Disruptions in taste and smell has emerged as a Covid-19 symptom. However, these sensory impairments can occur for several reasons other than Covid-19. Sometimes these impairments are temporary and not serious, as may occur with the common cold or flu. At other times, the causes of loss of smell and taste can be long term or even permanent. This does not always mean that it is serious. For example, the sense of taste and smell diminishes with age. However, there are instances where the loss of taste and smell may be symptoms of serious and even life-threatening diseases.
How do taste and smell work?
Like all senses, taste and smell are possible through specialized receptors. The olfactory receptors in the nose is stimulated by airborne chemical that enter the nose. These receptors then send signals to the brain where it is perceived as odors. Similarly chemicals that dissolve in the saliva within the mouth stimulate the taste buds. These receptors send signals to the brain which is perceived as certain tastes.
It is important to remember that smell and taste are linked. Chemicals that are released during chewing may not only stimulate the taste buds but also the smell receptors in the nose. This can heighten the sense of taste. Therefore when smell is compromised, taste sensitivity is reduced and vice versa. Therefore a problem with one sense impacts the other although there may be no concomitant disorder of the other sense.
There are different medical terms that describe loss of taste or smell.
- Anosmia is the loss of smell. This term describes a total loss of the sense of smell (olfaction). Hyposmia refers to a partial loss of the sense of smell. Another term is dysosmia which refers to any distortion of the sense of smell.
- Ageusia is a complete loss of taste. Hypogeusia is the partial loss of the sense of taste. Dysgeusia refers to any distortion of the sense of taste. Sometimes there are other disruptions to the sense of taste like a foul taste which is known as cacogeusia.
Read more on loss of sensation.
Causes of Loss of Taste and Smell
There are various causes of sensory impairments. The loss of taste and smell may be partial or complete. The latter is less common. Sometimes one sense may be diminished while there is complete loss of the other sense. For example, diminished taste can occur with a total loss of smell, or vice versa. Given the COVID-19 pandemic, it is important to seek immediate medical attention when loss of taste and/or smell arises. COVID-19 has to be excluded as a possible cause.
The olfactory receptors in the nose and taste buds in the mouth decrease in number as we age. This causes a gradual reduction in taste and smell. It is more noticeable in seniors. Sweet and salty tastes are often reported to be impaired. The dulling of these senses may not cause any major disruption but sometimes other chronic diseases can further compromise smell and taste.
Allergies are a common cause of loss of smell or taste. Inflammation arises when the body’s immune system overreacts to otherwise harmless substances (allergens). The inflammation disrupts the function of the taste and smell receptors. Excessive mucus production in the nose also occurs which is often seen as a runny nose or nasal congestion. This causes an obstruction which limits the interaction between airborne chemicals and smell receptors thereby diminishing the sense of smell.
Infections, and particularly upper respiratory tract infections, are one of the common causes of dulled taste and smell. It is frequently seen with the common cold and seasonal flu. This smell and taste impairment may also occur with a sinus infection or a throat infection. These infections cause inflammation and excessive mucus production which disrupt smell and taste.
Always consult with a medical professional if you have symptoms like loss of smell and taste especially when there is a fever. Covid-19 infection can cause these symptoms.
Trauma can disrupt the functioning of smell receptors and taste buds. This may include physical trauma, chemical trauma or electromagnetic trauma as may occur with radiation therapy to the head or neck. Even injury to the head without involving the nose or mouth can result in loss of taste or smell. This may be seen with traumatic head injury which can occur with vehicle accidents, falls and a blow to the head from any cause.
Several drugs can affect the senses like taste and smell. This is usually a side effect which means it is an unintended consequence. It can occur with prescription drugs or over-the-counter (OTC) medication. Among the drugs that can cause the loss of taste and smell, certain antibiotics and anti-hypertensives are common drugs that are known to result in this side effect. However, various other drugs may also be responsible including some antipsychotic drugs.
Various neurological disorders can compromise the senses. These diseases may affect the proper functioning of receptors, the nerves that carry signals from receptors to the brain as well as the brain centers where sensory impulses are processed. From Alzheimer’s and Bell’s palsy to multiple sclerosis and Parkinson’s disease, the loss of taste and smell may be a symptom of these neurological conditions. It can also occur with dementia. Sensory loss may also occur with other diseases like diabetes where the elevated blood glucose levels can damage nerves.
Any obstruction particularly of the nasal passages can affect the sense of smell. This may also impair the sense of taste. Mucus congestion in the nose is a common cause seen with injury, infections and allergies involving the nose. Foreign bodies in the nose (more common with children), nasal polyps or a deviated septum may also cause an obstruction of the nasal passage and impact the sense of smell.
- Alcohol and other substances that can dull senses.
- Benign tumors that may affect nerve or brain function.
- Certain cancers and cancer treatment like chemotherapy or radiation therapy to the head and neck.
- Cigarette smoking, tobacco chewing and snuff (inhaled tobacco).
- Illicit drug use particularly with snorting substances like cocaine.
- Inhaling toxic substances including industrial glues and solvents.
- Nutritional deficiencies like vitamin A, B6, B12 and zinc deficiency.