Mesial temporal lobe epilepsy
Epidemiology
Temporal lobe epilepsy (TLE) is considered to be the most common form of epilepsy, and it has been seen that most patients are refractory to antiepileptic drugs.
Drug resistant epilepsy is a major clinical challenge affecting about 30% of temporal lobe epilepsy (TLE) patients.
Mesial temporal lobe epilepsy with hippocampal sclerosis (mTLE-HS) is the most common type of focal epilepsy.
The incidence of temporal lobe epilepsy (TLE) due to mesial temporal sclerosis (MTS) can be high in developing countries. Current diagnosis of MTS relies on structural MRI, which is generally unavailable in developing world settings.
Classification
Mesial temporal lobe epilepsy.
Bilateral mesial temporal lobe epilepsy
Non-lesional mesial temporal lobe epilepsy
Etiology
see Temporal lobe epilepsy etiology.
Pathophysiology
In order to understand the pathophysiology of temporal lobe epilepsy (TLE), and thus to develop new pharmacological treatments, in vivo animal models that present features similar to those seen in TLE patients have been developed during the last four decades. Some of these models are based on the systemic administration of chemoconvulsants to induce an initial precipitating injury (status epilepticus) that is followed by the appearance of recurrent seizures originating from limbic structures.
Kainic acid and pilocarpine models, have been widely employed in basic epilepsy research. Their behavioral, electroencephalographic and neuropathologic features and response of these models to antiepileptic drugs and the impact they might have in developing new treatments are explained in the work of Lévesque et al. 1).
The transition to the ictal stage is accompanied by increasing global synchronization and a more ordered spectral content of the signals, indicated by lower spectral entropy. The interictal connectivity imbalance (lower ipsilateral connectivity) is sustained during the seizure, irrespective of any appreciable imbalance in the spectral entropy of the mesial recordings 2).
Clinical features
Mesial temporal lobe epilepsy clinical features
Diagnosis
neuro exam: normal except memory deficit
● MRI: hippocampal atrophy and signal alteration with ipsilateral dilatation of temporal horn of lateral ventricle
● unilateral or bilateral independent anterior temporal EEG spikes with maximal amplitude in basal electrodes
● external ictal EEG activity only with CPS, usually initial or delayed focal rhythmic onset pattern of 5–7 Hz, maximal in 1 basal temporal derivation
● interictal fluorodeoxyglucose PET scan: hypometabolism in temporal lobe and possibly ipsilateral thalamus and basal ganglia
● neuropsychological testing: memory dysfunction specific to involved temporal lobe
● Wada test : amnesia with contralateral amobarbital injection
A pilot study demonstrates that seizures in mesial temporal and temporal-plus epilepsies (i.e., temporoperisylvian) can be detected reliably in the anterior thalamic nucleus (ATN). Further studies are needed to validate these findings 3).
Fractional anisotropy asymmetry (FAA) values can be potentially used to identify the seizures of origin of TLE and to help understand the relationship between fiber tracts with the side of seizure origin of TLE 4).
The area of predominant perifocal 18F positron emission tomography hypometabolism and reduced [11C]flumazenil (11C-FMZ) -binding on PET scans is currently considered to contain the epileptogenic zone and corresponds anatomically to the area localizing epileptogenicity in patients with temporal lobe epilepsy (TLE).
Differential diagnosis
Mesial temporal lobe epilepsy differential diagnosis.
Treatment
Mesial temporal lobe epilepsy treatment.
Outcome
Mesial temporal lobe epilepsy outcome.
Randomized controlled trials
Three RCTs (two adult RCTs and one pediatric RCT) consistently supported the efficacy of resective surgery as treatment for epilepsy with semiology localized to the mesial temporal lobe. In these studies, 58-100% of the patients who underwent resective surgery achieved seizure freedom, in comparison to 0-13% of medically treated patients. In another RCT, the likelihood of seizure freedom after resective surgery was independent of the surgical approach (transSylvian [64%] versus subtemporal [62%]). Two other RCTs demonstrated that hippocampal resection is essential to optimize seizure control. But, no significant gain in seizure control was achieved beyond removing 2.5 cm of the hippocampus. Across RCTs, minor complications (deficit lasting < 3 months) and major complications (deficit > 3 months) ranged 2-5% and 5-11% respectively. However, non-incapacitating superior subquadrantic visual-field defects (not typically considered a minor or major complication) were noted in up to 55% of the surgical cohort. The available RCTs provide compelling support for resective surgery as a treatment for mesial temporal lobe epilepsy and offer insights toward optimal surgical strategy 5)
Case series
see Mesial temporal lobe epilepsy case series.