Could adult hippocampal neurogenesis be relevant for human behavior?

Although the function of adult neurogenesis is still unclear, tools for directly studying the behavioral role of new hippocampal neurons now exist in rodents. Since similar studies are impossible to do in humans, it is important to assess whether the role of new neurons in rodents is likely to be similar to that in humans. One feature of adult neurogenesis that varies tremendously across species is the number of neurons that are generated, so a key question is whether there are enough neurons generated in humans to impact function. In this review we examine neuroanatomy and circuit function in the hippocampus to ask how many granule neurons are needed to impact hippocampal function and then discuss what is known about numbers of new neurons produced in adult rats and humans. We conclude that relatively small numbers of neurons could affect hippocampal circuits and that the magnitude of adult neurogenesis in adult rats and humans is probably larger than generally believed.     

When should a resection sparing mesial structures be considered for temporal lobe epilepsy?

Anteromesial temporal lobectomy (AMTL) is an effective and safe treatment for refractory temporal lobe epilepsy (TLE) caused by hippocampal sclerosis (HS). It is possible that modifications to this procedure could offer improved seizure control or a reduction in functional consequences in some patients. Reviewed here is the issue of when it might be appropriate to perform a resection for TLE that spares the mesial structures, particularly the hippocampus and parahippocampal gyrus. This issue is particularly important for dominant hemipshere TLE and for patients without obvious HS, as these are the patients at greatest risk for verbal memory decline following AMTL. Current evidence suggests that mesial structure-sparing resections may be worth consideration for two types of patients: those with temporal lobe foreign tissue lesions outside the mesial structures, and those with temporal lobe hypometabolism on fluorodeoxyglucose positron emission tomography but a normal MRI. Patients with dual pathology (i.e., HS plus another epileptogenic lesion) are unlikely to benefit from a resection that spares the mesial temporal lobe. There is little evidence to state whether resections of this kind are worthwhile for cryptogenic TLE, or for mesial TLE with preserved memory function. There is a clear need to move beyond the field's present focus on the hippocampus and investigate new approaches to TLE that may minimize the risks of functional consequences in patients without HS.     

Hypertensive encephalopathy: antecedent to hippocampal sclerosis and temporal lobe epilepsy?

The authors describe three patients with refractory temporal lobe epilepsy (TLE) following an episode of hypertensive encephalopathy as their only identified antecedent event. All patients had typical MR features of hippocampal sclerosis (HS), and the two operated cases had typical HS histology and became seizure-free postoperatively. These cases suggest that hypertensive encephalopathy may be a rare form of initial precipitating injury, leading to TLE and HS.     

Is interictal EEG activity a biomarker for mood disorders in temporal lobe epilepsy?

Objective

Psychiatric comorbidities are frequent in temporal lobe epilepsy (TLE), and symptoms of these comorbidities may be related to epilepsy activity. Here we evaluated interictal EEG activity in TLE patients with or without psychiatric comorbidities.

Methods

A cohort study of 78 patients with TLE, with evaluation of wake/sleep interictal scalp EEG. All subjects were submitted to a psychiatric structured clinical interview (SCID) for the diagnosis of lifetime psychiatric comorbidities. Three major diagnostic categories were studied: mood disorders, anxiety disorders, and psychosis. We then evaluated differences in interictal EEG activity between patients with and without these psychiatric comorbidities.

Results

Infrequent EEG interictal spikes, defined as less than one event per minute, were significantly associated with mood disorders in TLE (p = 0.02).

Conclusions

Low intensity seizure disorder has been associated with a decrease in interictal EEG discharges and with an increase in psychiatric symptoms in TLE, a phenomenon known as forced normalization. In our study, we observed a low interictal spike frequency on EEG in TLE patients with mood disorders.

Significance

A low spike index might be a neurophysiological marker for depression in temporal lobe epilepsy.     

Do sphenoidal electrodes aid in surgical decision making in drug resistant temporal lobe epilepsy?

ObjectiveThe utility of sphenoidal electrodes (SPh) in analyzing interictal epileptiform discharges (IEDs) and ictal electrography remains controversial, despite its widespread use.MethodsOne hundred and twenty-two consecutive patients with presumed temporal lobe epilepsy (TLE) who underwent presurgical evaluation were prospectively studied. SPh and Silverman’s electrodes were placed, in addition to routine electrodes in 10–20 international system. IEDs and ictal electroencephalography (EEG) were analyzed separately in bipolar and referential montages. The proportion of patients selected for surgery after adjusting for SPh placement based on the earlier ictal onset and IEDs were analyzed.ResultsOf the 8701 IEDs in SPh, only 65% were seen over the scalp bipolar montage; 1392 (16%) IEDs were confined to SPh electrodes, and were not seen at scalp bipolar montage (p < 0.001). Spike amplitudes were highest at SPh (p < 0.001). Of the 592 seizures analyzed, 62 (61%) had simultaneous SPh and scalp onset, while in 26 (25%) SPh onset preceded the scalp.ConclusionsOut of the 35 patients with unilateral mesial temporal sclerosis (MTS) with additional neocortical changes and/or non-lateralized bitemporal IEDs and/or diffuse ictal onset (group 1), 27 were selected for surgery (77%). About 7% was selected for surgery in this group by SPh placement. Also, in patients with bilateral MTS (group 2), 25% (5/20) were chosen for anterior temporal lobectomy, SPh provided an additional benefit in 11% (p < 0.001). Patients with normal magnetic resonance imaging (group 3) and temporal plus epilepsy (group 4) had a lower surgical yield, only 12% and 9.5% could undergo surgery. They were denied surgical candidacy with SPh (p < 0.001).SignificanceOne-third of patients after SPh placement were selected for resective surgery obviating the need for invasive monitoring. The maximum yield was noted in unilateral MTS (associated with additional neocortical features or non-lateralized bilateral temporal interictal IEDs or diffuse ictal onset in scalp EEG) and in bilateral MTS. Those with normal MRI/temporal plus epilepsy could be excluded from direct resective surgery.     

