Loss of Smell and Taste: Epidemiology and Impact on Quality of Life

10.1055/b-0034-91128

Loss of Smell and Taste: Epidemiology and Impact on Quality of Life

Steven Nordin, Hely Tuorila

Introduction

From an evolutionary perspective, the chemical senses are the oldest of our senses, and therefore differ considerably with regards to role and function from the younger senses, such as vision and hearing. The life-preserving role of the chemical senses requires smell and taste loss to be taken seriously by the clinician.

The most important role of the human sense of smell is to guide our attention toward hazards (e.g., poisonous fumes and spoiled food) and toward items that in a general sense have positive connotations (e.g., nutritious food). This guidance is primarily driven by the hedonic value (i.e., pleasantness or unpleasantness of an odorous item [e.g., food]), which, to a large extent, is determined by the individual′s personal history with that item. The relatively strong positive or negative emotions often evoked by smells are also shaped by prior experience, and believed to enhance the appropriate behavioral response. Apart from being an important chemical warning system for safety issues and for regulating food intake, the sense of smell is involved in social communication.1 An infinite number of volatile chemical compounds can evoke a smell sensation, and a smell is, in many cases, an integrated sensation caused by a combination of such compounds.

The sense of taste functions as a gatekeeper of the internal milieu, acting to guide ingestive and avoidance behaviors.2 As with smell, hedonic value is an important perceptual dimension of taste, with close ties to motivational behavior. Taste qualities provide information about the presence of useful substances or dangers: carbohydrate energy sources in sweetness, sodium in saltiness, and proteins in umami (“meaty,” “brothy”) perception; and possible danger from acids and toxins in sour and bitter perceptions, respectively.

The most common cause of a loss in smell or taste is aging, a natural condition applying to an increasing number of people in the developed world. Typically, age-associated loss in the chemical senses appears progressively after the age of 60 years, and shows large individual variation, greatly afflicting some and leaving others intact. Pathological losses in smell and taste are caused by disease, trauma, working conditions, and lifestyle, and can appear either abruptly or progressively. Congenital losses in smell and taste are rare.

A complete smell or taste loss is called anosmia or ageusia, respectively, whereas diminished smell and taste sensitivity is called hyposmia and hypogeusia, respectively. The terms functional anosmia and functional ageusia are commonly used in clinical work, referring to some sensations being evoked, but not sufficiently for smell and taste to be useful in daily life (see also Chapter 5 Diagnostic and Clinical Work-Up). The smell loss can be temporary (e.g., during viral disease), causing some discomfort or loss of appetite, but with no major disturbance. In contrast, long-term or permanent smell or taste loss is a clinically challenging condition.

Persons may also seek medical attention for hypersensitivity to ambient odorants, with a symptom profile that can vary considerably between individuals. Those who consider themselves to be sensitive to odors report higher annoyance from odors than those who perceive themselves to be less sensitive.3 However, this hypersensitivity appears to be predominantly cognitive in origin, as persons with these complaints typically have normal smell detection sensitivity. It has been documented that practitioners request more knowledge and clinical guidelines for the management of this groups of patients.4

The objective of this chapter is to provide a review of what we know today about the epidemiology of loss in smell and taste, and its impact on quality of life (QoL). As aging accounts for much of these sensory losses, and as ear, nose, and throat (ENT) and related clinicians also meet patients with hypersensitivity to ambient odorants, these issues will also be reviewed briefly.

Epidemiology of Smell and Taste Loss

Smell Loss

The olfactory epithelium is directly exposed to potentially neurotoxic compounds in surrounding air, making this sensory system vulnerable to pathology. Furthermore, the system depends on only one cranial nerve to mediate information from the epithelium to the brain.

Smell loss is a common condition. Large-scale population-based studies with clinical examination show a prevalence of smell loss of 19% in Swedish adults (≥ 20 years),5 22% in German adults (25 to 75 years),6 and 24% in US adults (≥ 53 years).7 These prevalence rates per se may lead one astray if not specified by age factor. The typical increase in prevalence with aging, and the slightly higher prevalence in men than in women is illustrated in Fig. 1.1 . The prevalence of smell loss in the general adult population, when based on self-reports, has been reported to vary between 9.5 and 15.3%.7,8 Although self-reports of smell loss underestimate prevalence rates obtained by smell testing,7,9 it can be assumed that it is predominantly the individual′s self-evaluation of his or her sense of smell that will determine whether he or she will seek medical attention. The under-reporting of smell loss is likely to be related to unawareness of smell loss, which is common in old age.

