Excitation and inhibition of electrical activity in the brain are carefully balanced. Normally, neurons fire singly or in small groups to accomplish a task and then stop firing.
A seizure happens if many neurons fire at once in uncontrolled bursts. This firing results from a combination of factors that interfere with how the brain normally functions.
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Neuron excitation and inhibition become unbalanced; either there is too much excitation, or too little inhibition.
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A small group of neurons begins to fire all together.
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Other neurons nearby or throughout the brain also start firing together because of abnormal connections between neurons or groups of neurons. This firing is called hypersynchrony.
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The neurons involved in the seizure send instructions to the parts of the body that they control. Because the brain is the control centre for the body, the uncontrolled electrical activity in the brain has an effect on the body. The effects of the seizure depend on where the seizure occurs in the brain.
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If the electrical activity is confined to one part of the brain, this results in a partial seizure. If it spreads through the entire brain, this results in a generalized seizure.
Provoked and unprovoked seizures
Seizures may be provoked or unprovoked.
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Provoked seizures are the direct, immediate result of a cause such as a head injury, a high fever, an infection, drugs, withdrawal from medication, or low blood sugar. Anybody can have a single seizure under these conditions. Provoked seizures are less likely to happen again and are not considered to be epilepsy.
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When a seizure is not provoked by an immediate, acute cause such as the ones described above, it is called an unprovoked seizure. If a child has two or more unprovoked seizures, he is considered to have epilepsy.
About the seizure threshold
The seizure threshold is not a specific measurement. It is a concept, a way of thinking about the balance between neuron excitation and inhibition in the brain.
Sarah’s seizures are well controlled on anti-epileptic drugs. She has an important test tomorrow. She stays up three hours later than usual to study, and then takes her medication when she goes to bed. The next day, she has a seizure at breakfast.
Under normal circumstances, Sarah would not have had a seizure, because her medication and her regular sleep schedule keep her seizure threshold high. The combination of taking her medication late and losing three hours of sleep lowered her seizure threshold just enough to trigger a seizure.
Some factors can lower the seizure threshold of a person with epilepsy, including:
While one of these things by itself might not be enough to cause a seizure, a combination of them may lower the seizure threshold enough to cause a seizure. Factors such as illness and lack of sleep are sometimes known as seizure triggers.
Factors that can raise the seizure threshold include:
This means that a person with epilepsy may be able to lower her risk of seizures by getting enough sleep and taking her anti-epileptic drugs as prescribed. This is easier for adults than for children. Because children’s bodies are changing day by day, it can be difficult to find the right balance.