Overview

Ageusia is a rare disorder characterised by the complete loss of the sense of taste, including the inability to detect salty, sweet, bitter, sour, and umami (savoury) flavours.⁴ This condition can arise from damage to the nerves involved in taste perception, infections affecting the mouth or brain, the use of specific medications, or as a side effect of treatments such as chemotherapy or radiotherapy.¹

Although ageusia is not typically life-threatening, it can lead to significant challenges. Taste plays a crucial role in detecting dissolved chemicals and warning the body of harmful substances, so losing this sense can result in discomfort and inconvenience.² It may also lead to psychological effects, such as decreased enjoyment of food, and physiological consequences like reduced appetite, unintended weight loss, or the need to stop medications prescribed for other health conditions.²

Neural pathway of taste

Taste bud cells

There are approximately 10,000 taste buds (50 to 70 micrometers in size) occurring in multiple places, including the papillae of the tongue (small bumps on the surface), palate, i.e., roof of the mouth, throat and mucosa of the epiglottis (a tissue found at the base of your tongue).¹ At the apical (topmost) point of taste bud cells, there are 50-100 sensory receptors within each cell, which can detect stimuli.^6,7

There are different types of taste receptors, including G-protein coupled receptors and ion channels, including basal cells, type I dark cells, Type III intermediate cells.^1,6

Type I cells, comprising about half of the taste bud, are thought to have a supportive role, including neurotransmitter clearance.¹ Type II cells make up roughly one-third of the taste bud and function as chemosensory receptors for sweet, umami, and bitter tastes.⁶ They achieve this by expressing specific G-protein coupled receptors (GPCRs): T1R for sweet and umami, and T2R for bitter.⁶ Typically, each Type II cell expresses only one class of receptor, allowing them to respond to a single taste quality, though some co-expression can enable detection of multiple stimuli within a taste bud.⁶ Type III cells, which account for 2–20% of the taste bud, detect sour stimuli through mechanisms that do not involve GPCRs.⁶

The identity of the cells responsible for detecting salty taste remains uncertain.⁶ Basal cells, also present within the taste bud, are thought to be immature cells that do not directly contribute to taste detection.¹

The cranial nerves

Ions (H⁺, Na⁺), carbohydrates, inorganic molecules, and other chemicals stimulate taste.² Food contains substances that can stimulate sensory receptors within the taste buds and convert them into electrochemical signals.² These are passed along to sensory nerves within the cranium. There are three cranial nerves (CN) involved in this neural pathway: the facial nerve (CN VII), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). Their cell bodies are found within the ganglia: the geniculate ganglion, the inferior (petrosal) ganglion and the nodose ganglion respectively.⁵

The facial nerve consists of the chorda tympani nerve which innervates most of the anterior part of your tongue, and the greater petrosal nerve, which innervates the taste buds within the palate.⁷ The glossopharyngeal nerve innervates a small posterior part of the tongue, and the vagal nerve transmits impulses from the back part of mouth and the topmost part of the oesophagus.⁷

Transmission to the brain

All three nerves — the facial, glossopharyngeal, and vagus — carry taste signals to a region in the brainstem called the nucleus solitarius, specifically to its gustatory (taste-related) part, which is located in the medulla.^2,5 The facial nerve enters the brainstem at the pontomedullary junction, while the glossopharyngeal and vagus nerves enter at the level of the rostral medulla. All three nerves synapse in the gustatory nucleus, which acts as the brain’s first processing center for taste input.⁵

From this point, second-order neurons transmit the signals to the ventral posteromedial nucleus of the thalamus (VPMpc) — a region that relays sensory information.^2,5 Then, third-order neurons carry the taste signals to higher brain areas that interpret taste, including the frontal operculum, anterior insular cortex, and part of the somatosensory cortex.⁵ These regions help us identify and differentiate between taste qualities.⁵

In addition, some taste signals are passed on to the orbitofrontal cortex, where they are combined with smell information. This integration forms the overall perception of flavour.⁵

What exactly causes ageusia?

Ageusia can occur as a consequence of many illnesses.¹ When the chemosensory components are altered including the tongue, saliva, oral mucosa, neural pathways and neurotransmitters, the function of taste changes.² Common conditions related to ageusia include:

• Diabetes
• Hypothyroidism
• Pernicious anaemia
• Nutritional deficiency such as zinc¹
• Guillain-Barré Syndrome: Damage from immune system may cause issues in the chorda tympani thus reducing signalling of taste pathways^3,5
• SjögrenSsyndrome: Autoimmune disorder where the lacrimal and salivary glands are damaged by the person’s immune system. This prevents stimulants from reaching the taste bud cells⁵

Here are some other ways ageusia may develop:

• Damage to cranial nerves as a result of trauma, e.g., skull injury or surgery, e.g., operation on cervical arteries can cause severe damage in one of the taste-related nerves affecting the neural pathway of taste¹
• Invasive medical procedures such as laryngoscopy may cause neuralgia or polyneuropathies¹
• Inflammation and injury in surrounding structures¹
• Medications including antibiotics and other drugs may cause ageusia through changes in salivation^1,5
• The slow decline of the sensitivity of taste receptors with old age

Diagnosis

In order to identify ageusia, a comprehensive review of a patient’s medical history is required.² Another thing of note is that usually issues brought forward relating to taste are usually caused by olfactory dysfunction.

Tests that are frequently taken to determine ageusia include:

• Electrogustometry: Transmitting weak electric currents to different areas of the taste buds²
• Chemogustometry: Judging taste sensitivity using chemical solutions²
• Head and neck imaging: To recognise any sort of trauma or fracture⁴
• Suprathreshold taste test: Examine nerves involved with taste by using strips to judge whether a person can identify different taste sensations⁵

FAQs

How common is ageusia?

Complete dysfunction of taste (ageusia) is highly uncommon, and occurs only at a percentage of 0.001%. Most perceptual loss of taste is actually due to a reduced sense of smell.^1,2

Can ageusia be treated?

In some cases, ageusia is not required to be treated and may eventually resolve.³ Generally, the focus of treatment is on treating the cause rather than the symptom itself.⁴ When there is structural trauma or operation in parts of the pathway resulting in ageusia, depending on the healing of the nerve, taste may be restored. In this case it has no definitive cure, and it is uncertain whether the condition will change. Zinc supplementation proves useful in conditions caused by zinc deficiency, and in patients who have had taste bud cells destroyed through radio or chemotherapy.³

A lot of treatment is focused on helping the affected person to eat smaller meals, changing the variety and textures of foods eaten, and avoiding cold or spicy food.³

When drugs are the main cause once the person has stopped using the drug or reduced their intake, the effect of ageusia may decrease after a period of time. When illnesses are the cause, usually recovery helps remove the signs of ageusia.⁴

Summary

Taste is perceived at the taste buds, which are found on the tongue and various parts of the mouth. These contain receptor cells: Type II cells, which detect sweet, bitter, and umami; and Type III cells, which respond to sour tastes. Signals from these receptors are transmitted by three cranial nerves which are the facial, glossopharyngeal, and vagus nerves.

These taste signals travel to the nucleus solitarius in the brainstem, where they first synapse. From there, they are relayed to the thalamus and then passed on to taste-processing areas in the brain which help identify and interpret taste.

Ageusia, the complete loss of taste, can occur when any part of this pathway is disrupted. Causes include damage to taste buds, nerve injury, reduced saliva, or side effects from certain medications. Conditions such as inflammation, oral infections, or burns can also interfere with taste signal transmission. While ageusia is not usually dangerous, it can impact appetite, nutrition, and mental well-being.