Hippocampal sclerosis is a disorder in which nerve cells die off and scar tissue is formed in an area of the brain known as the hippocampus. The condition is known to be associated with up to around three quarters of cases of temporal lobe epilepsy, a disease in which people experience unusual sensations, along with altered emotions and behavior, muscle spasms and sometimes convulsions. In spite of this strong association, it is not known whether hippocampal sclerosis causes temporal lobe epilepsy or temporal lobe epilepsy causes hippocampal sclerosis. It is possible that both conditions are linked to another underlying abnormality.
The hippocampus is located in an area of the brain called the medial temporal lobe, where it forms part of what is known as the limbic system. As part of the limbic system, which is concerned with the sense of smell, the sensation of fear and the formation of long-term memory, the hippocampus is involved in forming new memories. It is also thought to play a part in what is called spatial navigation, a process which depends on individuals being able to remember and compare their current and past positions in the environment in order to find their way around. While sclerosis in this area of the brain is most frequently linked to temporal lobe epilepsy, other structures in the limbic system, such as the amygdala, which controls the sensation of fear, are often involved as well.
Using magnetic resonance imaging (MRI) scans, the main change seen in the hippocampus when it is affected by hippocampal sclerosis is a decrease in size. When viewing samples of the hippocampus under a microscope, it is possible to see that individual nerve cells have been lost and that scarring has developed. It is thought that this damage may be caused by excessive amounts of some neurotransmitters, the chemicals that carry signals between nerves, being released. These particular neurotransmitters bind to special receptors on nerve cells, causing calcium to enter; in excess, calcium overload leads to cell death.
Researchers have investigated whether epileptic seizures might cause hippocampal sclerosis. It has been suggested that fits occurring in childhood, such as those associated with fever, might injure the brain, damaging the hippocampus and leading to temporal lobe epilepsy in later life. While some children who experience such fits are found to have hippocampal sclerosis later, most of them do not, and it is not possible to know if the sclerosis developed before the first seizure. Another theory is that defects in the temporal lobe may predispose individuals to both hippocampal sclerosis and seizures. Alternatively, a combination of hereditary, environmental and developmental factors could make some people more prone to both epilepsy and sclerosis in the hippocampus.