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A focal seizure may spread to involve wider brain networks, resulting in a focal to bilateral tonic-clonic seizure. This was previously known as a 'secondary generalized seizure'. Consciousness is impaired. Motor components in such a situation include tonic and clonic features. This type of seizure may follow after other focal seizures types, such as focal motor seizures, focal cognitive seizures, focal sensory seizures or focal impaired awareness seizures. Alternatively, the spread in brain networks may be so rapid that no preceding focal seizure type is identified. The focal to bilateral tonic-clonic seizure may be differentiated from the generalised tonic-clonic seizure on the basis of other evidence that supports a focal epilepsy/etiology, or there may be asymmetry seen in the focal to bilateral tonic-clonic seizure (such as head/eye version to one side, asymmetric amplitude to the motor features), and this asymmetry is seen consistently from seizure to seizure.

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