If we know where a seizure starts, it is not called an "unknown onset" seizure. Instead, seizures are classified as "unknown onset" seizures specifically when the beginning or onset of the seizure is not known or witnessed. Unknown onset seizures are those where it is unclear whether they begin focally (in one area of the brain) or generalized (in both hemispheres) due to lack of information. Once more information is available, unknown onset seizures can be reclassified as focal-onset or generalized-onset seizures. Thus, if the area where a seizure starts is known, it is classified as either:
- A focal onset seizure if it starts in one localized brain area,
- Or a generalized onset seizure if it involves both sides of the brain from the outset.
Unknown onset seizures are initially categorized that way when the onset is not witnessed or identifiable, such as when a seizure occurs during sleep or when a person is alone. They may later be reclassified as focal or generalized based on further diagnostic tests like EEG and video monitoring.
