Anosmia is the inability or decreased ability to smell. It can be permanent or temporary, depending on the cause.

In this article, we review the definition, symptoms, and causes of anosmia, as well as how it may relate to COVID-19. We also discuss how doctors may treat this condition.

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Anosmia is the inability to smell. Some people haveTrusted Source anosmia from birth, whereas others lose their ability to smell over time.

Experts suggest that anosmia and hyposmia affect about 3–20%Trusted Source of the population. Hyposmia is a decreased ability to smell.

Sometimes people call anosmia smell blindness. The condition may be permanent or temporary.

Smell is a complex process that involvesTrusted Source communication between the brain and nose. When a person sniffs an odor, air travels into the nose, and the odor molecules attach to receptors on the nerve that sense smell, called the olfactory nerve.

These nerves line the olfactory epithelium, which is the tissue lining the nasal cavity. When odor molecules from the environment stimulate these nerves, they transmit signals to the brain.

The brain receives the olfactory information and processes it into a scent that a person can identify.

When people develop anosmia, they may notice that they can no longer smell identifiable odors.

On the other hand, people who were born with anosmia due to genetic differences may not recognize they have anosmia, because they never had the ability to smell.

Neurodegenerative diseases, such as Parkinson’s disease and Alzheimer’s disease, may causeTrusted Source a loss of the sense of smell.

If people living with neurodegenerative diseases recognize a loss of the sense of smell, they should seek medical evaluation as soon as possible. Sometimes anosmia is the earliest sign of neurodegenerative diseases.

When a person can no longer smell, they are unable to sense warning odors in foods and the environment. People living with anosmia may experience a lower quality of life related to social interactions, eating, and feelings of well-being.

Several factorsTrusted Source may contribute to anosmia, including:

  • inherited anosmia
  • older age
  • chronic sinonasal diseases, which affect the nasal cavity or the nasal sinuses
  • severe head injury
  • upper respiratory infections
  • neurodegenerative diseases

Inherited anosmia

Some people may inherit anosmia, which means they have it from birth. Healthcare professionals refer to anosmia not associated with any other condition as isolated congenital anosmia (ICA).

However, researchersTrusted Source have only identified two genes associated with ICA. Scientists need to do further research to find out more about the genetic basis for this condition.

Aging

As people age, their sense of smell decreases. People may also lose the ability to discriminate between different smells.

Experts suggest that possible reasonsTrusted Source some older adults may lose their sense of smell include:

  • decreased nerve fibers and receptors in the olfactory bulb, a part of the brain responsible for processing scent
  • loss of sensory cells in the nose
  • deterioration in the central nervous system cognitive processing functions

Chronic sinonasal diseases

People with chronic sinonasal diseases may have one of two categoriesTrusted Source of disease: conductive or neurosensorial.

Conductive sinonasal disease refers to conditions that affect airflow, such as:

People with neurosensorial sinonasal disease may have damage or dysfunction along the nerve pathway between the nose and brain. Some conditions that can damageTrusted Source this pathway include upper respiratory viral illnesses and toxin inhalation.

Upper respiratory infection may contribute to 43%Trusted Source of neurosensorial olfactory loss. Tumors can also cause neurosensorial anosmia.

Toxins that may contributeTrusted Source to anosmia include:

Neurodegenerative diseases

Some neurodegenerative diseases that may cause anosmia or some degree of loss of smell include:

Idiopathic anosmia

Sometimes doctors are unsure why a person developed anosmia. This is known as idiopathic anosmia.

General health exams do not typically include olfactory or smell testing. To detect whether a patient has anosmia, a doctor needs to rely on them to self-report a loss of or change in their ability to smell.

In general, experts suggestTrusted Source that self-reporting of the sense of smell is specific but not sensitive. This means that people may not be able to recognize anosmia if they experience it.

Improving health questionnaires may helpTrusted Source doctors diagnose anosmia better.

Diagnostic questionnaires may helpTrusted Source identify more people with anosmia by asking both younger people and older adults about any changes they have experienced in their smelling ability and the presence of phantom scent sensations.

The National Health and Nutrition Examination Survey (NHANES) self-reported olfactory function index offers a reliable methodTrusted Source to screen for anosmia.

Through this survey, doctors can arrangeTrusted Source further examination for a person based on what they report.