Regarding clinical populations, a review10 of studies focusing on consecutive cases at smell and taste clinics in the United States, Europe and Japan suggests that the most common etiologies of smell loss are postviral upper respiratory tract infection (URTI) (18 to 45% of the clinical population) and nasal or sinus disease (7 to 56%), followed by head trauma (8 to 20%), exposure to toxins or drugs (2 to 6%), and congenital loss (0 to 4%). Another study of ENT patients in Germany, Austria and Switzerland shows similar results.11 Conversely, the percentage of patients with these conditions who have clinically proven smell loss is rather high: 76 to 95% in postviral URTI, 72 to 98% in nasal/sinus disease, 86 to 94% in head trauma, 67% in exposure to toxins/drugs, and 100% in congenital cases.10 Loss due to postviral URTI, head trauma, and exposure to toxins or drugs are, in some cases, spontaneously reversible, whereas many cases of nasal or sinus disease can be treated with medication or with a combination of medical and surgical treatment.

Certain volatile compounds are perceived by some individuals, but not by others. An example of such a compound is androstenone, a hormonebased substance to which 20 to 40% of the population are anosmic.12 Such specific anosmia emphasizes the individuality of the perceptual world, but this type of loss is not likely to have a major consequence for QoL.

Taste Loss

Taste sensitivity is for several reasons robust compared with smell sensitivity. Not only are there three types of papilla and three cranial nerves on each side involved in mediating taste, but there is also so-called taste constancy that contributes to the robustness. This constancy is believed to be based on inhibitory feedback loops among the cranial nerves, so that damage to one taste nerve releases inhibition on others, resulting in intensification of sensations from other regions of the oral cavity. Despite this robustness, taste loss can result from damage to any location of the neural gustatory pathway, from the taste buds via the peripheral (facial, glossopharyngeal, and vagal nerve) and central nervous system (brainstem and thalamus) to the cerebral cortex.13

The inability to taste the bitter compounds phenylthiocarbamide and n-propylthiouracil is genetically determined and extensively studied,14 but, in general, there is a lack of population-based studies on taste loss with clinical examination. However, in a German population aged 25 to 75 years, 20% recognized three or fewer of the four tastants (sucrose, sodium chloride, citric acid, and quinine hydrochloride) when presented at suprathreshold concentrations.6 In contrast, only 0.14% of the US adult population indicated a loss in taste function.15 The large discrepancy in prevalence between these two studies (20 vs. 0.14%) may be attributable to a rather loose criterion for taste loss in the former study, and perhaps to unawareness of loss in the latter. A recent, large study examining over 700 individuals in Switzerland using psychophysical testing with two types of stimulus administration (taste spray and taste strips) revealed a 5% prevalence of taste disorder. However, none of the participants showed complete ageusia.16 Naming a taste, even in a cued task (multiple choice options provided), easily leads to confusion either between sour and bitter in both adults and children17,18 or between sour, bitter, and salty,16 because persons seldom receive training in chemosensory tasks. In any case, it is apparent that further epidemiological studies of taste loss and its consequences in the general population are needed.

Taste and chemesthetic perception, together with retronasal odor perception, constitute what we call the flavor of food. Owing to this integrated perception whereby retronasal odors are localized in the oral cavity, a considerable proportion of patients at ENT clinics who seek medical attention for taste loss do instead show smell loss. This is illustrated by data from the University of Pennsylvania Smell and Taste Center, showing that 66% of the patients complained of taste loss, but only 5% of these had a measured taste loss. The most common etiology was head trauma (30%), followed by postviral URTI, toxic exposure, medication induced, and idiopathic (13% for each etiology).19 The literature suggests a very large variability in the proportion of patients with measured taste loss (between 1 and 30%),19–22 probably because of the lack of consensus on appropriate clinical measures. Sparse documentation of recovery from taste loss indicates complete or incomplete recovery in 5 out of 10 cases.22

Summary

Smell loss is common in the general adult population (19 to 24%), but with a rather low self-awareness. Importantly, both prevalence and awareness are associated with increasing age. Common etiologies of smell loss among patients at smell and taste clinics include postviral URTI, nasal/sinus disease, and head trauma. A combination of too few population-based studies and large differences in the criteria used to diagnose taste loss makes it difficult to provide a good estimate of the prevalence of taste loss in the general population, although it has been reported at 5% in a non population–based study.16 Common etiologies of taste loss among patients at smell and taste clinics are head trauma, postviral URTI, toxic exposure, and use of medication.

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