Methodological factors in determining rates of dementia in transient ischemic attack and stroke: (I) impact of baseline selection bias.

BACKGROUND AND PURPOSE: Many previous studies on dementia in stroke have restrictive inclusion criteria, which may result in underestimation of dementia rates. We undertook a large prospective population-based study of all transient ischemic attack and stroke to determine the impact of study entry criteria on measured rates of pre- and postevent dementia. METHODS: All patients with acute transient ischemic attack or stroke from a defined population of 92 728 are referred from primary care or at hospital admission to the Oxford Vascular Study (2002-2007) and have baseline clinical and cognitive assessment and follow-up. We examined the impact of early death, other nonavailability, and commonly used selection criteria, on measured rates of dementia. RESULTS: Among 1236 patients (mean age/SD 75.2/12.1 years, 582 men, 403 transient ischemic attack), 139 died or were otherwise unavailable for baseline assessment, 319 had prior dependency, 425 had comorbidity, 512 were aged ≥80 years, 85 were dysphasic, and 502 were hospitalized. Pre-event dementia was 3-fold higher in patients dying preascertainment (10/47, 21%) and twice as high in other nonassessed (14/92, 15%) versus assessed patients (69/1097, 6%; P=0.0006 and P=0.002) and was several-fold higher in those with prior functional impairment (24% versus 3%; P<0.0001), age >80 years (13% versus 3%; P<0.0001), dysphasia (11% versus 7%; P<0.0001), and comorbidity (10% versus 6%; P=0.04). Findings for postevent dementia were similar: prior functional impairment (40% versus 13%; P<0.0001), age >80 years (28% versus 10%; P<0.0001), dysphasia (39% versus 15%; P<0.0001), and comorbidity (20% versus 15%; P=0.04). CONCLUSIONS: Exclusion of patients unavailable for assessment, and other widely used selection criteria, results in underestimation of the measured rate of dementia associated with transient ischemic attack and stroke.