Abstracts

Rationale: Periventricular nodular heterotopia (PNH) are associated with the development of focal seizures. These PNH may have functional connections with the neocortex that could propagate seizures that originate within the PNH and spread to neocortex. We report a case where electrical stimulation of a trigonal PNH produced formed visual hallucinations suggesting that PNH tissue could generate conscious sensations locally or by propagation. Methods: A 30 year old right handed man developed simple partial visual and complex partial seizures with or without secondarily generalized tonic clonic seizures at age 15. Investigation revealed bilateral PNH on MRI involving the trigones and temporal horns of the lateral ventricles as well as megacisterna magna. Scalp EEG and clinical seizures were consistent with a right temporooccipital ictal onset. On 2-27-2008, a responsive neurostimulation device (NeuroPace) was chronically implanted for a clinical trial with two 4-contact intracerebral depth electrodes, one implanted from a posterior approach to the right amygdala and hippocampus and the other to the largest of the trigonal PNH using MRI guidance and a CRW stereotactic frame. Interictal EEG showed spiking in the PNH, mesial temporal structures and occipital contacts. Routine afterdischarge threshold testing was part of the NeuroPace study protocol. Electrode stimulation consisted of 200 Hz biphasic 160 microsecond pulse width per phase, 100 msec trains at variable (0.5 to 6 mA) intensity. Any symptoms or signs noted with stimulation were recorded in the medical record. Electrode location was confirmed by coregistration of post-implant CT with pre-implant MRI and this demonstrated the targeted regions were implanted. Results: Stimulation of the right intracerebral depth electrode contacts for both the heterotopia electrodes and the hippocampal electrode produced contralateral hemifield visual hallucinations consisting of red and white squares or triangles. There were no afterdischarges up to 6 mA while the sensory functional threshold ranged from 0.5 mA to 2.5 mA. Occipital cortical contacts did not produce symptoms with stimulation and using these contacts as a reference electrode, there were still visual hallucinations from stimulation of the heterotopia electrodes. Conclusions: Stimulation of trigonal PNH can produce contralateral visual hallucinations. Whether these hallucinations originate in the heterotopia or result from propagation to neocortical structures is unclear, but these hallucinations occur at low stimulus intensities that activate small tissue regions. If the trigonal PNH are functioning neural tissues, it may explain why resection can sometimes produce visual deficits.