The International League against Epilepsy (ILAE) changed how it classifies seizures in 2017. The reason was to make it easier and more accurate to diagnose and classify the different types of seizures. More accurate diagnosis and classification in principle should lead to better response to treatment. The reason is seizures with specific features tend to respond better to certain forms of treatment. Also, because this new classification system is more practical and meaningful, it should aid in research as well.
Prior Classification Systems
For many decades the most common terms for the different types of seizures were grand mal and petit mal (absence) seizures. Then, in 1981 a classification system emerged which replaced those terms with ones based on where in the brain a seizure begins. Those beginning in one area or side of the brain bore the name partial seizures. Those starting on both sides of the brain at the same time were termed generalized seizures.
- Complex partial seizure – There is some degree of alteration of awareness that may range from slight confusion to no awareness at all.
The New Seizure Classification System
The old system worked fairly well for many years, but did not account for some of the different types of seizures. That prompted ILAE to develop the new system. It is more or less a revision of the system developed in 1981 and later revised. There are some similarities and some differences between the new and the older classification systems.
It is not necessary that the public know the ins and outs of the new classification system or all of the different types of seizures within it. But it is helpful for patients or persons who witness seizures to know the type of information they need to report to doctors so they can make the best use of it.
How the ILAE 2017 System Types Seizures
The new system consists of two versions. They are the basic version and the expanded version. The version used depends upon how specific one wants to be. The difference in the two versions is the specificity of the classifiers at the lowest level.
Basic Version of ILAE 2017 Seizure Classification System
In this new system, much like in the older system, the first level of classification is the location of the seizure onset. The seizure onset is the beginning of a seizure. It correlates with the initial abnormal electrical activity; the area of the brain where it occurs; and the sign(s) and/or symptom(s) that stimulation of that area results in. Thus, the initial manifestation is often a clue of where in the brain the seizure started and if it started in one side or both sides.
Focal onset means the seizure starts out in one side of the brain. If it has motor features, abnormal muscle movement in an opposite limb or the opposite side of the body is often a clue.
Generalized onset means it begins in both sides of the brain. In this case there is usually abnormal movement of both sides of the body if it has motor features. Many times the abnormal movement might involve all four limbs and other parts of the body.
When there are no clinical indicators of where a seizure starts, doctors often rely on other types of data to presume the location of the onset. They include the EEC tracing and imaging studies, namely CT scans and MRI scans. Evidence of a lesion in one hemisphere of the brain usually serves as proof of a focal onset. Diffuse bilateral abnormal patterns on an EEG suggest a generalized onset.
A focal seizure is one whose onset is in one side of the brain. It is the replacement term for partial seizure in the older system.
The next level of classification for a focal seizure in the new system is in accordance with the level of alertness, but it is optional. Alteration of consciousness was a key factor in classifying seizures in the older system. But to many outside of the medical community, the term consciousness is confusing and not well accepted. Therefore, the new naming system has replaced it with the term awareness. The terms consciousness and responsiveness are still in use however, in describing certain types of seizures. They are just not a part of the name.
The next level of classification is on the basis of whether or not motor or nonmotor signs and symptoms occur during the event. If so, the word motor or nonmotor is part of the name of the type of seizure.
The lowest level of basic classification of focal seizures in this new system is one for use when a seizure begins in one side of the brain and subsequently spreads to the other side as well. As the schematic above shows, it is termed a focal to bilateral tonic-clonic seizure.
Generalized seizures remain those in which the onset is in both sides of the brain. Most generalized seizures usually, but don’t always affect awareness to some degree. Thus, there is no awareness classifier for these types of seizures in this new system. Name expansion in this group is also on the basis of whether bilateral motor activity or nonmotor signs or symptoms occur at the beginning of the seizure. Here, absence is a synonym for nonmotor. Absence seizure is a replacement term for petit mal seizure in the older naming system.
Basic Classification of Seizures of Unknown Onset
ILAE added an onset unknown category to be used when the area(s) of the brain where a seizure begins is unclear. The word motor or nonmotor also denotes whether or not those types of symptoms occur during a seizure of this category. When there is insufficient information to subclassify such a seizure at this level it is proper to designate it as unclassified.
Expanded Version of ILAE 2017 Seizure Classification System
As the above diagram illustrates, expanding a focal seizure to the next level of classification is also optional. Expansion at this level consists of adding the term for the first prominent type of motor or nonmotor sign or symptom that occurs during the event to the name of the seizure type. This modifying word is a seizure classifier. It is important to note that the classifier is a type of sign or symptom and not the actual specific sign or symptom per se. Even though the actual signs and symptoms are not a part of the expanded name of a seizure they are still often useful for providing greater detail.
The specific types of signs and symptoms that are a part of the name of seizures of focal onset can be motor or nonmotor. They are the initial features of the seizure and are listed as follows:
The name expansion for focal seizures is not tiered since citing the level of awareness in the naming is optional. It is practical to omit the level of awareness when it is unknown or not applicable.
The specific types of signs and symptoms that are used to subclassify seizures of generalized onset are as follows:
myoclonic-atonic muscle activity
myoclonic-tonic-clonic muscle activity
tonic-clonic muscle activity
tonic muscle activity
For the unknown onset category seizure type the sub-classifying signs and symptoms are the following:
|tonic-clonic muscle activity
The unclassified subcategory is for use when it is unclear whether or not a seizure of unknown onset has motor or nonmotor features.
Signs and symptoms alone don’t define a seizure type across-the-board. The reasons are the different levels of classification and the fact that some of the different types of seizures have features in common. The sequence in which they manifest is often different though. For this reason doctors often look for patterns of how a seizure presents.
Doctors might also use other data to help decide how to properly classify a seizure. They include: EEG patterns; lab test findings; imaging studies (such as CT or MRI scans); videos taken by family members; a gene mutation; and a known epilepsy syndrome.
Seizure Classification Future Outlook
There is some degree of uncertainty with the use of any seizure classification system. But the flexibility of this new system and its goal of greater transparency between doctors and patients in the management of seizures are reasons for optimism.
Along the same lines, clinicians can reclassify seizures as information changes. The changes might be due to future observation, better describing, or a better understanding of what is important on the part of patients. Regardless, the end-result should be better patient care.
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