The Predictive Role of Hyposmia in Alzheimer's Disease

Loss of olfactory function starts at 60 years and become significantly worse after 70. In many cases olfactory disorders may be a consequence of a disease. Different types of olfactory deficit may be revealed by smell evaluation. Anosmia is defined as inability to perceive all odors (total) or some odors (partial). Hyposmia or microsmia is a decreased sensitivity to odors. Dysosmia is a distorted smell percpeption. Olfactory agnosia is defined as failure to identify odors in presence of normal detection and discrimination. Olfactory allucinations are named phantosmias. Many common diseases may compromise the sense of smell, permanently or temporaneally. The range of diseases causing olfactory disorders varies from the common cold to neurodegenerative diseases. Most common causes of olfactory loss are local nasal diseases (allergic rhinitis, nasal polyposis, sinus disease), head trauma, viral and bacterial infections of upper airways. Some neurodegenerative diseases like Alzheimer's disease (AD) and various forms of Parkinson’s disease (PD) are accompanied, even from their earliest stages, by olfactory disorders. Dementia is defined by the American Academy of Neurology as a progressive and permanent decline in cognitive function and affects nearly 15% of people who live up to 65 years and 35% of those who reach the age of 85. The Alzheimer's Disease International (ADI) in Alzheimer World Report published in 2010 provides that an aging population with dementia the most common form is AD which is currently estimated to affect 35,6 million of people will nearly double in 20 years to reach 66 million in 2030 with a higher concentration in poor countries leading to enormous social costs. The research on AD is now oriented to an early diagnosis which is essential before the development of the irreversible and typical changes due to AD. In AD patients, a reduced capacity for olfactory detection, discrimination and identification is usually found and confirmed by several studies (Mesholam et al., 1998; Hawkes, 2003; Kovacs, 2004; Albers et al., 2006; Westervelt et al., 2007). In this chapter we present a review on the predictive role of hyposmia in the early diagnosis of AD patients.