Abstracts

Although several series (reviewed in Blume and Wiebe, 2000) described clinical phenomena among patients with occipitally originating seizures, whether intraoccipital site specific semiologies occur is not clear. Moreover, Collins and Caston (1979) showed experimentally that a robust ictal discharge will propagate promptly throughout the occipital lobe, possibly obscuring ictal origin. Therefore, this study determined the proportion of intractable occipital epilepsies which could be localised to a single occipital surface, either mesial or lateral-inferior, utilising subdural EEG, neuroimaging, or both modalities. Secondly, whether ictal semiology or clinical examination can distinguish between seizures originating strictly mesially from those arising in other areas was sought.
We included all patients whose subdurally recorded seizures arose from a single occipital lobe, or those with an occipital lesion whose removal reduced seizure quantity by over 90%. Excluded were occipital seizure patients with a more active extra-occipital epileptogenic zone. Multiple surface occipital subdural electroencephalography was performed in 35 patients: 24/30 with focal occipital neuroimaging lesions and all 11 without such.
Of 41 patients meeting criteria, a principal epileptogenic zone was established in 29 (71%): mesial surface in 18, lateral surface in 11, and both surfaces in 12 patients. Of 30 patients with a focal occipital neuroimaging abnormality, seizures arose from that single surface in 23 (77%) while 6 of 11 patients (55%) without neuroimaging focality had single surface occipital originating seizures.
Visual aura occurred in 35 (85%) of 41 patients. Of 31 patients with unformed phenomena, seizures arose from the mesial surface in 11 (35%), lateral surface in 9 (29%) and simultaneously from both surfaces in 11 (35%). Of 18 patients with formed visual phenomena, seizures arose from the mesial surface in 7 (39%), lateral surface in 5 (28%), and both surfaces in 6 (33%). Dyscognitive (partial complex) attacks occurred in 33 (80%) patients and secondarily generalised tonic-clonic seizures in 35 (85%). Neither the visual aurae, dyscognitive or secondarily generalised phenomena distinguished patients whose seizures began from mesial, lateral, or both occipital surfaces.
Although all 9 patients with preoperative quadrantanopia or hemianopias had seizures arising from the mesial (6 patients) or both mesial and lateral surfaces (3 patients) as compared to 20 of 30 patients with normal preoperative Goldmann visual fields, this relationship did not achieve statistical significance (p = 0.0794, Fisher[rsquo]s Exact). Optic atrophy precluded visual field testing in 2 patients.
Unfortunately, ictal semiology and visual field testing failed to distinguish mesial, lateral and both surface occipital epileptogenesis groups. Nonetheless, single surface occipital seizure origin could be discerned in 29 (71%) of 41 of these medically intractable patients.