Introduction

Ageusia is a medical condition characterised by the complete loss of the taste function of the tongue. This condition can lead to a loss of appetite and weight reduction. There are various causes of ageusia, including nerve damage, dietary deficiencies, and certain systemic conditions, such as hypothyroidism, diabetes mellitus, Sjögren syndrome and Crohn’s disease.¹ Many of these diseases have an autoimmune component to them. In autoimmune diseases, proteins called autoantibodies, or antibodies, are produced, signalling the body to attack itself.²

In this article, we will examine the intricate relationship between autoimmune diseases and ageusia, whether it is direct or indirect. Furthermore, we will consider the clinical implications, diagnostic challenges and potential therapeutic strategies to manage this symptom in patients with autoimmune diseases.

Understanding ageusia

The sense of taste is controlled by three main cranial nerves, namely the facial, the glossopharyngeal and the vagus nerves. Moreover, taste transduction and perception are protected by the overlapping innervation of taste buds. The olfactory and gustatory neurons often target the orbitofrontal cortex, which suggests that they play a critical role in the taste-related experience. Various processes can influence taste-mediating components, some of them are:³ 

• The exposure of oral taste receptors to atypical-tasting substances as a result of oral health issues, such as dentures
• Difficulties in the transport of taste stimulus due to severe infections or taste bud destruction as a result of trauma, tumour, or burn
• Damage to the neurological pathways innervating taste buds
• Tumours, epilepsy, strokes and systemic metabolic disturbances, like hypothyroidism and diabetes, can affect the gustatory system through various mechanisms 

It is important to differentiate between taste disorders, as ageusia refers to the complete loss of taste, whereas dysgeusia—more commonly reported—is an alteration or distortion in taste perception. Individuals with dysgeusia mostly complain of diminished ability to perceive or distinguish taste quality. Both can be a source of mental distress and have an impact on the quality of life, as the loss or alteration in the sense of taste can lead to changes in nutrition and have an impact on the individual’s social interactions as well. In geriatric populations, it can lead to anorexia and incontinence, and it can be a risk factor for diseases, such as cardiovascular and cerebrovascular stroke, amongst other diseases which require a specific diet.¹

Mechanisms of autoimmune disease

Several mechanisms could lead to autoimmune disease, which occurs when there is a loss of self-tolerance. Immune tolerance is important because the persistent activation of immune cells causes significant tissue damage.⁴ This loss of tolerance can be targeted at any bodily tissue. However, the tissue targets that could lead to ageusia mainly include salivary glands, mucosal tissues and the nervous system. For example, studies have shown that infections, such as COVID-19, can alter the composition of saliva, leading to temporary ageusia.⁵

The pathophysiology of ageusia depends on the factor leading to its cause, and its link to autoimmune diseases could be understudied; however, there are various autoimmune diseases that, either through the production of pro-inflammatory cytokines, generate an environment of chronic inflammation in the body that can lead to tissue-specific damage.¹

Specific autoimmune diseases associated with ageusia

Sjögren’s syndrome

It is a condition where the glands that produce fluid in the body, such as tears and saliva, stop working properly, leading to symptoms which include dry eyes and mouth, among others. In primary Sjögren syndrome (pSS), the function of the salivary glands is often considerably reduced; this is mainly due to the destruction of salivary glands, which causes a direct link to taste loss.⁶ 

Systemic Lupus Erythematosus (SLE) 

It is a long-term condition that causes joint pain, skin rashes and tiredness.  The causes are not fully understood, but the systemic inflammation of SLE can lead to oral ulcers, gum disease, and dry mouth, which can affect taste perception. Moreover, lupus-induced inflammation can lead to cranial nerve damage, particularly those involved in taste perception, the facial and glossopharyngeal nerves.⁷

Multiple Sclerosis (MS) 

MS is a chronic disease of the central nervous system which causes the breakdown of the protective covering of nerves, called myelin; eventually, this disease can cause permanent damage to the nerve fibres. The nerve damage that occurs could damage taste-related neural pathways, which would lead to an impaired taste signal transmission, leading to ageusia.⁸ 

Hashimoto’s Thyroiditis 

This is an autoimmune thyroiditis that is one of the most common causes of hypothyroidism. Some research suggests that more severe forms of hypothyroidism can lead to a noticeable decline in olfactory function.⁹ Moreover, about 50% of patients with hypothyroidism experience some form of dysgeusia, which could be improved after treatment with levothyroxine.^9,10

Coeliac disease 

Coeliac disease is a condition where the immune system attacks cells in the small intestine, which affects the gut’s ability to absorb nutrients.¹¹ Nutrient malabsorption properly could lead to taste changes; for example, vitamin B12 deficiency can lead to progressive neurologic damage, leading to ageusia.¹²

Diagnostic considerations

The doctor will review the medical history to understand if there are any conditions, recent procedures or medications that may be triggering the loss of taste. Moreover, other tests in addition to the physical examination can also be performed:¹ 

• Taste tests: these include the application of different solutions, which include sweet, sour, salty, or bitter flavours applied to the tongue to assess the sense of taste
• Identifying autoimmune etiologies: autoantibody testing, imaging, and biopsy
• Imaging tests: in case the loss of taste is related to a structural issue in the head or neck
• Laboratory tests: these include blood tests that can help identify the condition causing the loss of taste, such as a vitamin or mineral deficiency

Treatment 

Addressing the underlying autoimmune disorder

Common treatments to manage autoimmune disease symptoms include pain relievers, immunosuppressants, physical therapy, and occupational therapy. However, the complexity of autoimmune diseases makes ageusia in these patients more likely to require a multidisciplinary approach, as autoimmune diseases are already tremendously complex, requiring medical advice from rheumatologists, neurologists, otorhinolaryngologists, nutritionists and mental health professionals.^1,2

Symptomatic relief for ageusia

In most cases, restoring the sense of taste simply requires treating the underlying acute inflammatory condition, for example, an infection. Therefore, targeted therapy, such as antihistamines, decongestants or antibiotics, depending on the specific infection, can be effective. Daily habits could also help, like quitting smoking or improving oral hygiene to treat gum disease.¹ 

Summary

The cause of ageusia can include various external factors, such as medication and dietary deficiencies; however, there is a connection between some autoimmune diseases and ageusia, which makes the recognition of taste loss as a potential autoimmune symptom. Ongoing research is crucial to gain a deeper insight into the nature, frequency, duration, and causes of ageusia, as well as its effects on food intake and malnutrition in patients. Given the diagnostic challenges that gustatory disorders pose for clinicians, further studies are needed to enhance diagnostic methods. If you or someone you know is experiencing ageusia, or a change in the sense of taste, you should consult your healthcare practitioner to understand any underlying causes and possible treatments.