Answer: Seizures are classified based on how widely affected the brain is during the seizure.
Seizures are a clinical symptom of concern. They may happen in people with epilepsy, following a head injury, as a result of a brain tumor, in the presence of drugs, or for other unknown reasons.
Seizures are temporary, but can range in intensity. Some may require immediate medical attention. Any seizures are signs of an unhealthy nervous system and should be followed up with a physician such as a neurologist.
On a cellular level, seizures are believed to begin with some atypical or otherwise unpredictable activity of neurons. The location of the unusual activity may account for why seizures are all different and have different symptoms.
Because seizures often result from unhealthy electrical activity of the brain, they can be diagnosed using an EEG, or an electroencephalogram. The EEG is a quick and painless procedure that uses sensors placed on the surface of the head to read the underlying changes in electrical signals. EEGs can help identify the location of seizure activity of the brain.
Broadly speaking, seizures can be classified as being either generalized or focal.
Generalized seizures
A generalized seizure is one where both hemispheres (halves) of the brain are affected. This could be because the underlying cellular pathology is found in neurons on both sides of the brain, or if the electrical activity starts on one side then spreads to the other across the white matter tracts (such as the corpus callosum) that communicate between hemispheres.
Within the category of generalized seizures, there are at least two subcategories depending on the symptoms observed during a seizure: motor and non-motor (also called absence) seizures.
Generalized motor seizures
These happen on both hemispheres of the brain at the same time (compared to focal seizures, described below).
The specifics of the seizure are identified by a series of adjectives which describe the nature of the symptoms.
Tonic-clonic: These seizures involve loss of consciousness and uncontrollable muscle contractions. Historically, these seizures have been called "grand mal seizures." The two muscle actions are named: tonic refers to muscle stiffening or increased muscle tone, while clonic refers to a jerking of the muscles.
Some generalized seizures can have only the tonic component, or only the clonic component, and should be differentiated if possible.
Non-motor generalized seizures may also be called "absence seizures", or historically, "petit mal seizures". During an absence seizure, the patient may appear to "space out", blinking rapidly or staring into space. They may be unaware or unresponsive during this time. When a person has an absence seizure, the people around them may not even notice.
Focal seizures
Focal seizures are when only one small part of the brain is affected at a time. These may also be called "partial seizures", but this naming convention is less popular today because it may be confusing ("the person is only experiencing part of a seizure" may be falsely implied by the term).
These seizures can also be either motor or nonmotor.
Another important consideration in diagnosing focal seizures is whether the patient is aware of their seizure or not. If they are aware, the seizure is categorized as a simple focal seizure. Among simple seizures, descriptive adjectives may be added to better characterize the experience, using terms like motor, sensory, and psychological.
If awareness is lost during the event, these are called complex focal seizures.
Different types of anti epileptic drugs (AEDs) may be more effective depending on the nature of the seizure. For example, carbamazepine may be effective for decreasing certain types of adult epilepsy, but may worsen childhood (febrile) seizures.