Alzheimer’s disease (AD) is the most common form of dementia, however a single cause is yet to be identified. It is thought that a combination of genetic and other factors are most likely to increase a person’s risk of developing the disease. Genetic research has confirmed that the ε4 allele of the apolipoprotein E (ApoE) gene, found on chromosome 19, is the strongest genetic risk factor for AD.

AD is usually associated with progressive memory loss and functional decline, however parts of the brain associated with sensory function, including smell, are also affected. 

New research published in Behavioral and Brain Functions by Charlie D Morgan and Claire Murphy from San Diego State University shows that the regions of the brain which respond to olfactory stimulus begin to decline sooner  in people with an increased risk of AD.

They compared event-related potentials in an odor identification test and a picture identification test in healthy adults in three different age groups (young, middle age and older).  Half the people in each age group were
positive for the ε4 allele of the ApoE gene.

Results of the study showed that younger participants correctly identified more odors than both the middle and older age groups.  Individuals at genetic risk of developing AD were not as good at recognizing odors and they demonstrated differing patterns of brain activity from the control group.  This indicates that processing of olfactory stimuli is affected by presence or absence of the ε4 allele across age groups.

These findings suggest that ApoE-related olfactory functional decline is taking place at much earlier ages than
previously observed, providing the potential for pre-clinical diagnosis of AD at an early stage.