A proposal for a new concept in psychiatric phenomenology: The "Yume-Utsutsu" (dreamy or half-asleep) phenomenon in patients with dementia with Lewy bodies.

Background: Patients with dementia with Lewy bodies (DLB) present with a variety of physical and mental symptoms, including visual hallucinations, delusions, depression, rapid eye sleep movement behavior disorder, and olfactory dysfunction. This study describes another interesting psychiatric symptom, which seems to be characteristic of DLB, in many patients who visited the author's psychogeriatrics department for neurocognitive disorders or psychogeriatric diseases.

Methods: The clinical courses of seven representative cases of DLB, each of which included episodes of this interesting psychiatric symptom, are described to clarify the commonalities they shared.

Results: After examining the characteristic symptoms of these cases, a novel term was proposed for a new concept in psychiatric phenomenology, the "Yume-Utsutsu" (dreamy or half-asleep) phenomenon. "Yume-Utsutsu" is a Japanese word that means a mental state in which dreams are indistinguishable from reality. This phenomenon is different from visual hallucinations, fluctuating cognition, rapid eye sleep movement behavior disorder, delirium, and epilepsy-related automatism.

Conclusions: The "Yume-Utsutsu" (dreamy or half-asleep) phenomenon proposed as a novel psychiatric concept in this article is useful for diagnosing and clarifying the psychopathology of DLB.