Tactile hallucinations in dementia with Lewy bodies.

Background: Visual hallucinations (VH) are one of the most common psychological symptoms of dementia with Lewy bodies (DLB). It is generally considered that the VH that occur in DLB usually disappear when patients try to touch imaginary objects. However, DLB patients also sometimes experience tactile hallucinations (TH). To the best of my knowledge, this is the first report to comprehensively describe the TH experienced by DLB patients.

Methods: I searched the literature for case reports that described TH in Parkinson's disease (PD), PD dementia, or DLB. There were four reports regarding TH in PD, one of which was a review article published in 2017. I described the clinical courses of five representative cases of DLB in which TH were associated with VH, and then compared the characteristics of the TH experienced in DLB with those of the TH encountered in previously reported cases of PD.

Results: The TH experienced by the DLB patients had very similar characteristics to the TH experienced by PD patients. However, the TH of the DLB patients differed from those of PD patients in the following respects: (i) when the DLB patients tried to touch the imaginary objects, they often did not disappear; (ii) the imaginary objects experienced by the DLB patients sometimes exhibited characteristic hardness and temperature; and (iii) the imaginary objects experienced by the DLB patients were often influenced by the patients' behaviour, which in turn led to further hallucinations.

Conclusions: Based on the abovementioned characteristics of the hallucinations of DLB patients, I propose that these phenomena represent a novel psychopathological entity, which I shall temporarily refer to as, 'strengthening of VH by TH'.