This page covers the different types of generalised onset seizures, which are caused by abnormal electrical activity affecting the whole brain.
What are generalised onset seizures?
In generalised onset seizures, abnormal electrical activity affects both hemispheres (sides) of the brain, causing unconsciousness.
Sometimes, the period of unconsciousness is so short it may not even be noticed.
Seeing a child – or anyone – have a seizure can be quite frightening.
But it’s important to remember that they’re unconscious during generalised onset seizures, so they’re not aware of what’s happening.
Before a generalised onset seizure, they may have unusual symptoms that alert them to the seizure starting. This warning is also known as an ‘aura’.
It’s likely that this warning is the beginning of the seizure in just one part of the brain, before it spreads to the whole brain.
If they do get this warning, try to help them get to a safe place, even if that’s just sitting on the floor.
Tonic-clonic seizures
Tonic-clonic seizures are what most people think of when they hear the word epilepsy.
Like all generalised onset seizures, they are caused by abnormal electrical activity affecting the whole brain.
Some seizures start in one side of the brain and spread, affecting both sides. This is called a focal to bilateral tonic-clonic seizure.
Tonic-clonic seizures used to be called 'grand mal', which means ‘great illness’.
Now, however, the name ‘tonic-clonic’ describes the physical symptoms that occur during the two phases of the seizure: the tonic phase followed by the clonic phase.
The seizure starts with the tonic phase, where the person becomes stiff and falls to the ground unconscious. They may cry out or groan as air is forced out of their lungs.
In the clonic phase the person's limbs begin to jerk.
After a few minutes, the jerking slows and stops, and the muscles relax.
After the seizure, which usually lasts around one to three minutes, the person can be drowsy, confused and agitated, and take some time to return to normal.
They may also have been incontinent or bitten their tongue. Often, they will feel exhausted and need to sleep.
A convulsive seizure that lasts for five minutes or more, or a cluster of seizures over a 30-minute period, is known as status epilepticus.
When this happens, the National Institute for Health and Care Excellence (NICE) recommends that the person be given emergency treatment, and an individualised emergency management plan should be followed.
Absence seizures
Absence seizures used to be called ‘petit mal’, which means small illness.
They often begin in childhood and do not usually continue into adulthood, although they do continue in juvenile absence epilepsy.
There are two types: typical and atypical.
In typical absence seizures, a child may:
- 'Shut off' and stare blankly into space for about 5–10 seconds
- Not respond to anything
- Stop talking in the middle of a sentence
- Be unaware of what's happening around them
- Flutter their eyelids, fidget with their hands or walk around aimlessly
This can happen many times a day.
Then, the child will snap back and continue what they were doing before, unaware of the seizure.
Often, other people might not even notice these episodes.
At school, it's possible that staff may mistake absence seizures for not paying attention or daydreaming if they don’t know how seizures can affect children.
Children with absence seizures may also find it hard to learn because they miss information.
Typical absence seizures are associated with characteristic changes in EEG test results.
Atypical absence seizures do not start or finish as abruptly as typical absence seizures.
They usually last longer and your child may also experience changes in muscle tone that cause small movements, such as nodding their head slightly or a chewing action.
The child may continue to respond to their surroundings.
Atypical absence seizures are often associated with other seizure types and/or learning disabilities.
Myoclonic seizures
‘Myo’ means ‘muscle’ and ‘clonic’ means ‘jerk’. Myoclonic seizures can be both generalised onset (affecting both sides of the brain) and focal onset seizure types (affecting one side).
Myoclonic seizures usually affect both sides of the body at the same time.
The seizures can also happen in clusters.
During myoclonic seizures, a burst of abnormal electrical activity in the part of the brain that controls the muscles causes sudden jerking in the arms, legs, neck or body.
The muscles tighten rapidly, as if the child has had a very brief shock-like contraction.
The child can lose consciousness but for such a short period of time, it’s hardly noticeable.
Seizures often happen just after waking (sometimes known as a ‘messy breakfast’) or when the child is tired before going to bed.
Myoclonic seizures occur in a variety of epilepsy syndromes, such as juvenile myoclonic epilepsy syndrome.
Tonic seizures
Tonic seizures can be generalised onset (affecting both sides of the brain from the start) or focal onset (they start in one side of the brain).
‘Tonic’ means an ‘increase in tone or tightness’ of the muscles.
Although tonic seizures are more common in childhood, they are considered relatively uncommon. They often happen at night.
In tonic seizures, the muscles in the chest, arms and legs constrict rapidly, causing your child’s limbs and body to become rigid.
Their back arches but there’s no jerking.
The child may let out a cry as air is pushed out of their lungs, but they are not in any pain.
They will lose consciousness and fall to the ground, if standing.
As they often fall backwards, this puts them at risk of injury.
Their breathing may become laboured or stop for a moment.
As their breathing pattern changes, they may lose colour and the skin around the mouth may appear blue, because the lungs are getting less oxygen.
Tonic seizures are generally short (less than a minute) and recovery is rapid.
Once the seizure has stopped, the child regains consciousness and their muscle tone returns to normal.
People with Lennox-Gastaut syndrome are more likely to have this type of seizure.
Atonic seizures
Atonic seizures can be generalised onset (affecting both sides of the brain from the start) or focal onset (they start in one side of the brain).
Atonic seizures are also known as drop attacks.
Atonic is the opposite of tonic, so in atonic seizures all muscle tone is lost, causing the child to suddenly flop forward and fall to the ground.
Atonic seizures are sometimes called ‘drop attacks’ or ‘astatic seizures’.
This type of seizure can be hard to recognise if a child is sitting or lying down, because they don’t fall.
In babies, atonic seizures most often appear as a ‘head drop’.
Often, in atonic seizures, the child loses consciousness for only a brief time and recovers quickly.
But they are at serious risk of injury, especially to the head and face.
So, if atonic seizures happen often, the child may need protective headgear.
Children who have other seizure types, such as tonic or myoclonic seizures, may also have atonic seizures.
Find out more about focal onset seizures.
