Seizures are divided into groups depending on:
- where they start in the brain (onset)
- whether or not a person's awareness is affected
- whether or not seizures involve other symptoms, such as movement
Depending on where they start, seizures are described as being focal onset, generalised onset or unknown onset.
What are focal onset seizures?
Focal onset seizures start in, and affect, just one part of the brain, sometimes called the 'focus' of the seizures. They might affect a large part of one hemisphere or just a small area in one of the lobes.
Sometimes a focal seizure can spread to both sides of the brain (bilateral). Some people view this focal seizure as a warning, that another seizure will happen. For more details, see below, 'Focal to bilateral tonic-clonic seizures'.
Levels of awareness
Seizures are also described depending on a person's level of awareness during their seizures; this means whether or not they are aware of the seizure and what is happening around them. These seizures are known as focal aware seizures or focal impaired awareness seizures.
What happens during focal seizures?
What happens during focal aware and focal impaired awareness seizures depends on where in the brain the seizure happens and what that part of the brain normally does.
Some focal seizures involve movements, called motor symptoms and some involve unusual feelings or sensations called, non-motor symptoms.
Motor symptoms can include:
- making lip-smacking or chewing movements,
- repeatedly picking up objects or pulling at clothes;
- suddenly losing muscle tone and limbs going limp or floppy, or limbs suddenly becoming stiff;
- repetitive jerking movements that affect one or both sides of the body;
- making a loud cry or scream; or
- making strange postures or repetitive movements such as cycling or kicking.
Non-motor symptoms can include:
- changes or a ‘rising’ feeling in the stomach or déjà vu (feeling like you’ve ‘been here before’);
- getting an unusual smell or taste;
- a sudden intense feeling of fear or joy;
- a strange feeling like a ‘wave’ going through the head;
- stiffness or twitching in part of the body, (such as an arm or hand);
- a feeling of numbness or tingling;
- a sensation that an arm or leg feels bigger or smaller than it actually is; or
- visual disturbances such as coloured or flashing lights or hallucinations (seeing something that isn’t actually there).
Focal to bilateral tonic-clonic seizure
Sometimes focal seizures spread from one side (hemisphere) to both sides of the brain. This is called a focal to bilateral tonic-clonic seizure.
When this happens the person becomes unconscious and will usually have a tonic clonic (jerking or shaking) seizure. When focal seizures spread very quickly, the person not be aware that it started as a focal seizure.
Focal aware seizures
In focal aware seizures (FAS), previously called simple partial seizures, the person is conscious (aware and alert) and will usually know that something is happening and will remember the seizure afterwards.
Some people find their focal aware seizures hard to put into words. During the seizure they may feel ‘strange’ but not able to describe the feeling afterwards. This may be upsetting or frustrating for them.
FAS are sometimes called ‘warnings’ or ‘auras’ because, for some people, a FAS develops into another type of seizure. The FAS is then a warning that another seizure will happen (see focal to bilateral tonic-clonic seizures).
Focal impaired awareness seizures
Focal impaired awareness seizures (FIAS), previously called complex partial seizures, affect a bigger part of one hemisphere (side) of the brain than focal aware seizures.
The person’s consciousness is affected and they may be confused. They might be able to hear you, but not fully understand what you say or be able to respond to you. They may not react as they would normally. If you speak loudly to them, they may think you are being aggressive and so they may react aggressively towards you.
Generalised onset seizures and unknown onset seizures
Generalised onset seizures affect both sides of the brain at once and happen without warning.
The person will be unconscious (except in myoclonic seizures), even if just for a few seconds, and afterwards will not remember what happened during the seizure.
Unknown onset seizures are sometimes used to describe a seizure if doctors are not sure where in the brain the seizure starts. This may happen if the person was asleep, alone, or the seizure was not witnessed.
If there is not enough information about a person's seizure, or if it is unusual, doctors may call it an unclassified seizure.
Clonic seizures involve repeated rhythmical jerking movements of one side or part of the body or both sides (the whole body) depending on where the seizure starts.
Seizures can start in one part of the brain (called focal motor) or affect both sides of the brain (called generalised clonic).
Absence seizures are more common in children than adults and can happen very frequently. During an absence a person becomes unconscious for a short time. They may look blank and stare, or their eyelids might flutter. They will not respond to what is happening around them. If they are walking they may carry on walking but will not be aware of what they are doing.
Myoclonic means ‘muscle jerk’. Muscle jerks are not always due to epilepsy (for example, some people have them as they fall asleep). Myoclonic seizures are brief but can happen in clusters (many happening close together in time) and often happen shortly after waking.
In an atonic seizure (or 'drop attack') the person’s muscles suddenly relax and they become floppy. If they are standing they often fall, usually forwards, and may injure the front of their head or face. Like tonic seizures, atonic seizures tend to be brief and happen without warning. With both tonic and atonic seizures people usually recover quickly, apart from possible injuries.
Tonic clonic seizures
These are the seizures that most people think of as epilepsy. At the start of the seizure:
- the person becomes unconscious;
- their body goes stiff, and if they are standing up they usually fall backwards;
- they may cry out; and
- they may bite their tongue or cheek.
The International League Against Epilepsy (ILAE) announced some new names for seizures in May 2017. Our Education, Information and Support Services Manager explains the new seizure classifications and the reasons behind the changes. In this blog, you’ll find the new general outline of basic seizure classification.
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