Background: Although most people with dementia experience an insidious onset of symptoms, in some cases onset can be acute. The importance of acute onset is unclear. Some reports suggest that it portends a worse course.

Methods: We performed a secondary analysis of the clinical examination cohort (n=2914) of the Canadian Study of Health and Aging (CSHA). We defined “acute onset of dementia” from the Cambridge Examination for Mental Disorders in the Elderly (CAMDEX) questionnaire, conducted with an informant. People with dementia of acute onset were compared to those with dementia of insidious onset for development of adverse outcomes of death and institutionalization over 5years.

Results: Of the 1132 people who had dementia, 130 (11.5%) met criteria for acute onset. Compared with gradual-onset dementia patients, those with acute-onset dementia were more often men (42% vs. 30%, p<0.05), resided in nursing homes (75% vs. 63%, p<0.05), had vascular risk factors (72% vs. 47%, p<0.05), and a Hachinski Ischemia Scale (HIS) score ≥7 (64% vs. 19%, p<0.05). More patients with dementia of acute onset than gradual onset were diagnosed with vascular dementia (55% vs. 13%; p<0.05). Adjusted hazard ratios (HRs) for survival and institutionalization in the acute-onset group were 0.93 [95% confidence interval (CI) 0.7–1.2] and 0.76 (95% CI 0.4–1.3), respectively, compared with the gradual-onset group.

Conclusions: People with acute-onset dementia had more vascular risk factors than those with gradual-onset dementia across all dementia diagnoses, and lower risks of institutionalization but worse survival. Routine inquiry about the onset of dementia might help to better clarify prognoses in patients with dementia.