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Seizures (or convulsions) are temporary alterations in brain functions due to abnormal electrical activity of a group of brain cells that present with apparent clinical symptoms and findings. An isolated abnormal electrical activity recorded by an electroencephalography examination without a clinical presentation is not called a seizure.

Seizures can cause involuntary changes in body movement or function, sensation, awareness, or behavior. A seizure can last from a few seconds to a few minutes. There are more than 20 different types of seizures.

Seizures are often associated with epilepsy and related seizure disorders.

Seizure is often associated with a sudden and involuntary contraction of a group of muscles. However, a seizure can also be as subtle as marching numbness of a part of body, a brief loss of memory, sparkling of flashes, sniffing an unpleasant odor, a strange epigastric sensation or a sensation of fear. Therefore, it is traditional to classify the seizures as motor, sensory, autonomic, emotional and cognitive.

Some seizure types are:

Symptoms experienced by a person during a seizure depend on where in the brain the disturbance in electrical activity occurs. Some seizures may be frightening to onlookers. A person having a tonic-clonic (grand mal) seizure may cry out, lose consciousness and fall to the ground, and have rigidity and muscle jerks. A person having a complex partial seizure may appear confused or dazed and will not be able to respond to questions or direction. Some people have seizures that are not noticeable to others. Sometimes, the only clue that a person is having an absence (petit mal) seizure is rapid blinking or a few seconds of staring into space.

It is still disputable whether a febrile seizure has to be regarded as an epileptic disorder or not. In general, a patient with two or more episodes of seizures is accepted to have epilepsy (a condition also known as a seizure disorder.) Many people with epilepsy percieve "auras", telltale sensations such as strange lights or unpleasant smells, before their seizures.

Major causes of seizures are head trauma, infection, tumor and metabolic alterations (e.g. low or high blood glucose levels). Many seizures have unknown causes.

Seizures in pregnancy can be a sign of eclampsia.

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In law, seizure can also refer to taking possession of an item: see search and seizure.