Seizure classification is a way of naming the many different types of epileptic seizures and putting them into groups.
In 2017, the International League Against Epilepsy revised its classification of seizures to make diagnosing and classifying seizures more accurate and easier. Some of the words you may have heard to describe seizures, such as ‘tonic-clonic’ are still used while others, such as ‘partial’ and ‘grand mal’ have been replaced with other terms.
Doctors look at the following three things when classifying a seizure:
- Where in the brain the seizure starts (e.g. the onset)
- If the person is aware or not during the seizure
- Whether the seizure involves movement.
Seizures can be divided into three major groups:
Most people will only have one or two seizure types. Sometimes a person with more complex or severe epilepsy may experience a number of different seizure types.
1. Focal Onset Seizures
Focal onset (formerly known as partial seizures) means the seizure starts in just one (focal) region of the brain. It may spread to other areas of the brain.
These seizures can often be subtle or unusual and may go unnoticed or be mistaken for anything from being intoxicated to daydreaming. About 60% of people with epilepsy have focal onset seizures – which are also simply known as focal seizures.
Focal onset seizures can be further divided into two groups relating to a person’s awareness during a seizure:
- Focal aware: the person is fully aware of what’s happening around them but may not be able to talk or respond (formerly known as simple partial seizures). They are usually brief, and are often called a warning or ‘aura’ (that a more significant seizure may develop) but are actually part of the seizure.
- Focal impaired awareness: awareness is affected (formerly known as a complex partial seizure) and the person may appear confused, vague or disorientated.
A focal seizure may progress to a bilateral tonic-clonic seizure meaning that it starts in one area of the brain and then spreads to both sides causing muscle stiffening and jerking.
2. Generalised Onset Seizures
Generalised onset means the seizure affects both hemispheres (sides) of the brain from the onset. Because of this, a person may lose consciousness at the start of the seizure. Generalised onset seizures almost always affect awareness in some way, so the terms ‘aware’ or ‘impaired awareness’ aren’t used. However, they can be classified further by movement:
- Generalised motor seizure: may involve stiffening (tonic) and jerking (clonic), known as tonic-clonic (previously known as grand mal) or other movements
- Generalised non-motor seizure: These seizures involve brief changes in awareness, staring, and some may have automatic or repeated movements like lip-smacking.
Types of Generalised Onset Seizures
There are many types of seizures in this classification. They include:
- Absence – a sudden lapse in awareness and responsiveness that look like brief staring spells or daydreaming
- Tonic-Clonic – the body stiffens (the tonic phase) and then the limbs begin to jerk rhythmically (the clonic phase)
- Myoclonic – sudden single jerks of a muscle or a group of muscles that may last no more than a second or two
- Tonic – Can occur when a person is asleep or awake and involves a brief stiffening of the body arms or legs. The person will suddenly fall if standing or sitting.
- Atonic – brief seizures that cause a sudden loss muscle tone and the person often falls to the ground or will have a sudden head nod if sitting.
- Clonic – although uncommon they cause jerking in various parts of the body
3. Unknown Onset Seizures
Unknown onset means the seizure cannot be diagnosed as either focal or generalised onset. Sometimes this classification is temporary and as more information becomes available over time or through further testing, the type of seizure may be changed to a generalised or focal onset seizure.
Rarely, doctors might be sure that someone has had an epileptic seizure, but can’t decide what type of seizure it is. This could be because they don’t have enough information about the seizure, or the symptoms of the seizure are unusual.
Most people will only have one or two seizure type(s), which may vary in severity. A person with severe or complex epilepsy or significant damage to the brain may experience a number of different seizure types.
Bilateral versus generalised: Bilateral is used for tonic–clonic seizures that spread to both hemispheres from a focal onset. Generalised is used for seizures that originate simultaneously in both hemispheres
Watch here for the most common types of seizures explained