Epilepsy is a disorder that causes a person to experience frequent seizures. The nervous system uses electrical impulses traveling across and between neurons to distribute messages within the brain, and between the brain, the muscles and the sensory organs. These electrical signals must generate in an orderly manner for the nervous system to work properly. Abnormal electrical activity in the brain leads to seizures.
Temporal Lobe Epilepsy
Epilepsy has several types. The types are differentiated by their point of origin within the brain and by the extent to which abnormal electrical activity takes over the brain during a seizure. In temporal lobe epilepsy, these disruptions originate in the temporal lobes of the brain and remain confined to nearby structures. Temporal lobe epilepsy causes "complex partial" seizures. Complex partial seizures are characterized by impaired consciousness and "automatisms," which are purposeless movements of the limbs. They do not cause a complete loss of consciousness or convulsions, however. While symptoms of the various types of epilepsy differ somewhat, causes of the condition pertain to all types.
Idiopathic Epilepsy
Most people who have a seizure never experience a second one. Isolated seizures that result from conditions such as a fever, drug withdrawal, oxygen deprivation or low blood sugar are not epilepsy. If a seizure remains a one-time occurrence for someone, that person does not have epilepsy. Sometimes the underlying irritant that triggers recurrent seizures cannot be found in the people who experience them. According to neurologist Dr. Michael Wong of Washington University in St. Louis, these seizures typically begin in childhood and are thought to have a genetic cause. When the available diagnostic tools cannot detect the causative abnormality, the patient is said to have idiopathic epilepsy.
Genetic Predisposition
Fever can cause seizures in some children. These children have a greater chance of developing temporal lobe epilepsy later in life than those who do not suffer from febrile seizures. Dr. J. Nakayama of the Ibaraki Prefectural University of Health Sciences in Japan states that genetic studies of these children have found mutations in the cell structures that regulate electrical conduction in neurons, causing them to malfunction. Epilepsy is one result of malfunctioning neurons.
Arteriovenous Malformations
Arteriovenous malformations result from improperly connected blood vessels. Normally, blood travels from the arteries to the capillaries and then to the veins. In an arteriovenous malformation, an artery connects directly to a vein with no capillaries between them. These malformations have been found in the brains of some epileptics. They do not directly cause epilepsy, but they do predispose a person to damage that can lead to the development of epilepsy. Blood-borne oxygen passes into the tissues through capillary walls, so one result of this condition may be a disrupted supply of oxygen to those tissues surrounding the malformation. Arteriovenous malformations are also prone to swelling, resulting in pressure on nearby tissues, and rupture, causing a hemorrhagic stroke.
Acquired Epilepsy
Traumatic injury, infection, oxygen deprivation, stroke and brain tumors can all cause damage that results in epilepsy. Among other things, these conditions can kill neurons. Neurons do not regenerate. Instead, a proliferation of astrocytes replace dead neurons. Astrocytes are cells that normally play a supporting role to neurons in the brain. When astrocytes proliferate after neuron death, they form a scar in a process called "gliosis." The idea that gliosis could provoke seizures dates back several decades. Now, Dr. F. Frohlich and his colleagues at the Salk Institute for Biological Studies have found abnormalities in the regulation of electrical activity in those areas where neurons contact glial scars.
References
- Merck Manuals: Brain, Spinal Cord and Nerve Disorders: Seizure Disorders
- Merck Manuals: Neurologic Disorders: Seizure Disorders
- Washington University in St. Louis School of Medicine Department of Neurology: Patient Care: Clinical Services: Pediatric Epilepsy Center: Patient/Family/Physician: Idiopathic Epilepsy Syndromes
- "Brain Development"; Progress in Searching for the Febrile Seizure Susceptibility Genes; J. Nakayama; May 2009
- "Neuroscientist"; Potassium Dynamics in the Epileptic Cortex: New Insights on an Old Topic; F. Frohlich, M. Bazhenov, V. Iragui-Madoz, T.J. Sejnowski; October 2008
