Impaired olfaction is associated with cognitive decline and neurodegeneration in the brain

We aimed to examine whether impaired olfaction is associated with cognitive decline and indicators of neurodegeneration in the brain of dementia-free older adults.

Within the Rush Memory and Aging Project, 380 dementia-free participants (mean age = 78 years) were followed for up to 15 years, and underwent MRI scans. Olfactory function was assessed using the Brief Smell Identification Test (B-SIT) at baseline, and categorized as anosmia (B-SIT <6), hyposmia (B-SIT 6-10 in men and 6-10.25 in women), and normal (B-SIT 10.25-12 in men and 10.5-12 in women). Cognitive function was annually assessed with a battery of 21 tests, from which composite scores were derived. Structural total and regional brain volumes were estimated. Data were analyzed using linear regression and mixed-effects models.

At study entry, 138 (36.3%) had normal olfactory function, 213 (56.1%) had hyposmia, and 29 (7.6%) had anosmia. In multiadjusted mixed-effects models, hyposmia (β = -0.03, 95% confidence interval [CI] -0.05 to -0.02) and anosmia (β = -0.13, 95% CI -0.16 to -0.09) were associated with faster rate of cognitive decline compared to normal olfaction. On MRI, impaired olfaction (hyposmia or anosmia) was related to smaller volumes of the hippocampus (β = -0.19, 95% CI -0.33 to -0.05), and in the entorhinal (β = -0.16, 95% CI -0.24 to -0.08), fusiform (β = -0.45, 95% CI -0.78 to -0.14), and middle temporal (β = -0.38, 95% CI -0.72 to -0.01) cortices.

Impaired olfaction predicts faster cognitive decline and might indicate neurodegeneration in the brain among dementia-free older adults.