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Extraventricular Long-Axis Cannulation of the Hippocampus: Technical Considerations

BACKGROUND: Patients with hippocampal epileptogenic foci may benefit from targeted intracranial monitoring of seizures and treatments such as hippocampal electrical stimulation, closed-loop stimulation, and stereotactic laser ablation. Each may benefit from a greater volume of hippocampal coverage with long-axis cannulation. Furthermore, an extraventricular trajectory avoids brain shift and reduces the risk of hemorrhage from ependymal breach. Unfortunately, detailed descriptions of the technical aspects of longitudinal cannulation of the hippocampus remain sparse. OBJECTIVE: To develop a standard protocol for extraventricular longitudinal hippocampal cannulation. METHODS: Images from 25 patients stereotactically implanted with 27 longitudinal hippocampal devices were retrospectively reviewed to determine the location of the burr hole or twist drill craniostomy. Simulated planning for bilateral occipital trajectories was then performed on a second cohort of 25 patients (50 trajectories) with mesial temporal sclerosis. An entry point derived from these 77 trajectories was subsequently validated on a third cohort of 25 patients (50 trajectories). RESULTS: Extraventricular long-axis hippocampal implantation necessitates a lateral-to-medial and cephalad-to-caudal trajectory that skirts the inferomedial border of the temporal horn. Measurements from 64 trajectories suggested a consensus entry point that successfully facilitated 50 test trajectories as well as frame placement on 4 patients requiring long-axis hippocampal cannulation. CONCLUSION: Although trajectories must be individually tailored for each patient, we recommend a starting entry point approximately 5.5 cm superior to the external occipital protuberance and 5.5 cm lateral to midline for extraventricular long-axis hippocampal cannulation in adult patients. Identification of this point is particularly important when positioning the stereotactic frame. ABBREVIATIONS: AC-PC, anterior commissure-posterior commissure CRW, Cosman-Roberts-Wells EEG, electroencephalographic EOP, external occipital protuberance SLA, stereotactic laser ablation