Long-term forgetting in temporal lobe epilepsy: Is this phenomenon a norm?

ObjectivesThis research investigated forgetting rates of patients with temporal lobe epilepsy (TLE) at brief and longer intervals.MethodsThe sample is formed by 5 patients with TLE and 10 healthy individuals. One of the patients received the diagnosis of transient epileptic amnesia (TEA). All patients underwent a standardized clinical protocol for diagnosis including a comprehensive neuropsychological assessment. In addition, two experimental tasks were used to assess the forgetting rates at 4 intervals (30 s, 10 min, 1 day, and 1 week): a story task to evaluate verbal cued recall and a route task to assess visuospatial cued recall.ResultsThere were no significant differences between groups in forgetting rates.ConclusionsThese findings suggest that forgetting patterns in patients with TLE may be heterogeneous, and the presence of accelerated long-term forgetting is not universal.     

Race/ethnicity: a predictor of temporal lobe epilepsy surgery outcome?

PURPOSE: The success of epilepsy surgery in temporal lobe epilepsy reaches a 64% rate of seizure freedom, based on a randomized control trial. Observational studies from epilepsy centers worldwide indicate seizure freedom rates up to 93% when the etiology is unilateral hippocampal sclerosis. Several risk factors are attributed to the recurrence of seizures following the surgical procedure. Nonetheless, whether race influences the outcome of temporal lobe surgery is unknown. The purpose of this study was to evaluate if race plays a role in outcome following surgery. METHODS: Data were obtained from the discharge database of the University of Alabama at Birmingham video/EEG monitoring unit, between 1998 and 2003, as well as the clinical charts. Seizure recurrence was evaluated 1 year following surgery. The sample consisted of all patients with a primary diagnosis of mesial temporal sclerosis (MTS) who underwent anterior temporal lobectomy. Multiple logistic regression analysis was used to model the presence of seizure recurrence after anterior temporal lobectomy for MTS. Two sets of logistic regression models were estimated to generate odds ratios (ORs) for seizure recurrence after an anterior temporal lobectomy for African-Americans or other possible ethnic/racial group present relative to non-Hispanic Caucasians. The first model incorporated only ethnicity as the independent variable and generated unadjusted ORs for seizure recurrence following the surgical procedure. The second set included the independent variables: duration of epilepsy, history of febrile seizures, lateralization of epileptogenic focus, handedness, and age. RESULTS: Seventy patients underwent surgical treatment and all of them had pathologic confirmation of MTS. Follow-up information for six was not available. Analysis of the remaining 64 patients revealed that African-Americans were more likely than non-Hispanic Caucasians to have seizure recurrence after surgery (OR=2.1, 95% CI=0.6-8.0). After potential confounders (duration of epilepsy, history of febrile seizures, lateralization of epileptogenic focus, handedness, and age) were controlled, this finding did not change (OR=1.7, 95% CI=0.3-10.7). CONCLUSION: Our data suggest that race may be an important factor related to seizure outcome following temporal lobectomy.     

Chronic encephalitis and temporal lobe epilepsy: a variant of Rasmussen's syndrome?

The authors report two adult patients with chronic temporal lobe epilepsy and pathologic features consistent with Rasmussen's encephalitis. Although seizures persisted after temporal lobe surgery no progressive cognitive or neurologic deficit has emerged. Prominent auditory auras in each suggested a persisting epileptogenic focus in the superior temporal gyrus. The current findings expand the clinical spectrum of Rasmussen's encephalitis and suggest that chronic nonprogressive encephalitis may serve as the pathologic substrate of medically intractable temporal lobe epilepsy.     

Subtypes of medial temporal lobe epilepsy: Influence on temporal lobectomy outcomes?

Surgical resection of the hippocampus is the most successful treatment for medication-refractory medial temporal lobe epilepsy (MTLE) due to hippocampal sclerosis. Unfortunately, at least one of four operated patients continue to have disabling seizures after surgery, and there is no existing method to predict individual surgical outcome. Prior to surgery, patients who become seizure free appear identical to those who continue to have seizures after surgery. Interestingly, newly converging presurgical data from magnetic resonance imaging (MRI) and intracranial electroencephalography (EEG) suggest that the entorhinal and perirhinal cortices may play an important role in seizure generation. These areas are not consistently resected with surgery and it is possible that they continue to generate seizures after surgery in some patients. Therefore, subtypes of MTLE patients can be considered according to the degree of extrahippocampal damage and epileptogenicity of the medial temporal cortex. The identification of these subtypes has the potential to drastically improve surgical results via optimized presurgical planning. In this review, we discuss the current data that suggests neural network damage in MTLE, focusing on the medial temporal cortex. We explore how this evidence may be applied to presurgical planning and suggest approaches for future investigation.