Olfactory Dysfunction in Neurodegenerative Diseases

INTRODUCTION Olfactory impairment is a common occurrence in aging and may be an early signal of neurodegenerative diseases. Olfactory loss caused by aging and diseases affects both quality of life and personal safety. Aging, as well as viral infections, neurodegenerative diseases, head injuries, chronic sinusitis infections and nasal obstructions are the most common causes of olfactory disorders (73). In addition, at least one third of older people report dissatisfaction with their sense of taste or smell (1, 70). The impact of sensory loss on elders is not only physiological but also emotional. The sense of smell and its capacity is critical for most mammals in terms of identification and evaluation of food, mates Interest in olfactory dysfunction has increased tremendously in the past decade, in part due to observations that chemosensory impairment is an early symptom of many neurodegenerative diseases. Several features of the olfactory system make it particularly relevant to understanding neurodegeneration/regeneration. First, while being vulnerable to environmental and infectious exposure, the olfactory system has the unique property of ongoing replacement of the olfactory sensory neurons under physiological conditions and following injury. Second, the receptor neurons residing in the periphery are developmentally related to the central nervous system, yet are accessible relatively noninvasively via biopsy from living subjects. Third, olfactory performance can easily be tested in a variety of ways that provide insight into the function of brain areas involved in detection, identification and memory. In addition to providing insights into the etiology or diagnosis of disease, a better understanding of olfactory neurobiology is needed to develop ways to treat olfactory dysfunction, which affects both quality of life and personal safety. At least 3,000,000 Americans suffer from chemosensory disorders and that is likely to grow as the aging segment of the population increases. Olfactory impairment represents a danger to the individuals resulting from inability to detect hazards as natural gas and spoiled food and threatens quality of life through the loss of enjoyment of foods and fragrances. The neurological systems responsible for olfactory function represent perhaps the most diverse, complex and adaptable components of the nervous system. Losses in olfaction result from changes at both the anatomical and molecular level. This loss can result from aging, toxins, infectious agents, environmental factors and a variety of diseases. Understanding the neurobiology of this sensory system may help us to develop new diagnostic measures and treatments for neurodegenerative disease as well as improving the quality of life for millions of people who are handicapped by the inability to detect the flavors and fragrances around them.