Clock drawing phenotypes in community-dwelling African Americans and Caucasians: Results from the University of Alabama at Birmingham study of aging

Background: Two dementia patterns have been described: 'type 1' dementia is characterized by executive function impairment and posterior cortical impairment, and 'type 2' dementia is characterized by executive impairment and relatively preserved posterior cortical function. The Executive Clock Drawing Task (CLOX) has been used to discriminate between type 1, type 2, and normal cognitive phenotypes. The aim of this study was to describe the prevalence of these phenotypes in community-dwelling African American and Caucasian elders. Methods: 433 African Americans and 477 Caucasians over the age of 65 were recruited. Executive function was assessed using CLOX1, a command-directed clock drawing task. Posterior cortical impairment was assessed using CLOX2, a clock copy task. CLOX scores were combined to estimate the prevalence of type 1 phenotype (those with poor CLOX1 and CLOX2 performance) versus type 2 phenotype (those with only poor CLOX1 performance). Results: 351 (39%) subjects had poor executive performance. Three hundred (33%) subjects had a type 1 phenotype and this pattern was 2.5 times more common among African Americans than Caucasians. One hundred and thirty-seven (15%) subjects had a type 2 phenotype with no significant difference between African Americans and Caucasians. African American ethnicity was independently associated with poor CLOX1 performance after adjusting for sociodemographic factors, posterior cortical function, and global cognition. CLOX2 performance was not associated with ethnicity. Conclusion: The prevalence of poor executive performance in community-dwelling elders is high. African Americans were more likely to have a type 1 phenotype and these ethnic differences were not explained by sociodemographic variables alone.