We aimed to investigate the association between very late-onset schizophrenia-like psychosis (VLOSLP), a schizophrenia spectrum disorder with an onset of ≥60 years, and Alzheimer’s disease (AD) using biomarkers.

Retrospective cross-sectional study.

Neuropsychology clinic of Osaka University Hospital in Japan.

Thirty-three participants were classified into three groups: eight AD biomarker-negative VLOSLP (VLOSLP−AD), nine AD biomarker-positive VLOSLP (VLOSLP+AD), and sixteen amnestic mild cognitive impairment due to AD without psychosis (aMCI−P+AD) participants.

Phosphorylated tau levels in the cerebrospinal fluid and 18F-Florbetapir positron emission tomography results were used as AD biomarkers. Several scales (e.g. the Mini-Mental State Examination (MMSE), Wechsler Memory Scale-Revised (WMS-R) Logical Memory (LM) I and II, and Neuropsychiatric Inventory (NPI)-plus) were conducted to assess clinical characteristics.

Those in both VLOSLP−AD and +AD groups scored higher than those in aMCI−P+AD in WMS-R LM I. On the other hand, VLOSLP+AD participants scored in between the other two groups in the WMS-R LM II, with only VLOSLP−AD participants scoring significantly higher than aMCI−P+AD participants. There were no significant differences in sex distribution and MMSE scores among the three groups or in the subtype of psychotic symptoms between VLOSLP−AD and +AD participants. Four VLOSLP−AD and five VLOSLP+AD participants harbored partition delusions. Delusion of theft was shown in two VLOSLP−AD patients and five VLOSLP+AD patients.

Some VLOSLP patients had AD pathology. Clinical characteristics were different between AD biomarker-positive and AD biomarker-negative VLOSLP, which may be helpful for detecting AD pathology in VLOSLP patients.

Dementia with Lewy bodies (DLB) is the second most common form of degenerative dementia after Alzheimer’s disease. In some patients with DLB, relatively rare delusions are known to emerge, such as Othello syndrome, delusional parasitosis and delusion of duplication. Erotomania, also known as de Clerambault’s syndrome, is characterized by the delusion that a person has fallen in love with the patient. It occasionally appears secondary to psychiatric disorders and organic brain diseases. However, there have been no reports on cases secondary to patients with DLB.

The patient was an 83-year-old woman who lived alone. Mild cognitive impairment appeared at the age of 82 years. Soon after, she had the delusional conviction that her family doctor was in love with her. Her symptoms, such as gradually progressive cognitive impairment, cognitive fluctuations, and parkinsonism, indicated DLB. Although small doses of quetiapine, brexpiprazole and risperidone were prescribed for the treatment of the delusion, each of them was discontinued soon because of the adverse reactions. Finally, the delusion was successfully treated with a small dose of blonanserin without sever side effects.

This case report suggests the possibility of de Clerambault’s syndrome during the early stages of DLB. Recently, psychiatric-onset DLB has increasingly gained attention in recent years. Further accumulation of knowledge about delusions in patients with DLB for an early diagnosis.