Gustation, better known as taste, is an important special sensation that affects diet and human pleasure. Loss of taste or altered taste can occur in many disorders of the peripheral or central nervous system, and gustatory testing allows us to better diagnose and manage these conditions. There is a complex interplay of gustatory, olfactory, somatosensory, and emotional stimuli in taste perception. This poses a difficult task for the clinician wishing to test gustation, and as such, a multidisciplinary approach must be adopted. A further issue is the lack of suitable objective tests. To date, there are no clinically useful objective tests of gustation. and the majority of tests rely on subjective measures of taste. This is unreliable in certain pathologies such as dementia and other neurodegenerative disease. There is also a lack of standardization for taste testing and no universally accepted method. This is not the case for olfactory function, where the University of Pennsylvania Smell Identification Test (UPSIT) and the Sniffin Sticks are regarded as robust and widely used tools for odor testing. Additionally, many patients report on loss of taste while they actually have an impairment of the sense of smell. Therefore gustatory testing is important clinically. Here we review the various methods of gustatory testing and their relevance in clinical practice.
The sensation of taste is complex, and although the gustatory pathway has been anatomically defined, the perception of flavor is not simply a product of this pathway alone. Chemicals known as tastants are detected by cells from taste buds, which lie within papillae. Fungiform papillae are the most numerous and are found in high concentration on the tongue's anterior two-thirds. Circumvallate papillae are located within the sulcus terminalis in the posterior third, and foliate papillae are located laterally. There are, however, taste buds throughout the entire oral cavity, the posterior wall of the pharynx, and the epiglottis. These signals are carried via cranial nerves VII, IX, and X to the gustatory area in the midbrain (the nucleus of the solitary tract) and thereafter to the thalamus in the central tegmental tract. The thalamus relays this information to the cortex, specifically the anterior insula frontal operculum. There is some crossing of fibers in the central tegmental tract and subsequent synapse in the ventral posteromedial nucleus of the thalamus.
A key principle in taste detection is that there are five basic tastes: sweet, salty, sour, bitter, and umami (more recently, it has been suggested that fat may also be a further basic taste). Each sensory cell is capable of detecting multiple tastes due to the presence of receptors to either glucose (sweet), sodium chloride (salty), Hions (sour), glutamate (umami), or bitter substances such as quinine or caffeine. Taste cells contain a variety of different receptors, allowing for different intensities of stimulation. Signal combinations from the five different basic qualities and varying levels of intensity create a vast palette of flavors.
This process is modulated by sensory input from cranial nerves I and V. Olfaction is an important part of taste, as demonstrated in wine tasting: opening the velum without swallowing allows volatile tastants to reach the olfactory epithelium, thus contributing to the perception of flavor. It is also well documented that the loss of olfactory function limits taste. Texture also informs how we perceive flavor; trigeminal pathways detect somatosensory information such as viscosity and hardness. Chemesthesis describes the sensations of stinging, heat, cooling, or tingling and is carried by fibers from cranial nerves V, IX, and X. There is some debate over which pathway is most important clinically. However, the majority of gustatory studies examine the anterior two-thirds of the tongue, supplied by the chorda tympani branch of the facial nerve (VII). Studies have also suggested that somatosensory trigeminal fibers anastomose with facial taste fibers peripherally, acting in concordance. This further blurs the lines of the gustatory pathway. We must also consider the emotional component of flavor perception. There is wide individual variability, driven by past experience, likes, and dislikes, allowing for cortical modulation of how taste is perceived. In one study, subjects described an altered taste of bland water after reading about events of moral transgression. Let us not forget the metaphorical connection between taste and emotion, as evidenced in everyday vernacular: “Home sweet home,” or being “dealt a bitter blow.”
Gustatory disorders are classified clinically as either quantitative or qualitative. The former comprises ageusia (a complete loss of taste), hypogeusia (diminished taste), or hypergeusia (a heightened sense of taste). Dysgeusia is a qualitative disorder where patients experience a distorted taste. This is further classified according to its relation to eating. If occurring while eating, it is described as a parageusia, but if permanently present or experienced in the absence of oral stimulation, we call it phantogeusia. A dysgeusia diagnosis is made following descriptions in the patient’s own words and cannot be reliably tested. However, quantitative gustatory disorders can be measured.