Abstracts

Rationale: Following intracranial studies for seizure localization, some patients will have a transient improvement in seizure frequency immediately following insertion of depth electrodes, grids and strips; this has been termed the “implantation effect”.¹ Long-term improvement in seizure control has also rarely been reported following intracranial monitoring.^2,4 We reviewed our institutional data to identify patients who became seizure free with implantation alone. Methods: Retrospective chart review of all intracranial studies performed at Yale University from 2002 to 2016. Results: A total of 190 intracranial EEG studies were completed during this time. Recording was accomplished primarily through a combination of grids, strips and depth electrodes. 43/190 were not candidates for any kind of surgery either due to onset pattern or being at risk for language impairment or memory decline with surgery. Of these 43 patients, three (7%) had long-term seizure freedom for > five years following implantation alone.Clinical and electrographic characteristics of each patient are described in table 1. Depth electrode placements are shown in Figure 1. Two patients had thalamic depths (out of a total of 3 patients with thalamic depths and no resection) and one had hippocampal depth electrodes. These electrodes were not involved in seizure onset or early propagation. Patient 1 was not offered surgery due to diffuse onset and because clinical onset preceded electrographic onset with a few seizures. Patient 2 was not offered surgery because of risk of language impairment. Surgery was not offered for patient 3 secondary to diffuse onset. Patients remained seizure free on antiepileptic medications post-implant for 6, 7 and 5 years respectively before seizures recurred. Conclusions: To our knowledge, there are two case series and a case report describing long-term seizure freedom following intracranial implant alone.^1,4,5 Interestingly, 2/3 patients in our case series had thalamic electrodes. Thalamic (anterior or centromedian nucleus) electrode implantation (and stimulation) resulting in seizure reduction and or remission is well known .⁶ However, long-term seizure freedom (= five years) with just implantation is quite uncommon. Our observations support the hypothesis that the microlesional effect of depth electrode placement in the thalamus and/or hippocampus may have neuromodulatory effects sufficient to disrupt epileptogenic networks. This may be true even when the thalamus or hippocampus do not appear to be directly involved in seizure onset or early propagation, as in our patients. Further studies of the microlesional effect of electrode placement in the presumed seizure propagation pathways of these regions (particularly thalamus) are required to better understand this unique phenomenon.1. Lane MA et al. The implantation effect: delay in seizure occurrence with implantation of intracranial electrodes. Acta Neurol Scand. 2017;135:115-121.2. Katariwala NM et al. Remission of intractable partial epilepsy following implantation of intracranial electrodes. Neurology. 2001;57:1505-1507.3. Andrade DM et al. Long-term follow-up of patients with thalamic deep brain stimulation for epilepsy. Neurology. 2006;66:1571-1573.4. Roth J et al. Epilepsy control following intracranial monitoring without resection in young children. Epilepsia. 2012;53(2):334-341.5. Schulze-Bonhage A et al. Seizure control resulting from intrahippocampal depth electrode insertion. J Neurol Neurosurg Psychiatry. 2010;81:352-353.6. Fisher R et al. Electrical stimulation of the anterior nucleus of the thalamus for treatment of refractory epilepsy. Epilepsia. 2010;51(5):899-908. Funding: None