Mapping Hippocampal Subfield Connectivity Through Super-Resolution Diffusion Imaging: Exploring Interactions Between Limbic Network Integrity, Autonomic Cardiovascular Regulation, and Arrhythmogenic Risk

Rachel Fernández Bravo, MD/FM Camilo Fernández Bravo, MD/PhD/FACC Department of Cardiology, University of UCM Carlos J Finlay, Cuba Correspondence: Camilo Fernández Bravo, MD/PhD/FACC, Email: camilo.fb@ub.edu.br Abstract Background: The hippocampus, a key component of the limbic system, plays a critical role in memory, emotion, and autonomic regulation. Disruptions in hippocampal subfield connectivity may influence autonomic cardiovascular control, potentially elevating arrhythmogenic risk. This study employs super-resolution diffusion imaging to map hippocampal subfield connectivity and explore its interactions with limbic network integrity, autonomic function, and arrhythmia susceptibility. Methods: We recruited 50 participants (25 with mild autonomic dysregulation and elevated arrhythmogenic risk factors, 25 healthy controls; mean age 55 ± 10 years). High-resolution hybrid diffusion imaging (HYDI) was acquired at 3T, enhanced to 0.625 mm isotropic resolution using collaborative locally adaptive super-resolution (CLASR). Hippocampal subfields (CA1-4, dentate gyrus [DG], subiculum [SUB]) were segmented via automatic segmentation of hippocampal subfields (ASHS). Tractography and connectometry analyses assessed structural connectivity. Autonomic cardiovascular regulation was evaluated via heart rate variability (HRV), baroreflex sensitivity, and electrocardiographic markers of arrhythmogenic risk (e.g., QTc interval). Correlations were examined using Pearson’s r and regression models, with false discovery rate (FDR) correction. Results: Super-resolution imaging revealed reduced fractional anisotropy (FA) and quantitative anisotropy (QA) in CA1, CA3, and DG subfields among participants with autonomic dysregulation (p < 0.05, FDR-corrected). Diminished connectivity between hippocampal subfields and limbic regions (amygdala, insula, cingulate gyrus) correlated with lower HRV (r = -0.62, p < 0.01) and prolonged QTc (r = 0.58, p < 0.01). Participants at higher arrhythmogenic risk showed hypoconnectivity in the central autonomic network (CAN), predicting major adverse cardiac events (MAEs) with 78% accuracy in Cox regression. Conclusions: Hippocampal subfield connectivity disruptions impair limbic network integrity, dysregulating autonomic cardiovascular control and increasing arrhythmogenic risk. Super-resolution diffusion imaging offers a novel tool for early detection and intervention in neurocardiac pathologies. Keywords: Hippocampus, Diffusion Imaging, Limbic System, Autonomic Regulation, Arrhythmia, Connectivity Introduction The hippocampus, integral to the limbic system, orchestrates cognitive, emotional, and autonomic functions through intricate subfield networks.¹ Subfields such as cornu ammonis (CA) regions 1-4, dentate gyrus (DG), and subiculum (SUB) exhibit distinct connectivity patterns, influencing memory consolidation and stress responses.² Aging and pathological states, including cardiovascular diseases, exacerbate hippocampal atrophy and connectivity loss, linking to cognitive decline and autonomic imbalances.³ Autonomic cardiovascular regulation, mediated by sympathetic and parasympathetic branches, maintains homeostasis via the central autonomic network (CAN), encompassing the insula, amygdala, cingulate gyrus, and hypothalamus.⁴ Limbic dysregulation, particularly in hippocampal-amygdalar circuits, can precipitate sympathetic overactivation, reducing heart rate variability (HRV) and heightening arrhythmogenic risk.⁵ Studies indicate that hippocampal injury correlates with prolonged QTc intervals and sudden cardiac death (SCD) in heart failure patients.⁶ Diffusion tensor imaging (DTI) and hybrid diffusion imaging (HYDI) have advanced structural connectivity mapping, but conventional resolutions (1-2 mm) suffer from partial volume effects, obscuring submillimeter subfields.⁷ Super-resolution techniques, like CLASR, enhance resolution to 0.625 mm, enabling precise tractography of perforant pathways and fornix bundles.⁸ Prior feasibility studies demonstrate age-related FA reductions in CA1 and DG, associating with limbic hypoconnectivity.⁹ This study hypothesizes that impaired hippocampal subfield connectivity disrupts limbic network integrity, altering autonomic cardiovascular regulation and elevating arrhythmogenic risk. Using super-resolution HYDI, we map these interactions in a cohort with varying autonomic profiles, aiming to identify neurocardiac biomarkers.¹⁰ Methods Participants Fifty adults (age 45-65 years; 28 females) were enrolled from cardiology clinics in Porto Seguro, Brazil, between January 2024 and December 2025. Inclusion criteria: 25 with mild autonomic dysregulation (defined by HRV