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What is Epilepsy?
Epilepsy is a brain disorder in which clusters of
nerve cells, or neurons, in the brain sometimes signal abnormally.
Neurons normally generate electrochemical impulses that act on other
neurons, glands, and muscles to produce human thoughts, feelings, and
actions. In epilepsy, the normal pattern of neuronal activity becomes
disturbed, causing strange sensations, emotions, and behavior, or
sometimes convulsions, muscle spasms, and loss of consciousness.
During a seizure, neurons may fire as many as 500 times a second, much
faster than the normal rate of about 80 times a second. In some people,
this happens only occasionally; for others, it may happen up to hundreds
of times a day.
More than 2 million people in the United States –
about 1 in 100 – have experienced an unprovoked seizure or been
diagnosed with epilepsy. For about 80 percent of those diagnosed with
epilepsy, seizures can be controlled with modern medicines and surgical
techniques. However, about 20 percent of people with epilepsy will
continue to experience seizures even with the best available treatment.
Doctors call this situation intractable epilepsy. Having a
seizure does not necessarily mean that a person has epilepsy. Only when
a person has had two or more seizures is he or she considered to have
epilepsy.
Epilepsy is not contagious and is not caused by mental
illness or mental retardation. Some people with mental retardation may
experience seizures, but seizures do not necessarily mean the person has
or will develop mental impairment. Many people with epilepsy have normal
or above-average intelligence. Famous people who are known or rumored to
have had epilepsy include the Russian writer Dostoyevsky, the
philosopher Socrates, the military general Napoleon, and the inventor of
dynamite, Alfred Nobel, who established the Nobel prize. Several Olympic
medalists and other athletes also have had epilepsy. Seizures sometimes
do cause brain damage, particularly if they are severe. However, most
seizures do not seem to have a detrimental effect on the brain. Any
changes that do occur are usually subtle, and it is often unclear
whether these changes are caused by the seizures themselves or by the
underlying problem that caused the seizures.
While epilepsy cannot currently be cured, for some
people it does eventually go away. One study found that children with idiopathic
epilepsy, or epilepsy with an unknown cause, had a 68 to 92 percent
chance of becoming seizure-free by 20 years after their diagnosis. The
odds of becoming seizure-free are not as good for adults, or for
children with severe epilepsy syndromes, but it is nonetheless possible
that seizures may decrease or even stop over time. This is more likely
if the epilepsy has been well-controlled by medication or if the person
has had epilepsy surgery.
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 neurons 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 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 (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 deficiency, tuberous sclerosis, Landau-Kleffner syndrome, and
autism. Epilepsy is just one of set of symptoms commonly found in people
with these disorders.
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.
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