When deciding on the most appropriate treatment for a person living with anosmia, doctors must choose a therapy that matchesTrusted Source the cause.

People with genetic disorders may want to look into treatment optionsTrusted Source, such as cell and genetic therapies.

If a person developed anosmia following an infection, doctors may suggestTrusted Source the supplement zinc gluconateTrusted Source or smell training.

People with post-traumatic olfactory disorders resulting from a head injury may alsoTrusted Source benefit from smell training.

Smell training involvesTrusted Source sniffing four different odors intensely twice per day for several seconds over at least 4 months. People may benefit more from this therapy when they sniff a variety of odors on rotation over a prolonged training period.

Surgery or corticosteroid drugs may beTrusted Source the only available options for people with anosmia resulting from sinonasal disorders. Doctors may recommend corticosteroid drugs to manage anosmia, taken either into the nose or by mouth.

Researchers have only shownTrusted Source that topical intranasal steroids are effective in allergic rhinitis, especially when a person takes them with intranasal antihistamine.

Oral corticosteroids can beTrusted Source effective for anosmia, although the dose and duration are still controversial.

Doctors may find it challenging to predict whether surgical methods can help with anosmia. However, endoscopic sinus surgery might improveTrusted Source anosmia in some people.

Some cases of anosmia may also resolve spontaneously without treatment. This can occur in about 32–66%Trusted Source of patients with an upper respiratory tract infection.

Some people living with anosmia reportTrusted Source they have difficulty finding and receiving appropriate care for their condition.

Part of the issue is that some people living with anosmia have difficultyTrusted Source detecting, identifying, and distinguishing the condition.

People living with anosmia may experienceTrusted Source a lower quality of life. Losing the sense of smell can affect romantic and sexual intimacy because smelling one’s intimate partner is part of the sensual experience.

Mealtime may also be less enjoyableTrusted Source when a person cannot smell their food.

Anosmia can also be dangerous. Normally, a person’s sense of smell can inform them of environmental dangers, such as a gas leak or the presence of smoke. The ability to smell spoiled food or poisonous liquids can prevent a person from accidentally ingesting toxins.

People living with Alzheimer’s or Parkinson’s disease may develop severe complications from anosmia, including malnutrition, safety problems, and increased mortality risk.

The novel coronavirus responsible for the COVID-19 pandemic causesTrusted Source reduction of smell and taste in many patients.

These symptoms occur early in the disease and may sometimes be the only recognizable symptom in people with the SARS-CoV-2 virus.

In studies researchers did earlier in the COVID-19 pandemic, they reported that anosmia occurred in about 5%Trusted Source of individuals with a SARS-CoV-2 infection. However, further studies throughout the pandemic reported higher rates of anosmia.

A hypothesis for why some people with a SARS-CoV-2 infection develop anosmia is genetic differencesTrusted Source in the ability of the virus to attach to receptors in the nasal cavity.

This may dictateTrusted Source the infectivity and spreading of the virus. Researchers need to conduct further studies to investigate these findings.

If research can support this hypothesis, it may helpTrusted Source clinicians better describe which populations are more at risk of developing COVID-19, and may help healthcare experts manage the pandemic more effectively.

People with a SARS-CoV-2 infection typicallyTrusted Source regain their sense of taste and smell after several days or weeks, but for some, it can take much longer than that.

Anyone who recognizes a loss of or change in their sense of smell should contact a doctor. Depending on the cause of anosmia, doctors may select different treatments. These may includeTrusted Source cell therapy, drugs, smell training, or surgery.

If a person thinks they may have been exposed to the novel coronavirus or are experiencing symptoms of COVID-19, they should self-quarantine and call their local public health office for instructions.

Although losing the sense of smell, or developing anosmia, does not appear to be life threatening, the complications can be dangerous.

Smell allows people to recognize a dangerous environment or spoiled food, and people living with anosmia lose this ability.

Smell also provides pleasure. Smelling the scent of delicious food, fresh air of the countryside, or the sensual scents of intimate moments enhances a person’s everyday experiences. People who recognize they have lost their sense of smell may experience a lower quality of life.

Doctors have identified several causes of anosmia. Some causes can lead to permanent or temporary loss of smell. Choosing the most appropriate treatment requires an accurate diagnosis of the cause of anosmia.