Focal seizures are conceptualized as originating within networks limited to one hemisphere. They may be discretely localized or more widely distributed. Focal seizures may originate in subcortical structures. For each seizure type, ictal onset is consistent from one seizure to another, with preferential propagation patterns that can involve the ipsilateral and/or contralateral hemisphere. The semiology (symptoms/signs) that occur during a seizure can allow identification of the discrete area of the brain, or the lobe or hemisphere that is involved in seizure onset and propagation.
Focal seizure classification should only occur if the seizure is a focal epileptic seizure and epilepsy imitators have been excluded. Focal seizure classification is undertaken at two levels. The seizure is first classified according to level of awareness, as this is of critical importance for safety and independence in day to day life. If awareness is impaired at any point in the seizure, the seizure is a focal impaired awareness seizure. Because the first symptom/sign of the seizure is the most useful feature for identification of the regional brain network in which the seizure arises, focal onset seizures are also classified by their initial onset feature, even if this is not the most prominent feature overall in the seizure.
The following seizure types are therefore recognized for classifying focal seizures:
Focal seizures can spread widely in the brain to engage bilateral networks, including cortical and subcortical structures, resulting in a tonic-clonic seizure with loss of consciousness. This seizure type is known as a focal to bilateral tonic-clonic seizure.
In addition to the initial onset feature of a seizure, further symptoms/signs may occur during the seizure, these also reflect the regional networks involved in the seizure origin or propagation. Once the seizure has been classified by level of awareness and onset feature, these additional relevant features are noted as descriptors of the seizure.
NOTE Information from video of seizures, and from tests such as EEG, MRI, laboratory or genetic tests, can be used to support optimized classification of seizures.