Introduction

Talking about taste always requires clarification between the medical term taste and “taste” as it is used in daily language. Taste used in everyday language means the sensations one experiences during eating or drinking. This is composed of texture, retronasal smell, and “pure” taste inputs. The physiological definition of taste is restricted to the five qualities sweet, sour, salty, bitter, and umami (the taste of glutamate). As a consequence, most people confuse olfaction and gustation. For example, most patients with olfactory dysfunction complain of smell and taste alteration, while testing reveals that olfaction but not the five basic tastes are affected.1

In clinical practice, patients rarely present with taste disorders restricted to the five basic tastes. The reason for this low incidence is not clear. One explanation could be that many taste disorders are transient, and patients only seek medical advice for long-lasting or permanent taste disorders. Another explanation could be that many patients with taste disorders have concomitant, more troublesome health problems (e.g., facial paresis, stroke) and forget to mention the taste disorder. Taste assessment is far from a routine procedure, and the low incidence of taste deficits may simply reflect the fact that this sense is rarely tested. Finally, taste is represented by three cranial nerves per side, making taste innervation redundant. This could lead to a higher resistance of taste to trauma and injury, thus explaining the few taste disorders observed in clinical practice. An intact sense of taste is significant in the regulation of nourishment, the recognition of poisonous and rotten food, and even for the experience of the hedonic aspects of a common meal.

Recent routine taste testing in a large sample of the general population showed that measurable taste impairment is more frequent than previously thought. In a cohort of 761 healthy individuals aged 5 to 89 years, reduced taste function was measured in 5%, despite the fact that these individuals considered their taste capacity to be normal. Most of these cases were hypogeusia; ageusia was observed rarely.2 While taking the patient’s history, it is therefore important to ask specific questions such as, “Do you have problems tasting sweet, sour, salty, or bitter flavors?” The isolated question, “Do you have taste problems?” is too vague and will be answered too often with no, even when taste problems exist.3

Taste problems can be an important sign or symptom in daily clinical practice, as several case vignettes report. Severe taste disorders can occur after laryngoscopy,4 stroke,5 temporal bone fracture,6 dissection of the brain-supplying arteries,7 in celiac disease,8 or even as a paraneoplastic symptom.9 Such severe taste disorders can lead to anorexia, weight loss, and impairment of well-being. An isolated impairment of gustatory sensitivity to salt was recently identified as a risk factor for arterial hypertension.10

Typical Clinical Findings

Taste disorders do not have a distinct pathognomonic clinical finding that can be identified easily. The clinical presentations are rather heterogeneous, and here we try to describe the different types of gustatory disorders that can be present and how they can be diagnosed.

Classification of Taste Disorders

Taste disorders can be clinically classified as quantitative or qualitative disorders. The most important quantitative taste disorders are ageusia, a complete loss of taste, and hypogeusia, a diminished taste. The most important qualitative taste disorders are dysgeusia and parageusia, characterized by a distorted sense of taste or an ongoing bad taste even in the absence of a taste stimulant. The dysgeusia taste is often described as bitter, sour, or metallic. Phantogeusia is a gustatory hallucination and is encountered in patients with epilepsy or psychotic disorders. Difficulty identifying a tastant that has been properly perceived is termed taste agnosia. It is assumed that the anteromedial temporal lobe has an important role in recognizing taste quality.14 Despite the dichotomous classification in clinical practice, quantitative and qualitative taste disorders often coexist. The main difference between qualitative and quantitative taste disorders is that the latter can be tested objectively and assessed psychophysically, whereas qualitative disorders cannot yet be assessed by any available technique. Qualitative taste disorders are comparable to tinnitus in hearing: a frequent, bothersome complaint that may occur with or without hypoacousis and cannot be measured. Qualitative taste disorders (dysgeusia, phantogeusia, parageusia) are much more frequently encountered in a clinical setting than quantitative taste disorders ( Table 15.1 ). An alternative way to classify taste disorders is to distinguish between central or peripheral taste disorders ( Fig. 15.1 ) according to the site of the lesion. Finally, taste disorders can be classified according to their cause ( Table 15.2 ). Congenital ageusia has never been reported to the best of our knowledge (unlike congenital anosmia, which is frequent15). Selective taste deficiencies such as ageusia for a single taste quality seem to be rare (except for bitter). Only a few reports of these exist, and it is not clear if this is a result of under-reporting or the particular biological importance of an intact taste function preventing such isolated and congenital ageusia.16,17

Pathophysiology

Often, we do not know the exact pathological mechanism underlying taste disorders, which is reflected by the classification systems. Two of the three classification systems presented here are clinical and observational classifications without any explanation about the underlying mechanism. We observe patients with medication side effects, but barely know any molecular details about the disorders. In contrast, the pathological mechanism seems clear in situations where a lesion is visible or documented (as seen in Fig. 15.1 ). In these cases, the taste pathway is directly injured.

Etiology of Taste Disorders

To determine etiology of taste disorders the following differential diagnostic approach has been shown to be useful:

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  Is the taste disorder caused by drugs or toxins?

  
  
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  Is the taste disorder caused by local factors in the oral cavity, for instance atrophy of the mucosa, physical or chemical injuries, alteration of the composition of saliva, or otorhinolaryngological or dental diseases (e.g., Candida albicans)?

  
  
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  Is the taste disorder caused by a disease of the peripheral nervous system?

  
  
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  Is the taste disorder caused by a disease of the central nervous system?

  
  
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  Is the taste disorder caused by a systemic disease?

  
  
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  Did the taste disorder occur after any distinct event (e.g., infection, surgery, etc.)?

Related posts:

Burning Mouth Syndrome and Qualitative Taste and Smell Disorders Providing Expert Opinion on Olfactory and Gustatory Disorders Flavor: Interaction Between the Chemical Senses Structural Imaging in Chemosensory Dysfunction Taste Testing Functional Anatomy of the Gustatory System: From the Taste Papilla to the Gustatory Cortex

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