Behavioural rigidity, the tendency to persist with inflexible patterns of thoughts or actions, is increasingly recognised as a transdiagnostic symptom across psychiatric, neurodevelopmental, and neurodegenerative disorders. Empirical studies exploring the prevalence and underlying neural mechanisms of behavioural rigidity in dementia, however, are lacking. This cross-sectional study sought to delineate the structural and functional neural correlates of behavioural rigidity using a transdiagnostic approach looking across the frontotemporal lobar degeneration (FTLD) spectrum and Alzheimer's disease. A total of 204 participants were recruited, including 110 frontotemporal dementia (FTD), 53 Alzheimer's disease (AD), and 41 healthy older control participants. Within the FTD group, 66 cases were diagnosed with clinically probable behavioural variant FTD (bvFTD), 26 presented with semantic dementia (SD), and 18 cases had progressive non-fluent aphasia (PNFA). Behavioural rigidity was assessed using the Stereotypical and Motor Behaviour subscale of the Cambridge Behavioural Inventory-Revised. Voxel-based morphometry (VBM) was performed to identify grey matter regions associated with behavioural rigidity transdiagnostically, the results of which informed subsequent seed-based voxel-wise functional connectivity analyses. All imaging analyses were adjusted for relevant demographic and technical covariates. Statistical thresholds were set at voxel-level p < 0.001 (uncorrected) and cluster-level p < 0.05 (FDR-corrected). Our main finding was that behavioural rigidity is pervasive across dementia subtypes, ranging from most pronounced in bvFTD to milder/relatively absent in PNFA, relative to Controls. Whole-brain VBM across the entire patient sample revealed a significant negative association between behavioural rigidity and grey matter intensity exclusively in the bilateral nucleus accumbens. Using the bilateral nucleus accumbens as seeds, resting-state functional connectivity analysis showed that higher levels of behavioural rigidity were associated with stronger connectivity between the left nucleus accumbens and the left supplementary motor area, paracentral lobule, and precuneus. This is the first study, to our knowledge, to examine the neural substrates of behavioural rigidity across FTLD syndromes transdiagnostically using structural and functional neuroimaging approaches. Our findings reveal a gradation of rigid and repetitive behaviours, most apparent in bvFTD, which in turn reflects pathological disruption of the nucleus accumbens. Taken together, our findings highlight the need to consider repetitive and rigid behaviours as a transdiagnostic feature in neurodegenerative disorders, and one which indexes underlying nucleus accumbens pathology. More broadly, this study underscores the importance of screening for rigid and repetitive behaviours in the clinic and identifies the nucleus accumbens as a promising neural target to ameliorate these symptoms.

Frontotemporal dementia (FTD) presents a complex spectrum of neurodegenerative disorders, encompassing distinct subtypes with varied clinical manifestations. This study investigates alterations in brain module organization associated with FTD subtypes using connectome analysis, aiming to identify potential biomarkers and enhance subtype prediction. Resting-state functional magnetic resonance imaging data were obtained from 41 individuals with behavioral variant frontotemporal dementia (BV-FTD), 32 with semantic variant frontotemporal dementia (SV-FTD), 28 with progressive non-fluent aphasia frontotemporal dementia (PNFA-FTD), and 94 healthy controls. Individual functional brain networks were constructed at the voxel level and binarized based on density thresholds. Modular segregation index (MSI) and participation coefficient (PC) were calculated to assess module integrity and identify regions with altered nodal properties. The relationship between modular measures and clinical scores was examined, and machine learning models were developed for subtype prediction. Both BV-FTD and SV-FTD groups exhibited decreased MSI in the subcortical module (SUB), default mode network (DMN), and ventral attention network (VAN) compared to healthy controls. Additionally, BV-FTD specifically displayed disrupted frontoparietal network (FPN) integrity compared to other FTD subtypes and controls. All FTD subtypes showed increased PC values in the insular region and reduced connections between the insular and VAN/FPN compared to controls. Moreover, significant associations between specific network alterations and clinical variables were observed. Machine learning models utilizing these matrices achieved high performance in differentiating FTD subtypes. This pilot study reveals diverse brain module organization across FTD subtypes, shedding light on both shared and distinct neurobiological underpinnings of the disorder.

Community-based surveys conducted in Japan investigating the prevalence of dementia and its underlying causes revealed that dementia of Alzheimer's type (DAT) is the most common, followed by vascular dementia (VaD), dementia with Lewy bodies (DLB), mixed dementia, and other conditions including frontotemporal lobar degeneration (FTLD). Accurate differential diagnosis of these disorders requires clarification of their clinical characteristics. The initial symptoms of DAT typically include recent memory loss, episodic memory impairment, and temporal disorientation. Behavioral and psychological symptoms often observed in DAT include delusions of theft, "saving appearance" responses, and head-turning signs. Vascular dementia develops in association with cerebrovascular disease and frequently exhibits a stepwise progression. DLB is characterized by core clinical features such as cognitive fluctuations, visual hallucinations, parkinsonism, and REM sleep behavior disorder. Diagnostic tools such as 123Iodine-metaiodobenzylguanidine (MIBG) myocardial scintigraphy and dopamine transporter (DAT) imaging may aid in diagnosis. In Parkinson's disease with dementia (PDD), cognitive impairment appears more than one year after the onset of parkinsonism. FTLD involves degeneration of the frontal and temporal lobes, leading to prominent changes in personality, behavior, and language function. Several subtypes of FTLD exist depending on the affected brain region, including the behavioral variant of frontotemporal dementia, semantic dementia, and progressive non-fluent aphasia. Idiopathic normal-pressure hydrocephalus (iNPH) is characterized by gait disturbance, urinary incontinence, and dementia, resulting from an abnormal accumulation of cerebrospinal fluid. Pathologically confirmed cases of DLB and progressive supranuclear palsy (PSP) may occasionally present with symptoms resembling iNPH.

Timely diagnosis is crucial for managing neurodegenerative conditions. This study investigated whether time from symptom onset to diagnosis differs by clinical syndrome and sex. This retrospective, cross-sectional study included 591 participants with Alzheimer's disease (AD), frontotemporal dementia (FTD) subtypes (behavioral variant FTD [bvFTD], semantic dementia [SD], and progressive non-fluent aphasia), logopenic progressive aphasia (LPA), and syndromes associated with movement disorders (corticobasal syndrome, FTD with motor neuron disease [FTD-MND], and progressive supranuclear palsy). Bayesian regression models were used to compute diagnostic timelines. Compared to AD (3.35 years; 95% credible interval [CrI]: 3.03-3.72), SD and bvFTD had additional delays of 9.7 (95% CrI: 1.96-20.64) and 14.82 months (95% CrI: 6.94-25.42), respectively, while FTD-MND was shorter by 11.62 months (95% CrI: -15.7 to -4.68). Men with bvFTD had 23.64 month longer delays than women (95% CrI: 10.35-44.33). Diagnostic delays may reflect syndrome-specific clinical features, diagnostic complexity, and sociocultural factors. Findings highlight the need for improved diagnostic pathways and pre-clinical biomarkers to facilitate earlier identification. Bayesian analyses revealed that diagnostic delays differ by syndrome and sex.Alzheimer's disease (AD) was diagnosed on average 3.35 years after symptom onset.Diagnoses were delayed in semantic and behavioral variant frontotemporal dementia (bvFTD) compared to AD.Men with bvFTD had longer delays than women.Findings support need for improved diagnostic pathways and pre-clinical biomarkers.