 18 patients with hypometabolism only in the ipsilateral temporal cortex were completely seizure free (Engel class Ia) after surgery. In contrast, five (45%) of the 11 patients with extratemporal cortical hypometabolism confined to the ipsilateral cerebral hemisphere and only four (22%) of the 18 patients with hypometabolism in the contralateral cerebral cortex were completely seizure free after surgery. The postoperative seizure-free rates were significantly different across the three groups of patients with different cortical metabolic patterns ( P<0.005). Furthermore, all of the nine patients with a non-class I outcome (Engel class II-IV) had extratemporal (including contralateral temporal) cortical hypometabolism. Thalamic hypometabolism was noted in 20 (43%) of the 47 patients (ipsilateral in 12, bilateral in 8). Sixteen (59%) of the 27 patients with normal thalamic metabolism were completely seizure free after surgery, while only seven (35%) of the 20 patients with thalamic hypometabolism became completely seizure free ( P<0.05). Multivariate analysis revealed that among variables including clinical, EEG, magnetic resonance imaging, pathological and FDG PET metabolic findings, only cortical metabolic pattern was an independent factor for the prediction of postoperative seizure outcome ( P<0.005). It is concluded that extratemporal cortical hypometabolism outside the seizure focus, in particular hypometabolism in the contralateral cerebral cortex, may be associated with a poorer postoperative seizure outcome in TLE and may represent underlying pathology that is potentially epileptogenic. Thalamic hypometabolism, which was associated, but not independently, with a higher likelihood of postoperative seizures, may be secondary to extratemporal or temporal pathology.

Relationship between EEG and positron emission tomography abnormalities in clinical epilepsy

Positron emission tomography (PET) is a relatively noninvasive neuroimaging method by means of which a large variety of human brain functions can be assessed. Localized neurochemical abnormalities detected by PET were found in patients with partial epilepsy and suggested the use of this modality for localizing epileptogenic regions of the brain. The clinical usefulness of PET is determined by its sensitivity and specificity for identifying epileptogenic areas as defined by ictal surface and intracranial EEG recordings. The findings obtained from comparative EEG and glucose PET data are reviewed with special emphasis on patients undergoing presurgical evaluation because of medically intractable temporal and extratemporal lobe epilepsy. The utility of glucose PET studies for identifying regions of seizure onset is presented, and the limited specificity of glucose metabolic abnormalities for the detection of various EEG patterns in clinical epilepsy is discussed. The authors review the available intracranial EEG and PET comparisons using [11C]flumazenil (FMZ) PET, a tracer for the assessment of tau-amino-butyric acid/benzodiazepine receptor function. They also summarize their experience with [11C]flumazenil PET in identifying cortical regions that show various ictal and interictal cortical EEG abnormalities in patients with extratemporal seizure origin. Finally, the authors demonstrate that further development of new PET tracers, such as alpha-[11C]methyl-L-tryptophan, is feasible and clinically useful and may increase the number of patients in whom PET studies can replace invasive EEG monitoring.

Hypothetical mechanisms for the cellular and neurophysiologic basis of secondary epileptogenesis: proposed role of synaptic reorganization

This review article evaluates the hypothetical cellular mechanisms responsible for chronic lesion-induced epilepsy. Emphasis is given to particular clinical characteristics of secondary epileptogenesis: (a) a temporal latency, (b) the involvement of distant but related sites, and (c) irreversibility. Although loss of GABAergic inhibitory interneurons or increased excitatory input to these interneurons may contribute to epileptogenesis, several studies have provided evidence that inhibition is not depressed in epileptogenic regions and may actually be enhanced. Axonal sprouting, synaptic reorganization, and formation of new recurrent excitatory circuits have been proposed to account for the increased seizure susceptibility of temporal lobe epilepsy. Recent data support the hypothesis that local inhibitory circuits mask the multisynaptic excitatory interactions that are associated with mossy fiber sprouting in the dentate gyrus and that physiological mechanisms that reduce inhibition or increase excitability unmask the new recurrent excitatory circuits responsible for seizures. A hypothesis based on axonal sprouting and synaptic reorganization can account for the essential clinical characteristics of secondary epileptogenesis and may have widespread applicability to the general phenomenon of lesion-induced epilepsy.