The sense of smell is the first sense that is lost in the course of the disease with Alzheimer's. Directly linked to this is the activity of so-called “grid cells”. The grid cells (https://en.wikipedia.org/wiki/Grid_cell) are located in the ERC, the entorhinal cortex (https://en.wikipedia.org/wiki/Entorhinal_cortex) and enable orientation in space. It is therefore not surprising that after the loss of the sense of smell in AD patients, the sense of orientation is the next to be lost.

As an early indicator of AD, even before the onset of initial cognitive deficits, a well-founded olfactory test is excellently suited, as used by Devanand et al.(2019) in a long cohort study (over 1000 participants) with 2 follow-ups (almost 800 participants) and examined for its predictive power with regard to the future occurrence of AD.

The B-SIT test used (12 odours, 5 min duration in total), a subset of UPSIT (https://en.wikipedia.org/wiki/University_of_Pennsylvania_Smell_Identification_Test), is inexpensive and would, according to Devanand (2019), if a memory test were also carried out with an intact sense of smell and acceptable memory, indicate the probability of a transition to AD in the next 4 years with only 3.4%. The current study showed that B-SIT in combination with a (simple, short) memory test was able to rule out the transition to AD for 96.6% of the subjects in the next 4 years due to its multimodality. It is assumed in Devanand (2019) that improved neuropsychological tests will be able to further improve this result. Of course, the personal context must be taken into account: history, nasal diseases, exposure to toxic substances, including smoking. Vasavada et al (2017) showed that the loss of the sense of smell is not caused by a deficit of the olfactory sensors but by the central olfactory system.

In the course of an extended literature search in QI 2020, the aim is to find out what other possibilities there are for using olfactory sense testing to predict AD as early as the preclinical stage.

References:

1. Devanand D.P., Lee S., Luchsinger J.A., Andrews H., Goldberg T., Huey E.D., Schupf N., Manley J., Stern Y., Kreisl W.C., Mayeux R. (2019). Intact global cognitive and olfactory ability predicts lack of transition to dementia. Alzheimer’s & Dementia - (2019) 1-9. Elsevier Inc. https://doi.org/10.1016/j.jalz.2019.08.200
2. Vasavada M.M., Martinez B., Wang J., Eslinger P.J., Gill D.J., Sun X., Karunanayaka P., Yang Q.X. (2017). Central Olfactory Dysfunction in Alzheimer´s Disease and Mild Cognitive Impairment: A Functional MRI Study. Journal of Alzheimer’s Disease 59 (2017) 359–368. DOI 10.3233/JAD-170310. IOS Press.