Taste Loss, Partial (Hypogeusia) or Complete (Ageusia)
What is loss of taste?
Taste loss is the partial or complete impairment of the sense of taste. A partial loss is known as hypogeusia whereas a complete loss is known as ageusia. It is one type of taste disorder (dysgeusia). There may be several other abnormalities with the sense of taste which can be an indication of local problems in the mouth, neurological (nerve) disturbances that carry the signals to the brain or disorders in the brain that affect the taste centers which processes the signals. Since odor (smell), texture (touch) and temperature can also contribute to the taste perception, disorders in these senses may also compromise the normal sense of taste.
The sense of taste is one of five ways that a person perceives the environment, along with touch, smell, sight and hearing. It is facilitated by the taste buds on the tongue which can detect individual chemicals and perceive it as sour, sweet, salty, bitter and a type of savory taste (umami). Often tastes are a combination of two or more of these tastes. Not only does the sense of taste make eating a pleasurable experience, it also helps with regulating the appetite. The cravings for certain foods and preference for specific tastes may not only be a learned process or a cultural factor but also a way for the body to source nutrients that are needed. The sense of taste is sometimes seen as non-essential but it plays an integral role in regulating the proper nutritional intake to maintain health and life. When taste disorders arise, it can ultimately affect the appetite and the body’s ability to signal for specific nutrients thereby affecting proper nutrition.
Reasons for Taste Loss
Taste buds on the tongue are the main site for the specialized receptors that perceive different tastes. However, to a lesser extent the taste buds on the palate (roof of the mouth), throat (pharynx) and even as far down as the epiglottis and larynx also help with the perception of taste. It is important to note that there are several other structures and substances that facilitate the taste sensation apart from the taste buds. This includes :
- Saliva from the salivary glands
- Sensory nerves
- Taste centers in the brain
Therefore a problem in one or more of these structures or substances can compromise the normal sense of taste. In order to understand the reasons behind partial or complete taste loss, it is necessary to have a basic knowledge of how the human body is able to perceive tastes. The process is as follows :
- Food enters the mouth and substances within it are dissolved in saliva.
- This solution then enters the pores in the taste buds.
- It then makes contact with the tiny protrusions from the taste cells (gustatory receptors).
- Certain cells are more strongly stimulated by specific tastes.
- The membrane of the taste cells depolarizes and causes a release of neurotransmitters.
- These neurotransmitters stimulate nerve fibers in the mouth.
- Signals are then sent to the brain by the cranial nerves VII, IX and X.
- These signals either directly or indirectly pass to the brainstem, thalamus and cerebral cortex.
Taste loss is a state where a person may not be able to perceive the full extent of a flavor or detect any taste at all. The latter is less common. In many instances, these dysfunctions with taste are actually a problem with olfaction (smell) first and then a problem with the actual sensation of taste. It is further compounded by a disturbance is detecting the texture of the substance and even temperature plays a role in perceiving the full flavor of a substance. It is also known that tongue movements help to spread around the food over the tongue surface and thereby contributes to perceiving the full flavor as well. A problem with tongue movement can also impair the taste sensation.
Causes of Taste Loss
Taste loss causes can be divided between the problems in the mouth and that involving the nerves and brain. Although the nerves are also located in the mouth, it has been discussed separately from other mouth problems. Some causes can affect structures in the mouth, the nerves and brain simultaneously. Another important consideration when assessing the causes of taste hypogeusia or ageusia is the loss of smell, whether partial or complete and common conditions that affect the sense of smell.
- Poor oral hygiene which increases the chances of mouth infections and compromises dentition (health of the teeth).
- Gastroesophageal reflux disorder (GERD) where the stomach acid rises up into the esophagus and sometimes as high as the mouth possibly damaging the taste buds.
- Dry mouth (xerostomia) is mainly due to reduced saliva secretion and mouth breathing.
- Mouth infections due to viruses, bacteria or fungi which may affect the taste buds. This includes conditions like herpetic stomatitis and oral candidiasis (thrush).
- Oral cancer which is a malignant tumor which may cause taste loss particularly when it affects the tongue.
- Glossitis which in inflammation of the tongue.
- Radiation to the mouth, neck or head may damage taste buds.
- Salivary gland problems which leads to mouth dryness.
- Burns to the mouth or tongue specifically may injure the taste buds.
- Chemical damage to the tongue and taste buds associated with ingesting caustic substances, excessive alcohol and tobacco use. Stomach acid due to GERD may also be responsible.
- Tongue and mouth disorders like oral lichen planus and geographic tongue.
Nerves and Brain
- Transient ischemic attack (TIA) is a partial and temporary disruption of the blood flow to the brain.
- Stroke where the the impaired blood flow to the brain causes death of a small portion of the brain tissue.
- Traumatic head injury which may disrupt the taste centers in the brain.
- Brain tumors which may compress or destroy the taste centers.
- Nerve problems with the nerves responsible for taste including tumors, myelin sheath disorders, diabetic neuropathy, surgical damage or resection, compression or radiation damage.
- Age-related changes in senses.
- Hormonal changes with pregnancy and menopause.
- Nutritional deficiencies of vitamin B12, zinc, copper and nickel.
- Heavy metal poisoning.
- Liver cirrhosis.
- Kidney failure.