Localization
Elementary somatosensory aura is localized to the contralateral primary sensory cortex and is located in the anterior part of the parietal lobe, which constitutes the postcentral gyrus. Somatosensory auras, which are more widespread or bilateral, are localized to the supplementary sensorimotor area, which is located anteriorly to the primary sensory cortex in the precentral gyrus. The secondary somatosensory area is another localization for widespread somatosensory aura and is located in the superior bank of the Sylvian fissure or the posterior insula. The presence of somatosensory aura in presumed temporal lobe epilepsy raises the question of potential extratemporal seizure onset, which compromises the chances of seizure freedom following standard anterior temporal resection (27).
Elementary visual aura arises from the contralateral primary visual cortex (V1), in close proximity above and below the calcarine fissures. The superior portion of the calcarine fissure corresponds to the inferior visual field, whereas the inferior portion corresponds to the superior field. The primary function of the visual cortex is to process visual information, and it is stratified into six layers: V1 to V5 and the inferior temporal cortex. V1 is the first stop for the visual inputs and, as the visual information is passed along subsequent layers of visual cortex, becomes more specialized. V1 is constituted by simple cells and responds to specific visual cues, such as orientation of edges and lines in a single receptive visual field. V1 through V3 have more complex cells, which respond to edges and orientations of summation of several receptive fields along with movement in specific directions (16). More complex visual hallucinations are localizable to visual association areas and their projections along the dorsal and ventral streams of the visual recognition pathway.
Elementary auditory auras arise from Heschl's gyrus in the superior temporal lobe, which is the primary auditory area involved in auditory processing. Leftward asymmetries in volume of Heschl's gyrus, which is a normal variation in the human adult brain, are due to greater myelination attributed to its specialization for handling fast temporal information (24). On the other hand, the dense, highly interconnected columnar structure in the right auditory cortex makes it effective in fine frequency distinctions (31), thus, the left Heschl's gyrus processes rapidly changing auditory stimuli and the right cortex processes spectrally complex stimuli (40). Also, the processing complexity increases in the superficial layers of Heschl's gyrus (22). Diffusion tensor imaging studies have identified five tracts that pass through Heschl's gyrus fiber intersection area, including the anterior segment of the arcuate fasciculus, middle longitudinal fasciculus, acoustic radiation, inferior fronto-occipital fasciculus, and optic radiation, which are important in higher-order auditory processing, auditory visual association, and auditory learning (12). The affection of some of these fibers results in complex auditory perceptual alterations as seen in frontal opercular epilepsies (36).
Elementary olfactory auras are localized to the anterior olfactory nucleus, piriform cortex, olfactory tubercle, or lateral entorhinal cortex, which are the components of the primary olfactory cortex. Three olfactory areas have been identified in the human brain. The medial olfactory area is the primitive olfactory system located in the midbasal portions of the frontal lobe anterior to the hypothalamus. It connects to the hypothalamus and limbic system and is concerned with basic olfactory reflexes. The lateral olfactory area consists of the prepyriform and pyriform cortex and amygdala and is important in liking healthy foods and disliking unhealthy foods. The newer olfactory area, which is concerned with conscious perception and analysis of olfaction, is connected to the orbitofrontal cortex via the thalamus. The first and second olfactory areas are concerned with elementary olfactory, and the third area with complex auras (34).
Gustatory auras are less well studied and mostly arise from insulo-opercular regions.
Etiology and pathogenesis
The International League Against Epilepsy classified the etiology of seizures and epilepsy syndromes into six categories (13). Etiologies for focal seizures with preserved awareness under these categories include:
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(i) Structural causes like stroke, trauma, infection, tumor, or focal cortical dysplasia. |
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(ii) Genetic causes like mutations in RELN1 or LGI1 genes and often dysregulation in the mechanistic target of rapamycin complex 1 (mTORC1) pathway are associated with autosomal dominant lateral temporal epilepsy. |
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(iii) Infectious etiologies include tuberculosis, neurocysticercosis, and cytomegalovirus. |
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(iv) Metabolic causes for focal epilepsies commonly include mitochondrial encephalopathy with lactic acidosis and stroke-like episodes, peroxisomal disorders, and pyridoxine-dependent epilepsy. |
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(v) Immune-mediated focal epilepsy syndromes include Rasmussen encephalitis, antineuronal nuclear antibody type 1 (anti-Hu) encephalitis, anti-N methyl-D-aspartate (anti-NMDA) receptor encephalitis, anti-leucine-rich glioma inactivated 1 (anti-LG1) encephalitis, contactin-associated protein-like 2 (CASPR2) encephalitis, and glutamic acid decarboxylase 65-antibody (GAD65) encephalitis. |
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(vi) The etiology is unknown in a third of focal epilepsies, as seen in self-limited focal epilepsies of childhood. |
Pathophysiology
Epileptogenesis results from the transformation of a normal neuronal network into a chronically hyperexcitable one is a dynamic process that establishes critical interconnections and results in structural changes like neurogenesis, gliosis, axonal damage, sprouting, dendritic plasticity, blood-brain barrier damage, recruitment of inflammatory cells into brain tissue, reorganization of the extracellular matrix, and reorganization of the molecular architecture of individual neuronal cells. Excitatory potentials result in ictogenesis by the generation of paroxysmal depolarization shifts in individual neurons, synchronization of epileptiform discharges, and setting off slow depolarization and burst discharges. The major neurotransmitter involved is glutamate.