Smell and Taste Dysfunction as Early Markers for Neurodegenerative and Neuropsychiatric Diseases

During the last few decades a significant literature has evolved, suggesting that sensory dysfunction, particularly smell and taste dysfunction, can be early markers for neurodegenerative diseases such as Parkinson’s and Alzheimer’s and neuropsychiatric diseases including ADHD and Schizophrenia, all diseases that involve dopaminergic pathology. Smell loss and taste dysfunction appear in clinical versus non-clinical groups, and in longitudinal studies these symptoms have been noted years earlier than motor signs in the first degree relatives of individuals who already have the diseases. This paper is a review of the recent literature on empirical studies and reviews that have documented the results of sensory screenings of several groups with neurodegenerative and neuropsychiatric diseases and those first-degree relatives at risk for those diseases. Although early biomarkers could be useful in identifying those needing preventive intervention, the treatment literature is very limited. *Corresponding author: Tiffany Fie, University of Miami School of Medicine, Tel: 305 975 5029; E-mail: [email protected] Received February 12, 2015; Accepted March 18, 2015; Published April 25, 2015 Citation: Field T (2015) Smell and Taste Dysfunction as Early Markers for Neurodegenerative and Neuropsychiatric Diseases. J Alzheimers Dis Parkinsonism 5: 186. doi: 10.4172/2161-0460.1000186 Copyright: © 2015 Field T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Because of cross-cultural differences in smell identification, researchers from other countries have developed alternative versions that feature smells that are prevalent in their cultures including Brazil [30], Japan [31] and South Korea (who call theirs the cross-cultural smell test) [32]. Although most of the smell tests were designed for adults, a child’s version exists called the Sensory Identification Score [15], and infants with developmental delays are also being tested for sensory integration problems [33]. Others have evaluated the relationships between smell identification, taste threshold, dopamine transporter scan (DaTSCAN) and motor function and their diagnostic accuracy in early Parkinson’s disease and have suggested that a basic smell test is as sensitive as the DaTSCAN in the diagnosis of Parkinson’s [34],and, still others claim that they have not been as sensitive as the self-report measures [8]. Tests for taste have also been developed including sweetness, creaminess and pleasantness [35]. In that study, pleasantness identification was the most reliable of the three tests for taste. Liquid taste solutions for sweet, sour, salty and bitter have also been developed and have acceptable reliability [36]. Less conclusive data have been documented for taste dysfunction, although smell and taste disorders might be expected to be comorbid as those senses are often interactive, and many patients who have lost their sense of smell complain that their sense of taste is also blunted [9]. Some have noted that a damaged olfactory system reduces taste perception including sweet, sour, bitter and salty via the facial, glossopharyngeal and vagus nerves [37]. Comorbid smell and taste dysfunction has been reported for eating disorders, both bulimia nervosa and anorexia nervosa [22]. These were determined using the “Sniffin Sticks” method and the “Taste Strip” kit. Taste has been tested less often, probably because its assessment has been more difficult and aversive for research participants [35]. Journal of Alzheimer’s Disease & Parkinsonism J o u r n a l o f A lzh eim ers ease & Prkin s o n i s m