Seizure (cont.)
What Causes Epilepsy?
Epilepsy is a disorder with many possible causes. Anything that disturbs the
normal pattern of neuron activity -- from
illness to brain damage to abnormal
brain development -- can lead to seizures.
Epilepsy may develop because of an abnormality in brain wiring, an imbalance
of nerve signaling chemicals called neurotransmitters, or some combination of
these factors. Researchers believe that some people with epilepsy have an
abnormally high level of excitatory neurotransmitters that increase neuronal
activity, while others have an abnormally low level of inhibitory
neurotransmitters that decrease neuronal activity in the brain. Either situation
can result in too much neuronal activity and cause epilepsy. One of the
most-studied neurotransmitters that plays a role in epilepsy is GABA, or
gamma-aminobutyric acid, which is an inhibitory neurotransmitter. Research on
GABA has led to drugs that alter the amount of this neurotransmitter in the
brain or change how the brain responds to it. Researchers also are studying
excitatory neurotransmitters such as glutamate.
In some cases, the brain's attempts to repair itself
after a head injury,
stroke, or other problem may inadvertently generate abnormal nerve connections
that lead to epilepsy. Abnormalities in brain wiring that occur during brain
development also may disturb neuronal activity and lead to epilepsy.
Research has shown that the cell membrane that surrounds each neuron plays an
important role in epilepsy. Cell membranes are crucial for a neuron to generate
electrical impulses. For this reason, researchers are studying details of the
membrane structure, how molecules move in and out of membranes, and how the cell
nourishes and repairs the membrane. A disruption in any of these processes may
lead to epilepsy. Studies in animals have shown that, because the brain
continually adapts to changes in stimuli, a small change in neuronal activity,
if repeated, may eventually lead to full-blown epilepsy. Researchers are
investigating whether this phenomenon, called kindling, may also occur in
humans.
In some cases, epilepsy may result from changes in non-neuronal brain cells
called glia. These cells regulate concentrations of chemicals in the brain that
can affect neuronal signaling.
About half of all seizures have no known cause. However,
in other cases, the seizures are clearly linked to infection, trauma, or other identifiable
problems.
Genetic Factors
Research suggests that genetic abnormalities may be some
of the most important factors contributing to epilepsy. Some types of epilepsy
have been traced to an abnormality in a specific gene. Many other types of
epilepsy tend to run in families, which suggests that genes influence epilepsy. Some
researchers estimate that more than 500 genes could play a role in this
disorder. However, it is increasingly clear that, for many forms of epilepsy,
genetic abnormalities play only a partial role, perhaps by increasing a person's
susceptibility to seizures that are triggered by an environmental factor.
Several types of epilepsy have now been linked to
defective genes for ion channels, the "gates" that control the flow of ions in
and out of cells and regulate neuron signaling. Another gene, which is missing
in people with progressive myoclonus epilepsy, codes for a protein called cystatin B. This
protein regulates enzymes that break down other proteins. Another gene, which is
altered in a severe form of epilepsy called LaFora's disease, has been linked to
a gene that helps to break down carbohydrates.
While abnormal genes sometimes cause epilepsy, they also
may influence the disorder in subtler ways. For example, one study showed that
many people with epilepsy have an abnormally active version of a gene that
increases resistance to drugs. This may help explain why anticonvulsant drugs do not work for some
people. Genes also may control other aspects of the body's response to
medications and each person's susceptibility to seizures, or seizure threshold.
Abnormalities in the genes that control neuronal migration -- a critical step in
brain development -- can lead to areas of misplaced or abnormally formed
neurons, or dysplasia, in the brain that can cause epilepsy. In some cases,
genes may contribute to development of epilepsy even in people with no family
history of the disorder. These people may have a newly developed abnormality, or
mutation, in an epilepsy-related gene.
Other Disorders
In many cases, epilepsy develops as a result of brain damage from other
disorders. For example, brain tumors, alcoholism, and Alzheimer's disease
frequently lead to epilepsy because they alter the normal workings of the brain.
Strokes, heart attacks, and other conditions that deprive the brain of oxygen
also can cause epilepsy in some cases. About 32 percent of all cases of newly
developed epilepsy in elderly people appears to be due to cerebrovascular
disease, which reduces the supply of oxygen to brain cells.
Meningitis,
AIDS,
viral encephalitis, and other infectious diseases can lead to epilepsy, as can
hydrocephalus -- a
condition in which excess fluid builds up in the brain. Epilepsy also can result
from intolerance to wheat gluten (also known as
celiac
disease), or from a parasitic infection of the brain called neurocysticercosis.
Seizures may stop once these disorders are treated successfully. However, the
odds of becoming seizure-free after the primary disorder is treated are
uncertain and vary depending on the type of disorder, the brain region that is
affected, and how much brain damage occurred prior to treatment.
Epilepsy is associated with a variety of developmental
and metabolic
disorders, including cerebral palsy, neurofibromatosis, pyruvate dependency,
tuberous sclerosis, Landau-Kleffner syndrome, and autism. Epilepsy is just one
of a set of symptoms commonly found in people with these disorders.
Head Injury
In some cases, head injury can lead to seizures or epilepsy. Safety measures
such as wearing seat belts in cars and using helmets when riding a motorcycle or
playing competitive sports can protect people from epilepsy and other problems
that result from head injury.
Prenatal Injury and Developmental Problems
The developing brain is susceptible to many kinds of
injury. Maternal
infections, poor nutrition, and oxygen deficiencies are just some of the
conditions that may take a toll on the brain of a developing baby. These
conditions may lead to cerebral palsy, which often is associated with epilepsy,
or they may cause epilepsy that is unrelated to any other disorders. About 20
percent of seizures in children are due to cerebral palsy or other neurological
abnormalities. Abnormalities in genes that control development also may
contribute to epilepsy. Advanced brain imaging has revealed that some cases of
epilepsy that occur with no obvious cause may be associated with areas of
dysplasia in the brain that probably develop before birth.
Poisoning
Seizures can result from exposure to lead, carbon
monoxide, and many other poisons. They also can result from exposure to street drugs and from overdoses
of antidepressants and other medications.
Seizures are often triggered by factors such as lack of sleep, alcohol
consumption, stress, or hormonal changes associated with the menstrual cycle.
These seizure triggers do not cause epilepsy but can provoke first seizures or
cause breakthrough seizures in people who otherwise experience good seizure
control with their medication. Sleep deprivation in particular is a universal
and powerful trigger of seizures. For this reason, people with epilepsy should
make sure to get enough sleep and should try to stay on a regular sleep schedule
as much as possible. For some people, light flashing at a certain speed or the
flicker of a computer monitor can trigger a seizure; this problem is called
photosensitive epilepsy. Smoking cigarettes also can trigger seizures. The
nicotine in cigarettes acts on receptors for the excitatory neurotransmitter
acetylcholine in the brain, which increases neuronal firing. Seizures are not
triggered by sexual activity except in very rare instances.
Next: What Are the Different Kinds of Seizures? »
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