Multimodal fMRI and DTI correlates of affective symptoms in diarrhea-predominant irritable bowel syndrome: a subtype-focused neuroimaging study

Objective To investigate brain structural and functional alterations in diarrhea-predominant irritable bowel syndrome (IBS-D) and their associations with affective symptoms using multimodal magnetic resonance imaging (MRI). Methods 53 IBS-D patients and 55 healthy controls (HCs) underwent resting-state functional MRI (rs-fMRI) and diffusion tensor imaging (DTI). Rs-fMRI data were analyzed using fractional amplitude of low-frequency fluctuation (fALFF), z-transformed regional homogeneity (zReHo), and functional connectivity (FC). For DTI data, we extracted fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) under the tract-based spatial statistics (TBSS) framework. Inter-group comparisons of rs-fMRI metrics used FDR correction ( q < 0.05), whereas DTI metrics were compared using non-parametric permutation testing with TFCE correction ( p < 0.05). Abnormal imaging metrics were correlated with scale scores of symptoms, anxiety, depression and sleep quality. Results Compared with HCs, IBS-D patients showed increased fALFF in the left cuneus, left superior and middle occipital gyri, and left pericalcarine cortex; decreased fALFF in the right insula and bilateral precuneus/posterior cingulate cortex; and increased zReHo in the left thalamus/insula and sensorimotor regions. FC was altered, characterized by reduced insula-prefrontal connectivity, decreased sensorimotor-SMA connectivity, and enhanced thalamus/insula-hippocampal connectivity. DTI revealed lower FA in the left corticospinal tract, left cingulum bundle and right uncinate fasciculus with widespread MD and RD increases. Correlation analyses showed that fALFF in precuneus/posterior cingulate cortex was positively correlated with SAS ( r = 0.422, q = 0.045), zReHo in thalamus/insula with SDS ( r = 0.489, q = 0.013), and MD with PSQI ( r = 0.360, q = 0.047). Conclusion IBS-D is associated with multi-network, multi-level alterations involving the default mode network, thalamus-insula circuit, and sensorimotor network, with exploratory associations observed between select imaging dimensions and particular affective symptom dimensions. These findings provide subtype-focused, hypothesis-generating neuroimaging evidence for brain alterations in IBS-D.