The human brain’s hippocampus, a structure involved in memory, has a complex relationship with the neurological disorder epilepsy. This connection is a two-way street, as the hippocampus can be both a source of epileptic seizures and a victim of the damage they cause. Understanding this dynamic is necessary for diagnosing and managing certain types of epilepsy, because the health of the hippocampus influences the condition’s manifestation and treatment.
The Hippocampus: A Vital Brain Region
Deep within the brain’s temporal lobes lies the hippocampus, named from the Greek for “seahorse” due to its curved shape. This structure is a component of the limbic system, which is involved in emotion, learning, and memory. The hippocampus is a formation that includes the hippocampus proper, the dentate gyrus, and the subiculum.
The primary role of the hippocampus is in memory consolidation. It is associated with declarative memories, like facts and events, and the spatial memory used for navigation. It acts like the brain’s “flash drive,” holding and organizing new information before it is filed for long-term storage elsewhere. The hippocampus also works with other brain regions, such as the amygdala, to link memories with emotions.
Its location in the medial temporal lobe means it receives sensory information from other cortical areas. This input is processed through an internal circuit involving the entorhinal cortex, dentate gyrus, and the CA fields of the cornu ammonis. The hippocampus is also a highly excitable structure, a characteristic that aids learning but also makes it susceptible to the abnormal electrical activity of epilepsy.
Epilepsy: Understanding Seizure Disorders
Epilepsy is a neurological condition defined by a predisposition to generate seizures, which are sudden, uncontrolled electrical disturbances in the brain. A seizure is the event, while epilepsy is the disorder characterized by two or more unprovoked seizures. These “electrical storms” occur when neurons signal abnormally, disrupting the brain’s function and causing symptoms ranging from convulsions to momentary lapses in awareness.
Seizures are broadly categorized as either focal or generalized. Focal seizures originate in one specific brain area and can cause motor, sensory, or psychological symptoms, sometimes with impaired awareness. Generalized seizures appear to involve both sides of the brain from the outset and include absence seizures (staring spells) and tonic-clonic seizures (muscle stiffening and jerking).
The causes of epilepsy are varied, though often unknown. Known causes include:
- Genetic factors
- Brain injury from trauma or stroke
- Infections
- Developmental abnormalities
One of the most common types of focal epilepsy is temporal lobe epilepsy (TLE), which is particularly relevant to the hippocampus.
How Epilepsy Impacts the Hippocampus
The characteristics that make the hippocampus an effective memory processor also render it vulnerable to seizures. Recurrent or prolonged seizures, especially those in the temporal lobes, can damage this structure through a process called excitotoxicity. This occurs when intense electrical activity over-activates glutamate receptors, leading to neuron damage and death.
This seizure-induced damage can result in hippocampal sclerosis, or scarring of the hippocampus. It is the most common pathology found in people with drug-resistant temporal lobe epilepsy. Hippocampal sclerosis is characterized by significant neuronal loss and reactive gliosis (the proliferation of support cells), which causes the hippocampus to shrink and harden.
The damage is not just cell loss but a reorganization of the brain’s circuitry. Surviving neurons can form new, abnormal connections, creating faulty circuits that are more prone to generating seizures. This establishes a cycle where seizures cause hippocampal damage, and that damage makes future seizures more likely, contributing to memory problems.
The Hippocampus as an Epilepsy Origin
While seizures can damage the hippocampus, the relationship also works in reverse, as an abnormal hippocampus can be the origin of epilepsy. This is common in Mesial Temporal Lobe Epilepsy (MTLE), the most frequent form of focal epilepsy, where seizures start in the inner temporal lobe structures like the hippocampus.
The initial abnormality can stem from various causes, including:
- Congenital malformations during brain development
- Acquired damage from head trauma, infections like encephalitis, or lack of oxygen at birth
- Prolonged febrile seizures (seizures with high fever) in early childhood
These initial injuries can create a hyperexcitable focus within the hippocampal tissue. The damaged area, often characterized by hippocampal sclerosis, develops faulty wiring and an imbalance between excitatory and inhibitory signals. This unstable network can generate the electrical discharges that manifest as seizures, meaning the sclerosis is the cause, not the result, of the epilepsy.
Addressing Hippocampal Epilepsy
Diagnosing epilepsy involving the hippocampus requires identifying specific structural and electrical abnormalities. The primary diagnostic tool is Magnetic Resonance Imaging (MRI). A high-resolution MRI can detect signs of hippocampal sclerosis, such as shrinkage and abnormal signal changes, which helps confirm the diagnosis and pinpoint the seizure origin.
Another diagnostic test is the electroencephalogram (EEG), which records the brain’s electrical activity. A long-term video-EEG can capture a seizure and show the electrical discharge starting in the temporal lobe, confirming the focus. Neuropsychological testing is also performed to assess memory and other cognitive functions to help plan treatment and predict outcomes.
Treatment often begins with anti-seizure medications, but MTLE with hippocampal sclerosis is frequently drug-resistant. For these individuals, surgery to remove the affected tissue, including the hippocampus (anterior temporal lobectomy), can be highly effective. Newer, more targeted therapies like laser ablation or neurostimulation devices are also available as less invasive alternatives to traditional surgery.