Purpose Delirium and dementia have overlapping features that complicate differential diagnosis. Delirium symptoms overshadow dementia symptoms when they co-occur, but delirium phenomenology in comorbid cases has not been compared to both conditions alone.

Methods Consecutive adults with DSM-IV delirium, dementia, comorbid delirium-dementia and cognitively intact controls were assessed using the Revised Delirium Rating Scale (DRS-R98) and Cognitive Test for Delirium (CTD).

Results Delirium and comorbid delirium-dementia groups had comparable DRS-R98 and CTD total scores, which were greater than in dementia or control groups. On the DRS-R98, multiple non-cognitive symptoms, inattention and disorientation were more severe in delirium groups compared with dementia-alone. Patients with dementia differed from both delirium groups on the CTD test of attention. Spatial span backwards was significantly lower in all patients with cognitive impairment (delirium, comorbid delirium-dementia, dementia alone) compared to controls, whereas spatial span forwards distinguished delirium groups from dementia.

Conclusions Delirium phenomenology is similar with or without comorbid dementia. A wide range of neuropsychiatric symptoms distinguish delirium from dementia. Spatial span forward is disproportionately diminished in delirium suggesting usefulness as a differentiating screening test.