Epileptogenesis

Epileptogenesis is a process by which a normal brain develops epilepsy, a chronic condition in which seizures occur. The process, which is gradual, occurs in symptomatic epilepsy, in which seizures are caused by an identifiable lesion in the brain. It results from acute brain insults such as traumatic brain injury (physical trauma to the brain), stroke, or infection. Epileptogenesis is a series of events that occur between the event that causes epilepsy and the first spontaneous seizure. In epileptogenesis, an array of events occur on molecular and cellular levels that cause neurons to fire in a disordered manner, resulting in seizures.

Causes

A variety of conditions can result in epileptogenesis, including neurodegenerative diseases, traumatic brain injury, stroke, brain tumor, infections of the central nervous system, and status epilepticus (a prolonged seizure or a series of seizures occurring in quick succession).

Latent period

After a brain injury occurs, there is frequently a "silent" or "latent period" lasting months or years in which seizures do not occur; Canadian neurosurgeon Wilder Penfield called this time between injury and seizure "a silent period of strange ripening". During this latent period, changes occur in the structure and physiology of the brain that result in the development of epilepsy. It is this process in which hyperexciatble neural networks form that is referred to as epileptogenesis, and it is during this latent period that symptoms of epilepsy first occur. If researchers come to better understand epileptogenesis, the latent period may provide a chance for healthcare providers to interfere with the development of epilepsy or reduce its severity.

Pathophysiology

Changes that occur during epileptogenesis are poorly understood but are thought to include cell death, axonal sprouting, reorganization of neural networks, alterations in the release of neurotransmitters, neurogenesis. These changes cause neurons to become hyperexcitable and can lead to spontaneous seizures.

Brain regions that are highly sensitive to insults and can cause epileptogenesis include temporal lobe structures such as the hippocampus, the amygdala, and the piriform cortex.

Neural reorganization

In addition to chemical processes, the physical structure of neurons in the brain may be altered. In acquired epilepsy in both humans and animal models, pyramidal neurons are lost, and new synapses are formed.

Hyperexcitability, a characteristic feature of epileptogenesis in which the likelihood that neural networks will be activated is increased, may be due to loss of inhibitory neurons that would normally balance out the excitability of other neurons, such as GABAergic interneurons. Neuronal circuits that are epileptic are known for being hyperexcitable and for lacking the normal balance of glutamatergic neurons (those that usually increase excitation) and GABAergic ones (those that decrease it). In addition, the levels of GABA and the sensitivity of GABA_A receptors to the neurotransmitter may decrease, resulting in less inhibition.

Another proposed mechanism for epileptogenesis in TBI is that damage to white matter causes hyperexcitability by effectively undercutting the cerebral cortex.

Receptor activation

It is believed that activation of biochemical receptors on the surfaces of neurons is involved in epileptogenesis; these include the TrkB neurotrophin receptor and both ionotropic glutamate receptors and metabotropic glutamate receptors (those that are directly linked to an ion channel and those that are not, respectively). Each of these types of receptor may, when activated, cause an increase in the concentration of calcium ions (Ca²⁺) within the area of the cell on which the receptors are located, and this Ca²⁺ can activate enzymes such as Src and Fyn that may lead to epileptogenesis.

Glutamate

Excessive release of the neurotransmitter glutamate is widely recognized as an important part of epileptogenesis early after a brain injury, including in humans. Excessive release of glutamate results in excitotoxicity, in which neurons are excessively depolarized, intracellular Ca²⁺ concentrations increase sharply, and cellular damage or death results. Excessive glutamatergic activity is also a feature of neuronal circuts after epilepsy has developed, but glutamate does not appear to play an important role in epileptogenesis during the latent period. Another factor in hyperexcitability may include a decrease in the concentration of Ca²⁺ outside cells (i.e. in the extracellular space) and a decrease in the activity of ATPase in glial cells.

Blood

Blood that spills into brain tissue (as may occur in traumatic brain injury or stroke) may play a role in the damage that results in epilepsy, perhaps by depositing hemosiderin or iron into the tissue. Iron from hemoglobin, a molecule in red blood cells, can lead to the formation of free radicals that damage cell membranes; this process has been linked to epileptogenesis.

Research directions

Epileptogenesis that occurs in human brains has been modeled in a variety of animal models and cell culture models. Epileptogenesis is poorly understood, and increasing understanding of the process may aid researchers in preventing seizures, diagnosing epilepsy, and developing treatments to prevent it.

History

Throughout most of history for which written records exist on the subject, it was probably generally believed that epilepsy came about through a supernatural process. Even within the medical profession, it was not until the 18th century that ideas of epileptogenesis as a supernatural phenomenon were abandoned. However, biological explanations have also long existed, and sometimes explanations contained both biological and supernatural elements.

See also

• Kindling model
• Post-traumatic epilepsy
• Post-traumatic seizure

References