Oral health and olfactory function : what can they tell us about cognitive ageing?

Abstract

The objective of this thesis was to advance our understanding of whether oral health and olfactory function may predict accelerated cognitive ageing. Data from two Swedish study populations and one from the United States were applied to investigate the relationship of oral health and olfactory function with cognitive decline and brain ageing in late life.

Study I examined the association of self-reported tooth loss with cognitive decline, and brain volume differences in older adults (n= 2715) from the Swedish National study of Aging and Care-Kungsholmen (SNAC-K). A subsample (n= 394) underwent magnetic resonance imaging (MRI). Tooth loss was associated with a steeper global cognitive decline (β: -0.18, 95% confidence interval [CI]: -0.24 to -0.11). Participants with complete or partial tooth loss had significantly lower total brain volume (β: -28.89, 95% CI: -49.33 to -8.45) and grey matter volume (β: -22.60, 95% CI: -38.26 to -6.94). Thus, tooth loss may be a risk factor for accelerated cognitive ageing. Study II Investigated the effect of poor masticatory ability on cognitive trajectories and dementia risk in 544 cognitively intact adults aged ≥50 from the Swedish Adoption/Twin Study of Aging (SATSA) with 22 years of follow-up. Masticatory ability was assessed using the Eichner Index and categorised according to the number of posterior occlusal zones: A (all four), B (3-1), and C (none). After the age of 65, participants in Eichner category B and C showed an accelerated decline in spatial/fluid abilities compared to those in category A (β: -0.16, 95% CI: -0.30 to -0.03 and β: -0.15, 95% CI: -0.28 to -0.02, respectively). Hence, poor masticatory ability is associated with an accelerated cognitive decline in fluid/spatial abilities. Study III examined whether impaired olfaction is associated with cognitive decline and indicators of neurodegeneration in 380 participants (mean age = 78 years) from the Memory and Aging Project (MAP). Participants with hyposmia (β = −0.03, 95% CI: −0.05 to −0.02) or anosmia (β = −0.13, 95% CI −0.16 to −0.09) had a faster global cognitive decline than those with normal olfaction. Impaired olfaction was related to smaller volumes of primarily the medial temporal cortex (β = −0.38, 95% CI −0.72 to −0.01). Olfactory deficits predict faster cognitive decline and indicate neurodegeneration in older adults. Study IV identified age-related trajectories in episodic memory and odour identification, as well as determinants of the trajectories. 1023 MAP participants were followed for up to 8 years with annual assessments. Three joint trajectories were identified; Class 1- stable performance in both functions; Class 2- stable episodic memory and declining odour identification; and Class 3- decline in both functions. Predictors of class membership were age, sex, APOE ε4 carrier status, cognitive activity, and BMI. Episodic memory and olfactory function often show similar trajectories in ageing, reflecting their shared vulnerability to changes in the medial-temporal lobes.

Conclusions: Both poor oral health and olfactory deficits may predict cognitive decline and indicate neurodegeneration in the brain. Poor oral health is associated with accelerated cognitive decline and brain ageing, whereas, olfactory deficits may reflect loss of brain integrity in old age.