Typical absence seizures and related epileptic syndromes: assessment of current state and directions for future research

The current state of absence epilepsy: can we have your attention?

Absence seizures are common within many different epilepsies and span all the ages. Even though absence seizures were described more than three centuries ago advances associated with its classification, pathophysiology, genetics, treatment, prognosis, and associated co-morbidities continue to be made.

Persistence of cortical sensory processing during absence seizures in human and an animal model: evidence from EEG and intracellular recordings

Absence seizures are caused by brief periods of abnormal synchronized oscillations in the thalamocortical loops, resulting in widespread spike-and-wave discharges (SWDs) in the electroencephalogram (EEG). SWDs are concomitant with a complete or partial impairment of consciousness, notably expressed by an interruption of ongoing behaviour together with a lack of conscious perception of external stimuli. It is largely considered that the paroxysmal synchronizations during the epileptic episode transiently render the thalamocortical system incapable of transmitting primary sensory information to the cortex. Here, we examined in young patients and in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well-established genetic model of absence epilepsy, how sensory inputs are processed in the related cortical areas during SWDs. In epileptic patients, visual event-related potentials (ERPs) were still present in the occipital EEG when the stimuli were delivered during seizures, with a significant increase in amplitude compared to interictal periods and a decrease in latency compared to that measured from non-epileptic subjects. Using simultaneous in vivo EEG and intracellular recordings from the primary somatosensory cortex of GAERS and non-epileptic rats, we found that ERPs and firing responses of related pyramidal neurons to whisker deflection were not significantly modified during SWDs. However, the intracellular subthreshold synaptic responses in somatosensory cortical neurons during seizures had larger amplitude compared to quiescent situations. These convergent findings from human patients and a rodent genetic model show the persistence of cortical responses to sensory stimulations during SWDs, indicating that the brain can still process external stimuli during absence seizures. They also demonstrate that the disruption of conscious perception during absences is not due to an obliteration of information transfer in the thalamocortical system. The possible mechanisms rendering the cortical operation ineffective for conscious perception are discussed, but their definite elucidation will require further investigations.

Vision-based absence seizure detection

In order to diagnose epilepsy, neurologists rely on their experience, performing an equal assessment of the electroencephalogram and the clinical image. Since misdiagnosis reaches a rate of 30% and more than one-third of all epilepsies are poorly understood, a need for leveraging diagnostic precision is obvious. With the aim at enhancing the clinical image assessment procedure, this paper evaluates the suitability of certain facial expression features for detecting and quantifying absence seizures. These features are extracted by means of time-varying signal analysis from signals that are gained by applying computer vision techniques, such as face detection, dense optical flow computation and averaging background subtraction. For the evaluation, video sequences of four patients with absence seizures are used. The classification performance of a C4.5 decision tree shows accuracies of up to 99.96% with a worst percentage of incorrectly classified instances of 0.14%.

Absence seizures in the first 3 years of life: an electroclinical study of 46 cases

PURPOSE

Early onset absence seizures have been considered a rare heterogeneous group with a poor prognosis. Only few patients may be categorized into well-known syndromes. We have evaluated electroclinical features, evolution, and the nosologic boundaries of early onset absence seizures.

METHODS

Forty-six neurologically normal patients with absence seizures associated with bilateral, synchronic, or asynchronic, and symmetric or asymmetric spike-and-wave paroxysms with onset in the first 3 years of life were included. Patients with abnormal neurologic examination and brain imaging were excluded from the study.

KEY FINDINGS

In our study, 39 patients met the clinical and electroencephalography (EEG) criteria of well-defined epileptic syndromes. Childhood absence epilepsy was found in 11 patients, benign myoclonic epilepsy in infancy in 18 patients, eyelid myoclonic epilepsy in 4, and epilepsy with myoclonic absences in 6. We did not find clinical and EEG criteria of well-recognized epileptic syndromes in seven children. Nine of 11 patients with simple absence seizures became seizure free. All these patients had normal neurologic and neuropsychological evaluations. Of the 35 patients who had absence seizures associated with myoclonic seizures, 20 became seizure free. Fifteen of 35 children continue having seizures. At the last visit, 20 of these 35 patients had normal neurologic and neuropsychological evaluations, 11 presented with mild mental retardation, and 4 with severe mental retardation.

SIGNIFICANCE

Epilepsies with absence seizures of early onset are relatively uncommon. Most of the patients had well-defined epileptic syndromes with a variable evolution. The evolution depended on the epileptic syndromes.

