Hippocampal sclerosis is the loss of neurons in the field CA1 of Ammon's horn in the hippocampus due to temporal lobe epilepsy. The typical magnetic resonance imaging (MRI) features of hippocampal sclerosis are unilateral volume loss and increased signal intensity on T2-weighted images. There is no sex or side preference as a small proportion of cases are bilateral. Macroscopically the hippocampus is firm and shrunken, sometimes with visible collapse of the CA1 field. In hippocampal sclerosis, microscopic findings include a characteristic pattern of neuronal loss and reactive gliosis which varies in severity from case to case.
The mechanism of neuronal loss in hippocampal sclerosis is likely to be excessive release of the excitatory neurotransmitters glutamate and aspartate, acting at calcium-permeable NMDA and AMPA receptors. This leads to intense depolarization and calcium overload which triggers multiple cell death pathways. In addition to neuronal loss and gliosis, two common associated findings are mossy fiber sprouting and granule cell dispersion.
A- Normal hippocampus. B- hippocampus with sclerosis and a collapse in field CA1 (arrows)
From research conducted by Dr. Paul Johns, from National Hospital for Neurology and Neurosurgery, Queen Square; and Dr. Maria Thom, a Senior Lecturer at the Institute of Neurology, Queen Square.