Authors :
Presenting Author: Kalina Misiolek, MD – University of Virginia
Matthew Rumschlag, MD – Resident, PGY-4, Neurology, University of Virginia; Ifrah Zawar, MD – Attending, Neurology, Epilepsy Division, University of Virginia
Rationale:
Implantation effect is the phenomenon of transient decrease in clinical seizure frequency following placement of intracranial electrodes which is thought to last no longer than five to six months. Although its pathophysiology is unknown, it has been hypothesized that the insertion of intracranial leads may create microlesions which disrupt epileptic networks. Implantation effect has been described in people undergoing resection and intracranial implantation with both subdural grids and stereo-EEG electrodes. More recently, responsive neurostimulation (RNS) electrode placement has also demonstrated similar transient improvement in clinical seizure frequency. Electrocortigraphic (ECoG) spectral power changes and changes in spike rates have also been described for up to five months of chronic ECoG in RNS patients. Exactly how much of these changes are due to RNS stimulation is less clear.
Methods:
To our knowledge, no previous studies have investigated the impact of electrode implantation on long episodes and subclinical EEG seizures on RNS in patients not receiving RNS stimulation. Here we describe a patient with bilateral independent mesial temporal lobe drug resistant focal epilepsy (DRE) who underwent bilateral RNS implantation. Following implantation, she was free of clinical seizures for 18 months without RNS stimulation during the initial eight month period.
Results:
A 19-year-old female presented with nine years of DRE of unknown etiology. She experienced focal seizures with impaired awareness with or without secondary generalization. Baseline frequency was five to nine seizures per month. Brain MRI demonstrated periventricular heterotopia in right frontal region and possible focal cortical dysplasia in right temporal region. EEGs demonstrated left temporal slowing and seizures originating in the left frontotemporal region. Ictal SPECT showed left frontoparietal hyperperfusion. PET showed hypometabolism in bilateral temporal regions. Stereo-EEG confirmed independent seizures arising from bilateral hippocampi. Subsequently, she underwent RNS placement with bilateral hippocampal electrodes. After RNS, she has remained free of clinical seizures for 18 months. She also remained free of EEG seizures and long episodes for 14 months following RNS implantation. Rare long episodes and EEG seizures (Fig 1) were noted in the setting of significant medication reduction after 14 months which resolved after increasing medication doses to pre-RNS doses. For the initial eight months of implantation, RNS stimulation was not on.
Conclusions:
Here we describe an extreme case of implant effect of an RNS patient who has remained free of clinical seizures for 1.5 years, which is significantly longer than the typical implant effect of up to 6 months. We also describe the impact of RNS implantation on EcOG findings including no electrographic seizures or long episodes. This case is significant because it highlights the extended impact of RNS electrodes purely from the implantation without the confounding effect of RNS stimulation.
Funding: None.