Absence seizures: individual patterns revealed by EEG-fMRI

PURPOSE

Absences are characterized by an abrupt onset and end of generalized 3-4 Hz spike and wave discharges (GSWs), accompanied by unresponsiveness. Although previous electroencephalography-functional magnetic resonance imaging (EEG-fMRI) studies showed that thalamus, default mode areas, and caudate nuclei are involved in absence seizures, the contribution of these regions throughout the ictal evolution of absences remains unclear. Furthermore, animal models provide evidence that absences are initiated by a cortical focus with a secondary involvement of the thalamus. The aim of this study was to investigate dynamic changes during absences.

METHODS

Seventeen absences from nine patients with absence epilepsy and classical pattern of 3-4 Hz GSWs during EEG-fMRI recording were included in the study. The absences were studied in a sliding window analysis, providing a temporal sequence of blood oxygen-level dependent (BOLD) response maps.

RESULTS

Thalamic activation was found in 16 absences (94%), deactivation in default mode areas in 15 (88%), deactivation of the caudate nuclei in 10 (59%), and cortical activation in patient-specific areas in 10 (59%) of the absences. Cortical activations and deactivations in default mode areas and caudate nucleus occurred significantly earlier than thalamic responses.

DISCUSSION

Like a fingerprint, patient-specific BOLD signal changes were remarkably consistent in space and time across different absences of one patient but were quite different from patient to patient, despite having similar EEG pattern and clinical semiology. Early frontal activations could support the cortical focus theory, but with an addition: This early activation is patient specific.

Dysfunction of executive and related processes in childhood absence epilepsy

The nature and extent of the neuropsychological difficulties associated with childhood absence epilepsy (CAE) remain unclear. Because aberrant thalamocortical rhythms have been implicated in the pathogenesis of CAE, it was hypothesized that children with CAE would show greater difficulties in neuropsychological domains that are thought to be subserved by basal ganglia-thalamocortical circuits. Multivariate analysis of variance was used to compare the neuropsychological functioning of 16 children with CAE with that of 14 children with type 1 diabetes mellitus and 15 healthy children. The CAE group did not perform differently from the other groups on measures of intellectual functioning, memory, academic achievement, fine motor speed, or processing speed. In contrast, significant differences were found in problem solving, letter fluency, complex motor control, attention/behavioral inhibition, and psychosocial functioning. These results suggest that children with CAE show difficulties in neuropsychological functions thought to be subserved by the same regions implicated in the pathogenesis of the disorder.

Absence epilepsy and periventricular nodular heterotopia

We report a case of a girl who presented with typical absence seizures at age of 4.5 years. EEG showed absence seizures of sudden onset with 3 Hz spike-and-waves that also correlated with the clinical absences. The seizure semiology included subtle deviation of the eyes which prompted MRI investigation of the brain. This showed a periventricular nodular heterotopia in the mid to anterior horn of the right lateral ventricle. Although possibly coincidental, periventricular heterotopia are considered to be epileptogenic and this association has been reported once before. Migration disorders, such as in the periventricular heterotopia of our patient, may influence the formation and excitability of the striato-thalamo-cortical network involved in the generation of 3 Hz spike-waves.

Electroencephalographic differences between WAG/Rij and GAERS rat models of absence epilepsy

The inbred Wistar Albino Glaxo Rats from Rijswijk (WAG/Rij) and the Genetic Absence Epilepsy Rats from Strasbourg (GAERS) are well-validated genetic models of absence epilepsy. Although they share similar characteristics including the spike-and-wave discharges (SWDs) in the EEG, some differences have been reported between both strains. This study aimed a systematic and detailed comparison of the SWD patterns of both strains in terms of the intensity, frequency and waveform morphology of the discharges by using exactly the same measurement and analysis techniques. The number, cumulative total duration and mean duration of SWDs were significantly higher in GAERS compared to WAG/Rij, while the discharge frequency was higher in the WAG/Rij. Furthermore, SWDs spectra and average SWD waveforms indicated that a single cycle of the SWD contains more energy in faster components such as spike and late positive transient in the GAERS. Additionally, WAG/Rij exhibited a significantly higher power between 8 and 14 Hz during the pre-SWD period. These clear phenomenological differences in the EEGs of both animal models suggest that these variables may represent basic phenotypic features of SWDs that should be sought after in the future studies that explore the genetic and molecular basis of absence epilepsy.