Characteristics and clinical significance of subclinical seizures.

Purpose: To define characteristics of subclinical seizures (SCS) and their prognostic significance after epilepsy surgery.

Methods: Reports from intracranial video-EEG monitoring were reviewed for patients who had epilepsy surgery between 1989 and 2003. Relationships between SCS and clinical seizures were categorized as either: complete colocalization (Group A), when both SCS and clinical seizures originated from the same single focus, or incomplete and no colocalization (Group B), when some or all SCS and clinical seizures originated from different foci in different lobes or hemispheres.

Results: A total of 111 patients were included in this review. Seventy-one (64%) patients had 2,821 SCS and most SCS came from the mesial temporal lobe. The mean duration of SCS was shorter than complex partial seizures and generalized tonic-clonic seizures but similar to simple partial seizures. SCS rarely propagated beyond the site of origin and the majority of SCS had the same area of origin as clinical seizures. Sixty-five patients had both SCS and clinical seizures and underwent resective surgery. Group A patients had a higher seizure-free outcome rate (77.5%) than Group B patients (37.5%). The colocalization rate of SCS and clinical seizures may impact seizure-free outcome. The presence or absence of SCS, SCS duration, and extent of propagation of SCS did not influence surgical outcome.

Conclusion: SCS commonly originate from the same cortical area as clinical seizures and are related to postsurgical outcome. These findings suggest they should be viewed as having similar significance in the surgical decision process as clinical seizures.