The cause of a seizure is determined in about 28% of partial epilepsy patients. In the rest, however, epilepsy is deemed idiopathic, which means that the cause is unknown. The age of seizure onset can sometimes offer a clue. Idiopathic epilepsy is rare in children and young adults.
Epileptic seizures are triggered by abnormalities in the brain that cause a group of nerve cells in the cerebral cortex to become activated simultaneously, emitting sudden and excessive bursts of electrical energy. A seizure's effect depends on the location in the brain where this electrical hyperactivity occurs. Effects range from brief moments of confusion to minor spasms to loss of consciousness.
Ion Channels. Sodium, potassium, and calcium act as ions in the brain. They produce electric charges that must fire regularly in order for a steady current to pass from one nerve cell in the brain to another. If the ion channels that carry them are genetically damaged, a chemical imbalance occurs. This can cause nerve signals to misfire, leading to seizures. Abnormalities in the ion channels are believed to be responsible for absence and many other generalized seizures.
Neurotransmitters. Abnormalities may occur in neurotransmitters, the chemicals that act as messengers between nerve cells. Three neurotransmitters are of particular interest:
Dozens of genetic syndromes representing a variety of seizure patterns may account for the different forms epilepsy.
A genetic cause has been identified for at least some cases of juvenile myoclonic epilepsy, which represents 10% of all epilepsy cases. (Such research and other studies have pointed to the GABA signaling system as an important player in many cases of epilepsy.)
Febrile seizures are caused by high fever and occur in 2 - 5% of children ages 6 months to 5 years. They usually have no long-lasting effect. There are two types of febrile seizures: simple and complex. Simple febrile seizures last for less than 15 minutes and occur once in a 24-hour period. Complex febrile seizures last longer than 15 minutes and occur more than once in 24 hours. Most children who experience simple febrile seizures have a low risk of developing epilepsy.
In young children, high fever from a vaccination can, in rare instances, trigger seizures. These seizures are almost always temporary and have no serious consequences.
Some controversy arose a few years ago over the possibility that the DTP (diphtheria-tetanus-pertussis) vaccine might trigger epilepsy or other neurologic diseases. Some research suggests that children who have neurologic events following their DTP shot already have a preexisting impairment such as epilepsy, which is revealed, but not caused by, the vaccine. Children with existing epilepsy may be at risk for seizures 2 or 3 days after the vaccination. Infants with suspected neurologic problems may have their vaccinations delayed until their neurologic situation is clarified, but not beyond their first birthday. Also, a newer version of the DTP vaccine reduces the risk of any seizure.
Brain Tumors. Both cancerous and noncancerous brain tumors can cause seizures in all patients.
Hydrocephalus and Shunts. Hydrocephalus occurs when cerebrospinal fluid (CSF) accumulates in the brain, leading to excessive swelling of the brain ventricles. The resulting pressure can damage the brain's tissue. Hydrocephalus itself is not commonly known to cause seizures, but its treatment, which involves insertion of a shunt, may cause them. The shunt is a device that drains the excess fluid from the brain. Up to half of children who receive shunts may experience epileptic seizures, particularly if the shunt is placed before 2 years of age. More research on its relationship to epileptic seizures is needed.
Focal Cortical Dysplasia. This is an abnormality in fetal development in which the normal migration of nerve cells is altered. It can cause very severe epilepsy that is difficult to treat.
Hippocampal Sclerosis. Hardened tissue (sclerosis) in the brain's hippocampus is the most commonly identified abnormality in patients with partial epilepsy. Such abnormal brain tissue leads to structural reorganization, and both the loss and regeneration of nerve cells.
Cavernous Angiomas. Cavernous angiomas are blood vessels that grow abnormally and, like a tumor, can put pressure on nerve tissue.
Other Causes of Seizures in Children. Seizures in infants and children may be due to birth defects, difficulties during delivery, or poisoning.
Alcohol Abuse. Alcohol abuse is one of the most common causes of adolescent- and adult-onset seizures. Seizures, nearly always generalized tonic-clonic, occur in about 10% of adults during withdrawal. Multiple seizures happen in about 60% of these patients. The first seizure occurs 7 hours to 2 days after the last drink, and the time between the first and last seizure is usually 6 hours or less. [For more information, see In-Depth Report #56: Alcoholism.]
Sudden withdrawal from certain antianxiety or antidepressant drugs such as benzodiazepines, barbiturates, and tricyclic antidepressants can also contribute to seizures.
Head Injuries in Adults. Head injuries to adults can cause seizures, with the risk highest in severe head trauma. A first seizure related to the injury can occur years later, but only very rarely. People with mild head injuries, which involve loss of consciousness for fewer than 30 minutes, have only a slight risk that lasts up to 5 years after the injury.
Head Injuries in Infants and Children. Infants are at high risk for head trauma, and the severity of injury may be difficult to determine. The risk of even one seizure is generally only a concern after severe head trauma. Most children who have had a minor or not very serious head injury do not need to have medications to prevent seizures, especially when an evaluation in the emergency department was unnecessary.
Stroke. Seizure is one possible symptom of a major stroke. Even injury to the brain from small strokes may cause seizures. Patients who have had a severe stroke are 5 times more likely to develop epilepsy than patients who have had a mild stroke.
Seizures in adults can also be caused by:
Between 20 - 45% of cases of untreatable seizures have a psychologic rather than physical origin. In this form of epilepsy, known as psychogenic seizures, the patient has no conscious intent of forcing a seizure and does not show unusual emotional behavior or signs of hysteria. It is very difficult to treat and can be very disabling. Psychogenic seizures can usually be distinguished from true epilepsy by using an electroencephalogram (EEG), which measures brain waves. The cause of psychogenic seizures is unknown.
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