Taste Testing
General Remarks on Gustatory Testing
In the general population, isolated taste disturbances are rare. Using two different psychophysical taste tests, hypogeusia was found in ~5% of a group of 761 healthy subjects.1 Complete ageusia is very rare. Taste testing in clinical practice is mainly restricted to departments of otorhinolaryngology and neurology, or to taste and smell centers involved in the treatment of diseases that may cause taste disorders. Ear, nose, and throat (ENT)-related causes of taste disturbance include middle ear surgery, tonsillectomy, oropharyngeal surgery (including oncological treatment), and procedures associated with lingual compression.2
Taste in humans begins on the tongue, where taste cells of different gustatory papillae (fungiform, foliate, and circumvallate papillae with taste buds) detect contact-chemical cues. The three different morphological types of taste-detecting papillae are topographically arranged on the tongue. Fungiform papillae are located on the anterior two-thirds of the tongue, while foliate papillae and circumvallate papillae are found on the lateral edges and on the posterior third of the tongue, respectively. Sensitivity to all tastes (sweet, sour, salty, bitter, and umami) is found in all areas of the human tongue,3 and there is no evidence that there is a tongue map.4 However, some studies have revealed that some tongue regions are more sensitive to natural stimuli than others.5,6 Similar findings were reported in electrogustometric studies.7,8 In electrogustometry (electric stimulation of small regions of the tongue) in healthy humans, electric thresholds are higher on the tongue edge than on the tongue tip.8 This should be kept in mind when comparing gustatory results in healthy humans with those in patients with taste disorders.
This chapter describes common clinical taste tests, including questionnaires mainly in Europe and Japan.
Isolated taste disturbances are rare.
Patient History
In more than two-thirds of patients, a detailed patient history provides diagnostic evidence of disturbances of the olfactory and gustatory system.9 In most cases, patients suffering from smell and taste loss have isolated smell disorders or a combined olfactory and gustatory impairment rather than an isolated taste disturbance.10 Loss of smell presenting as taste loss, with normal findings on gustatory testing, is described as a “flavor disturbance.”10 Taste and flavor are generally used as synonyms. Retronasal olfaction has a strong influence on flavor perception and supports the need for a standardized psychophysical taste and smell test, rather than relying on the patients’ symptoms alone. Additionally, patients with anosmia have poorer sensitivity to all taste qualities than normosmics.11
Another aspect that is important to consider is innervation of the gustatory system. On both sides, different nerves (the chorda tympani for the anterior two-thirds of the tongue and the glossopharyngeal nerve for the posterior part of the tongue) provide a gustatory supply for the tongue; the petrosal nerve supplies the soft palate.12 Somatosensory supply is provided by the trigeminal nerve. Patients reporting numbness or tactile dysgeusia13 of the tongue are often unaware whether the trigeminal or the gustatory system is affected.
The following aspects need to be clarified in the patient history:
Surgery in the head and neck region14: middle ear,15,16 including stapes, surgery17; dental procedures; tonsillectomy18; oropharyngeal surgery; oncological therapy; procedures with lingual compression, such as surgery in general anesthesia; microlaryngoscopy18; tracheal intubation; and surgery of the nasal septum or paranasal cavities
Trauma
Medication: numerous drugs19 and toxic agents20 are known to cause taste disturbances
Infections: influenza-like infections of the upper respiratory airways may cause loss of smell and tend to lead to taste disturbances, but may also cause flavor disturbances; infections within the oral cavity, and also of the middle ear, may damage the gustatory system21
Burning mouth syndrome (primary and secondary): oral symptoms such as burning, tingling, pain, and dryness may affect the gustatory and trigeminal system of the oral cavity22
Systemic diseases: renal insufficiency, diabetes, diseases of the lower digestive tract, metabolic disorders, Sjögren syndrome23
Neurological or psychiatric diseases
Questionnaire
Taste disorders are less common than olfactory impairments and most actually reflect olfactory disorders. Taste disorders are classified as qualitative or quantitative.24 The latter includes ageusia (inability to taste) and hypogeusia (decreased ability to taste). Quantitative taste disorders are less frequent than qualitative taste disorders and can be total (all tastes involved), partial (affecting several tastes), or specific (only one or a select few tastes). Qualitative disorders, or dysgeusia, describe a distorted ability to taste, and include parageusia (unpleasant taste sensation, usually metallic, on taste stimulation) and phantogeusia (often permanent taste in the mouth, e.g., metallic or salty, for which no external stimulus can be found).
The accuracy of self-reporting in detecting taste disorders is poor.25 The relatively low prevalence of taste disorders in the general population, and the tendency to confuse olfactory loss with taste impairment, contributes to this low accuracy.25 However, questionnaires have been shown to be accurate in detecting patients with no taste disorders: patients who reported no problems with the ability to taste sweet, sour, bitter, and salty demonstrated normal taste sensitivity.
In contrast to qualitative taste disorders, quantitative taste disorders can be measured by psychophysical taste tests.
Psychophysical Taste Testing
Many psychophysical taste tests have been introduced to assess peripheral gustatory sensitivity. In general, two test tools can be distinguished. The first tool uses natural stimuli to measure the ability to taste sweet, sour, salty, bitter, and umami. This tool may be used as a whole-mouth method or as regional taste test to detect side differences.
The second tool uses electric currents applied to the tongue surface to elicit taste perceptions, and is named electrogustometry (EGM). EGM is an excellent method for measuring side differences in patients with taste disorders, provided that electric thresholds can be measured. It is assumed that electrogustometry also costimulates the trigeminal system. Recent studies revealed that gustatory and trigeminal fibers act together peripherally.26,27 In clinical practice, particularly in patients with symptoms such as numbness and burning of the tongue, it can be helpful to assess both the gustatory and the trigeminal system. Therefore, a somatosensory test will also be presented.
