It’s important to identify dementia as early as possible, and identifying the beginning of the transition from healthy aging to dementia has become the focus of recent research.

Loss of episodic memory (remembering events in one's life that can be explicitly stated) is commonly linked to Alzheimer's disease, but it is not the only aspect of cognition (thinking, learning and memory) that is affected.

A study of 444 participants suggests that cognitive abilities other than memory, including visuospatial skills needed to perceive relationships between objects, could decline years prior to a clinical diagnosis in patients with Alzheimer's disease.

Researchers assessed individuals who were dementia free when enrolling in the study. Each participant underwent a clinical evaluation and a psychometric assessment including tests of four cognitive factors: global cognition, verbal memory, visuospatial skill and working memory.

Over an average follow-up of 5.9 years, 134 individuals developed dementia and 310 did not; 44 with dementia died and brain autopsies confirmed a diagnosis of Alzheimer's disease. Using data from the psychometric assessments, the researchers constructed models to evaluate the decline in various cognitive areas before individuals were diagnosed with dementia. The researchers found that visuospatial abilities demonstrated an inflection point [sudden change to a steeper slope of decline] three years before clinical diagnosis.

Declines in overall cognitive abilities followed in the next year, whereas inflection points for verbal and working memory were not seen until one year before clinical diagnosis. Similar results occurred in only the subgroup of individuals with Alzheimer's disease diagnosis confirmed by autopsy.

"There are several implications of this study," the authors conclude. "Some of the earliest signs of preclinical disease may occur on tests of visuospatial and speeded psychomotor skills. Furthermore, the greatest rate of preclinical decline may occur on executive and attention tasks. These findings suggest that research into early detection of cognitive disorders using only episodic memory tasks, such as word lists or paragraph recall, may not be sensitive to either all of the earliest manifestations of disease or the most rapidly changing domain."

"In summary, converging longitudinal evidence suggests that after a sharp departure from the relatively flat course of normal aging there is a preclinical period in Alzheimer's disease with insufficient cognitive decline to warrant clinical diagnosis using conventional criteria but that can be seen with longitudinal data from multiple domains of cognition and not just memory," they conclude.
References:
1. David K. Johnson, et al. Longitudinal Study of the Transition From Healthy Aging to Alzheimer Disease. Arch Neurol, Oct 2009; 66: 1254 - 1259.