Diffusion Spectrum Imaging Maps Early Axonal Loss and a Unique Progressive Signal in Neuronal Intranuclear Inclusion Disease

OBJECTIVE: To delineate specific in vivo white matter pathology in neuronal intranuclear inclusion disease (NIID) using diffusion spectrum imaging (DSI) and define its clinical relevance. METHODS: DSI was performed on 42 NIID patients and 38 matched controls. Microstructure was assessed via quantitative anisotropy (QA), generalized fractional anisotropy (GFA), and isotropy (ISO) across 48 white matter tracts. Group comparisons and clinical correlations used stringent multiple comparison corrections. RESULTS: Our analysis revealed a tripartite signature that constitutes a novel pathophysiological model for NIID: (1) Universal QA reduction (all Bonferroni-p < 0.05), indicating a pervasive loss of axonal integrity as an early event, detectable even before radiological abnormalities (FDR-p < 0.05); (2) GFA reductions in commissural/projection fibers (Bonferroni-p < 0.05) implicated network disconnection, correlating with cognitive/functional deficits (FDR-p < 0.1); (3) ISO decrease in brainstem-cerebellar pathways (Bonferroni-p < 0.05) identified a unique intracellular pathology, uniquely predicting disease duration (FDR-p < 0.05). INTERPRETATION: Our DSI framework resolves distinct pathological processes in NIID: QA serves as a biomarker for early detection, GFA maps to symptom heterogeneity, and ISO tracks disease progression, collectively advancing the pathophysiological model and clinical assessment of NIID.