In this section, we describe widely accepted taste tests and categorize them as tests using natural chemicals (sweet, sour, salty, bitter, and umami), or those using non-natural stimuli (electric current). Chemical taste tests are further divided into whole-mouth tests or regional tests.
Taste Tests with Natural Chemicals
Whole-mouth Test
Three-drop Method
This taste test using liquid taste solutions was first introduced by Henkin et al28 A modified test procedure and normative data from the clinical taste test were presented in 2007 by Gudziol and Hummel.29 With a 10-µL pipette, a series of three drops of a liquid solution (one drop of tastant and two drops of solvent) is placed in a pseudorandomized order on the midline of the patient’s extended tongue. After swishing the drop in the closed mouth, the subject is asked to identify its taste quality. Four verbal gustatory descriptors are presented (sweet, sour, salty, and bitter). A multiple forced-choice procedure is used. Eight triplets are presented to the patient in ascending order, starting with the lowest concentration. Between each triplet presentation, rinsing the mouth with tap water is allowed. Steps of increased concentration are presented until the subject recognizes the taste solution correctly in two consecutive trials. Thus, scores for each taste quality range between 1 and 9. To obtain the overall gustatory test result, the scores of all taste qualities are added to provide a total score. Age- and gender-related normative data are available for each taste quality and for the total score (whole mouth).29
Tasting Tablets and Wafers
An alternative procedure is to use tablets of four tastants30 (sweet, sour, salty, and bitter) instead of liquid solutions. Four tasteless tablets and tablets with six concentrations of four tastants (sucrose, citric acid, sodium chloride, and caffeine) are presented. They are administered in ascending and pseudorandomized order, starting with the lowest concentration. The taste test provides a good test–retest reliability (r = 0.69, p < 0.001). Similar results were achieved with wafers.31 The flavored wafers can be used for a regional or whole-mouth taste test.31 The main advantage of taste tests with tablets or wafers over liquid solutions is the long shelf-life of the test kits. The main disadvantage is the lack of age- and gender-related normative data.
Regional Tests
Taste Strips
The taste strip test is a well-established quantitative test using impregnated taste strips. This test was first introduced in 2003.32 Filter paper strips (taste strips) with four concentrations of four tastants (sucrose, citric acid, sodium chloride, and quinine hydrochloride) are used ( Table 14.1 ). The strips are placed on the left or right side of the anterior third of the extended tongue. Before taste testing and between each application, the mouth is rinsed with water. Taste strips are presented in a randomized and ascending order, starting with the lowest concentration of each tastant and alternating the side of presentation. This procedure results in 32 trials (16 trials of each tongue side). Leaving the tongue extended, the patient has to identify the taste quality using gustatory descriptors (sweet, sour, salty, bitter) in a multiple forced-choice procedure. Besides side-related scores, overall gustatory sensitivity is calculated by adding up the scores of both sides to create a total score. With the taste strip method, a test–retest reliability of r = 0.68 was achieved (three-drop method r = 0.69).32 Age- and gender-related normative data for the taste strip test were presented in 2009.33 This test is useful in routine clinical practice. The aim of further studies is to add monosodium glutamate (umami taste) and to extend the validated test.
Filter Paper Disc Method
The filter paper disc (FPD) test ( Fig. 14.1 ) is a qualitative clinical tasting test developed by Tomita et al34 It measures four (sweet, sour, salty, and bitter) taste qualities at five different concentrations ( Table 14.1 ). The FPD test uses a round filter paper disc, 5 mm in diameter, impregnated with a drop of tastant solution. Sucrose is used for the sweet taste, sodium chloride for the salty taste, tartaric acid for the sour taste, and quinine hydrochloride for the bitter taste. The impregnated filter paper discs are placed at the same points on the tongue as for EGM. The taste solutions are presented in ascending order from the lowest concentration. The taste qualities and area, however, are presented in a randomized order. The bitter taste should be presented last. When different taste solutions are presented, the mouth needs to be rinsed with water. Patients have to identify the taste from a list of four descriptors (sweet, salty, sour, and bitter) while their tongue is extended. When patients cannot identify the taste clearly, they can choose “Undefined taste” or “No taste,” unlike in the taste strip or three-drop methods. The scores of the FPD test range from 1 to 5 for each taste quality, and the regions are lateralized. When the taste can be perceived in the third concentration, taste function is assessed as “within normal limits.” When the taste is not identified in the fifth concentration, the score is calculated as “6.” The average score for people over 60 years old is ~1 point higher than that for young people. The FPD score for the area innervated by the greater petrosal nerve is particularly influenced by age. When high scores are observed in this area, this finding might be of little significance. The age-related changes start at the age of 30 years.
Tomita et al34 demonstrated a significant correlation between EGM and FPD tests in an area innervated by the chorda tympani nerve in patients with facial palsy. Ellegård et al35 reported that there was a significant correlation between EGM threshold and the whole-mouth perception of all tastes, and it was no stronger for sour taste perception than for other taste qualities.
Related posts:
Burning Mouth Syndrome and Qualitative Taste and Smell Disorders
Taste Disorders
Providing Expert Opinion on Olfactory and Gustatory Disorders
Flavor: Interaction Between the Chemical Senses
Structural Imaging in Chemosensory Dysfunction
Functional Anatomy of the Gustatory System: From the Taste Papilla to the Gustatory Cortex
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