Dysfunction of olfactory structures: A conserved mechanism of neurodegeneration?

The olfactory system participates in the reception, integration, and interpretation of olfactory signals. This chemical sense is essential for survival since it is involved in basic behaviors and physiological processes. Olfactory function decreases with age; however, olfactory impairments are also observed in several neurodegenerative and psychiatric pathologies. It is widely described that olfactory dysfunction is an early symptom in Parkinson (PD) and Alzheimer (AD) diseases, furthermore, olfactory brain areas are affected by the pathological hallmarks of these diseases before the brain areas involved in the motor and cognitive impairments, respectively. This information suggests a key role of olfactory system damage in the beginning of a neurodegenerative process. Not only does the early injury of the olfactory system occur in AD and PD, but also in other pathologies since increasing evidence indicate the presence of olfactory impairments in other neurogenerative and psychiatric diseases namely, depression, schizophrenia, and autism among others. In addition, people with systemic chronic diseases that promote central nervous system damage such as type 2 diabetes and obesity also show olfactory dysfunction, which also suggests that olfactory alterations in these individuals could be an early manifestation of a neurodegenerative process. Then, the aim of the manuscript is to describe the information which supports that olfactory system impairment is a prodromal factor for the development of several neurodegenerative and psychiatric disorders, to recognize it as a shared mechanism of degeneration among diverse neuropathologies, and discuss the relevance of the assessment of the olfactory function in the diagnosis and improvement of neurodegeneration.