Evaluation of taste disorders

Summary

Taste (gustation) is attributed to sensory information received from the oral cavity and oropharynx and is the perception accompanying oral intake. Taste comprises five basic taste qualities: sweet, bitter, salty, sour, and umami.[1] Umami (pleasant, good, desirable taste) is the perception of monosodium glutamate; its taste resembles that of chicken bouillon.[2]

Additional taste functions have been identified, including fat "taste" that may be mediated by receptor transduction and nonspecific transport across the cell membrane and spicy "sensation" (e.g., capsaicin, ginger) mediated by sensory afferent fibers.[3] A receptor resulting in kokumi taste that enhances other flavor sensations, and a "water" receptor have also been described.[4] [5] [6] [7]

However, the common sense of the word "taste" often means "flavor," which in turn is a composite of several (nongustatory) chemosensory afferents including taste, spice, texture, temperature (both mediated by the trigeminal nerve - i.e., cranial nerve V), vision and importantly, olfaction (more precisely, retro-olfaction) perceived while eating.[1] [8] [9] As such, a patient's complaints about taste loss do not always reflect the underlying pathology. Taste testing is mandatory to exclude a primary olfactory or trigeminal nerve pathology that manifests clinically to the patient as a problem with taste.

According to testing with taste strips, 5.3% of people considered as healthy have hypogeusia although very few have complete ageusia.[10] The evaluation of a patient presenting with taste dysfunction comprises the patient's history (including medical history, drug intake and dietary/nutritional elements), a detailed clinical exam (including oral exam), and investigations to determine the underlying etiology.[1] [11]

Applied anatomy and physiology

Taste-receptor cells are located in the taste buds, primarily within the oral cavity. The taste system is highly redundant, with bilateral distribution and bilateral transmission along multiple cranial nerves. There are 4 types of papillae: fungiform, foliate, circumvallate, and filiform. All, apart from filiform, have taste buds.[12]

Taste is a neurologic process with tastants delivered to neuroepithelial receptors on taste buds.[13]

G-protein coupled receptors are expressed in three types of taste receptor cells.[3] [4] [5]

Type I glial-like supportive cells: may be heterogeneous in function; recognizes low salt taste
Type II: recognizes sweet, umami and bitter tastes
Type III: recognizes sour taste.

Kokumi taste (enhancing continuity, thickness, and mouthfeel) is mediated by another G-protein coupled receptor, the calcium-sensing receptor.[5] [7]

The free fatty acid receptor is a G-protein coupled receptor in the oral cavity and the gastrointestinal tract that has a role in energy regulation and appetite and in taste/texture preferences.[14]

Afferent gustatory fibers run from the taste buds through gustatory pathways. The anterior two-thirds of the tongue is supplied by the chorda tympani, a branch of the facial nerve (cranial nerve VII). The gustatory fibers from the chorda tympani enter the brainstem via the nervus intermedius nerve (cranial nerve VII bis). At its distal end, the gustatory fibers with the chorda tympani join the lingual nerve, a part of the mandibular branch of the trigeminal nerve (cranial nerve V). Surgical procedures involving the posterior mandible may damage the lingual nerve, causing a transient or permanent loss of taste. The posterior third of the tongue is innervated by the lingual branch of the glossopharyngeal nerve (cranial nerve IX) and the base of the tongue and epiglottis by the vagus nerve (cranial nerve X). From the cranial nerves, the fibers converge in the nucleus tractus solitarius (NTS) in the brainstem. Leaving the NTS, the taste fibers still run ipsilaterally and cross partially at midbrain level, projecting in both thalami and insulae. Gustation may be altered at several levels within these pathways. Taste-receptor neurons, like olfactory-receptor neurons, are capable of regeneration.

Although it may be generally anticipated that taste complaints are related to the taste of food, some patients may report taste change in the mouth when no food is present. In some cases of taste complaint, food may taste normal and cover the abnormal taste reported when not eating. These may be reported as taste phantoms. It is important to distinguish the nature of the complaint.

Humans have at least 5 basic taste qualities (sweet, sour, salty, bitter, and umami) that can be distinguished by taste receptors. Current understanding is that the entire tongue is able to perceive each of the taste qualities.[13] Nevertheless, there are areas, such as the tip and the rear of the tongue, with higher global taste sensibilities. Some data suggest that there might be a gustotopy, similar to the cortical tonotopy related to hearing.[15] However, studies in humans concluded that taste identification involves temporal coding without well-defined, taste-specific "hot spots."[16]

Saliva

Taste is an important stimulus for production of saliva and, vice versa, saliva is necessary for the perception of taste and is not replaced with water intake while eating. Saliva is the natural oral solvent that dissolves food components to reach the taste receptors.[17] Taste is also influenced by salivary composition, as a specific quality must be above normal saliva concentration in order to be perceived. Epidermal growth factor and nerve growth factor in saliva may play a role in taste function. Thus, both saliva quantity and quality can influence gustatory sensation.[18] Three pairs of major salivary glands and hundreds of minor glands distributed throughout the oral and pharyngeal mucosa produce saliva, which contains minerals, enzymes, and immune globulin (IgA). The system is regulated by the autonomic sympathetic and parasympathetic pathways. Physiologic, pathologic, and iatrogenic processes may interfere with salivary output and thus induce dysgeusia.

