Exploring Olfactory Bulb Volume and Its Shrinkage in Aging and Neurodegeneration: A Systematic Review and...

The olfactory bulb (OB) plays a crucial role in processing smells and has significant neuroplasticity throughout life. Age-related changes in OB volume (OBV) are associated with declining olfactory function, potentially impacting quality of life and serving as an early marker of neurodegenerative diseases. This study conducted a meta-analysis to assess OBV changes across diverse age groups in healthy individuals, explored its association with olfactory function, and further examined OB atrophy in Parkinson's disease (PD) and Alzheimer's disease (AD).

A systematic review and meta-analysis were conducted following PRISMA guidelines. Studies evaluating OBV through MRI in healthy individuals and patients with PD or AD were included. Data were extracted on age, sex, olfactory function, and OBV. Meta-regression was performed to assess the correlation between OBV and age, while subgroup analyses examined the effects of sex and laterality.

Twenty-nine studies were analyzed, including 12 on healthy individuals, 7 on AD, and 11 on PD. The pooled mean OBV was 54.5 mm ³ (95% CI, 42.03-66.98) for the right OB and 55.56 mm ³ (95% CI, 42.96-68.15) for the left OB, with no significant sex or laterality differences. OBV showed a moderate negative correlation with age ( r  = -0.53 to -0.59, p  < 0.05), suggesting progressive atrophy with aging. Olfactory function, assessed through the Threshold, Differentiation, and Identification (TDI) and the University of Pennsylvania Smell Identification Test (UPSIT) scores, was also moderately correlated with OBV ( r  = 0.48, p  < 0.01). In neurodegenerative diseases, OBV reduction was greater, with shrinkage of 0.9 to 0.93 SD in PD and 1 to 1.05 SD in AD, primarily attributed to pathological degeneration.

Age-related OBV reduction is a normal physiological process with a moderate impact on olfactory function. While neurodegenerative diseases exacerbate OB atrophy, at least 40% of OB shrinkage observed in PD appears to be age-related. OBV could serve as a potential biomarker for aging and early neurodegeneration.