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.
Anosmia is the inability to smell. Some people have anosmia from birth, whereas others lose their ability to smell over time.
Sometimes people call anosmia smell blindness. The condition may be permanent or temporary.
Smell is a complex process that involves 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.
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 factors 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
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, researchers 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.
As people age, their sense of smell decreases. People may also lose the ability to discriminate between different smells.
Experts suggest that possible reasons 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 categories 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 damage this pathway include upper respiratory viral illnesses and toxin inhalation.
Upper respiratory infection may contribute to 43% of neurosensorial olfactory loss. Tumors can also cause neurosensorial anosmia.
Toxins that may contribute to anosmia include:
- illicit drugs
- toxic vapor
Some neurodegenerative diseases that may cause anosmia or some degree of loss of smell include:
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 suggest 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 help doctors diagnose anosmia better.
Diagnostic questionnaires may help 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 method to screen for anosmia.
Through this survey, doctors can arrange 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 matches the cause.
People with genetic disorders may want to look into treatment options, such as cell and genetic therapies.
People with post-traumatic olfactory disorders resulting from a head injury may also benefit from smell training.
Smell training involves 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 be 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 shown that topical intranasal steroids are effective in allergic rhinitis, especially when a person takes them with intranasal antihistamine.
Oral corticosteroids can be 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 improve anosmia in some people.
Some people living with anosmia report they have difficulty finding and receiving appropriate care for their condition.
Part of the issue is that some people living with anosmia have difficulty detecting, identifying, and distinguishing the condition.
People living with anosmia may experience 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 enjoyable 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 causes 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% 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 differences in the ability of the virus to attach to receptors in the nasal cavity.
This may dictate 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 help 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 typically 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 include 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.