Causes of taste change

Local causes can be due to local/regional conditions, including poor oral hygiene, oral/dental/sinus disease, oral infection (e.g., candidiasis), oropharyngeal pathosis, diet, and tobacco use.

Types of taste loss

There are several ways of classifying taste disorder. The method most commonly applied in clinical practice is to distinguish qualitative from quantitative taste disturbance as follows:[1]

Dysgeusia: the general terminology for any kind of taste disorder
Parageusia: qualitative taste impairment, which delineates a triggered taste distortion (e.g., bitter, metallic, or other taste perception occurs with eating/drinking)
Phantogeusia: qualitative taste impairment, which delineates a non-triggered, permanent or intermittent taste distortion, which includes several complaints, such as metallic taste or permanent bitter, sour, salty, or (even rarer) sweet taste
Hypogeusia: a quantitative taste disturbance producing reduced taste function
Ageusia: a quantitative taste disturbance producing absence of taste.

Any of these, except ageusia, can be lowered, enhanced, or unaffected by eating/drinking. These quantitative and qualitative disorders can occur together or alone (e.g., a patient with a bitter parageusia can have a normal or altered measured taste function).

Within the literature, "dysgeusia" is often used variably either as qualitative, quantitative, or general taste disorder, which can cause confusion.

Other classifications are based on the anatomic site of the lesion (e.g., peripheral nerve lesion, brainstem lesion, thalamic or fronto-orbital lesion).

Epidemiology

The epidemiology of taste disorders is unclear. Prevalence rates for taste disorders have been reported as ranging from 0.6% through 20% in the literature.[13] [19] [20] [21]

Taste disorders may not be noticed by the patient or the physician (e.g., in stroke) and, if testing is not performed, may be unrecognized.[22] The lack of recognition and the physiologic particularities concerning taste, such as the capacity of touch to be interpreted as taste perception, suggest that taste impairments are underreported. Data suggest that around 5% of the general population have lowered taste function without necessarily being bothered by it or recognizing it.[10] Taste change is necessarily primarily based upon patient reports, which may underestimate the epidemiology of taste changes. Taste changes are associated with aging.[23] [24]

During the covid-19 pandemic, 40% to 50% of people globally reported a change in the sense of smell and taste and these symptoms were often the strongest predictors of SARS-CoV-2 infection.[25] In one meta-analysis, around 5% of patients develop persistent taste dysfunction.[25]

Taste disorders are very common in head and neck cancer, with over 70% of patients reporting taste loss.[26] In some patients, full recovery does not occur and, in patients treated with radiation therapy, the umami taste takes longer to recover than other qualities.[27]

Taste changes occur with regional radiation therapy, and with systemic chemotherapy and targeted therapies in patients having solid-tumor cancer or stem cell transplant therapy.[28] [29] [30] Approximately 60% of patients receiving systemic chemotherapy will report some dysgeusia, which has been associated with the type of cytotoxic agent and the presence of oral mucositis.[31] Systemic chemotherapy may cause taste change that is usually reversed on cessation of treatment and may be related to direct toxicity and from secretion of medication in saliva.[30] [32] Alteration of sweet and salty perception in these patients may persist up to 3 years after cessation of cancer therapy.[33]

Citations

Key Articles

Hsieh JW, Daskalou D, Macario S, et al. How to manage taste disorders. Curr Otorhinolaryngol Rep. 2022;10(4):385-92.[Abstract][Full Text]
Wrobel BB, Leopold DA. Clinical assessment of patients with smell and taste disorders. Otolaryngol Clin North Am. 2004 Dec;37(6):1127-42.[Abstract][Full Text]
Thomas DC, Chablani D, Parekh S, et al. Dysgeusia: a review in the context of COVID-19. J Am Dent Assoc. 2022 Mar;153(3):251-64.[Abstract][Full Text]
Heckmann JG, Heckmann SM, Lang CJ, et al. Neurological aspects of taste disorders. Arch Neurol. 2003 May;60(5):667-71.[Abstract][Full Text]

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