Clinical Trajectory of Late‐Onset Delusional Disorder Toward Evolving Dementia With Lewy Bodies

ABSTRACT

Recent clinical observations indicate that late‐onset psychosis can precede the onset of dementia with Lewy bodies (DLB). However, its longitudinal clinical trajectory remains poorly understood. Delusional disorder (DD) in older adults is relatively uncommon and poor insight frequently leads to refusal of psychiatric care, limiting available data despite its long‐standing diagnostic recognition. This article focuses on late‐onset DD in relation to subsequent development of DLB. We revisited our previously reported cases in which delusions were the initial manifestation of Lewy body disease (LBD) and aimed to identify the shared clinical features to help delineate more homogeneous subgroups within the heterogeneous spectrum of late‐onset psychosis. These cases with late‐onset DD revealed a recurring phenotype characterised by onset in the 50s and 60s, reluctance to seek psychiatric care, eventual hospitalisation, poor antipsychotic tolerance and delayed emergence of dementia accompanied by core features of DLB. Clinicopathological examination of four cases raises the hypothesis that persistent delusions may emerge in the presence of relatively restricted Lewy pathology, preceding the development of widespread cortical involvement and overt dementia. Relative preservation of neuronal populations in the substantia nigra may account for the absence of parkinsonism during the early phases dominated by isolated delusions. Despite long‐standing delusions beginning in the 50s and 60s, a common age range for DLB onset (mid‐70s and older) was observed, indicating an exceptionally prolonged prodromal period. These findings suggest that late‐onset DD associated with LBD may follow a clinical trajectory distinct from that of mild cognitive impairment with Lewy bodies (MCI‐LB), the most common clinical phenotype of prodromal DLB. Differentiating DD from broader late‐onset psychosis categories may facilitate future research by reducing heterogeneity and stratifying cases with potentially prolonged periods without dementia or parkinsonism. Clinical and pathological reports of late‐onset DD remain scarce and further case documentation is warranted.