In temporal lobe epilepsy, the hippocampus, amygdala, and other temporal lobe structures are resected. In the hippocampus, lesions present as hippocampal sclerosis (HS). HS is also called Ammon's horn sclerosis. CA1, CA3, and CA4 are prone to damage, which results in neuronal loss, gliosis, and associated pyramidal cell layer atrophy. Conversely, CA2 tends to be preserved relatively well in this condition, and the subiculum of the hippocampus is usually almost completely normal.
Varying degrees of neuronal (i.e., granule cells) loss are seen outside of the CA region, specifically in the dentate granule cell layer. However, even when there is less obvious loss, the width of the layer increases, which causes the neurons (i.e., granule cells) to loosen. This phenomenon is called dispersion, and it is considered a pathological change that occurs relatively early in HS. Dispersion progression can appear to separate the granule cell layer into two layers (i.e., duplication). Moreover, the granule cell layer can also present as waving undulations, which, when occurring with duplication, can be regarded as microdysplasia; however, no theory explaining this phenomenon has been established.